Xaudia

MOTM - RCA Selmer RL1

2012-01-26T10:37:00.000Z

The first "microphone of the month" for 2012 is the RSA RL1... also known as the Selmer RL1.

RSA Selmer RL1 microphones

So what's in a name? Some of these microphones have two manufacturers' names on them: RSA is proudly embossed on the aluminium body of the mic, whereas the switch plate reads 'Type RL1, H. Selmer & Co. Ltd., London, Made in England". Others have RSA on the switch plate too, although these seem to be less common, and I have only come across one example of this.

According to the Vintage Hofner website, Selmer took over the R.S. Amplifiers Ltd in 1947, and presumably these schizophrenic examples come from around that time, when the new owners were using up the cast casings. They are are made from a big slab cast aluminium, with a chrome grill. They are mounted by a yoke with a switch in the base, and the black paint with bare aluminium stripes gives it a very distinctive Bauhaus* / Art Deco appearance.

Inside the RCA Selmer RL1 microphone

The main body of the mic contains the ribbon motor assembly, hidden inside a cotton bag, and a large iron core transformer with a very thick double (parallel) primary winding. The secondary winding connects to a short length of cable, which goes off to the switch.

One thing to note about these mics is that many of them are designed for a very low impedance. The relatively thick piston style ribbon and low ratio transformer combine to give an output impedance of around 15 ohms (measured at 1kHz), and so they can seem very weak when plugged into a modern preamp. In this case a matching transformer is essential to get a decent level from the mic.

Also, these mics do not generally have a secure earth connections, and may hum unless one is provided. Fortunately, it is a simple job to add a terminal to the inside of the body.

Piston style corrugated ribbon.

In terms of sound, these have a very vintage tone and limited bandwidth, rolling off rapidly above around 4kHz - perfect for an old AM radio broadcast effect.

RSA RL1 frequency plot

The RL1 was eventually replaced by the fully Selmer-ised RL2, which used the same grill insert. The RL2 seems to be a very rare thing indeed.

Selmer RL2, from Vintage Hofner website

* The design school, rather than the goth band. Although either might apply.

Fender Telemaster build

2012-01-05T09:24:00.000Z

My holiday project was assembling this Telemaster guitar, from parts acquired that I have accumulated over the past six months or so.

'Fender-ish' Telemaster

The 'Telemaster' is an imagined product that Fender might have made in the 1960s or 70s, but never did - it is essentially a Jazzmaster body with Telecaster hardware. In this case I used a neck with a Strat-style headstock, and added a Fender-badged Bigsby vibrato, simply because I love the Bigsby sound.

The body was made and sprayed by John Manuel of Carlisle who did an amazing job with a transparent white which really shows the grain below. The neck and locking tuners were from Vanson guitars, both of whom trade on ebay. The neck is pretty nice quality, although I did need to rub it down with some very fine abrasive paper to remove a couple of rough spots in the varnish.

Bigsby with super-tremola upgrade

One thing that I don't love about Bigsbys is re-stringing them. The ball ends of the strings have a tendency to slip off the metal posts, which can be very frustrating, and one really needs four hands for the job - to hold the string at both ends, keep in under tension, and turn the tuner! Two things help with this - firstly locking tuners clamp the string at that end, and also I have fitted a Duesenberg super tremola kit, from Rockinger, which replaces the posts with a through-hole system. Much less frustrating!

The pickups are of course home made, and I tried to get as close to the original Telecaster specs as I could. Rather than using a bobbin/former to wind the coil, the core of the pickups were made using top and bottom 'flatwork', glued to Alnico rods. Enamelled wire was then wound directly onto the magnets, which avoids having a gap between the magnets and the wire which is inherent with a plastic former.

The bridge pickup was wound with 8500 turns of 42 gauge wire, and the bridge was reverse wound with around 6500 turns of 43 gauge. Once wound, the pickups are wrapped with string to protect the delicate coil, and then dipped in wax to stop microphonics. The whole thing is very solid, and rather heavy.

I did briefly consider adding a headstock decal to complete the fake Fender look, but decided that it was too good to give anyone else the credit - so it has a nice Xaudia badge. I am rather proud of it! :)

Happy 2012!

2012-01-01T08:00:00.000Z

2011 was a big year, with lots of changes.

Although we have been fixing ribbon mics for our own studio and a few customers for several years, 2011 was the first year that we went public and began to advertise the re-ribboning service. The mic repairs were moved to their own special room, with a dedicated testing chamber. The other big development was the acquisition of our Meteor coil winder, and the decision to do transformer repairs and re-winds in-house. This has vastly expanded the services that we can offer.

In 2011 we repaired some 186 microphones, along with a few guitars, amplifiers, reverbs, DI boxes and so on.

Xaudia - Distribution of microphones serviced in 2012, by manufacturer.

The various models of Reslosound mics have been the most popular brand - there are still a lot of these around kicking around in Europe, and we serviced 37 of these in 2011. As one would expect, there were also quite a few RCA ribbon mics - 23 passed through our hands this year.

We would like to thank all of our customers for helping to make this such an excellent and fun year, and we look forward to even more exciting things in 2012.

Happy New Year!
Stewart & Jane
Xaudia.com

Microphone of the month - Index of posts

2011-12-29T10:40:00.000Z

Here is a list of all the Microphone of the Month articles up to the end of 2011.

2010
Sep: Old Czech tube mic
Nov: RCA 77b
Dec: Philips ribbon mic

2011
Jan: Sony FV300 dynamic
Feb: Zephyr 30RA
Mar: Shaftesbury Velodyne
Apr: Framez ribbon mic
May: Melodium 42b
Jul & Aug: GEC BCS 2370 & 2373
Sep: Cadenza 'rocket' mic
Nov: Lustraphone ribbon mic
Dec: LEM 305 and 306

Looking back, there are a couple of gaps. October was swallowed up by installation work at York Maze, and June saw the arrival and installation of our coil winding facility, so I found little time for blogging.

I hope to be a little more efficient in 2012!

MOTM - LEM 305 ribbon mics

2011-12-23T12:47:00.000Z

December's Microphones of the Month are these impressive art deco-style LEM ribbon mics. LEM are a French manufacturer who still make reporter-style dynamic microphones. They used to make really cool looking ribbons!

LEM 305 / 306 microphones

These are all high impedance mics, and although they are substantially similar, there are some subtle differences between the models.

Inside the LEMs

The microphones all use large, 6 mm wide ribbon which sit between two steel pole-pieces. One of the mics arrived with fully corrugated ribbons, whereas the others had thicker, half-corrgated ribbons.

LEM 306 ribbon motor with holes in the pole-pieces.

The magnets are different - in the one, a pair of block magnets are connected at the rear by a welded steel plate, whereas two have a pair of horsehoe magnets. The smaller, more powerful magnets in the later models allow holes to be made in the pole-pieces, which (in theory at least) should give better high frequency response.

LEM 305 magnets

The other obvious difference between the microphones is the transformer, with the older models having a larger, iron core, whereas the newer ones have smaller mu-metal laminations, and these transformers are housed in screening cans.

Sadly all three transformers were faulty - the old rubber insulation around them had become brittle with time and the wires were broken. We were able to re-wind the two, but the third had suffered from an earlier bodged repair attempt, and had to be replaced completely. In this case we took the opportunity to give it a 300 ohm output impedance.

LEM transformers in metal screening cans

LEM transformer with larger core

Once serviced, these mics work well, with fair output for old ribbon mics, and a good vintage tone.

Frequency plots for LEM ribbon microphones, after servicing.

STC 4136 condenser mic - phantom power

2011-12-17T18:30:00.000Z

Here is a little curiosity. STC are mostly known for making high quality ribbon and dynamic mics, inclduing the much loved 4033 and 4038 models. But they also made condenser mics, and this nice little STC condenser mic came in for service recently....

It is model number 4136, and according to its specification sheet would originally have had its own power supply, which ran on either mains voltage or five 9 Volt batteries. Unfortunately the mic was bought without the power supply. The power would have been supplied to one pin of the XLR connector, ground to another, and (unbalanced) audio to the third. The capsule has a sintered glass, metallised backplate.

The battery supply suggests that 45V should be sufficient to power the mic. There was some corrosion on the circuit board from leaky capacitors, and without the original PSU, or knowledge of the operating voltage, it seemed sensible to create a balanced phantom powered circuit, loosely based on the Neumann KM84 - this is shown below along with the original circuit.

In practice, the conversion worked nicely, and despite its age and rough looks, the capsule gave a relatively even response, with a broadly cardioid pattern.

Measured frequency response plot for STC 4136

Strange Reslo microphone

2011-12-14T08:59:00.000Z

I found this ribbon / magnet / motor assembly inside a Reslo RBL microphone...

..which is very different from the usual Reslo motor design.

I have never seen this type, and I don't know if it is an attempt by Reslo to upgrade or modernise the old design, a transplant from another make of microphone, or a good quality DIY repair job. It remains a mystery!

When good magnets go bad

2011-12-10T18:23:00.001Z

This is the inside of a client's Avantone dual ribbon microphone, which was in pretty bad shape.

The metal plating on the neodynium magnets has suffered a catastrophic failure, and in doing so has been pushed out, crunching the ribbons and covering the whole assembly with powdered neodynium alloy. Not good.

The cause of the failure is still a mystery - perhaps it could be due to a process failure in a batch of magnets, or maybe the mic was exposed to an excessively humid environment.

I have seen other mics with signs of flaking on the magnets, but nothing this serious. Neodynes are still a relatively new magnet technology, and how they will stand up to years of studio use and abuse remains to be seen.

Happily, we were able to bring the mic back to life by replacing the magnets and of course re-ribboning the mic.

T-bone RB500 reribbon & upgrade

2011-12-07T09:30:00.001Z

Ronnie at Diesel Studios in Italy sent in a pair of T-bone RB500 ribbon mics for service and upgrade. Ronnie asked for some photos of the process, and I thought they would make a nice photo-blog.

Here is one of the mics on arrival...

Inside the body, there is further screening to protect the ribbon, and the transformer sits in a metal can.

One of the old ribbons.

This one is not so great!

New 1.8 micrometer ribbons....

The old transformer has primary DC resistance of 90 milliohms, and 30 ohms for the secondary. Those laminations are not very well packed.

Winding new transformers for the mics.

Potting the transformers in the wax bath keeps the coils and laminations from vibrating....

The finished transformers. We may as well re-use the metal can for some extra screening.

Back together again

Sony C38b stand mount repair

2011-12-07T07:47:00.001Z

Here's a fairly low-tech solution to a problem.

The Sony C38b is held by a yoke, and the stand mount contains a pair of rubber diaphragms which provide a little bit of shock absorption for the mic.

With time the rubber ion this one has perished, leaving the mic to flop around, rattle, and - worst of all - fall off the mic stand.

Here is how i fixed one with a strain relief rubber grommet, three rubber rings, and a jack plug bushing. The rubber rings are 23mm OD, 16 mm ID, and are the kind available (at least here in the UK) for fitting metal boxes for a ring mains, to stop the cables rubbing. The grommet measured 14 mm OD, 5 mm ID, with a recessed ridge diameter of 9 mm, and is cut off at the bottom to fit.

Firstly, unscrew the large grey knurled nut and take the mount apart. All the old rubber needs to be cut away. Then push the new strain relief grommet into the centre hole in the large grey knurled nut. Remove the bottom nut from the centre screw that is attached to the yoke, and push this into the centre of the grommet. It should look like this....

Then, pack the barrel of the mount with three rubber o-rings.

I then used a bushing from a Neutrik jack plug and inserted this into the centre.

Finally, push the re-rubbered yoke into the centre, and firmly hand-tighten the knurled nut. It should look like this.

The new assembly doesn't give as much 'bounce' as the original, but it holds well, doesn't rattle, and most importantly, it doesn't fall off!

SJT

"Big Al" - old German bottle mic

2011-12-04T10:36:00.001Z

Big old German bottle microphone (flaschenmikrofon)

This time-capsule condition, stunning bottle mic is a recent ebay find, but we know very little about it! So, if you recognise this one, or have any further information, please get in touch.

