Last week I posted about repairing some vintage Decca London ribbon tweeters. I was confident that I had done a good job but was unable to do listening tests in context as I only had the tweeters to play with. These are now back with Andy, their owner, who kindly sent in his impressions and some photos. Overall I think we have done a good job and it’s a nice feeling to have helped get these back up and running.
Decca Volt speaker with ribbon tweeter
“The [new] ribbon sounds smooth, crisp and clear with no tizz or distortion. I ran it for a while at different volumes and with different music and it sounds excellent with everything I gave it. It is very close in sound to the original in the other speaker with the soundstage being in the middle and correct. I had to try hard to notice a difference, Bob Marley’s Is This Love has some high frequency cow bell drum noises and you could hear they were slightly crisper and louder from the new ribbon. The crossover point in the Volt’s is about 1200hz so any imbalance between the two tweeters is easily heard as the singer tends to move off centre.
New ribbon installation at Xaudia
“I think the differences could be classed as being between an old and new ribbon the sound from them is so close. I assume as the new ribbon breaks in it will soften in sound a bit. I am tempted to switch the other new ribbon in as well.
Andy’s impressive hi-fi setup.
“Considering the lack of info on the ribbons I thought getting a close match sonically was not good but it’s turned out to be very close indeed. It is a huge relief to know that if a ribbon gets damaged again I can get help.
“I am not sure but you might get a lot of interest, when I was trying to find someone to repair my ribbons I came across a huge amount of forum posts on the same issue dating back about 5 years. Nobody had managed to find anyone so in theory there are a lot of ribbons to repair out there.”
Decca London ribbon tweeters with huge transformers.
I have not repaired speakers in the past, but I recently had my arm twisted to look at some vintage Decca London ribbon tweeters. In theory they work in a similar manner to a ribbon microphone and should not present too much of a challenge…. right???? Anyway, it is nice to see something different on the bench and maybe I will learn something in the process.
The horn of a Decca ribbon tweeter
The first challenge was to take the speakers apart and work out what is going on inside. They have a large horn with two vents, which is is held in place with six screws. With these out of the way it is possible to remove the large, powerful magnet assembly and ribbon frame.
Brass mess behind the horn,
I received three of these ribbon tweeters but they are not all the same. Two have cast alloy horns and the third had a plastic horn. And the alloy horns have a brass grill that sits in front of the ribbon (shown above), whereas the plastic one has a nylon mesh, which would better protect against incoming metal particles.
These have large, powerful magnets
The ribbon is mounted in a square frame made from two slices of stiff material. All three frames were different and used combinations of cardboard, Tuffnol and what looks to be copper-less circuit board material. Some have silver plated ribbon mounts and others are bare copper.
Old ribbon (left) and new one waiting to be soldered.
The ribbon material in these speakers is stiffer and thicker than one would use in a microphone. The thickest foil I had in stock is 6.5 micrometer thick, supplied by Advent Research Materials, so that is what is going into these. That is about three to four times thicker than would be used in a microphone. The gap is 10mm and I cut the ribbons at 8.6mm, based on measuring a ‘good’ ribbon. Corrugating is routine, but the challenge here is making the electrical connection from the ribbon to the mounts. There are no ribbon clamps but I can see traces of old solder.
New ribbon in position.
Soldering thin aluminium needs a special solder and I used Alusol, which worked well after a little practice. All the contacts need to be cleaned before soldering. The solder needs quite a high temperature and you need to work quickly. To make this easier, I lightly glued the ends of the ribbon in place with nail varnish, which helped keep everything aligned. The top layer of the frame are then replaced – I used double sided tape to hold them together, and then the speaker can be re-assembled.
New ribbon as seen through the horn.
I was only had the tweeters, not the cabs or woofers, and so could not test with full range music. Instead I ran a sign wave through the speakers. They sound clear and without distortion down to about 500 Hz which is a good indicator that I have done a good job. In use they should only be working above about 7 kHz. Now they will go back to the owner for proper testing.
Some time ago I posted about GEC models BCS 2370 and 2373, and how they were essentially the same design in different body shapes for different applications.
Left to right – unknown, unknown. GEC BCS2370 and 2373 mics. All have the same motor.
Since then I have come across some other ribbon microphones which may be earlier GEC models, prototypes. They are perhaps rather ugly in style, but I find their functional utilitarian style rather charming.
The two unbadged mics share some parts including a twinaxial connector at the base and rubber yoke mounts. The mic on the left is made of folded steel, whereas the one on the right is brass, with a more open grill.
All four mics are essentially the same design inside. The mic on the far left has a smaller transformer and different brand of magnet, but the pole pieces and ribbon assembly are the same. The mic in the middle of the photograph above is identical inside to the known GEC mics – with the same magnet and transformer and crumbling plastic ribbon mounts.
Remarkably, both of these mics are still working with a nice tone, although a little noisy. A good clean and service should sort that out.
