Toshiba are well known for making mid-range consumer electronic equipment. Once upon a time they used to make some fine ribbon microphones too!
Toshiba A (left) and RCA PB144 ribbon microphones
The microphone on the left is generally know in internet-land as the Toshiba ‘Type A’, although it is labelled SN-1631. It is a very close copy of the RCA 44A and its relatives. I have been informed by a Japanese expert that this was made under license by RCA, most likely in the post-WW2 era. Having looked carefully at the Type A and compared it to my own RCA PB144 (right), I have no reason to doubt this assertion.
The mics look similar outside – my PB 144 has a film set style hanger mounting, whist the Toshiba has a simple cast yoke, and the grill hole size differs.
Inside the microphones there are many similarities and few differences. Both use three large horseshoe magnets to provide the magnetic field, and and the ribbon dimensions are very close.
Perhaps the biggest difference is that the RCA uses cast pole pieces, whereas in the Japanese version they are milled. This probably reflects the tooling and machinery available at the time. Casting is an expensive process for low quantity products.
Beyond the cosmetics, the Toshiba has a 200 ohm output transformer whereas my PB 144 is a 50 ohm microphone, with these no doubt being in line with the broadcast standards of Japan and USA at that time.
Here is Tannoy’s cardioid ribbon microphone – the MD422.
Tannoy MD422 ribbon mic, front
Firstly, Tannoy lose a point for the name. ‘MD’ should surely mean “microphone dynamic” in any sensible society! Perhaps the D stands for “directional”? Who knows, but it puts them at odds with other the Sennheiser MD421, and it is just plain confusing.
Tannoy MD422 ribbon mic, rear
Whatever the D stands for, the mic itself has an industrial look, and this one is finished in a bronze-ish coloured paint. To the best of my knowledge this is the only cardioid ribbon model that Tannoy ever produced, and it uses an acoustic labyrinth to provide the necessary back pressure to the rear of the ribbon. The chamber is the black cylinder in the photo below.
Tannoy MD422 ribbon mic, chamber
Some of the parts were made to a budget, or perhaps a short production run, with thin stamped metal for the base and top cap, along with two layers of off-the-shelf mesh to protect the mic from dirt and wind.
The ribbon assembly and magnets are the same as found in the type 2 Tannoy ‘pitchfork’ microphone, which would have saved costs by sharing components.
Tannoy MD422 inside, showing ribbon motor
The MD422 was subject of a BBC technical report, which concluded:”..this microphone has nothing to commend it for use in the Television Service“. That assessment seems rather harsh, but at the time the Corporation’s selection criteria were flat, uncoloured responses and good signal/noise performance.
In the report, the bi-directional BBC-Marconi AXBT was used as a comparison, which was much more expensive, a different pattern, and therefore a tough benchmark. Cardioid (and non-directional) ribbon mics generally use an acoustic chamber on one side of the ribbon to apply pressure, and this damps the ribbon motion, reducing the output compared with its natural bi-directional response.
Drawing of the Tannoy MD422 from BBC technical document.
Unlike my mic, the BBC’s example had a yoke mount rather than a fixed base, although there are mounting screw holes in the side of mine.
Despite my irritation with the model number, and the BBCs damning assessment, I like this microphone very much. It has a gentle warm tone. More importantly, it was an attempt at technical innovation, which is always to be celebrated.
This was Reslo’s attempt at a commentator’s lip microphone, and appears to be cobbled together from spare RB parts. The upper body and head are from an RB, but the head is rotated through 90 degrees and screwed to another chopped-down head. The lower body-stroke-handle is a straight aluminium tube with a switch which terminates in a normal Reslo output socket.
Inside the head is a rotated RB-style motor. The magnets face the speaker, to provide some pop protection for the ribbon at the rear.
Behind the ribbon I found this metal baffle, which should control the pickup pattern and tweak the frequency response. There is also fibre glass and felt wadding for more protection.
Overall it is a strange little mic. The ribbon runs horizontally, which is generally considered a bad plan as, if it sags, it will droop into the metal pole pieces. They must have been a way of producing a lip mic without the expense of re-tooling for a completely new design. I have only ever seen this one, although I know of a customer who owns another.
Sadly we don’t have the mount, but as this shares a connector with the Reslosound mics, we can use a Reslo stand mount for it. None of the other parts are common to the Reslos, so they must have just sourced the connector from them.