In looks, this is very much in the style of an RFT CM7049 or a Neumann CMV3, but doesn't match any of the models that I am aware of by those manufacturers. The mic stands around 320 mm tall by 80 mm diameter, and is beautifully machined from aluminium, so we'll call him 'Big Al'.

The bottom bell is secured by two thumbnuts, which make access to the tube and battery compartment very easy.

The capsule is held in place by a clamping ring with 12 screws, and the diaphragm looks to be either nickel, or some kind of metallised film. It is not possible to get the capsule out of the head without removing these screws - not something I really want to be doing. It is even possible to work out the backplate hole pattern from the dents in the diaphragm.

Bottle microphone capsule

The tube is a Telefunken DAF11 which dates back to the 1940s. I haven't met one of these before, but the datasheet is available at Frank's, and shows it to be a diode and pentode in the same shell, with a common heater & cathode.

DAF11 tube

The diode part is not used in this mic. The heater supply is a modest 1.2V at 50 mA, and is designed to run from a battery cell.

DAF11 bottle mic schematic

I traced out the circuit - the heater supply does indeed come from a battery, and there is a space inside the mic for a large cell. The switch on the top of the mic breaks the filament supply, saving battery power and (eventually) muting the mic, and there is a Neumann / RFT style indicator. in the top.

The capacitors in the rectangular metal cans are not labelled, but each can contains a pair of caps with a common negative terminal. On the bench, all four caps measure 1.0 ± 0.2 uF, and the different can sizes must reflect different voltage ratings.

There is no grid resistor present in the mic - either it has been removed for some reason, or the design relies on grid leak to set the bias.

Update 7/12/2011

We had a nice little discussion about this mic over at GroupDIY.

Stellar RM3 upgrades

2011-12-02T09:00:00.000Z

This Stellar RM3 ribbon microphone recently came into the Xaudia workshop for a transformer upgrade and new ribbon, which of course gives us a chance to poke around inside and take some photos.

Stellar RM3 with new Xaudia transformer and ribbon. The old transformer in in the foreground

The design closely follows the Speiden / B&O / Royer style of microphones - possibly a little too closely for comfort! Like the B&O mics, the ribbon frame slides out of the top, along with the transformer and lots of synthetic woolly packing. The body forms part of the magnetic return circuit, so it was good to see that this one was made of a magnetic steel, rather than brass or aluminium. In my opinion this is one of the better made of the budget ribbon mics.

As is commonly the case with budget microphones, the design is let down a little by the transformer, which measured 47 mΩ (milliohms) on the primary winding, and 90 Ω for the secondary, with a turns ratio of 1:54. Overall the mic gave a measured output impedance of about 650 Ω, of which over 200 ohms is pure DC resistance from the transformer. And in ribbon mics, resistance equals noise.

The new Xaudia 595/40 transformer (in this case a 1:40 ratio) has DC resistances of 0.02 Ω for the primary and 16 ohms for the secondary - much lower than the stock trannie. Although the lower transformer ratio would be expected drop the output level of -2.6 dB, the thinner, lighter ribbon gives an almost equal but opposite effect, and the output levels before and after are very similar. However, the lower DC resistance of the replacement transformer gives much lower noise, and the larger core allows higher sound pressure levels before the onset of saturation.

MOTM - Lustraphone (a.k.a. Grundig) ribbon mic.

2011-11-30T08:52:00.000Z

November's microphone of the month is a British long-format ribbon that was sold under the names "Lustraphone" and 'Grundig".

A Lustraphone-badged ribbon microphone in glorious brushed stainless steel finish

This mic was available in at least three different impedances, and I have come across 30, 200, and high impedance models. Unfortunately the badge often falls off so you don't always know what you are getting! Most of the models I have seen are finished in a bronze hammerite colour, although there is a deluxe low impedance model which has a gorgeous brushed stainless steel finish. Despite the different badges and finishes, the mics are exactly the same on the inside (transformer aside).

Lustraphone ribbon microphones on the bench

One of the ribbon clamps sits on a spring-tensioned screw thread, which allows fine adjustment of the ribbon tension. This makes tuning the ribbon very straight-forward, and allows the owner a little bit of grace if the ribbon becomes a little stretched over time. This feature should probably be mandatory on all ribbonmics!

Grundig badged lustraphone mic, opened up for service

The magnetic field is supplied by a pair of horseshoe magnets (which unfortunately sometimes age with time, losing their strength). Connection to the rest of the world is made by a balanced three-pin paxolin plug, which are hard to come by now. The middle pin is ground, with the audio on the outer two pins.

Rear connector and original plug.

Fortunately, a male XLR connector can be modified to fit by slicing off part of the barrel.

XLR connector modified to fit the mic.

The long ribbon and motor design gives these mics a full bottom end and a pronounced proximity effect. Here are the frequency plots for three of these mics that we have serviced recently:

Lustraphone ribbon mic frequency plots.

(Thanks to Mark Stevens for additional information).

Update 23/1/12. These microphones were also sold under the brand Pamphonic. One appeared recently on ebay:

The shape of things to come....

2011-11-28T20:09:00.001Z

Here's a sneak preview....

Can you tell what it is yet? :)

Ed Laurie with a Melodium 42b at Kore Studios

2011-11-26T07:41:00.001Z

It is very gratifying to know that the mics we service and repair go out and get used to make great recordings.

Of course it is not just the microphone - to make something special you also need good songwriting, a strong performance, and an engineer that knows what to do.
Here we have it all...

This beautiful song by Ed Laurie was recorded by George Apsion at Kore Studios.

George and Kore are good customers of ours, and we recently serviced a pair of Melodium 42bs and a little Cadenza microphone for them. The vocals on the video were recorded using one of the Melodiums, running through an Xaudia impedance matching transformer box. A Neumann U67 was used for the guitar amp, and a pair of AKG 414s as room mics, all into some vintage Neve preamps. I think it sounds stunning!

Ed has a new album out in January, and is currently out on the road.

The Devil's Own Multi-meter

2011-11-25T20:33:00.001Z

I used to own a multi-meter, which I inherited from my Grandfather sometime back in the early 1990's. It was a nothing fancy, but had a function for quickly measuring the HFE value, which I found very useful.

Said meter died a few months ago. My newer multimeter lacked this function, so I bought a cheap meter on ebay to fill in the gap.

But it is the devil's multimeter. Whenever I turn it on, the display briefly flashes up the number '666' before reading zero. Here's a video....

Nothing is attached, and the number comes up regardless of the function selected.

Spooky!

Halloween again

2011-10-31T12:27:00.000Z

Once again Xaudia spent most of October at York Maze, getting ready for Halloween. The Maze has two haunted houses, tractor rides through the haunted fields of corn, and pig racing. All of which need sound systems. Here's the Hearse, before getting a shiny new paint job....

These two goats took great interest in the wiring of a PA system for the pig racing. Unfortunately by the end of the season they had eaten through some of the cable, causing a couple of speakers to go down. The goats were fine.

There were also owls, including Florence, a beautiful Eagle owl.

Anyway, we survived Halloween, so back to the world of microphones for another year!

Purple Cauliflower Chemistry in the Kitchen

2011-10-02T21:35:00.000+01:00

And now for something completely different...

Purple Cauliflower

We found some beautiful purple cauliflower at a farm shop today. I have boiled this stuff in the past and had trouble keeping the intense colour when cooked. This time I tried steaming it, and the colour stayed put. Having studied physical chemistry (some years ago now), I asked myself why this should be the case.

Cauliflower steaming water (left), and with two drops of vinegar (right)

I noticed that the cooking water in the pan had turned a rather delightful shade of emerald green, which at first seems odd. Recalling that organic colouring molecules often have pH dependence, I added two drops of vinegar to the liquid to acidify it - within ten seconds the colour had switched to pink-purple.

Our kettle here tends to coat with limescale pretty quickly here - there is certainly calcium carbonate dissolved in our water supply, which is basic (alkali) in character. So, the dye is green in alkali conditions, and pink/purple when acidic.

So, if one wants to keep the nice purple colour, we need to keep the cauliflower acidic whilst it is cooking,. Steaming it, or addition of vinegar or lemon juice to the boiling water would seem to be the way to go. Although the latter would make it taste of vinegar.

This may well apply to other vegetables!

MOTM - Cadenza Rocket ribbon mic

2011-09-30T22:19:00.000+01:00

Sometimes I see patterns or trends in what arrives on the bench. A year ago it was RCA 74s, and in spring 2011 it was Electrovoice ribbon mics. And then the summer brought Melodium 42bs. Of course these are just statistical anomalies or 'blips' in the random noise of what my customers send me to repair, but it does at least suggest ideas for the regular 'Microphone of the Month' column.

Cadenza microphone set with box, documents and stand

This month these little Cadenza ribbon microphones are in vogue - these were made by Simon SoundService Ltd in London, and were designed by Eric Tomson, Stanley Kelly Peter Bell. The mics have a very 1950s styling, and are often called 'rocket' mics. However, having spoken to a couple of customers, the consensus was that many of these microphones no longer sound as good as they should - or at least as good as they look. The complaints are that they are noisy, with low output.

The Cadenzas have dual impedance outputs, and can be wired for either 30 ohm or high impedance (80 KΩ) output by changing the wiring at the connector. Neither option is really ideal for modern studio use, where something between 200 and 600 ohms is much more common for a mic output.

The ribbon itself is slightly unusual in that it has a fixed or nodal point half way along, where the ribbon is glued to an insulated support. This may well have been designed to reduce the likelihood of the ribbon being stretched, and one can imagine this either as two ribbons in series, or like a guitar string where the octave harmonic has been struck.

Cadenza mic transformer under the knife

From a technician's viewpoint (i.e., my opinion :p), there are a some weak points in the design which all relate to the transformer. Firstly, the ribbon clamps are connected to the transformer simply by winding the wire under a nut and tightening it - really not a reliable long term approach. Secondly, the transformer wires are extremely thin, and half a century later the insulation becomes brittle and tends to break, with disastrous results. It is more common practice to use thick gauge wire for the fly-leads to the primary, to keep resistance and noise to a minimum. And finally, as these thin wires become old and oxidised, the mics become noisy.

This all points to a transformer upgrade or rewind. However, it seems that Simon Sound Services Ltd. did a better job of encapsulating the transformer than they did of connecting it to the ribbon. The tranny is glued inside a mu-metal can with a hard, clear resin that resists removal. It was necessary cut the metal shell in half and then to soak the resin in dichloromethane for 48 hours to remove the resin. This allows removal of the laminations and a better inspection of the transformer windings.

Cadenza windings - primary (left),
and 30 ohm output (right)

The outer high impedance winding is a very fine gauge (approx. 0.07 mm), below which is a single layer of 0.4 mm wire which makes up the primary. The innermost winding is the 30 ohm output, which is approximately 0.2 mm in diameter. Unfortunately the solvent also damaged the bobbin, and and a replacement was found to make a new transformer using the vintage laminations.

The new transformer has a single 600 ohm output, with thicker gauge wires to give lower resistance. It took some effort but the result is a higher output with a lower noise floor, and the mic is much more usable. I'll post back with a 'before' and 'after' frequency plot for comparison.

The manual for the Cadenza mic is here.

Reslo transformers vs the WEE monster

2011-09-28T10:46:00.001+01:00

This little dinosaur sculpture was made out of waste winding coils, stripped mostly from old Reslo and other microphone transformers. :)

So why are we stripping transformer coils?

Some older ribbon mics were originally wound for 30 or 50 ohm output impedance, and tend to give a low output level when connected to modern recording equipment.

Many of these low impedance mics, including Reslo and Tannoy ribbons, can be rewound for a modern 200 or 600 ohm input, raising the output to a more useable level, and avoiding noise from having to crank up the preamps.

Bobbin from Reslo transformer with secondary winding removed.

The old Relso 30/50 ohm transformers have an inner (primary) winding consisting of just 12 or 13 turns of thick (0.8 mm) enamelled wire, and a secondary winding of 152 turns of 0.4 mm wire. The thick wire of the inner winding ensures that the primary resistance is low, which keeps noise to a minimum.

