MOTM - Grampian GR1 and GR2

It is time again for microphone of the month, and for May we have these lovely Grampian ribbons.

Grampian ribbon mics

Grampian mics come in a variety of model names and numbers which we will try to decypher. These were made in the 1960s and 70s by Grampian Reproducers Ltd, of Feltham in Middlesex, England.

Grampian GR1/L and GR2/L ribbon mics

The mics are labeled GR ("Grampian Ribbon"), followed by a number and a letter. The GR1 designation was used for the semi-cardioid version, and the GR2 is the 'normal' figure-8 pattern. The letter shows the impedance of the mic, set by the output transformer winding. They come in Low, Medium, High, and 'X' flavours - the letter codes are as follows:

GR1/L and GR2/L   ... 30 ohms
GR1/X and GR2/X  ... 200 ohms
GR1/M and GR2/M ... 600 ohms
GR1/H and GR2/H  ... 50K ohms

Here is a rather wobbly scan of the original Grampian data sheet.

The GR1 usually has a silver grill at the front, and a black one at the rear to show its asymmetry, whereas the GR2 has two silver grills. However, inside there is very little real difference between the two models, and any cardioidness* is imposed by additional foam padding around the rear of the ribbon. This foam usually depolymerises over the 40 years or so since manufacture, and the mics will typically fall apart when opened. If the mics have been stored for a long time they will require careful cleaning and new foam suspension before being put back into use.

The mics have a small thin ribbon, held in a removable plastic frame for ease of service. In that respect they are similar to the Reslos and Film Industries mics with which they undoubtedly competed. Unlike the Reslo and FI mics, the symmetrical magnet arrangement means that the GR2 has a true figure-8 response, and are suitable for Blumlein pair or a side mic in a mid-side array.

The internal transformers have a tendency to break, particularly on the H models which have very thin wires. If the break is in the right place the transformer can be repaired, but a rewind may be necessary.

Grampian plugs and connectors are a bit of a pain - they are getting scarce now, and rely on the barrel to make the ground connection, which can lead to hum if the connector is worn or oxidised. I have blogged previously about XLR conversions for these.

Grampian plugs

External matching transformers were also available, in case one needed to connect to an input of a different impedance. These are still handy for getting a bit more level out of the 30 ohm models.

These were mid-price microphones when they were made. I have a Grampian price list from June 1976, and the GR1 & 2 microphones were priced at £32.05 (plus tax) in a wooden instrument case, and £27.55 in a cardboard box. This would translate today as £180 to £334 plus tax, which would buy one of the better Chinese ribbons or a Beyer M260.

* I made this word up.

UFO over York

Strange cylinders were spotted today in the skies above the Xaudia workshop!

Actually it was the Good Year airship, but Aliens and Time Travellers are welcome to visit.

The kettle is always on, and there is a good pub round the corner.

Reslo RB 30 ohm & Hi Z mic wiring

Some Reslo RB mics have a transformer with a split secondary that gives the owner the choice of either a 30 ohm or high impedance output. Often these mics get dismantled and rewired, so here is where the internal transformer wires go....

For a 30 ohm output, yellow is the 'hot' output, and should eventually end up at XLR pin 2. White is cold, and goes to pin 3.

The green wire should not be grounded, but this sometimes happens by accident if the plug uses one of the pins for ground. The result is a high impedance path to ground from the output, which can act as a filter and give a weak output and weird frequency response.

Update May 2021

One reader reported that his microphone was reverse polarity after following this wiring diagram.

It is always a good idea to check polarity after any repair or modification. The polarity can be corrected by simply swapping over the two transformer output wires. 

Mystery mic motor

One of our readers sent in these photos of an unidentified ribbon mic motor. It looks like an attempt at a cardioid pattern, with the magnet and transformer located behind the ribbon. Presumably the original mic was a circular, end address arrangement.

If you recognise this or have any further information, we would love to hear from you.

Thanks to David for sending this in.

Reslo Black & Red Labels continued...