Inside, the mic is a fairly conventional figure-8 ribbon mic, but has added baffles both sides of the ribbon, and thick wires run from the top ribbon clamp, giving some blast protection along the way.
The ribbon motor uses tapered triangular pole pieces, which compensate for the decrease in magnetic field with increasing distance away from the two block magents, and ensure a more even magnetism along the length of the ribbon.
And of course there is a suitable transformer in the base to convert to a sensible impedance – in this case 50 ohms. The mic sounds rather nice, rich and dark close up – it may make a vocal session very soon!
Although much much smaller in size than the Melodium 42B, the RM6 takes some of its design philosophy from its predecessor. The motor assembly has horseshoe magnets above and below the ribbon (like a tiny RCA 44BX), and an inductive filter with multiple cut-off frequencies. It also has an obsolete and hard to find proprietary connector, albeit a different one from the 42B.
The body is made from cast metal which has a tendency to crack, and the first challenge is to get inside the mic without damaging it. There are three tiny metal pins pushed through the mic that fix the grill to the lower body of the mic. These need to be pushed all the way through the mic so that the grill can be removed, and usually these can be found stuck to the magnet, although one will inevitably go missing!
To remove the mic completely, the connector must be unsoldered and the switch tip removed. Once inside, we see something that looks like the Easter Island statues!
The ribbon is hiding behind the baffles, and the motor requires quite a lot of disassembly before the ribbon can be accessed. The ribbon itself is about 1.8 mm wide, so a bit fiddly to fit. Like most mics of a certain age, half the problem is that the ribbon has become oxidised and stiff, and the other half is small particles of wild iron that that have become stuck between the pole pieces, preventing the ribbon from moving freely.
Removing the strong magnets made cleaning and re-ribboning a lot easier! The transformer and filter inductor are housed in a mumetal can, screwed beneath the motor assembly and above the filter switch.
As usual, the mic sounds best without the high pass filter engaged, although I can imagine it being useful to compensate for proximity effect when close micing some instruments.
Our microphone of the month for September is this a rare and lovely BBC-Marconi ‘type B’ ribbon microphone. The type B is closely related to the ‘A’ series of mics (AX, AXB and AXBT) which were made from around 1935 onwards. The model B was made from 1937 onwards and used the same motor assembly, but with a smaller (yet still massive) magnet and without the ribbon tension adjustment facility.
BBC Marconi type B, side view
The smaller magnet format allowed the mic to be packaged into a smaller cylindrical body, and these were apparently were often used for outside broadcast, attached to the chest of the broadcaster! One would have to be pretty strong to carry that around all day.
Although this is widely know as the model ‘B’, the nameplate calls it 8559A.
BBC Marconi type B badge
As well as the outer grill, there are two extra windshields inside the mic, which would have protected the ribbon from wind blasts and dirt. These windshields did their job well – in this example, the ribbon is unbroken, but this has oxidised and stiffened over the years and will need to be replaced.
BBC Marconi type B ribbon motor
The magnet and motor assembly are held in place with foam rubber, which has mostly survived the years. The output transformer is hidden behind the magnet. This had a break in the secondary winding, but luckily I was able to unwind it by a single turn and bring the mic back to life.
BBC Marconi type B, transformer
At the rear of the mic, connection to the outside world is made by three screw terminals, and square bulge at the bottom shows the location of the transformer.
BBC-Marconi ribbon mic, rear
Overall, the mic sounds warm and rich, although the metal can gives it a certain ‘boxy’ sound that instantly defines it as a very old microphone!
There is some more info about the BBC-Marconi ribbon mics at the Coutant and Orbem websites.
The MR1 was an attempt by Reslosound to re-invigorate the classic RB model for a 1970s market, whilst trimming down the cost of manufacturing parts and assembly.
Reslo MR1 and RB models
The MR1 (left) has the same format as the RB, with the ribbon element and magnets housed in a tilting head attached to a cylindrical body that contains the transformer. The grills are now flat, rather than domed, and the custom pivot screw is replaced with a standard hex socket bolt.
Reslo MR1 and RB
The MR1 still usesd Reslo’s custom 3-pin connector, and still makes ground contact through the body only. One assumes that this was to ensure backward compatibility with their older mics and cables, but it seems like madness that they did not take the opportunity to switch to using XLR connectors, which were very well established by this time.