Reslo transformer rewound for 600 ohm output

Re-winding the transformer involves removing the outer winding from the original transformer and replacing it with sufficient turns of a thinner gauge to reach the desired turns ratio and output impedance. Usually the original primary winding can be kept in place. The transformer is then reassembled and dipped in wax to fix the windings and lams in place.

This makes the microphone much more usable in a modern studio - transformers can be would for 250Ω, 600Ω or any other desired output impedance.

Syncron AU7a revisited - Phantom power

2011-09-21T07:31:00.003+01:00

A while back I wrote about the Syncron AU7a FET condenser microphones, which are sometimes badged as the Fairchild F22.

In that post I had sketched out the schematic. I have since converted one for a customer to run on phantom power, and spotted a glaring error in the schematic. Here is the revised version...

The transistor is of course a P-channel JFET, and the battery polarity is reversed, giving a positive ground. The batteries are switched off when the plug is disconnected, and the routing through the plug makes tracing a little tricky - that was my excuse anyway.

All of this means that some small modifications are needed for phantom power use, because negative ground is by far easier to implement. Using an N-channel JFET makes things much more straightforward - something like this...

The 'adjust' resistor is tweaked for best response to a sine wave applied across the head amplifier, and in this case the result was around 1kΩ. JFETs can vary quite a lot, and it is sensible to adjust this individually for each mic.

I built a small breakout board to supply the required voltages from the phantom power. The board fits neatly in the battery compartment.

The "110K" is again adjusted on the bench to ensure that the voltage is correct under load.

There is one more thing to note - now we have switched to negative-ground and an n-channel device, the output cap needs to be flipped round.

Here's a measured frequency response plot for the modified mic (the dips at around 150 Hz and 600 Hz are likely to be room modes)...

The microphone works perfectly, and it is nice to hear one brought back to life after all these years!

MOTM: GEC BCS2370 & 2373 ribbon mics

2011-08-05T08:35:00.009+01:00

GEC BCS 2373 and 2370 microphones

This pair of rather handsome ribbon microphones by General Electrical Company are our microphones of the month for July & August.

The mic on the left is labelled with the model number BCS2373, and was the 'studio' model, with a single layer mesh grill, and thumb screw terminals. Like many early studio ribbons, this one has a 30 ohm output impedance.

On the right is the 2370, with a more robust housing and a curved body. These were probably used as 'lip' microphones for sports commentary and broadcasting in noisier environments. Indeed, one of our customers sent in a very nice example that came complete with its original handle.

GEC 2370 with handle

These mics were available with different output transformers for different applications. I've seen two examples of the 2370 - one had a 10K ohm output, the other measured 600 ohms impedance.

Although they look rather different from the outside, they are twins under the skin - both have identical motor assemblies, with cylindrical pole pieces attached to a large horseshoe magnet, held in place by the magnetic field along The design of the microphone is described in this patent from 1947. All of the GECs that I've seen have held their magnetic field well over time. The only real differences between these mics are the output transformer and the body.

As usual, after all these years they benefit from a good clean and a new ribbon. Particularly as this one arrived with a ribbon made from a fag-packet! Both mics are now working well and should provide some good service for years to come.

* Thanks to Santiago Ramos for additional information.

Mullard mustard capacitors

2011-07-26T21:48:00.000+01:00

Mullard mustard capacitors

:-D

Falling Spikes!

2011-07-01T09:31:00.003+01:00

Today was spent recording three tracks with The Falling Spikes, a seven piece newgaze outfit, with hints of Mogwai, the Beta Band and Spiritualised.

Falling Spikes - Corbett, Moz, Alex, green lizard, orange robot, Ray

The Falling Spikes are Corbett (bass), Moz, (keyboards), Alex (guitars & vocals), Ray (drums), Mikey (guitars), Maria (percussion and vocals), and Chris (percussion and vocals). With Stew and James pushing the buttons, and Andy joining us in the post-meridiem, we had a record ten souls in the Xaudia studio.

... Ray, lots of guitars, Mikey, Maria, Chris

The mic lineup included lots of ribbons - we used a Melodium 42b on keyboards, RCA 44 on guitars, and the Cowley and Tripp 'Naked Eye' Roswellites on both snare and Alex's guitar. The room mic is a Josephson C720, set up for stereo. The Melodium came out again for the main vocals, and running it through the Altec vari-mu compressor gave a smooth rich tone that works well with the reverb bath that is beginning to take shape.

Mixing takes place next week - all is good!

Coil winder 3 - guitar pickups

2011-06-30T07:41:00.000+01:00

There is no 'Mic of the Month' article this month - normal service will resume in July. You can read the archive HERE.

Since the Meteor ME307 coil winder arrived, things have been rather hectic and we've been busy getting it working, and having some fun winding transformers, inductors and pickups. The friction wheel / clutch that drives the lateral action was worn, but we were able to get a good used replacement part from Poland, and everything is working well.

Re-wound ribbon mic transformers

Hungry? P-p-p-pickup a P90...

James from Hungry Ghosts came down to the Xaudia workshop for an afternoon, which we spent rolling our own guitar pickups. James wanted a pair of P90s to replace the stacked humbuckers on his 'Vintage' guitar, and I was still playing about with options for my Reverend Slingshot.

P90s are pretty simple things. We made the base plates by drilling and tapping a bar of 5mm square mild steel, and two "Alnico-5" magnets were glued to this, side by side, like this.

We wound four bobbins, at 7000, 8000, 8500 and 9500 turns, reverse winding some so that they could combine to give a humbucking effect....

Dipping the magnets in hot wax prevents microphonics, and we rigged up a 'redneck' wax potting system using a £10 slow cooker from Tesco. This works really well and I'll describe it in a separate blog post. That done the pickups could be assembled and wired.

I found some nice deep headed screws which made perfect pole pieces for the pickups. Here is the finished P90...

And the new pickups imbued James' guitar with a spooky pink glow!

OK, we made the last bit up but they do sound astonishingly good. 9500 is quite a lot of turns for a P90, but the pickups all had plenty of clarity and no shortage of bottom end either.

Redneck wax potting bath

2011-06-29T22:25:00.002+01:00

We needed a better way of wax potting our pickups and transformers. Here's what we did on a budget...

Redneck wax potting bath for guitar pickups

It's a hacked 300 Watt slow cooker, acquired from Tesco for the grand sum of £10, which contains an inner ceramic liner and holds about 3 litres. The probe is simply a cook's thermometer (£14 from Barnitts of York).

It takes a little while to warm up, but sits at about 90 degrees all day, which is just about perfect for potting. The temperature can be further tweaked by running the bath on a variac.
Here's a newly dipped transformer coming out of the soup - a mix of bees wax and parafin wax...

Update: 1 December 2011

The batteries in the cook's thermometer died, and we really wanted to regulate the temperature and avoid the possibility of overheating the wax, so I built a little temperature controller into an old power supply box. The digital controller was 'new old stock' at £30 from ebay, and a 5 amp solid state relay and resistance thermometer came from Farnell to complete the setup. It works very well!

Coil winding part 2

2011-06-12T08:58:00.002+01:00

The new ME307 coil winder is up and running, and we're jumping straight in to winding some guitar pickups. The idea was that this should be an easy place to start as they can be made with a single winding, and bobbins, wire etc. are all readily available. There is a big pickup winding community in interweb-land, so help and advice should be available.

The winder in action, with the pickup mounted on a metal plate that we made for the job...

Pickups typically use circa 42 AWG wire (0.0633 mm), which is a very fine gauge and requires care whilst winding and soldering to avoid breakages.

The wire tensioners that came with the winder were a little out of calibration, but with a little practice at setting the correct tension and winding speed, we could easily get 7000 turns onto a P90 bobbin with room to spare. It's probably possible to get 8000 or 9000 turns with practice, to wind a really hot pickup. Here's the first winding from the Meteor....

Looks nice! The next step will be to rewind some of the broken microphone transformers that we have accumulated over the past few years. But before that we need to mend the clutch plate, which is slipping.

One Note virtual piano project

2011-06-09T22:53:00.002+01:00

Christian Borngraeber has launched this brilliant project...

He is asking for help to record single notes from a huge number of pianos, and will combine them into a single virtual instrument made up of all the little piano notes.

What will it sound like? Well, we don't know yet but probably a bit like a piano.

You can email contributions in to his website, so go and grab a microphone, pick your favourite note, and record away. And there's a free party pack for everyone who helps.

Here's how you can get involved.

Our New Coil Winder - Meteor ME307

2011-06-07T09:15:00.002+01:00

The 'new'* Meteor ME307 coil winder has just arrived at the Xaudia workshop, after a 500 mile round trip to collect it...

We are very excited about this as it will expand our repair abilities and let us rebuild and rewind broken microphone transformers and guitar pickups. It has a counter to set the number of turns, foot actuated motor control, and of course the all important set of tensioners to allow the wire to be spooled at the correct tension.

The coil winder was probably built sometime in the 80's and is built like a tank, with some beautiful Swiss engineering inside. Everything needed a little lubrication and a good clean and, although the machine has clearly seen some years of use, everything is still working perfectly.

ME307 internal gears and drive train.

We hope to be doing our first windings later this week...

Direct Inject!

2011-05-28T18:50:00.000+01:00

My friends in The Savoy Ballroom needed some stage DI boxes for their numerous acoustic instruments. They wanted to be able to mute the signal whilst tuning up or switching instruments, so I build these nice little boxes for them.

They are based on Bo Hansen's easy to make design, but with added bells and whistles - well, a switch, a lamp and an extra output.

Job done - now back to recording!

Transformer assembly line!

2011-05-22T11:12:00.003+01:00

This week we have been building lots of little transformer moxes to match vintage 30 ohm ribbon mics to modern mic preamps. Correct impedance matching can deliver a +12 dB increase in level without noise penalty or loss of frequency response, which is welcome for many older microphones. Here is how a Reslo RV microphone behaves with and without the transformer:

Frequency response plots for Reslo mic with and without an impedance matching transformer

They are suitable for many old microphones including the following:

Reslo RBL and RB 30 ohm models
Melodium 42b
Grampian ribbon mics (including GR1/L and GR2/L)
Cadenza ribbon microphones (wired for low-Z use)
RSA & Selmer ribbon mics
STC 4033
Altec and Western Electric ribbon microphones

More information.

Mic of the month - Melodium 42b

2011-05-15T22:30:00.002+01:00

Our Microphone of the Month for May is the magnificent Melodium 42B ribbon microphone.

Evolving Melodium 42b - serial no. in the 1700s, 4300s and 6600s (L to R)

The Melodium 42B was conceived as a French alternative to the RCA 44 family of microphones, and is in its own way every bit as good as the more celebrated RCA mics. At 32 cm high and 14 cm wide, and weighing 2.65 Kg, the 42b is a huge microphone with a large ribbon (68 mm long by 4.2 mm wide) and big powerful magnets*, to give a strong output with low noise.

We have been lucky enough to have four of these through the workshop, and have had the opportunity to observe some for the finer details that have evolved during the production of these beauties. Over the years the grill of the 42b has been refined, with the holes becoming larger, and a solid unperforated band appearing across the bottom of the microphone. Later microphones have a three pin connector at the rear, whist early models are hard-wired.

The magnet structure has also changed over time - earlier models have U-shaped magnets above and below, whereas the later microphones have four block magnets, arranged in pairs and connected by metal plates to complete the magnetic circuit.
The photo on the left shows the later style of magnets. The transformer and inductor are in the circular metal can below. Some models have a square can, but the transformer and inductor inside are similar, at least in the ones we studied. The transformer has a ratio of 1:14, giving an output impedance of 50 ohms**

There is also a facility to adjust the ribbon tension, which should be mandatory on all ribbon mics!

Here's a short summary of the changes with serial number:

1700 - Small grill holes, no connector, U-magnets, circular transformer case
3400 - Small grill holes, connector, U-magnets (unmarked), rectangular transformer case
3700 - Big grill holes, connector, U magnets, rectangular transformer case
4300 - Big grill holes, connector, U magnets, rectangular transformer case
6100 - Big grill holes, connector, U magnets (North is marked), rectangular transformer case
6600 - Big grill holes, connector, block magnets, circular transformer case

The 42b has a three-position switch on the front which is used to select "Speaker", "Voix", and "Musique" modes. In the first two positions an inductor is switched in parallel with the output transformer which causes the lower frequencies to be rolled off.