Drawing from a ribbon mic patent by GEC and Thomas Julian, 1947
Which in turn is obviously inspired by the BBC-Marconi type AX ribbon mic…
Drawing from BBC-Marconi type AX ribbon mic manual.
As an aside, the GEC patent is slightly odd, in that the major innovation is that the pole pieces are held in place by the magnetic field alone, with no mechanical fasteners. That is to say the major innovation is something that they have left out, rather than something they have added to the system. The implication being that other manufacturers MUST use a screw, bolt or other fastener, or else risk infringing the patent. It would have been interesting to see how that one would stand up in court!
Altec 670B (left) and 670A (right) ribbon microphones
Q. What is the difference between Altec 670A and 670B ribbon microphones?
I had not realised that there was a difference until I re-ribboned this 670A for a client, which gave me an opportunity to compare it to our studio 670B. The two microphones look identical from the outside apart from the label. Both are housed in some kind of blue-grey plastic, and the label hides an impedance switch cryptically marked ‘1’ and ‘2’.
Inside, the mics look similar at first glance, but on close inspection the ‘B’ model is a significant evolution of the 670A.
The ribbon in the earlier 670A is twice as wide as the later 670B, measuring 3.8mm and 1.9 mm wide respectively. The narrower gap between the magnets would also give a higher magnetic field, although I forgot to measure that.
Altec 670A ribbon and 670B (right) ribbons.
Both ribbons are the same length, so if the same thickness of foil is used, the 670B ribbon would have (about) twice the impedance of the 670A. This has implications for the transformer design, as a different ratio and different inductance would be required.
A second difference is in how the transformer primary wire is routed. In the 670A (left) the thick wire is routed directly behind the ribbon, which would affect the pickup pattern of the mic. This is redesigned in the 670A and passed to the side of the ribbon.
Altec 670A and 670B pattern selectors
Both mics were ready for new ribbons so I replaced both with 1.8 µm foil and measured the impedances (in position 2). The 670A had an impedance of 600Ω whereas the 670B measured 350Ω, which shows that the transformers are indeed different ratios.
It also suggests that the 670A would have originally been fitted with substantially thicker foil – perhaps 4 or 5 µm. The feel and stiffness of the corroded old ribbon also suggests this to be the case.
Here is a frequency plot of the two mics. The 670A is the red line, and the 670B, in blue has a slightly higher overall output. Both mics sound very good and quite similar in tone despite the differences.
This month’s MOTM is the Reslosound SR1 Studio microphone, which was a big brother to the RB models, and was Reslosound’s top of the range ribbon microphone. They are nice sounding figure-8 ribbon microphones, and are very much less common than the RBs, and have become quite desirable amongst collectors and studio engineers. Often they came in a stunning bronze colour with matching plug, like the one above.
The advertising at the time claimed 30Hz to 20kHz – although doesn’t qualify that with a dB range.
Reslo SR1 (top) and RB microphones
Extra sensitivity comes from using a longer ribbon and an extra magnet, and the mic is consequently about 1/3 as long again compared with the more common RB. By necessity the SR uses a different ribbon frame from other Reslo microphones. The mic below came complete with its original fibreglass stuffing – which protects the ribbon, damps any ringing of the shell, but seems to muffle the sound a little.
Inside the Reslo SR1
Reslo SR1 output connector
The RB microphones are quite notorious for grounding problems, as the mic body is only grounded through mechanical contact at the plug. The SR1 deals with grounding in a better way. The output socket has a hole drilled in the centre with a small wire pushed down and soldered to one of the lugs (see photo, by the green wire). When the mic is reassembled, the long screw that secures the connector makes firm contact with both the mic body and the central wire, giving a good solid ground. This works well, and I now use this approach to ground troublesome Reslo RBs too.
Reslo SR1 transformer, being re-wound
winding arrangement for SR1 transformer
The SR1 was available in both 30/50 ohm and 250 ohm versions, and used the same laminations and bobbin as the later RB transformers, but with a more sophisticated winding configuration. The SR1 transformer has a 5-winding construction, with two primary windings alternating between three secondary windings.
This transformer came out of a faulty Beyerdynamic M260.
Beyerdynamic M260 transformer
The mic was giving no output, but the ribbon was OK. The problem turned out to be a short in the primary winding, which gave an excuse to take a peek inside.
I was surprised how roughly the primary coil is wound – it really looks like it has been done by hand. Perhaps it is to minimise parasitic capacitance, although that is not usually such a big problem with low impedance windings as they only have a few turns.
In this case the short was located and the original transformer was repaired, but that is not always possible, and sometimes a rewind or replacement is required.
Some of the Beyerdynamic ribbon mics, such as the M500 and some models of M260 have a ‘built in’ high pass filter. This is done by manipulating the transformer inductance, allowing some of the bass frequencies to pass to ground. Some people like this, but others (including myself) prefer a full frequency response.