The classic stamped brass badge on the RB is replaced by a printed foil sticker that hides the join between the barrel of the mic and the cast base of the pivoting head. The sticker shouts ‘Reslo MR1’ – note that it no longer says ‘Reslosound’ – in a very trendy 1970’s style font, with fattened letters typical of the era. I am also 1970s vintage, and the label reminds me of watching programmes like Magpie on TV in my childhood!
Magpie – classic 70s font!
The ribbon and motor assembly (top in the picture below) is exactly the same as the RB, but the transformer is smaller. Over time, better understanding gave rise to alloys with higher magnetic permeability, enabling smaller transformers to be made with the same inductance properties. The RB mics used a EE24/25 size core (pictured below the motor). The MR1 has a narrower body and a smaller transformer marked 4414.
Top to bottom… Reslo MR1 motor, transformers from RB, MR1 and Xaudia
Xaudia, as you might expect, make a suitable replacement transformer (bottom) for upgrade and impedance matching duties. 😉
Compared to the RB, I see very few MR1s, and can only conclude that they were not a great success. By the 1970s, good dynamic mics were becoming the first choice for home recording, and professional recording studios would probably favour condenser microphones for many applications. Reslosound Ltd. disappeared sometime in the 1970s, and this must have been one of the last models that they produced.
Here is a rare and magnificent microphone: the Melodium type R (number 12).
The microphone is dated 31.12.40, which makes it 72 years old at the time of writing! The mic was made by Melodium for the national broadcaster “Radiodiffusion Nationale”, which would have been the French equivalent of the BBC. This is particularly fascinating as it dates the manufacture to during the period of occupation in the Second World War. I was surprised that microphones were still being manufactured during that period, but I suppose that the government needed to broadcast their propaganda, or perhaps this one was stolen and used by Le Resistance!
Inside, the mic is very similar to the 42B, with the same magnets and motor assembly, but the internal grills are different, using a wider mesh and cloth.
The high pass filter section of the mic is also different to its square cousin. Instead of the three way rotary switch on the 42B, this mic uses a metal bar to bridge a terminal inside the base of the mic, and it just as ‘Music’ and ‘voice’ settings. A hole in the bottom plate lets you see whether the jumper bar is engaged or not. It is a more reliable arrangement, but you need a screwdriver and spanner to make the change. I guess the guys at the radio station did not want the filter to be engaged accidentally, causing them to wonder where the bass had gone!
Remarkably, the ribbon on this one was still intact, and after cleaning the motor the mic sounded very good indeed, with a big bottom end and nice proximity effect to make voices sound richer and fuller.
New parts for the old microphone.
The mic arrived without some parts, so we made replacement brass washers and thumb nuts on the lathe. They look really smart, and in a few months will be a lot less shiny!
Melodium 12 side by side with the 42B
In comparison to the 42B, the model R has a bit more bass, although this could simply be due to the differences in tension and mass of the ribbons. There are other small differences to the high frequency sonic signatures, primarily due to the differences in grill shape and mesh.
Frequency sweeps for Melodium R (red) and 42B (Blue)
The mic is now back home at Kore studios, in good company!
We have TWO microphones-of-the-month for May. Both are cardioid ribbon microphones from Beyerdynamic. We see a lot of M260 models, but the M320 and M360 are bigger, uglier and much less common too.
Beyer M320. Do not stick it in your kick drum!
The M320 makes good use of plastic for the body, and has all the style of a brick! A Beyer catalogue* from 1965 called it a “heavy-duty dynamic type for show business use”, and the switch allows the use to select ‘music’ or ‘speech’ settings, the latter giving a bass attenuation of 12dB at 50Hz. Because of its appearance and a vague resemblance to the AKG D12, it is tempting to thing that this is designed for inside a bass drum. It is not.
The M320 has the same motor that is found in the M260, but is connected to a large rectangular chamber below the mic via a tube. I have not been brave / stupid enough to cut open the chamber to see what is going on inside, but one assumes a series of interconnected tubes forming an acoustic labyrinth.
Beyer M320 inside
Although the M320 is called ‘heavy duty’, the M360 is much more sturdy, with a metal housing and grill. The catalog calls it a “dynamic unidirectional studio microphone with ultra-modern styling“. Again, it is as elegant as a 1980s Volvo, but at least form follows function in these mics!
Beyer M360. Ultra-modern?
… or just ugly?