Above is a frequency plot that I recorded in our anechoic mic testing chamber (well, more of a walk-in cupboard really!). This was recorded at 40 cm distance from the sound source, which is a concentric full range speaker, using a swept sine wave technique.

The bottom end boost due to proximity effect is pronounced, even at this distance, and the effect on the sound is very musical when capturing acoustic instruments. Of course sometimes you don't want or need the proximity boost, and the switch conveniently corrects for this at speaking and singing distances.

If you are lucky enough to come across an old Melodium 42b but are disappointed with the sound, it may well have an oxidised ribbon and will need cleaning. The strong magnets are prone to attracting little shards of iron, which stick to the magnets and interfere with free motion of the ribbon.

* It is worth noting that the magnets on these microphones can fade with time - a healthy 42b should have a magentic field between the pole pieces of around 4000 Gauss. One of our microphones measured at just 1500 gauss and it was necessary to replace the magnets with suitable modern alternatives. If you have a Melodium with a weak output it may be worth having the field measured and the ribbon checked. Xaudia can of course help with all those things.

** For optimum results with modern equipment a matching transformer is recommended.

Here are some Melodium links on the web:

Mattéo and Axel at Infernale Machine studios in Beziers, France are both Melodium enthusiasts and there are some good photos and some information in French about servicing and cleaning these microphones on their website.

George at Kore Studios is also a fan - here is a rather lovely song that was recorded at Kore using a 42b for vocals: East Wind by Ed Laurie.

The original Melodium brochure is available to download from Professor Stan Coutant's excellent website.

Meazzi and Framez ribbon mics revisited

2011-05-15T21:48:00.003+01:00

April's MOTM was a nice old Framez ribbon mic, and its chunky Meazzi sister also made an appearance. Here's the article... and here are the microphones:

I've had the opportunity to compare these in the anechoic box - here's how they look (click to enlarge).

Frequency plots for Meazzi and Framez mics

The Meazzi has an overall flatter frequency response, but in both cases there is a significant peak around 4KHz, which should add attack to recorded percussion and clarity & presence to vocals. The Framez is slightly more sensitive in the mid range, but falls away somewhat below 150 Hz.

Whilst revisiting these mics, it is worth looking at the Meazzi's perforated backplate resonator, which contributes to the pickup pattern and frequency response.

(Nb. Both microphones were fitted with 1.8 micrometer ribbons and wired for low impedance.)

Summer is here, and it is Reslo season.

2011-04-23T08:28:00.003+01:00

Another typical day at the Xaudia studio & workshop...

Outside the studio .... flowers, blue sky and cows.

And inside, in the basement workshop.... a Reslo production line!
We see a lot of these little British microphones. The original ribbons are quite thick and have an unusual 'square wave' corrugation. Very often these have oxidised and become noisy, and after 50 years they usually benefit from a clean and a fresh ribbon.

Microphone of the Month - Framez ribbon microphone.

2011-04-14T11:53:00.009+01:00

April's microphone of the month (MOTM) is this glamorous shiny blue ribbon microphone by Framez (not to be confused with Framus!)

This has no model or serial number, but is probably from later 1950s or early 60s. Framez were an Italian brand related to Meazzi - according to one source the name is a contraction of Fratelli Meazzi (trans. Meazzi Brothers), which does sound plausible. Framez / Meazzi also made some cool oddball guitars, and were associated with Wandré Pioli. Fetish guitars have much more information about these guitars.

Back to the microphone! This looks very much like a copy of the RCA 74b 'junior' microphone, but us physically somewhat smaller. And it is a pretty good microphone in its own right. The magnets have retained their strength over the years, measuring a healthy 4500 gauss between the pole pieces. It has a hefty transformer with taps for both low and high impedance, making it suitable for both recording and PA use.

This one arrived with a thick flat ribbon - probably a DIY 'kitchen foil' repair. With a proper 2.5 micrometer ribbon installed, and the mic rewired for balanced, low impedance operation, the sound is clear with a strong output, just lacking a little of the low end proximity boost that you find with many ribbon microphones. The 'low' impedance tap is 1:45 ratio, which gives around a 450 ohm output with the 2.5 micron ribbon.

I'm actually very impressed with this little microphone, and it looks great too!

Update 23/4/11

We managed to get hold of a Meazzi ribbon microphone for comparison. It's a little less glamorous, and has a similar ribbon dimensions, but a very different motor assembly.

Strange things you find inside mics, part 2

2011-04-14T10:37:00.005+01:00

A few months ago I wrote this blog post about the strange things that I had seen inside ribbon microphones. In those cases the 'strange things' in question were put in there deliberately by previous owners or techs trying to repair or improve the microphone.

Since then I have come across a couple of microphones which contained even weirder things - insects!

Before modern foams, felted wool was widely used in microphones for shock mounts, wind shields and the like. Unfortunately, moths love this stuff too.

Here's an old STC4033. You can see the moth eggs on the lower block of green felt.

Worse still, one of the moths had become lodged behind the ribbon:

Moths also seem to like the wool lining and felt mounts on AKG D12s - here you can see the eggs and damage to the lining in the inside corner of the grill:

Anyway, I guess the lesson to be learned is that microphones should be stored in dry, clean places and not in the garage, or at the bottom of the wardrobe.

:-D

(thanks to Steve Parry )

DYNAMICKÉ MIKROFONY

2011-04-13T21:35:00.000+01:00

I love Czech!

MOTM: Shaftesbury Velodyne Supreme

2011-03-31T09:05:00.006+01:00

As you may know, I study quite a lot of vintage ribbon microphones. In general these are simple devices, with just a ribbon, motor, magnets and output transformer . The details and quality of the parts may vary, but most have the same mode of operation. But just once in a while something surprising comes along. Like this Shaftesbury Velodyne Supreme microphone, which is a ribbon microphone with a twist.

The output of a ribbon mic will scale in proportion to the length of the ribbon, at least up to a point. The idea behind the Velodyne was to give increased output by using a super-long ribbon. Normally that wouldn't give you as high an output as it should, because of 'rippling', or other incoherent vibrational modes. But by fixing the ribbon in multiple places they claim to avoid this problem. The microphone was sufficiently novel at the time for the inventors to apply for protection.

The full patent application is here.

Well that's the theory. In practice the microphone has one giant ribbon that goes round corners, and each length of ribbon has a 'node' in the middle, so in total it has 8 elements, each at 45 mm x 4 mm. So 360 mm of vibrating ribbon!

What is more, the microphone has no transformer! I guess the designer thought it had enough impedance already and did not need one.

I wonder how well it worked? Sadly we may never know. The ribbon is broken in many places, and is glued down, so it will net be an easy thing to replace. I'm still struggling over whether to try and get this working or not. I suspect it really ought to be left in its historic condition, but I am curious about how it would have sounded.

Postscript: History shows that this design was not a success. Whether this was for sonic or economic reasons, we can only speculate, but Shaftesbury appear to have abandoned the concept. Their later ribbon microphones were a much more conventional affair, like this Shaftesbury RT model - ribbon, magnets and transformer.

Server issues - and we're back again!

2011-03-05T09:22:00.007Z

Update 8 March 2011:

It looks like all the changes to our web settings have gone through now, and everything should be back on line.

Hi!

You may be wondering what has happened to the Xaudia website.

We are currently upgrading our web hosting, and things haven't gone as smoothly as we would have hoped.

The Xaudia website will be back on line as soon as possible - hopefully on Tuesday. In the mean time, do have a look around the blog whilst we get things up and running again.

We are still in business and we are still repairing microphones.

If you need to get in touch, please contact us at

xaudia@gmail.com

:-)

Mic of the Month - Zephyr 30RA high impedance ribbon mic

2011-02-22T10:53:00.001Z

February's MOTM is a ribbon microphone made by Zephyr, of Australia. It's nice to see a microphone that begins with the letter Z!

The mic has a very "retro sci-fi" look, with hammerite paint and a punched metal grill. The name plate reads "High fidelity velocity microphone type 30RA, but despite the misleading model number, the mic is actually a high impedance mode, and the output transformer is marked "50K'.

The magnets are still healthy on this one, kicking out around 2500 Gauss between the pole pieces. The ribbon is 2.5 mm wide and 28 mm long, similar in scale to a Reslo RBL or Grampian microphone.

The back of the ribbon element is covered in lots of felt, to stop air blasts and brighten the mic for vocal use. Removing some of this opens the sound up a bit. This beast was probably intended for use with a home tape recorder, and would have worked very nicely in that application.

The ribbon itself is mounted on a removable plastic saddle, which makes servicing very simple. Which is a good thing, because this one is stretched. This idea is also seen on Reslo and other microphones.

With a new ribbon, and a suitable impedance-matching buffer, the microphone sounds rich and full - well worth the time spent to get it up and running again.

New workshop part 2: Construction Time Again

2011-02-21T09:21:00.001Z

After a morning with glue and a staple gun, sticking acoustic foam to the soft walls, our new microphone testing chamber is up and running. Here are a couple of photos....

The chamber is hooked up to pink noise and swept-sine equipment, and can collect a frequency plot in a matter of seconds. I'm still working on validation, testing different speakers and mics for best performance, but the facility is already proving its worth.

New workshop! (part 1)

2011-02-14T21:58:00.004Z

It's been a quiet month so far for blogging, but a busy one at Xaudia.

We've spend the last week building a new microphone workshop in the basement below the studio, including an 'anechoic' isolation booth for mic testing. Ralph & Jane have been helping out with building and painting.

Here's the empty room, waiting to realise its potential:

Wall going up...

A door, window and a slap of paint

Isolation booth and test and measurement area...

And with the test gear going in:

The door on the left is an isolation / semi-anechoic box for testing mics or punishing drummers. The sound treatment is on order.

Anyway, loads left to do....

Click here for part 2.

The Syndicate

2011-01-24T10:36:00.001Z

Yesterday I once again had the pleasure of recording The Syndicate, a three-man indie/rock band who came out of the York scene a couple of years ago, but who are currently living and working in Manchester, trying to push the band onwards in a bigger scene.

But it sounds like a bigger pond means bigger sharks. Singer & bassist Barny told me some horror stories about a band being charged £2000 to record one song in a studio, and they didn't even like the result! For contrast, we set up the drums on Saturday night, then tracked three songs with the band playing together 'live', then recorded the vocals, and a bit of mixing, in a single day. The tracks sound raw, and that's how they like it.

The vocals were tracked on a Josephson C720, through API preamps, Fatso compressor plus a little digital compression and 'plate' reverb.

Three hours later, the band had the tracks up on myspace!

Thread Adapters for RCA, Sony and other microphones

2011-01-11T21:54:00.003Z

RCA to modern mic stand thread adapter

A few months ago I was trying to find some thread adapters for a customer with an old RCA junior ribbon microphone, who wanted to use it with a modern standard 5/8" microphone stand. Most suppliers were charging around $25 dollars, plus post and the inevitable import and handling duties.

So we decided to make some.

These fit most of the 'big' vintage RCA microphones, including the 44, 74b and 77 ranges, and also are perfect for Sony professional mics, including the C38b, FV300 and C48 microphones. They also fit Syncron AU7A / Fairchild F22 microphones, and several other American and Japanese microphones with a large thread.

Update:
These are now for sale on the Xaudia website priced at £6.00.

If you want one, please get in touch.

Japanese Auction Poetry

2011-01-09T19:55:00.006Z

Unintended poetry, via translation of a Japanese auction.

It is a rare thing. It is a microphone,
I kept it as collection with a thing handed over before,
The movement had sound then.

But the measurement performance in the new thing is unidentified,
Because I do not make the measurement whether you appear,
Attached to the measurement data of 43 years and 44 years.

The one that has, thanking you in advance,
The one that I want even as for such thing, thanking you in advance,
Because there are a wound and a scratch.

Please understand used goods and ask after above contents,
Understanding in clay,
I do the question as far as I understand it, but answer.

It is communication after the end, but contacts the first on the next morning,
Sorry by a private affair, but thanking you in advance,
Of the one that I am very sorry, but new one and evaluation remarkably have worse.