Last month I wrote a post discussing reslo microphones with red and black labels, and spent some time describing the transformers inside. Since that post I have measured more transformers, and the larger data set is beginning to show some definite trends.

Transformers from Black label mics - type 10202 

Perhaps the most revealing transformer characteristic is the primary inductance. The DC resistance is also important in terms of noise, but seems pretty consistent across the measured transformers - usually around 50 mΩ for the primary and 1.2 ohms for the secondary. DC resistance should depend on the thickness of the wire and the number of turns used for the winding, and this seems to be common to the transformers although occasionally the leads may become oxidised.

Measuring the inductance is a little hazardous as it is dependent on the frequency at which the measurement is made. For most of the transformers I have measured at 1kHz and also 100 Hz, but the first few were just measured at 1kHz. The inductance at 100 Hz is usually around 3 times that measured at 1KHz.

The graph shows the measured inductances at 1kHz and 100Hz when available (circles). The transformers marked with crosses were just measured at 1kHz, and the 100Hz value extrapolated from behaviour of the others.

Another way of looking at the transformer data is the relative cut-off frequency (fc) - the ribbon and transformer primary winding form a high pass filter. Assuming the ribbon has an impedance of about 0.3 ohms, then we can calculate fc. We use the value measured at 100Hz as this is closer to the frequencies of interest.

The transformers have a wide range of inductance / frequency values, but fall broadly into two classes. All but one of the transformers from the black label microphones show fc values of between 20 and 50Hz, whereas the red label ones have a much wider spread. About half of the reds are very similar to the blacks, but the others have a much higher fc value - between 70 and 110 Hz.

Many of the black label transformers are marked "10202", and are occasionally painted in blue, pink or purple. These are sometimes found in red label mics too. The transformers with lower inductance have a 'sandwich' of laminations with darker ones in the centre, and these are often marked "SE 4402".

SE 4402 type Reslo transformers

So it seems that we are somewhat closer to the truth about the red and black Reslos. The black ones are more consistent, and have a better chance of having a deeper bass response. Some of the red ones are just like the blacks, but about half the reds have a different, lower inductance transformer. These were probably designed for speech, and are not inherently better or worse than the others. However, if you are trying to record the lower frequencies of a bass instrument, or a fat electric guitar, then you may feel that the mics with the lower value for fc are more suited to the task.

We have spare 10202 black label transformers in stock, and also make replacement full range 300 ohm transformers for Reslo RB mics. So if your Reslo doesn't sound up to scratch, then get in touch!

Steampunk tube mic!

Here is a tube microphone that we put together for Jørn Christensen of Rodeløkka studio in Oslo, Norway. Jørn wanted a steampunk vibe... and that's exactly what he got!

BBC R&D reports on ribbon mics

The good old BBC have made many of their research and development reports available through their website, spanning 1944 through 1996.

Below are links to some of the reports that relate to ribbon microphones. In many cases there is a lot of interesting technical data. The rest of the articles can be accessed here.

1945
The Crosley ribbon velocity microphone.

1947
Tannoy Ribbon Microphone

1953
Lustraphone VR53 ribbon microphone

The design of the PGD and PGS ribbon microphones.

The EMI microphone Type 2351E.

Tannoy microphone Type MD. 422.

1956
Design of the PGD and PGS Ribbon microphone Part 2

1958
RCA BK5

1961
Reslo RB

1965
Beyer M160


(Thanks to Santiago Ramos for sending the links.)

More bad magnets

These rather disturbing photographs are of bad magnets inside a T-Bone RB500 ribbon microphone.

Failed magnets in a T-Bone microphone

The coating around the magnets had peeled away, allowing the rare earth magnets to oxidise and expand. This process of course destroys the ribbon too. The magnets can be replaced, but may not be worth the expense.

I have seen a few microphones that look like this, and the worrying aspect is that it seems to happen spontaneously, possibly because the coating on the magnets was not of high quality, or perhaps they were scratched or cracked on installation.

Nearly all manufacturers now use neodynium magnets for their microphones, and I fear a epidemic in the future.