The M360 is slightly larger and like the M320 it also has an acoustic chamber below the ribbon motor. I was fully expecting to see the same motor in this mic as is found in the other Beyerdynamic ribbon microphones of the same era, so was suprised to find something quite different. Rather than being assembled from four small magnets glued together, the M260 has a larger cast magnet with two copper coloured pole pieces which may also be a magnetic alloy.
Beyer M360 motor
Although the motor is wider and deeper, it still accommodates the same size of ribbon. And the magnetic field measures around 5000 Gauss, which is similar to a ‘normal’ healthy Beyerdynamic motor. So I am really not sure why this uses a different motor. Perhaps it was an early production run. The transformer and switch (on/off) are housed below the acoustic chamber below the microphone.
In the photo above, the ribbon is fully corrugated, but another specimen (below) had a standard Beyer ribbon, which seems more likely to be the original.
Both mics sound good when working properly, with a cardioid pickup, a relatively flat response and a bit less proximity effect than you would find in a figure-8 ribbon. They use the typical tiny Beyer transformers, which in the case of the M60 at least is wound for a full range response.
The Toshiba model K was one of the later Japanese ribbon mics. By this point they had really mastered the technology and were producing high quality microphones to rival to the American mics. It is slightly shorter than an RCA 77DX, but is equally heavy and well built. I very much like its stubby looks and slotted grill holes.
Rear of the model K, with pattern control
The model K was designed to broadcast standards, and this specimen was obviously made for NHK (Nippon Hōsō Kyōkai, or Japan Broadcasting Corporation), which is Japan’s equivalent of the BBC. Like most Japanese ribbon mics, this has a 600 ohm impedance and gives a strong output.
Inside the mic, the motor is based around a single strong horseshoe magnet, with the ribbon held between two chunky pole pieces.
Like the RCA 77DX, there is an acoustic labyrinth made from a series of holes with connecting channels, which goes up and down the centre of the mic. Two thick wires take the signal from the ribbon, through the labyrinth, down to the transformer below.
Acoustic labyrinth in the middle of the mic
The pattern control uses a choice of baffles to partly or entirely redirect the rear of the ribbon into the acoustic labyrinth. This turns the mic into a pressure transducer when the rear baffle is closed, giving a more omnidirection pattern.
Pattern control on the Toshiba type G.
It differs from the RCA design: the 77DX has a a cam shaped copper plate that allows the rear vent to be opened by incremental amounts, whereas the Toshiba has three discrete positions, which are labelled…
N (fully closed – non-directional)
B (fully open – bidirectional or figure 8), and
U (a small opening – unidirectional or cardioid)
And in an attempt to beat the Americans, on the bottom of the mic there is a switch for a 6 position variable frequency high pass filter – the RCA77DX only has three!
High pass filter switch
Update 27/5/13… this is how the filter affects the frequency response….
Lustraphone was a British manufacturer of audio and hi-fi equipment based on Regent’s Park Road in North West London. They traded from 1942 until sometime in the 1970s, when the company was dissolved. This month’s MOTM is the Lustraphone VR64 pencil ribbon mic, known s the ‘ribbonette’ because of its small size. It was the successor to the VR53. They were also sold under the EAP / Elizabethan brand.
The VR64 is an attractive looking small ribbon mic, with a slightly wider grill at the front than the rear. Like its competition from Reslo and Film Industries, the magnets are positioned to the rear of the ribbon making it an imperfect figure-8 pattern. It was produced in several colours – I have see brushed chrome, blue paint, and grey hammertone examples.
Add caption
I always thought that Lustraphone had one of the coolest logos, a bit reminiscent of The Man From UNCLE TV series. Different models have different styles of badges, either applied as a decal or a sticker.
When working properly, the VR64 sounds rather good, with a higher output and better top end response that the VR53. Like most of the British ribbons of the time, the Lustraphone mics were available with different impedances. The mics come badged with either ‘High’ or ‘Low’ impedance, but that doesn’t always tell the whole story. Two examples pictured here were both marked low impedance. However, one measured 15 ohms and the other 600 ohms, so it is important to know what you have.
The output transformers have a slight quirkiness, in that the bobbin is pushed halfway through the frame of the mic before the laminations are added, which neatly solves the problem of mounting the tranny.
Lustraphone VR64 inside
Lustraphone VR64 motor – rear view showing magnets
I’m not sure if having a layer of insulator between the laminations has any sonic effect, beyond the fact that the transformer has one fewer layer of laminations in the stack, and so a lower inductance and higher cutoff frequency. Probably little or none.