I will delete, although being really selfish,
In addition, if communication seem to be three days late, please let me know,
I cancel it and I do it.

It is measured for there to be many successful bids of the one,
Where I cannot totally accept communication recently,
Be heated, and there is not it with the state of things really anymore.

But please understand it, and that which I never want,
Please let me know because I do not mind a question and a business,
In addition, please let me know when communication becomes slow.

I am really selfish, but approve it.

Unintentionally written by Coldbreak9, 2011

False Economy a.k.a. "I can't afford to buy cheap."

2011-01-09T10:45:00.002Z

Here's a little lesson that I learned (again) recently. Buying cheap, used equipment can cost just as much, or more, than buying new.

RE20 microphone in the Xaudia studio

I recently found an ElectroVoice PL20 microphone for sale on a forum for $200, which translates as about £130. The PL20 is exactly the same (apart from the colour) as the much loved RE20, which is a perfect choice for recording drums, bass, vocals, brass and many other instruments. Considering that these microphones generally go for around £400 here in the UK, I thought I would grab a bargain for our studio. Even with transatlantic postage, duty, and maybe some small repairs, I should come out ahead of the game, right?

WRONG!

The first thing that arrived was a letter demanding a whopping payment of duty and handling for the import.

On unpacking I noticed that the microphone rattled - the capsule was loose. This is actually a common fault with these microphones - the foam disintegrates and the capsule becomes loose. Luckily, replacement foam is available for a small charge from Shuttlesound. I ordered a kit and it arrived the next day.

Time for surgery: Here is an excellent photo-journal showing how to open up an RE20.

The mic comes apart easily - you need to unscrew the XLR connector at the bottom (note the screw turns counter-intuitively), and unsolder it , then undo the hex nut behind it. This reveals the wiring which can be fed through the mic body so the capsule may be withdrawn.

Also remove the grill, which will unscrew once a small grub screw has been removed. On this mic the hexagonal hole had been stripped round, and so I had to carefully drill this out and re-tap the thread.

With the mic apart I unsoldered the capsule, cleaned out dirt and deteriorated foam, and then put it all back together again with the new foam for testing.

The capsule was no longer loose, but the mic still rattled! Something was broken inside the capsule assembly.

I took it apart again and took the top cover off the capsule. Inside I could see that the small black baffle behind the diaphragm had become unglued. This is not good - it can't be reached without removing the diaphragm, and this is like brain surgery.

With nothing to lose, I carefully unsoldered and removed the diaphragm with a scalpel, then re-glued the baffle and reversed the process, and tested. Sadly the 'repaired' capsule had low bass response. It was time to admit defeat and order a new capsule, which did indeed solve the problem. The mic is now back to full working order, and sounds just like the other RE20s.

Total cost:
'Bargain' microphone: £130
International postage: £25
Duty and handling £49!
New foam set + delivery + VAT: £23
New capsule + VAT: £167
+ a couple of hours ordering and fitting bits.

Total = £394

For reference, Thomann.de sell a package for £399 which includes an RE20, a stand and 10M cable.

Lesson learned.... at least for now.

Oh, and the mic stand clip is missing....

Mic Of The Month: Sony F-V300 Dynamic microphone

2011-01-08T09:38:00.003Z

Sony F-V300 and C38b microphones

This unusual Sony microphone (left) looks like the younger brother of a C38b (right) or maybe a C48 condenser mic, and shares the same high build quality and some hardware components with these mics. But in actual fact this is a high quality cardioid pattern dynamic microphone, model F-V300.

Inside, the microphone is very simple, with just a capsule in the top compartment, and the on/off switch and transformer wired to a printed circuit board below. The fixed grey output cable looks to be the same type as found on the C38b.

The capsule is suspended from 8 small springs to act as an internal shockmount, which seems to work well, in conjunction with the yoke, to acoustically decouple the microphone element.
This example has some traces of foam around the element, indicating that an internal windshield has at some time been removed.

The label states that the mic is nominally of 1kΩ impedance, although in practice seems lower than this and it has no problems driving standard low-Z microphone preamps. The output is strong and clear across the range, and the output is similar in level and detail to a healthy EV RE20, although with a more pronounced proximity effect.

As far as we know, this microphone was only marketed in Japan, but if you have any information to confirm or deny this, or in fact any information about this mic at all, please let us know!

UPDATE 22/2/11
This microphone has become a bit of a favourite for guitar cabs - it seems to have just the right amount of proximity boost for that application, giving clear and solid presence to the lower end. It also makes a decent tom mic.

Strange things you find inside ribbon mics (part 1)

2011-01-07T23:51:00.002Z

Microphone ribbons are generally made from very thin metal foil, and aluminium is the ideal material as it is very light but also very conductive. The output of the microphone is inversely proportional to mass, and so a thicker, heavier ribbon will give a lower output, and a thin light ribbon will be more sensitive. Many manufacturers use something typically around 0.0001 inch or 2 microns in thickness. The ribbon is also typically corrugated either along the full length to prevent lateral motion, or at the ends to give a 'piston' style of ribbon. Well, that is how it should be.

However, ultra-thin aluminium is hard to get hold of, and the non-specialist may be tempted to make repairs using materials that are more readily available. Here are some things I have found inside microphones masquerading as ribbons - needless to say they were all replaced with good quality aluminium foil of an appropriate thickness!

1. Cigarette Paper.

This microphone actually worked, to an extent! It at least made a sound. The ribbon was made from an old fag packet.
Cigarette packs used to come lined with paper-backed foil - I've never been a smoker so I don't really know why, but I imagine for freshness or something. The foil is thin and already textured - it just needs to be separated from the paper. Actually this last part seems to be optional, and sometimes bits of paper are still attached, making the ribbon heavy and noisy.

1.b I've heard that chewing gum wrappers were also used for redneck ribbons, if you want a minty fresh microphone.

1.c Here's a lovely example of cigarette paper being used for a ribbon in an old GEC microphone.

2. Kitchen Foil

Kitchen foil is easy to handle, yet much too thick to make a decent ribbon. But that doesn't stop people trying. This is a common ebay trick... the ribbon looks in good condition, but when the mic arrives the output is low and sounds crunchy.

3. Sweet Wrappers

Plastic coated foil or metallised plastic, like that found in sweet wrappers is an interesting innovation, but is generally too heavy and has too high a resistance to make a decent ribbon. Also the plastic doesn't conduct. This microphone gave almost no signal, and it isn't hard to see why.

Microphone of the Month - Philips Ribbon Mic

2010-12-21T10:19:00.001Z

This old microphone by Philips came from a seller in Egypt - I have a vision of it being used back in the 1940s and 50's, broadcasting out in the desert, near the Pyramids and Sphinx....

The mic was in pretty bad shape and in need of a full restoration. The ribbon was broken, and it was missing a yoke and several other parts. However, it's a pretty interesting microphone and so gets to be our microphone of the month for December.

This microphone appears to be based closely on Harry F. Olson's drawings in early patents and presented in the Journal of the Society of Motion Picture Engineers, back in 1931.

The magnetic field is provided by one large permanent barrel magnet. This microphone had a measured field of about 1200 Gauss between the poles, with ribbon dimensions of 5.5 mm wide by 67 mm long.

The original ribbon - sadly very oxidised - was of the piston type, with corrugations at each end and a flat section in the middle region. On closer inspection, the ribbon appears to have been designed for in-field replacement: each end is terminated in a thicker, silver-plated fold of foil, with a hole drilled for 'easy' mounting (easy being a relative term in this case). The ribbon is held in place with two brass clamps, each mounted held in place with a singe screw. The disadvantage of using a single screw rather than a pair for the ribbon clamps is that the clamp has a tendency to rotate as it is tightened, which can distort or wreck the ribbon. The clamps are soldered to wires which run to the transformer primary, and these wires are doubled (or tripled) in each case, presumably to keep resistance noise to a minimum.

With a rewire and a new (corrugated) ribbon the microphone works and sounds rather full and rich. However, the output transformer is wound for high impedance output, and won't drive a standard mic preamp - so the microphone benefits from using an active buffer or an impedance matching transformer. Hum is also an issue with this, despite the massive brass housing.

I haven't seen another one like this - either in life or on the web. If you have any further information on this, I'd love to hear from you.

Fun with magnets and an Electrovoice V1 velocity ribbon mic

2010-12-02T12:23:00.000Z

Here's an early Electrovoice velocity ribbon mic, model V1. These are great looking microphones, but the early versions are rather crudely made and this one, like many others, suffered from low output due to weakened magnets.

Bob Crowley has a few things to say about these mics - not all of them nice!

The motor of this model is based on a single cylindrical permanent magnet, clamped to a pair of metal plates which make up the pole pieces of the assembly. Because of the positioning of the magnet, the magnetic field is uneven, with a significant difference in field between the top and bottom of the motor assembly. In our example we found that the field varied from around 700 gauss at the bottom to 1000 gauss at the strongest point. This is very low for a ribbon mic, and, along with the oxidised ribbon is responsible for a low, noisy output.

Fortunately, we have sourced some very powerful cylindrical N42 neodynium magnets of a suitable size and shape, which are a perfect replacement for the original weak magnet.

With the new magnet the field is increased by a factor of around four, to about 3000-3200 gauss, a much healthier figure which should lead to an increased output and much improved signal-to-noise performance.

Now it's time to cut a new ribbon, reassemble the microphone, and do some listening tests. In the meantime, we made a rather attractive bracelet from some of the spare magnets.

Cat Mac Attack!

2010-12-01T13:00:00.000Z

Pansy the cat has got it in for the MacBook, and launches surprise attacks like this, usually whilst I'm writing an important email, a blog entry, or leaving feedback on ebay.

At least it explains the spelling!

Winter is here! And so are Modern Day Chicane.

2010-11-28T08:58:00.003Z

This was the scene at Xaudia studio yesterday - a beautiful cold, crisp and bright day in North Yorkshire, and it's times like this you really appreciate the peace & tranquility we get here. Ideal for creativity and recording. And this was the view from the studio over the surrounding fields.

We're a little isolated, not quite in the middle of nowhere, but certainly just down the road from the middle. They don't grit the road here, and sometimes the pipes freeze, but yesterday everything was perfect.

Modern Day Chicane were here to record some drum tracks for a future EP, in what turned out to be a productive, if somewhat frantic session. I arrived early to mic up the kit as drummer Pete was coming up from London on the train, which of course arrived a little late due to the weather, which left us about 4 hours to get the drum takes. We tracked at a blistering pace, cutting six live versions that may (or may not) see the light of day as bonus tracks, then we did five of the songs again to a click track, with Ryan and Nicola (pictured) in the control room providing guide vocals, guitar and bass.

Once we had a couple of decent takes of each drum track, we abandoned the editing and packed up early because it was getting cold and dark, and the talent had to get to a gig. It had already dropped to -4︒C by the time we had to scrape the ice from the cars. Brrr!

I went home and lit the fire!

Mic of the Month - RCA Junior ribbon mics

2010-11-25T14:35:00.001Z

October was a bad month for blogging - I was busy with the haunted house sound installation, and this was compounded by a fault with my Macbook, which took the Apple repair centre three weeks to find and fix, a long time to track down a faulty cable. With a microphone, that would be the first thing to check! Amongst all the chaos I completely forgot to do the 'mic of the month' column.

Back in the real world, I have chosen the RCA Junior ribbon for November's Mic of the Month. This is because they seem popular at the moment, and we've seen four at the workshop for service or repair. The fun thing about this family of microphones is that they vary somewhat in construction, so it is possible to compare and contrast versions from different eras. They tend to be a bit more affordable than the bigger RCA 44 and 77 mics, but still have a good tone that is very usable in a modern studio, especially if the ribbon is in good condition and the transformer is healthy and wired correctly.

The 'Junior' was created as a budget version of the RCA44, with a similar motor assembly but smaller magnets and housing. The most commonly seen models are the 'black badge' and 'red badge' versions, and these are actually quite different inside - the black badge model has a 3.0 mm x 55 mm ribbon, whilst the red badge version I examined has a wider, 4.5 mm ribbon and a stronger magnetic field.