Many thanks to Andrea Cappellato for sharing the photos of his microphone.

MOTM: very old LEM ribbon

April has been hectic! We have been repairing microphones, installing a new coil winder, and developing some new & exciting products. And suddenly it is already the 30th and just time for a very brief MOTM.

This month's mic is a beautiful and very old French LEM ribbon mic, probably from the 1930s....

Very old LEM ribbon mic

Inside, it is very similar to the smaller Amperite ribbon mics, with a large fibreboard frame supporting the ribbon, and a pair of (rather weak) magnets behind.

Old LEM mic deconstructed

The mic has a classic design, with brass sides, a folded steel grill and a cast steel yoke. These solid-sided microphones look strange now, but I guess the thinking at the time was that figure-8 microphones didn't pick up sound (or reflections) from the sides, which allowed for a very simple construction. The ribbon itself is positioned right at the front next to the grill, with a metal plate across the magnets at the rear, so there is some attempt at making the mic more directional.

The transformer had failed due to insulation breakdown, but with new magnets, a fresh ribbon and repaired transformer, the mic has a reasonable output and nice vintage tone. And it looks great!

Cadenza mic XLR modification

Cadenza ribbon microphones are quite common, but there seem to be more microphones than there are connectors for them. The mics were originally supplied with an integrated stand & connector, which was ideal for desk recording, but not very effective for hanging over a drum kit.

This mic was missing its connector, so here is a chop-job to convert to XLR output....

The connector was removed from the mic and the bottom thread cut off and filed flat. Then a piece of brass rod was machined to fit snugly into the base of the mic, and this was bored to accept a standard three pin XLR insert.

The XLR has the added advantages of making a good earth connection, and also gives a way of mounting the mic on a stand as it can be slipped into a standard mic clip. I think elongating the base  makes the look more elegant too.

Reslo black and red label microphones

Disclaimer

Firstly let me say that this is a study in progress, and should by no means be taken as definitive. Hopefully we will eventually have a big enough data set to be able to speak with confidence, but it will take a while!

Red and black badges on Reslo ribbon mics.

Are black label Reslos better, or even different from red ones?

There is a rumour that occasionally appears on the internet concerning the relative merits of Reslosound RB microphones. Some of the mics have red labels, and others have black ones, which has led to speculation that the mics must be different, and one type must sound better than the other.

Normally it is stated that the black badged ones are better. Most rumours have some basis in fact, so let's investigate!

Reslosound RB microphone dissected

Over the past couple of years I have serviced around 50 Reslo mics, with both colours of badges. Here are some of my empirical observations...

1. The black ones are less common than the red ones, but they are by no means rare. I don't have exact figures but perhaps 75% are red, and 25% black. I will be keeping note from now on!

Edit 29/11/2013: I wanted to correct this figure as I have seen it regurgitated on ebay a couple of times. Having seen a hundred or so more since I wrote this, I really can't say that one is more rare than the other. I would probably guess that they are equally common.

2. There are at least three styles of red badges from different periods.

3. Some later mics (red and black) have a white plastic ribbon holder. The older mics have black bakelite holders. This should not affect the sound.

4. The transformers vary greatly in both looks and specs. This will affect the sound!

So, the only real differences between the red and black label microphones are the transformers (and possibly the state of the ribbons).

Recently, I had seven 30/50 ohm Reslo RB microphones on the bench, and I took the opportunity to examine the transformers. Although the basic construction is the same, the transformers are quite different in looks, and have different inductance values! Some have a striped core with two metals, the middle often being darker or rusty, suggesting a higher iron content. 

Reslo transformers (left to right) A, B, D, E, F

Impedance and resistance values
This is hardly a statistically significant data set, but here goes...