The 15 ohm transformer is made up from 15 turns of 0.85 mm wire for the primary, and about 145 turns of 0.3 mm for the secondary, giving a ratio just under 1:10. This one was re-wound for 300 ohms using the original lams, which gives a better output level into a modern preamp, whilst retaining the vintage character.
Lustraphone transformer autopsy
This example had a quick release mounting system. A bullet shaped part screws onto the mic stand, and then the mic is pushed onto it. Very cunning!
I have no idea who made October’s microphone of the month. But it is quite a beast, dwarfing the SM57 that I have used as a geologist’s hammer*.
Italian ribbon microphone by an unknown maker
This one came from Italian ebay, and has a whiff of DIY about it. Except that if it is, the work quality is very high. It could well have been a project for an engineering degree. Or perhaps a prototype from a professional workshop? Some parts of the mic are well thought out, but ultimately it is let down by the transformer and low magnetic field.
The microphone body is based around brass tubing with an imposing chromed grill and chromed bells at each end. The base is fitted with a (horrible) 4 pin CB-radio style plug. The yoke is nicely bent aluminium with a turned base and wingnuts.
Italian ribbon mic – motor and magnets
Inside, four large cylinder magnets are clamped between two heavy blocks of mild steel, and the ribbon motor sits at the centre. There is a hint of the RCA 44BX in this approach, and the ribbon is of similar dimensions to that mic. The ribbon itself is clamped with sturdy brass blocks. These magnets here are not really strong enough for the job and the measured field in the ribbon gap is around 1000 gauss – a bit low really. Perhaps they were once stronger than this.
The transformer has its own internal can for extra screening, and the connections to the tranny primary are made with heavy copper for very low resistance. Good thinking!
Ribbon mic transformer
The transformer itself looks home-made from recycled laminations. Although the thick copper wire for the primary is a sensible choice, the inductance is a rather low 24 microHenries and there is no chance of reproducing a full frequency range. These lams look familiar – I have seen similar ones in Thiele microphones for the power transformer, and also, I think, in Geloso amplifiers
Domed end cap of the mic, with striped output leads
The maker, whoever he was, has used brass, mild and stainless steels, copper and aluminium for the construction. Parts are turned, milled, brazed, folded and domed. It looks more and more like an engineering workshop project designed to showcase the maximum number of skills, and in that respect it does a very good job.
Better laminations or a new transformer would show the true potential of this mic, as would some stronger magnets to boost the field. I will try that!
If anyone out there knows more about this mic, I would love to hear from you.
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.
It is said that imitation is the sincerest form of flattery. If that’s true then the RCA 74b junior ribbon was one of the most flattered microphones ever made!
In the middle is a ‘real’ RCA 74b and on the right is a 74b made under license in Italy by Magneti Marelli. But what’s that on the left? It’s our microphone of the month, and I don’t know what it is.*
When I first saw this I thought it must be another rebadged or licensed 74b. The outer dimensions are almost identical to the real thing, and it even has a red ‘meatball’ badge, although this time with some unidentified Eastern-looking symbols – perhaps from somewhere in South East Asia.** It even has a good copy of the swivel stand mount.
Inside the 74b clone
But inside the mic there are some differences. The ribbon has similar dimensions, but the magnet assembly is quite different, with two large, strong horseshoe magnets providing the field through a pair of U-shaped pole pieces.
Rear view with horseshoe magnets
In a ‘real’ RCA 74b, the magnets are at the front and bevelled, with a rectangular frame at the rear.
‘real’ RCA 74b microphones inside
In the photos above the transformer is a replacement. The original transformer, below, was faulty, probably due to insulation breakdown. Again, the marking on the transformer looks very Eastern in style, and appeared to have been written with a brush, in eastern style.
Perhaps the oddest thing about this microphone is that it has been shot! There is a pellet embedded in the side of the mic. Perhaps it was used as target practice. Or just maybe it was in use at a radio station in the last days of Saigon….
*Thanks to John Gooding!
** I have tried to identify these but no luck yet!
—————-
Update, 13 August 2012
A couple of readers have sent in messages about the provenance of this microphone….