The output transformers on these microphones can be set for 50 Ohm, 250 Ohm or 10KOhm output impedance, and it is worth checking that the mic is wired correctly to get the best performance with modern studio equipment. Normally that will be the 250 ohm setting.

The earliest and rarest version, the MI-4010-A, is shown on the right in the picture below. It is slightly larger than the later versions, with a different ribbon assembly which has horseshoe style magnets around the back of the ribbon. The magnetic field in this example is weaker, and the output lower than the more modern versions, although the tone with a new 2 micrometer ribbon is very pleasing indeed.

Finally, some RCA mics were actually made in Europe, and it would seem that some appear under different names. The microphone on the left is badged as 'Magneti Marelli, Milano, Italy' but is almost identical to the black badge RCA 74b. The only difference is that the Magneti has an alternative transformer, but still with high and low impedance options. The sound is every bit as good.

We'd love to hear from anyone who knows more about the Magneti Marelli microphones and their relationship with RCA.

(Thanks to Jules at DADA Studios in Belgium and Jørn Christensen at Rodeløkka Studio in Norway.)

Errors and Omissions Expected

2010-11-19T15:21:00.003Z

I've had a couple of messages recently pointing out that some of the schematics on the website may be contain errors or omissions, and I've even had a suggestion that we shouldn't publish stuff unless we know it to be correct.

I'm happy to correct verified errors, but am reluctant to remove schematics completely unless we have a known better version. If the original documents are unavailable, surely it is better to have something traced from a mic on the bench than nothing at all? This at least gives the microphone technician a starting point, even if the mic in front of him/her is a bit different. Of course the mic under scrutiny may be incomplete, modified, adapted, or made on a Friday afternoon after a couple of pints. Manufacturers in the past evolved their designs and substituted parts, so really we end up with an 'example' schematic, rather than something that is true in every case.

For example, here is a 19a19 schematic on the site which a kind reader sent in a while ago:

Yesterday a different reader kindly sent in another schematic which looks completely different - different values, different tube, different power supply.

Which schematic is correct? Well, either would be useful if it helps someone get their vintage mic up and running.

So, if you find an error and have a correct schematic that you would like to share, or if you have a different version of the same microphone we would be delighted to hear from you.

Thanks to Roddy and Omar for sharing!

Drew Stephenson - A Driven Man is (Finally) Here

2010-11-16T10:05:00.003Z

Almost two years ago, singer-songwriter Drew Stephenson and Cellist Hilary Jones started recording an album in our studio. Anyway, after what seems like a very long time, Drew's album "A Driven Man is Coming" will be released on November 19th at a free launch gig in York. The gig will be at 1331, on Grape Lane. More details can be found at Drew's website.

Support comes from Dan Webster.

Do come and join us if you can.

Sounds from the halloween maze

2010-11-02T09:25:00.011Z

The big 'Hallowscream' event at York Maze is now over for the year, and the guys and girls are now hurridly getting ready for Guy Fawkes night. It's a quick turn around so good luck!

Several people asked to hear the sounds that we 'created' for the two Halloween haunted houses. I took most of the sounds from the Freesound Project, which is a very handy searchable library of sounds, most of which are available through the Creative Commons license scheme. Other sounds used were my own creations.

The creepy clown theme for the 'CarnEvil' haunted house was based on a loop from The Midnight Syndicate, who specialise in music for haunted house and similar attractions. Some of their tracks were also used for background music in the theatre area.

I'll leave these tracks up for a week or so, so grab them quickly. Enjoy!

Carnevil Haunted House

Main Clown Theme

Upside down room music

Meat Locker

Boiler Room

Barnegeddon

Toilet Lobby
(A mash-up of Beethoven, Mozart, and some C&W in the background!)

Steampunk 1

Steampunk 3

Halloween? Hallowscream

2010-10-31T10:33:00.001Z

For a change, I have spent most of October fitting speakers and providing the sounds for the Halloween events at York Maze close to our studio, and now it's all up and running - we had over 1000 people through the gate last night and are expecting even more this evening. The two haunted houses are excellent scary fun, and the creepy clown music will worm its way into your brain and keep you up all night.

A couple of people have asked for clips of the music from the haunted houses, and I'll try to post those up tomorrow.

Going slow

2010-10-28T12:19:00.003+01:00

Blogging is going a little slowly this month. The xaudia macbook pro had a logic board failure two weeks ago, and is being repaired very slowly under warrantee. I've resorted to blogging from my ipod, which is somewhat tedious.

On another note, the York maze Hallowscream event starts tonight. Xaudia has installed the sound for the two haunted house attractions. Scary!

http://www.yorkmazehallowscream.co.uk/

Microphone Re-ribboning Service

2010-10-03T09:51:00.002+01:00

Xaudia is pleased to announce its re-ribboning service, to help get those treasured vintage microphones up and running again.

MICROPHONE RE-RIBBONING SERVICE

We have precision cutting and corrugating equipment based on the original methods used by RCA, to ensure the best possible replacement ribbon for your microphone.

Precision motorised ribbon corrugator and cutter.

The aim of the service is to provide microphone owners in the UK and Europe with an alternative to sending your microphone off to the USA for repair, and to give an economically viable service for less expensive microphones.

We offer a personalised service. Each microphone is examined on receipt, and before we start work we provide an estimate of what needs doing, how long it will take and how much it will cost.

Grundig microphone being restored

So if you have an old broken Reslo, Grampian, Tannoy or other microphone, then we can help.

For more information please contact us through out website.

Microphone of the month - Old Czech tube mic: Tesla?

2010-09-19T10:20:00.003+01:00

This is the first 'Microphone of the Month' blog, featuring classic or unusual microphones. Hopefully I'll manage to find time each month for this!

This old Czech tube microphone - a recent ebay find - may well have been made by Tesla. The capsule is connected using a connector that can also be found on old Tesla and Phillips microphones. Some of the capacitors are also made by Tesla, who were a large state owned electronics company in communist Czechoslovakia.

The 'Tesla' looks very much like an imitation of the Neumann / Gefell CMV563 bottle mic. In fact the microphone is smaller in diameter than the CMV, has no output transformer and has an unbalanced output. Like the CMV, the capsule may be swapped, and presumably other polar patterns were available. This one is marked with a red circle, which probably means omnidirectional. (I have yet to test the capsule).

The amplifier is a very simple grounded cathode amplifier, based around a Soviet 6Ж1Л (6Z1P) tube, which is a small signal pentode similar to EF95. These are also found in some Lomo and Oktava microphones, including the Lomo 19a9 and Oktava MKL2500.

Without the original power supply we can only speculate on the operating voltages. However, a B+ supply of 90V would be a good place for experiments to start - this would give a voltage on the capsule of around 60, and a sensible current through the tube circuit.

Tube mic circuits - Connecting the capsule 2

2010-08-26T15:09:00.003+01:00

Last week's technical article talked about different methods of connecting a condenser capsule to the grid of a tube amplifier, in order to build a tube mic. In this part we consider how to connect a capsule with two diaphragms in order to get a multi-pattern mic.

First let’s examine the different pickup patterns available. There are three extremes: Omni, where the microphone hears sounds equally in all directions. Cardioid (heart shaped*), where sensitivity is greatest in the direction in which the microphone is pointing, falling off to a null point behind. And Figure-of-Eight, with equal (but opposite) pickup in front and behind, and null pickup to the sides. To complicate things further, the pickup pattern may depend upon the frequency, and some mics will have good directionality at higher frequencies, but become less directional as the frequency drops.

But what if we want a microphone with selectable pattern? This can be achieved by arranging a pair of cardioid capsules back-to-back, and combining there signals in different ways. We’ll call these capsules front and back, although of course they could be pointing in any direction. If we require a cardioid signal, we just take the front capsule and for omnidirectional pickup, we mix both signals equally. If we want figure of eight, we subtract the output of the rear from the front: where the signals overlap at the sides of the microphone, they cancel each other out producing null points. Other patterns such as hyper-cardioid and super-cardioid may be considered as in-between positions of these extremes.

So, what is the best way to achieve this practically in our hypothetical tube microphone? Two of the earliest commercial mics with more than one pattern were the Neumann U47, which offered cardioid and omni, and the U48, with cardioid & fig. 8. Let’s look at the U47, as this is probably the simplest way to combine the two capsules.

In the U47 the front diaphragm is grounded through a 100 Meg grid resistor, and the backplate of the microphone’s dual diaphragm is polarised with about 60V, providing the potential difference required. The rear diaphragm is connected to a switch. When the switch is open, the rear capsule is left floating and only the front cardioid diaphragm is active. When the switch is closed, the rear capsule adds its contribution to the front, making an omnidirectional microphone.

What about the U48? We have seen above that if we require figure of 8 instead of omnidirectional, we must subtract, rather than add, the sounds from the rear. To do this we must invert the polarity of the rear diaphragm by reversing its relative charge. So, rather than grounding the rear diaphragm, we must raise the potential by 60V** above the backplate, and 120V above the front capsule! This is easily achieved by using the HT supply to the anode of the tube, but creates another problem. We can’t simply connect the two diaphragms because they are now at different potentials, and so a blocking capacitor must be used. The circuit looks like this:

Finally, to make the microphone have variable pattern, we simply need to make a supply that is adjustable from 0V to 120V, and apply that to the backplate. Alternatively, the signal may be taken from the backplate, through a capacitor to the tube grid. The Neumann-Gefell UM57 does it exactly this way, with the pattern selector in the power supply.

*Really more kidney shaped, and in some languages this is the word used.
** In fact the U48 operates around 50V / 100V.

Further reading: The G7 page at Gyraf.dk

Tube mic circuits - Connecting the capsule part 1.

2010-08-18T09:25:00.012+01:00

Even in the simplest of tube microphone circuits, there are different approaches to connecting the microphone capsule to the tube. Let's use a single-sided microphone capsule as our starting point.

The capsule behaves as a variable capacitor, changing its capacitance in response to changes in air pressure (i.e. sound). In order to generate a signal, the capsule needs to be polarised by some voltage, creating a difference in potential between the diaphragm and the back plate. This is the first decision that needs to be made - should the polarising voltage be applied to the diaphragm or the capsule backplate?

In the circuit shown on the left, the backplate of the microphone is polarised at 60V, which is obtained from the B+ supply, via a resistive divider and a small capacitor to stabilise and filter the polarising voltage. The membrane is connected directly to the tube grid, and a high value resistor (Rg, typically 100 kΩ to 1000 kΩ) connects both the grid and the membrane to ground. We have our potential difference across the membrane, and the sensitivity of the mic may be adjusted by increasing and decreasing the polarisation voltage. As the capacitance of the capsule changes in response to sound, a tiny current will flow through Rg, and this signal is amplified by the tube.

An example of this arrangement may be seen in the Neumann Gefell M582.

In some cases the grid resistor may be omitted. In the circuit below, which appeared in an article in Tape Op magazine by Dave Royer, the capsule diaphragm is grounded by grid leakage rather than a 'real' resistor. It works perfectly.

This simple arrangement is not possible when the capsule backplate is mechanically (and electrically) connected to the body of the microphone. In this case the diaphragm must be polarised directly.

However, having a voltage of around 60V on the tube grid this would adversely change the operating points of the tube circuit, and so a capacitor must be used to block the DC voltage (left). Some listeners claim to hear the difference between different types of capacitors, and so normally a very high quality type should be used in this position. An additional high value polarising resistor is also required, otherwise the high impedance audio signal would be attenuated through the stabilisation cap.

An example of this method of connection is the Neumann-Gefell CMV563, which is designed to be used with bayonette style capsules such as the M7, M8 and M9.

Sometimes it is the membrane which is connected directly to the body, such as in this Teladi K120. The approach is the same as the circuit above.

In my next post I'll consider how to connect mic capsules with two membranes, and how to combine them to generate different polar patterns.

Unknown German Prototype Tube Microphone - 'The Unbekannt"

2010-07-21T11:19:00.007+01:00

Unknown German tube microphone

Here is a recent Ebay find. It's an unbranded tube microphone, and judging from the components probably from the 1960s, in what was formerly West Germany. We've called it the 'Unbekannt', which is simply German for 'unknown'. The amplifier circuit is a 3 stage unbalanced transformerless design, using EF40 pentode and an ECC81 twin triode. The final stage is a cathode follower.