Black labels
A. Lp = 0.463 mH, Rp = 84 mΩ,  Ratio = 1:12, fc = 103 Hz (purple)
B. Lp = 0.434 mH, Rp = 56 mΩ,  Ratio = 1:12, fc = 110 Hz (pink)
C. Lp = 0.470 mH, Rp = 56 mΩ,  Ratio = 1:12, fc = 102Hz

H. Lp = 0.514 mH, Rp = 52 mΩ,  Ratio = 1:12, (purple)
I.   Lp = 0.441 mH, Rp = 45 mΩ,  Ratio = 1:12, (pink)

Red Labels
D. Lp = 0.533 mH, Rp = 52 mΩ,  Ratio = 1:12, fc = 89 Hz
E. Lp = 0.204 mH, Rp = 63 mΩ,  Ratio = 1:13, fc = 234 Hz
F. Lp = 0.214 mH, Rp = 63 mΩ,  Ratio = 1:13, fc = 223 Hz
G. Lp = 0.454 mH, Rp = 49 mΩ,  Ratio = 1:12, fc = 105 Hz

Where Lp is the inductance at 1KHz, and Rp the DC resistance of the primary winding.

The mics are supposed to be 30 to 50 ohms output, and so from the ratio we can estimate the impedance of the ribbon and transformer itself to be around 0.3 ohms. The ribbon impedance and transformer inductance form a high pass filter, and so we can calculate the frequency, fc, at which the bottom end response drops away.* This handy tool means that we don't have to get out our calculators.

* It must be noted that the inductance of a metal core rises and frequency drops, so the cut-off frequencies will in reality be somewhat lower than these values. However, they should be comparable to one another.

What we can say for now, from our very limited data set, is that the three black label transformers, and two of the red ones, have substantially higher inductances and lower cut-off frequencies than the other two red ones. This difference in bass response is likely to be what some users hear as 'better'. However, it cannot be said that a red label mic always has less bass response than a black one.

The two transformers with purple paint have higher values than the ones with pink paint!

My feeling is that the later Reslos have 'better' transformers than the early mics, and that the colour is more cosmetic than diagnostic. But I shall keep adding to this list as more Reslos come into the workshop, and it will be interesting to see what trends develop.

And finally, if you are reading this and once worked for Reslo (or Grampian), we would love to hear from you.


Update 12 May 2012...

In 1961 the BBC R&D group studied the Reslosound RB microphone and recommended that the transformer be replaced with one of higher inductance. It seems plausible that the later Reslos were revised to use a different transformer following that study. You can read the BBC report here.

Stewart Tavener, Xaudia, First posted 24 April 2012, Latest update 12 May 2012

Marvin the Meteor

We have just installed a second Meteor ME307 coil winder at Xaudia, for rewinding ribbon mic transformers, pickups and making new parts.

Meteor ME307 with electronic controller

In fact we have were recently given a pair of used machines, and combined the best parts to make one good one. We call this one Marvin, because of his striking resemblance to the robot of that name.

Marvin the paranoid android from the original TV series of Hitchhiker's Guide to the Galaxy

Marvin the Meteor coil winder

Marvin even has a brain, although perhaps not the size of a planet. It has more sophisticated controller than our existing Meteor winder, which just has manual controls and a foot pedal. The newer model has preset adjustable ramp, speed, idle, and also a reverse wind setting, which is very handy and removes much of the human element from the winding. Let's hope he isn't bad tempered like his namesake !

Reslos in action 2 - Pete Gardiner

Pete Gardiner is a singer/songwriter and acoustic guitar player based in Newtownards, Northern Ireland and his new album 'Songs at Sunset' was produced by Paul Steen, who used a Reslo mic on the vocals alongside a Shure SM7b.

Paul bought one of our Xaudia upgrade transformers for his Reslo RB ribbon microphone, to give the mic an output of around 250 ohms. Paul fitted the transformer himself and used Ward Beck preamps for the recording. Paul said....

"I recorded with the reslo and an sm7b but the final edit ending up being 99% reslo. The sm7b is heavily compressed and dialled in on occasion for choruses etc."

You can find out more about Pete's album here.

Producer Paul's website is here.

Kolster Brandes KS651 speakers

I didn't have space for a Melodium sculpture, but here is something that I do have space for! I was delighted to find a pair of Kolster Brandes KS651 speakers at our local car boot sale this Easter weekend.