“My friends here in Thailand says the symbol is almost definately Chinese. It is certainly not Thai, Vietnamese or Cambodian. By most stuff here has been made in China since China started making things, so your Saigon theory is still the best.” James Browne, Hungry Ghosts
“Hi Stewart. Sent picture to my factory in H.K, the marking is definitely Chinese,” Andy Hearn, ECM
—————
Update, 15 June 2022 (ten years later…)
Thanks to Adam Pan from Taiwan for providing this additional information and photographs.
“The first photo left one ribbon mic is make by Shanghai Microphone Factory in 1940’s.
After a long time looking for some China made microphone. when I move to live in Shanghai China from 2001, I found two ribbon microphones in China, one is the same as yours, the logo is the word “Ding”(鼎) in chinese, that is “tripod” the meaning is “one word is as heavy as nine tripods”. Reminder to “keep your word”. And the other one’s logo is “He-Ping”(和平)，That meaning is “Peace”.
A few years ago, I helped Superlux develop some products, and I met an older engineer who is retired, He was involved in the manufacture of Type 74 microphones at the Shanghai Microphone Factory.”
I have a soft spot for Oktava microphones. They look cool, come from Russia, are fairly priced, and everyone has fun modifying them. That’s enough to make the ML52 microphone of the month for June.
Oktava have a long tradition of ribbon microphones dating back (at least) to the ML11 in the 1950s. The ML52 is one of the most recent models, with a slightly odd double-ribbon motor assembly. I had some in to service recently, and noticed something odd….
Here are two ML52s – can you see any differences?
The answer is “Yes”. The bottom mic has a logo and serial number, and an XLR that is machined into the bottom bell. It also has slotted screws rather than pozidrive. The grill spacing is also slightly different, but that is harder to see on the photos.
Under the grill, both have twin ribbons, but the motor assemblies are totally different. The one on the left is the same as the one found in Avantone microphones, the right hand one looks typically Russian, with silver contacts and big baffles to protect the ribbons.
The transformers are also very different. The right hand one is the mic with the logo, and it uses a toroidal transformer. The left hand transformer can looks typical of the China mics, and is connected to a (pointless) circuit board.
Sound-wise, the mics actually sound quite similar, which goes to show just how much the body and grill influence the sound of these mics. The Chinese one has a longer ribbon and stronger (neodynium) magnets, and has slightly stronger output.
Response plot for Russian (Red) and Chinese (Blue) ML52 mics
I don’t want to say the word “fake”, but if forced to jump, the obvious conclusion would be that one mic is from the Oktava factory in Tula, Russia, and the other is made in China – either under license, or perhaps not. 🙁
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.
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!
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.
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.
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…
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.
Our Microphone of the Month for May is the magnificent Melodium 42B ribbon microphone.
Evolving Melodium 42b – serial no. in the 1700s, 4300s and 6600s (L to R)
The Melodium 42B was conceived as a French alternative to the RCA 44 family of microphones, and is in its own way every bit as good as the more celebrated RCA mics. At 32 cm high and 14 cm wide, and weighing 2.65 Kg, the 42b is a huge microphone with a large ribbon (68 mm long by 4.2 mm wide) and big powerful magnets*, to give a strong output with low noise.
We have been lucky enough to have four of these through the workshop, and have had the opportunity to observe some for the finer details that have evolved during the production of these beauties. Over the years the grill of the 42b has been refined, with the holes becoming larger, and a solid unperforated band appearing across the bottom of the microphone. Later microphones have a three pin connector at the rear, whist early models are hard-wired.
The magnet structure has also changed over time – earlier models have U-shaped magnets above and below, whereas the later microphones have four block magnets, arranged in pairs and connected by metal plates to complete the magnetic circuit.
The photo on the left shows the later style of magnets. The transformer and inductor are in the circular metal can below. Some models have a rectangular can, but the transformer and inductor inside are similar, at least in the ones we studied. The transformer has a ratio of 1:14, giving an output impedance of 50 ohms**
There is also a facility to adjust the ribbon tension, which should be mandatory on all ribbon mics!