The schematic is here.

As is so often the case, the microphone has been separated from the original power supply, so it is not possible to say what the exact operating voltages would have been. However, the voltage divider for the capsule polarisation may give us a clue - 2 Meg and 400K would be a simple way of using a 240V supply to put 40V on the capsule.

The metalwork is nicely done, and is comparable to Reissmann, Thiele and Teladi microphones of similar age. It seems too well constructed to be a DIY mic, but the oddball range of parts makes us think that it is some prototype from one of the microphone makers of the era.

The capsule is quite unusual, but sadly is missing a diaphragm at the moment. It uses springs and screws to adjust the tension and space to the back plate, so this can be adjusted after installation. We'll try to get that up and running very soon so we can see what it sounds like!

Fraudulent vintage mic listings on ebay

2010-06-18T07:08:00.000+01:00

Things like this keep cropping up in vintage microphone auctions on ebay, often for Neumann U47 or U67 microphones. This one was embedded in an auction for a vintage AKG C12, which would normally be worth at least five times that value. There are a few dodgy things about this auction. Most obviously, it's asking for a deal away from the ebay system, which circumnavigates fees, but also the buyer protection that ebay provides. Also, this one was a US auction, but the request is for Australian dollars.

These are almost always hacked accounts. I'm not sure if they are an attempt to con you into parting with your hard earned cash, or simply to get your email for spam lists,. Either way, if something sounds too good to be true, it probably is.

If you see them, please use the 'report item' button. Also, the wording is usually embedded in a jpeg, so the ebay bots don't see it straight away, which is why it's doubly important for us to be vigilant.

Syncron AU7A microphones Part 2

2010-06-07T21:46:00.019+01:00

Last time I wrote about a pair of Syncron AU7A microphones. The capsules were in good condition, but the batteries had leaked, causing corrosion and damage to the circuit. For one of these mics I decided to fit a tube circuit based on a 6205 subminiature tube (5840* would do just as well or better)**.

Tube modified Syncron microphone circuit, 6V regulated heater supply omitted.

The Syncron capsule operates happily between about 40V and 60V, and a simple voltage divider was used to supply the backplate with a suitable polarising voltage. As the capsule is cardioid only, the circuit can be made as simple as possible, and there is no need for a capacitor between the diaphragm and the tube grid.

With a little creative hacking I was able to reuse the circuit board to construct a valve circuit, which avoids damaging the microphone further. Although physically larger, the tube sits where the transistor was (I even used the same PCB pads as the FET), and there is room on the underside of the board for a couple of capacitors. An added bonus is that the original transformer is quite suitable for use in a tube circuit, and was rewired in 10:1 configuration. The rest of the circuit - 5 resistors and another cap - fit on the 'wrong' side of the board in the cavity below the capsule housing. Then it is a simple case of wiring the connector to the circuit and connecting the capsule, taking care not to damage the diaphragm.

One thing to look out for with this arrangement is that the amphenol cable plug & connector on the microphone are the reverse of the normal gender, which means that there can be 110V DC on the exposed pins. Consequently care must be taken to connect the microphone before the power supply is turned on, otherwise a short sharp shock can happen. Of course this isn't really an acceptable acceptable solution from a safety point of view.

In practice the microphone works very nicely and is suitably quiet for recording vocals. We tracked some female vocals with it yesterday and it performed very well in that application.

Meanwhile, I have managed to track down some 22V batteries from Farnell, which should be suitable for the capsule polarisation, so I'll attempt to restore the second mic to its original state. More on that soon.

** With hindsight the 5840 may be a better bet as there is an internal connection between the cathode and grid 3. This allows you to cut off two of the leads, which means using up one less precious pad on the circuit board inside - space is tight!

**Readers familiar with the 'Royer' tube circuit will recognise the topology, although a few of the component values are different.

Xaudia blog post on phantom power for these mics.

Syncron AU7A (Fairchild F/22) microphones Part 1

2010-06-06T08:13:00.014+01:00

I was lucky enough to come across a pair of Syncron AU7A microphones (aka Fairchild F/22) on ebay. On arrival from the US I found that all the foam lining in the boxes had decomposed and spread black dust everywhere. Luckily the capsules appeared to be in fine condition, and the mics came with the original cables, so the should be a good chance of getting them back to working condition.

That's easier said than done! The mics run on 4 batteries - 2 x 4.2V for the amplifier and 2 x 21V for the polarisation. Unfortunately, our microphones came complete with the original vintage batteries inside, which had inevitably leaked and caused corrosion throughout. The batteries are now pretty much unobtainable, so I used a bench voltage supply to simulate the batteries. Microphone number one gave a very weak and noisy (hiss) signal - I suspected the FET had somehow become contaminated by the battery acids. Mic 2 was slightly better, but certainly not something you could use as a serious recording tool.

These are reported to be the first commercially available FET microphone, and searching the internet didn't throw up any schematics so I traced out the circuit, which is very very simple - capsule -> field effect transistor -> DC blocking cap -> transformer.

EDIT 21/9/2011 : please note that the schematic posted here contained errors. A revised version is here!

The transformer may be wired either for 200 or 50 ohms, and measurement showed it has a voltage ratio of 5:1 in series or 10:1 in parallel mode.

At this stage I needed to make a decision on how to get the best out of the microphone. More on that very soon, but for now here are some web links to Syncron information - there's not a lot of it about!

http://www.coutant.org/syncron/index.html

http://www.prodigy-pro.com/diy/index.php?topic=30014.0

http://recforums.prosoundweb.com/index.php/m/191308/0/

Neumann Gefell UM57 experiments

2010-02-16T13:21:00.005Z

Lately I've had the opportunity to play around with several vintage Neumann Gefell tube microphones - a CMV563 (below with UM70 capsule) , a M582 and a pair of UM57s.

These all have broadly similar circuits, with a EC92 tube and transformer coupled feedback. The UM57 is configured for different polar patterns, whereas with the CMV563 and M582 you have to swap the capsule. There are other differences - the schematics are shown here.

One particularly common fault with examples of these microphones is that the original electrolytic output capacitor can dry out with age. This is by no means always the case, and the capacitor in the UM57 on the left above was in perfect condition after nearly 50 years! The one on the right has been replaced with an orange modern metalised film capacitor.

So what is the effect of ageing of this capacitor? As the electrolyte dries out, the absolute value of the capacitance drops, which will affect the frequency response of the valve amplifier inside the microphone. To simulate this, a capacitance decade box was wired in place of the output capacitor (C3), and the chart below shows how the frequency response changes as the capacitance decreases in 0.2 uF steps.*

Part of the circuit is shown inset within the chart. Although intuitively we expect the smaller capacitor to give us less bottom end, the network of the capacitor, transformer primary winding and resistor acts as a resonant filter, producing a peak in the bass region just above a sharp drop off. The human ear can perceive this as more bass - although not necessarily in a good way: the microphone may seem muddy or lack clarity.

So, having a good quality capacitor here is vital, and the value of this can be used to tweak the bass response if desired. Of course this analysis is just for the tube circuit inside the mic and does not consider the effects of ageing on the capsule itself - that's a story for another day.

SJT Feb 2010

* Measured using a swept-sine wave from 1Hz to 48KHz.

Beautiful girls in Milton, Wisconsin

2010-02-12T09:36:00.007Z

This is the label on a reverb pan found inside a vintage Roland Space Echo SE555. “Manufactured by beautiful girls in Milton, Wisconsin, under controlled atmosphere conditions." Right! And you thought they were made in Japan by robots, didn’t you?

Under Construction

2010-02-12T09:10:00.002Z

If you’ve been here before, you’ll have noticed that the site looks different.

A couple of years ago we moved servers and renamed the site. I’d learned to used GoLive fairly well, but that had been discontinued so I rebuilt the xaudia website using dreamweaver. But I never really got the hang of that program, and was never happy with the layout of the site.

When the time came for a paid update to that software, it was decided not to continue with it. iWeb came free with the most recent macs, and this seems much more intuitive, at least to this brain. So I’ve decided to scrap the old website, and am in the process of completely rebuilding from scratch, bringing across the best of the content from previous incarnations, and adding many more schematics and documents which I’ve collected over the past couple of years.

The standard iweb blog doesn't have too much interactivity, so I've brought incorporated our existing blogger page into the site - you're on that now. The iweb 'blog' has been kept for things like brochures and documents - stuff that doesn't really require comments.

This should also be a lot easier to keep up to date, so with luck you’ll see a more dynamic site from now on. Let’s see...

Recording with the Josephson C720

2010-02-12T08:22:00.004Z

Back in August we were lucky enough to get our hands on a special edition Josephson C720 microphone for the studio, one of only 20 made in the first production run, and having lived with this for nearly half a year it’s probably about time we reviewed it.

The first thing you notice about the C720 is its size - this is one big mic, U47-big, and makes a big bold statement in the live room or vocal booth - it’s built like a tank and it definitely has that ‘wow’ factor which gets the artists talking.

The next thing that grabs your attention is its radical aluminium metal-foam headbasket, which looks like something you could scrub your pots and pans with. The non-periodic headbasket is actually designed to eliminate internal acoustic reflections and refraction of soundwaves, avoiding comb-filtering effects and so giving a more realistic recording.

The third thing you notice is that it has two XLR outputs. Like many condenser microphones, the capsule has dual diaphragms which can be combined in different combinations to give cardioid, omni, figure of eight patterns. Unlike most microphones, the two transducers have separate head amplifiers and separate outputs which means that the signals from opposite sides of the microphone can be recorded separately and mixed together, either in phase or with the polarity reversed, at a later stage. What this innovation means in practice is that you can pick the polar pattern after recording, which might be used to eliminate unwanted sound sources by rotating the null point of the microphone. Also, and more importantly in our studio use, with close-mic’ed sources such as vocals you can dial in more or less proximity effect, making a singer seem bigger, darker or lighter and brighter.

So what does it sound like? Possibly because of the lack of head basket comb filtering, and the facility to tune the pattern and proximity effect, this is a very versatile microphone. The one word that sprang to my mind in describing the sound, whichever pattern you pick, is ‘solid’.*

Another thing we’ve noticed is it sounds pretty damned good on pretty much anything you can throw at it. Because of it’s ability to handle high sound pressure levels and become a go-to mic for low end stuff, and is rock-solid on bass guitar and front of kick drum, but it’s just as handy on both male and female vocals - especially those with a tendency to get loud when they belt it out. Vocal recordings are up front, sound as they should, but without the top end brightness of, say, our TLM49.

Furthermore, it can be used for more radical effects by compressing or reverberating one side of the capsule only. For example, I had some interesting results on a male rock vocal by compressing the signal from the front capsule, mixing together with the phase reversed rear capsule, then compressing the sum of these. The effect was that as the vocals push harder, the relative amount of rear capsule included becomes greater giving it more of a hypercardioid pattern, balancing the tendency of the vocalist to step back from the mic when belting it out. With a bit of creativity the possibilities are endless.

Overall this is a brilliant, radical piece of thinking and and one of those bits of gear that comes into your life and is there to stay.

*After writing this I read another review of this mic which used exactly the same word - ‘solid’ - to sum up the sound.

Vintage Vandals

2010-02-12T08:18:00.007Z

I seem to spend half my life looking for unusual bits to get old things working again, either for my projects or for other people. Which is why seeing this kind of thing drives me nuts.

Can I make a plea for all sane people out there not to break up vintage microphones and other equipment unnecessarily?

On ebay at the moment there's a nice classic RFT CV7049 tube microphone sold without the hard-to-find connector, and then the connector for the mic sold separately just to try and make an extra couple of bucks (same seller of course). It's like selling a classic car and the wheels separately. And it's not the only example - you can find half a dozen similar things going on at any time on ebay. What’s the obsession with breaking up sets of vintage equipment?

I'm not even sure the sellers strategy works - Surely the two things belong together, and are ultimately more valuable as a pair? I certainly would bid more for a bit of vintage equipment knowing that at least the bits are there for us to get it up and running. And I would shy away from buying something like that, only to lose the connector to another bidder.