Xaudia B-series (top) and Beyer transformer (below)
Our new Reslo upgrade transformers are the result of this work. They are made here in our workshop in York, UK using high quality German laminations and teflon insulated silver plated lead-out wires.
They outperform the original Reslo units in terms of resistance noise and frequency response, giving less noise, an extended bottom end, and better impedance matching into modern equipment.
Typical Characteristics
Primary DC resistance 22 milliohms.
Primary Inductance >620 µH at 1KHz, >2.0 mH at 100Hz.
Frequency range <20Hz to >65 KHz.
Ratio 1:32 (available in 1:12 to 1:40, as required).
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.
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!
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
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”.
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.
Toshiba are better known these days as a giant manufacturer of consumer electronics goods, so it is perhaps surprising to find a ribbon microphone with their name on. In fact back in the 1960s Toshiba made some pretty decent models, some of which were good copies of RCA mics.
Beneath the outer grills lies a perforated metal baffle backed by a finer mesh screen, which protects the full length of the ribbon against air blasts and pops. The top and bottom of the shield have a tendency to go ‘ping’ – I could actually hear this ringing when speaking into the mic, so a little bit of sticky foam was used to damp this.
Rear of the Toshiba showing transformer and magnets
From the rear, we can see that the field is suppled by a pair of strong horseshoe magnets glued to the pole pieces, which give a measured field of about 3000 Gauss between the poles. The transformer is a twin core ‘humbucking’ type, in this case wound for high impedance.
Once the inner screen is removed, two features stand out as unusual. Firstly, there are no ribbon clamps! The ribbon is simply glued to the supports, and then soldered to the terminals. The arrangement works well enough, but you only get one go at fixing the ribbon.
Toshiba ribbon
The second interesting thing is the small cross bar that bridges the pole pieces. This is actually glued to both the pole pieces and the ribbon itself, dividing the 3.6 mm wide, 60 mm long ribbon into two sections in a 3:2 ratio. I can imagine two purposes for this – to stop the long thin ribbon from travelling to far, and to minimise overtones from harmonic motion.
I have also seen this ‘node’ on another Toshiba ribbon model, so it does seem like a little trick of theirs. The nearest thing I have seen in other microphones is in the Cadenza mics, where the ribbon is glued to a support half way along.
Microphone cleaning
This mic was rather dirty inside. Lots of little bits of iron were interfering with the ribbons movement, making it sound like it was scraping against the sides – which it was! These were easily cleaned with some sticky tape, but the ribbon had to be sacrificed first.
New ribbon in the Toshiba mic
A new ribbon was fitted – soldering 1.8 μm aluminium foil is a bit tricky, but I got the hang of it after a couple of tries. And gluing the cross bar to the ribbon also requires a steady hand!
Once re-ribboned and reassembled (and fitted with a low impedance transformer), the microphone sounds nice, with a relatively flat response up to around 6KHz, where it begins to roll away.
At Xaudia, one of the most common enquiries that we get is for replacement cables and connectors for vintage microphones. Very often we can help, but some of the connectors are becoming impossible to find in good condition and at reasonable prices.
Grampian GR1 ribbon mic with connector & cable
One example is the Grampian plug that was used for their GR1 ribbon and other microphones. These connectors are hard to find – they can be obtained by buying a less valuable Grampian dynamic mic, but we have found that the plugs come with various slightly different threads, which means that one plug cannot be guaranteed to fit another microphone. And the connector is not ideal in the first place – it has two prongs and for balanced use the screen of the cable is connected to the plug casing (and therefore the microphone) by pressure only. Grounding issues are therefore common.
There is, however, another approach, which is to dispense with the connector entirely, and replace it with a modern, industry standard XLR connector. This microphone arrived without a plug and with a rather battered and bent connector at the bottom of the mic.
The decision was made to replace it with a silver-plated XLR output, which also solves any grounding issues as the third pin may be connected to the body of the mic.
The old connector was cut off, and the housing threaded to accept the new connector.
The new XLR output is then simply screwed in place… and the mic rewired and put back together.
XLR modified Grampian
Everything works nicely – just plug a standard XLR mic cable and it is ready to record.
Some may see this as vandalism, and perhaps from a collectors point of view, it is. However, the conversion is sympathetic, and it is far better for the mic to be making recordings, than to be languishing in a box unused.
Last month we were lucky enough to see this rare and beautiful B&O BM2 microphone pass through the workshop.
According to the BeoPhile.com website, these were made from 1951, and was replaced by the very different looking BM3 in 1962. Although different in appearance, the motor assembly and ribbon are similar in both cases.
The BM2 also has a switchable high pass filter to select between the music and voice modes, and also shares a hard-to-find connector with the BM3 & BM4.
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.
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 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…
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!
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.
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.
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.
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:
Update 29/2/12. And here is one with a Mimco badge!
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.
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.
Addenda – one of these came up recently for sale complete with original box. The owner was kind enough to share this photo of the label.
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:
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 1.8 micrometer ribbon is very pleasing.
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.)
