The Xaudia Blog

28-05-2012

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.

27-05-2012

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.

25-05-2012

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!

22-05-2012

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!

06-05-2012

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.)

04-05-2012

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.

30-04-2012

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.

24-04-2012

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

21-04-2012

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 !