Once these things get broken up it's really hard and expensive to bring them back together again. It's just destroying antiques really.

The other thing that occurs to me as I write this, is that if a microphone or some other thing is broken up in this manner, one should assume that it is also broken in a functional sense. The equipment cannot be tested by the new owner and verified as working if, when purchased, it lacks a capsule, tube, connector or some other hard-to-find yet vital organ. So if it is broken up, it’s broken down. And one should bid accordingly.

New Toy!

2009-10-08T10:37:00.006+01:00

Here is our new pride and joy - a 1960's AVO mkIII valve characteristic meter. It's built like a tank, with wonderful vintage knobs and dials. And comes in handy for getting those amps and mics up and running.

Your New Drummer

2009-09-12T10:03:00.006+01:00

This was the sight that greeted me when I arrived at the studio at 9am yesterday morning. Drummer Tom from Your New Antique had got a lift down early, and was sleeping off the night before.

We tracked drums for 4 songs, stopping only for cups of tea, a sandwich run, and a brief emergency trip into sunny York to grab a replacement snare skin. The session gave me a chance to try out the Josephson C720 as a 'stereo' drum room mic, using the two cardioid capsules as left and right channels. It sounds pretty damn good actually - in fact I've yet to find an application where it hasn't shone.

The control room window could do with a good clean!

Getting Wood

2009-08-23T10:14:00.008+01:00

Probably the best £100 invested at the studio recently wasn't a gear purchase at all. We bought 60 metres of spruce floorboards and decked out the drum area. It took a day of hard labour sawing, screwing and varnishing, but this was well worth the effort. The sound now has a lovely early-reflection 'snap' that makes the drums sound bigger and brighter.

Sawing the planks made me think of my woodwork teacher, Mr Barker, who used to say..."measure twice, cut once".*

*He also used to say "as smooth as a baby's bum", but let's not go there!

Sunny Days

2009-08-21T21:03:00.006+01:00

It's sunny! But we still get wet on the way home! And there was a double rainbow landing in the field outside when Your New Antique left the studio tonight. Let's hope there's a crock of gold at the end of it all... as opposed to a crock of shit! No real danger of that - the tracks are sounding great so far.

What's going on?

2009-08-16T09:15:00.008+01:00

It's been busy here!

Dead Rebellion have been recording a three track single, which is now complete and ready to go to pressing as soon as the artwork is ready. The tracks are "Forever Burn", "Falling Down" and "Rottweiler", along with a couple of bonus tracks.

The latest single from TWO is now available from iTunes. It's called 'Summer' and it's very pretty, so check it out.

Today we start production on the first studio album from Your New Antique, which should hit the streets & interweb sometime early in 2010. We'll be blogging this project as we go along, so stay tuned.

Also, we have some new toys in the studio, of which the star is a Josephson C720 microphone, one of only 20 made. The headbasket is designed to eliminate internal acoustic reflections, and the two transducers have separate outputs so you can pick the polar pattern & proximity effect after recording. The sound is simply stunning!

The Day The Music Dies...

2009-06-27T19:22:00.007+01:00

7th of August, apparently!

Your New Antique on iTunes and youtube

2009-04-06T09:58:00.003+01:00

The Your New Antique EP "Open Your Eyes" is now available from iTunes. And you can watch the video for Open Your Eyes on youtube....

Don't forget the album launch on Friday!

Your New Antique

2008-11-19T12:37:00.006Z

Your New Antique have just finished recording their debut EP "Open Your Eyes" which will be released on the SOUP label shortly before Christmas.

The track list is: Open Your Eyes; Black Seas; Point of View; Safe and Sound

YNA hail from Huddersfield, and the artwork is based on images of the old Holiday Dyes and Chemicals plant that used to be a major emplyer in the town. The artwork was adapted from 'Core Activities', Bismarck Brown Series, by artist & print maker Julia Clegg.

www.printmakingonline.co.uk

The EP will be available next month on both iTunes, and from the XAUDIA web store.

Studio news

2008-11-19T12:24:00.002Z

We have been so busy! So what's new?

The long awaited Slash Bikini second album is finished!

The Universals album is finished and has been sent off for mastering.

Drew Stephenson is back in the studio with Dr Hilary Jones on cello and flute, to record a full album as a follow up to the acclaimed debut EP, 'Digging for Bones'.

And following their successful tour of the summer festivals, Hungry Ghosts are recording 4 tracks for an EP.

Very exciting!

Studio tracks on iTunes

2008-09-01T15:59:00.003+01:00

Some of the tracks from the studio have made their way, via bad bear records and AWAL, onto iTunes.
Hungry Ghosts and TWO both have releases there now, and we're expecting more in the very near future.

Noise!

2008-08-28T16:58:00.005+01:00

This week's special prize goes to LEEDS CITY COUNCIL, who called us to say there had been another complaint aboout noise levels from the studio.

To put this in context we moved the studio 30 miles from Leeds 18 months ago, and wrote to both environmental health and business rates departments at the council at the time to tell them of our move.

The old premises now sells water features. Perhaps they bubble loudly?

Teladi tube mic

2008-08-28T16:51:00.004+01:00

And here's something altogether nicer. It's an old Teladi end-address tube mic, probably from the 1950s. Construction wise, it feels very Gefell, with a small metal (nickel?) diaphragm and a U47 style connector. I'll sketch out the schematic shortly.

Cheap!

2008-08-28T15:57:00.005+01:00

Broadly speaking, you get what you pay for.

Over the past 5 years or so the market has become saturated wth inexpensive made-in-china microphones which have brought condenser mics within the budget of home studios and, along with computer based recording, have arguably revolutionised the recording industry. But there has to be some corner cutting or trade-off in component quality or quality control, doesn't there? I picked this SE electronics 'miniSE' up on ebay for £40, which brings it in at about 15 times cheaper than the ex-demo Neumann TLM49 we bought earlier in the year. Like the neumann it is a cardioid-only transformerless mic. And unlike the Neumann, this came in a nice aluminium carrying case (as opposed to cardboard!)

On opening the case, I found the mic was rattling. 2 minutes with a screwdriver revealed that the capsule had not been properly screwed down. Also you can see that the circuit board is covered in flux. The capsule looks exactly the same as the CK12 copies found in MXL2001 mics and others. I've seen several of these now with bumps and wrinkles in the membrane. The metalwork is actually quite nice, so it may get one of the spare AKG capsules and a new circuit transplanted.

One by TWO

2008-08-25T12:37:00.003+01:00

TWO will be hosting a a launch part for their new album 'One', which will be released shortly on the SOUP label. The gig will take place on Saturday 6th September at St Michael-Le-Belfrey Hall, Stonegate, York, from 8pm.

The album was recorded in our studio and sounds great. Drew Stephenson is playing too so it should be a great evening. We are really looking forward to it! If you want to come along please contact either us, or TWO directly via their website.

Its a free gig but at an unlicensed venue - so do bring your own drinks. You can hear snippets and find out about other gigs at their website.

Hungry Ghosts at Leeds and Reading

2008-08-25T11:55:00.008+01:00

Hungry Ghosts were recently in our studio recording songs for their debut release, and will be back soon to track songs for their album. Last weekend they played storming sets at the Reading and Leeds festivals. You can see them here performing a couple of tracks at the Reading festival, courtesy of the BBC website.

'Snap Shut' and three other tracks will be available soon at iTunes, but for now you can download tracks from the web store.

Hungry Ghosts Website

Universals in the studio

2008-08-20T09:52:00.002+01:00

We spent the weekend recording 11 tracks of drums and bass for the new album by The Universals, which gave me an opportunity to try out our new Crowley and Tripp Roswellite ribbon mics as drum overheads. They're fig-8 pattern and I find them a little harder to position than cardioids, but using the brighter side the tone is sweet, giving a good image of the kit without too much harshness on the cymbals.

On Monday we tracked some guitars and did some vocal tests. We tried singer Craig on 4 mics - the restored Thiele M4 tube mic in cardioid, Oktava ML11 ribbon, an RE20, and finally a Neumann TLM49. The Thiele was warmest, but the ML11 is so easy to sit in the mix so that's our go-to mic for this project. I'll post up some sound clips when we have some mixes to share.

Hungry Ghosts!

2008-08-19T20:59:00.001+01:00

Good luck to Hungry Ghosts, who will be appearing at Leeds and Reading festivals this weekend. We've just finished recording a couple of tracks for them - you can hear samples on their website.

Wetzel microphone on the bench

2008-08-09T17:34:00.002+01:00

Here's a nice little tube mic for restoration. The manufacturer is Joachim Wetzel of Liepzig and looks to date from the 50s or 60s.

It is currently undergoing a recap and the capsule will be reskinned. The head amp is based on an ECC83, with two triode gain stages. There's something funny going on with this one. The power supply voltages are somewhat low, and it seems like the old diode rectifiers are causing problems. More on this later.

Framus Television

2008-08-05T10:46:00.000+01:00

Framus Television

It is quite sad to think that the success of the big American guitar manufacturers (F--, G--, you know who you are!) led not only to some excellent instruments and high standards of construction, but also to the demise of several smaller builders of quality guitars. Cheap imports from the East added to the problem. One such victim was the German manufacturer Framus, who went bankrupt and ceased production at the end of the '70s. (The company has since been reborn).

Framus made guitars of generally high quality with some interesting features. Here is a 1968 Framus Television semi-acoustic, the shape of which is clearly influenced by Fender's Jaguar/Jazzmaster (or was it the other way round?) . It has a pleasing asymmetry, with a single f-hole (as far as we can tell, most Televisions had two), three pickups, and a tremolo. The headstock shape is whacky - like a big hand - but we like it. Colour of this one is a beautiful stained carrot-orange-yellow, rather like certain Gretschs - we think they call this 'aniline yellow'. It has a laminate top and back.

This guitar arrived on the bench with toggle switches in place of the original sliders, as well as a couple of loose wires, which we of course replaced/fixed. The circuit is slightly unusual (which is why it is on this website at all!), so we've sketched a circuit diagram to help other owners.

Framus Television circuit

The circuit consists of two parallel output stages each with volume and tone pots (marked T&V in the diagram) , which the player may use to switch between 'rhythm' and 'lead' settings. This approach predates the days when everyone has booster pedals on the floor, and similar systems are also found on Jaguars and Jazzmasters. In the 'rhythm' position, only the neck pickup is active. In the 'lead' position any combination of the three pickups may be switched on - the pickups in combination give some audible phase cancellation. There is also a bass cut filter switch. A summary of the switches:

S1 = Bridge pickup on/off
S2 = Middle pickup on/off
S3 = Neck pickup on/off
S4 = Bass cut (lead circuit only)
S5a = Sends neck pickup to rhythm or lead tone/volume circuit.
S5b = Switches between lead and rhythm tone/volume circuits.

As the tone and volume pots are tricky to remove and function well, they were not investigated or measured for value. I think they are just standard.
There's a good chance that the circuit is similar to Strato Delux models of similar vintage, which also have up to 5 switches. Do let us know if you can confirm or deny this.

2008-03-05T10:54:00.000Z

Jakob Erland’s Gyraf G7 DIY tube mic project has proved one of the most popular microphone projects. Several years ago I built a pair of these from scratch, and have since built several variations using different tubes, transformers etc. Below are a few notes on the project.

1. Using a single sided capsule

When using a single sided capsule, the circuit can be simplified somewhat and several parts omitted. The the capsule may be wired straight to the tube grid, avoiding use of a coupling cap. Note that this affects the polarity of the mic, so reverse the output wires and be sure to check against an SM57 or similar know microphone! In this arrangement, the backplate polarisation resistor can be lower than 1 gig.

2. Some measured voltages:

I built a version of the microphone by etching Gyraf's layout. Wired it up and checked some voltages - and found that the supply is rather low under load. I got about 176 Volts without the mic connected, and was down to 136 V with the mic in the loop. Heater supply dropped from 6.3V (set whilst unloaded) to 6.08V. Here are some voltages for reference.

3. Better matching of the capsule polarisation voltages.

Note in the diagram above that one side of the capsule has a slightly higher voltage than the other - no problem in omni or cardioid but noticeable in figure8 mode when recording Blumlein pairs. Here's a quick fix!