Reslo wiring guide

This is the 'official' wiring information from the Reslo RB brochure...

As you can see, different models had different wiring conventions and required different colour coded cables. This can cause confusion, particularly if the mic has been separated from its original cable in the 40 or so years since manufacture. The most important thing is to check the mic and cable wiring and make sure that they match!

Unfortunately, grounding the mic by connecting the cable screen to the body of the plug does not work well, and these mics are prone to hum. For the low impedance models, it seems sensible to use pins A and B for the balanced output, and pin C for ground. The ground must also be connected to somewhere to the body of the mic and/or to the shell of the cable.

The dual impedance models are particularly problematic, in that pins B and C were used to select either low or high impedance output. This means that if one pin is used as a ground then one part of the transformer will be grounded and some of the signal lost. It is therefore important to disconnect the high impedance leg inside the mic and rewire to match the cable.

Here is how we wire our Reslo microphones and their cables at Xaudia - with equivalent XLR numbers.

Most 21st century users want a balanced, low impedance output with good ground connection, preferably with an XLR connector at the other end of the cable. Pin A (hot) becomes Pin 2 at the XLR, Pin B becomes Pin 3, and Pin C (ground) becomes Pin 1.

Roddy Bell has written a very useful guide to Relso wiring, which can be found here. 

And here is a full scan of the original Reslosound user manual in PDF format.

(link corrected 28 Jan 2013)

MOTM - RCA Selmer RL1

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

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 guitar, Xaudia

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 ash body & scratch plate were made and sprayed by John Manuel of Carlisle, who did an amazing job with a nitrocellulose transparent white - it really shows the lovely grain below. He can be contacted through ebay.

The neck and locking tuners were from Vanson guitars. 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. I found it slightly chunkier in profile than an original Jazzmaster neck.

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!

home made telecaster pickup

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 the gap between magnets and wire that 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, and of course reverse polarity for the magnets.  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!

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

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

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.

Update: Here is  a product sheet for the LEM305, in French and English. Thanks to Microphone Online Museum for sending the link.

Update 5/1/15: Thanks to Philippe Le Gourdiol for sending in this picture of his LEM ribbon microphone, which is a low impedance model with a high-pass filter. The filter inductor is located below the motor assembly…

STC 4136 condenser mic - phantom power

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

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

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

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

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

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.