Kolster Brandes speakers

Kolster Brandes KS651 speakers

They were made sometime around 1970, and have an unusual but very chic cylindrical design, with mahogan-ish veneer and aluminium and gold trim.  The bottom acts a bass chamber, and a horn mounted on the top projects the higher frequencies upwards, giving an omnidirectional behaviour in the horizontal plane.

Inside, there is just a single six inch driver, rated at 15 watts, and a lot of foam padding.

It is always a risk buying used speakers, but they both work perfectly and I am rather charmed by the sound. They rather soften and round the edges of the program material, but maintain a good stereo image. Perfect for relaxing in the evening, and an antidote to a day of precise listening on headphones and studio monitors. Although only rated for 15 watts they are plenty loud enough for home use, and well worth the £40 paid.

Gramophone magazine reviewed these in 1970, and they retailed at £31 when new. They recommend placing them on a shelf, but I really can't agree with that. They seem perfectly suited to sitting on the floor of an elegant room and filling it with music - because of their omnidirectional behaviour they have a wide 'sweet spot', and are very easy to position.

They also make good cat pedestals, apparently!

Fantastic sculpture from Melodium mics

Here is something stylish to have in your living room! A wonderful sculpture made from several Melodium dynamic microphones.

At the time of posting, this was still for sale on French Ebay. I wish I had the space.

Xaudia Frankenphone ribbon mic

Here is a nice Frankenphone ribbon mic that I put together from spare parts ....

The mic body was salvaged from a broken SE condenser mic, repainted with some nice hammer finish paint and given one of our shiny brass nameplates.

The ribbon motor frame was an unidentified part that was found in an old Reslo mic, but fitted with new neodynium magnets to give a good strong magnetic field. And the transformer was, of course, wound here at Xaudia.

Ribbon motor with new magnets, and transformer

The microphone is rather nice sounding, with a full bottom, big proximity effect and a and good top end response too, for a ribbon. We'll be putting it through its paces with Silent Fears at the weekend, along with some other new toys.

MOTM: The Australian

This rather lovely RCA44 style ribbon is our microphone of the month for March. He was found on Australian ebay, and so will be known as 'The Australian' until his true identity is discovered!

'The Australian' Vintage RCA style ribbon mic

The mic has a chrome plated steel bottom, a steel yoke, and brass grills, painted black. Connection to the rear is via a pair of screw terminals for balanced output, but with no ground connection.

Although the body shape is very similar to an RCA44, the interior is more like the early Harry Olson prototypes, with a single large horseshoe magnet and broad flat pole pieces, giving a magnetic field strength across the poles of around 1000 Gauss.

The transformer is a dual bobbin type, with a primary inductance of 255 µH at 1 KHz, and 770 µH at 100 Hz. The thick primary wire gives a measured DC resistance of 27 mΩ, and the overall turns ratio is 1:50. With a 2.4 micrometer ribbon, the Australian has an output impedance of about 600 ohms.

There is no maker's name plate, and no sign that there ever was once. However, next to the ribbon there is some writing in pencil. This is a little tricky to capture on film, but it reads "RIBBON 23825 B1154".

Could this be a clue? Perhaps the 23/8/25 is a date from a previous ribbon installation? Sadly, I don't think so. 1925 seems a bit too early - although ribbon mics were invented in the 1920s, the RCA PB31 (first commercial ribbon and forerunner to the RCA44A) wasn't introduced until 1931. It seems more likely that this is a copy or prototype made by a small engineering firm, based around patent drawings.

Sound-wise, the mic has a rich warm tone with a decent output for its age, and the 600 ohm output makes it very usable with modern equipment. The lack of an earth connection makes hum an issue, and I am contemplating adding a third terminal to the rear, if I can find one that matches.

Perhaps it was made by an Australian manufacturer? AWA made copies of RCA microphones, and Zephyr were another Australian company that made some nice ribbon mics. For now it is a bit of a mystery, but we would love to hear from you if you know more about this.