Here’s a short summary of the changes with serial number:
1500 – Small grill holes, fixed cable (no connector), U-magnets with North marked, rectangular transformer case
1700 – Small grill holes, fixed cable (no connector), U-magnets, circular transformer case
1900 – Small grill holes, fixed cable (no connector)r, U-magnets, square transformer case
3400 – Small grill holes, connector, U-magnets (unmarked), rectangular transformer case
3700 – Big grill holes, connector, U magnets, rectangular transformer case
4300 – Big grill holes, connector, U magnets, rectangular transformer case
5100 – Big grill holes, fixed cable (no connector), U-magnets, rectangular transformer case
5300 – Big grill holes, connector, U magnets, rectangular transformer case
6100 – Big grill holes, connector, U magnets (North is marked), rectangular transformer case
6600 – Big grill holes, connector, block magnets, circular transformer case
8000 – Big grill holes, connector, block magnets, circular transformer case
The latest serial number that I know of is number 9250, on the Coutant website, which has large grill holes and a connector.
The 42b has a three-position switch on the front which is used to select “Speaker”, “Voix”, and “Musique” modes. In the first two positions an inductor is switched in parallel with the output transformer which causes the lower frequencies to be rolled off.
Above is a frequency plot that I recorded in our anechoic mic testing chamber (well, more of a walk-in cupboard really!). This was recorded at 40 cm distance from the sound source, which is a concentric full range speaker, using a swept sine wave technique.
The bottom end boost due to proximity effect is pronounced, even at this distance, and the effect on the sound is very musical when capturing acoustic instruments. Of course sometimes you don’t want or need the proximity boost, and the switch conveniently corrects for this at speaking and singing distances.
If you are lucky enough to come across an old Melodium 42b but are disappointed with the sound, it may well have an oxidised ribbon and will need cleaning. The strong magnets are prone to attracting little shards of iron, which stick to the magnets and interfere with free motion of the ribbon.
* It is worth noting that the magnets on these microphones can fade with time – a healthy 42b should have a magentic field between the pole pieces of around 4000 Gauss. One of our microphones measured at just 1500 gauss and it was necessary to replace the magnets with suitable modern alternatives. If you have a Melodium with a weak output it may be worth having the field measured and the ribbon checked. Xaudia can of course help with all those things.
April’s microphone of the month (MOTM) is this glamorous shiny blue ribbon microphone by Framez (not to be confused with Framus!)
This has no model or serial number, but is probably from later 1950s or early 60s. Framez were an Italian brand related to Meazzi – according to one source the name is a contraction of Fratelli Meazzi (trans. Meazzi Brothers), which does sound plausible. Framez / Meazzi also made some cool oddball guitars, and were associated with Wandré Pioli. Fetish guitars have much more information about these guitars.
Back to the microphone! This looks very much like a copy of the RCA 74b ‘junior’ microphone, but us physically somewhat smaller. And it is a pretty good microphone in its own right. The magnets have retained their strength over the years, measuring a healthy 4500 gauss between the pole pieces. It has a hefty transformer with taps for both low and high impedance, making it suitable for both recording and PA use.
This one arrived with a thick flat ribbon – probably a DIY ‘kitchen foil’ repair. With a proper 2.5 micrometer ribbon installed, and the mic rewired for balanced, low impedance operation, the sound is clear with a strong output, just lacking a little of the low end proximity boost that you find with many ribbon microphones. The ‘low’ impedance tap is 1:45 ratio, which gives around a 450 ohm output with the 2.5 micron ribbon.
I’m actually very impressed with this little microphone, and it looks great too!
Update 23/4/11
We managed to get hold of a Meazzi ribbon microphone for comparison. It’s a little less glamorous, and has a similar ribbon dimensions, but a very different motor assembly.
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.
This unusual Sony microphone (left) looks like the younger brother of a C38b (right) or maybe a C48 condenser mic, and shares the same high build quality and some hardware components with these mics. But in actual fact this is a high quality cardioid pattern dynamic microphone, model F-V300.
Inside, the microphone is very simple, with just a capsule in the top compartment, and the on/off switch and transformer wired to a printed circuit board below. The fixed grey output cable looks to be the same type as found on the C38b.
The capsule is suspended from 8 small springs to act as an internal shockmount, which seems to work well, in conjunction with the yoke, to acoustically decouple the microphone element.
This example has some traces of foam around the element, indicating that an internal windshield has at some time been removed.
The label states that the mic is nominally of 1kΩ impedance, although in practice seems lower than this and it has no problems driving standard low-Z microphone preamps. The output is strong and clear across the range, and the output is similar in level and detail to a healthy EV RE20, although with a more pronounced proximity effect.
As far as we know, this microphone was only marketed in Japan, but if you have any information to confirm or deny this, or in fact any information about this mic at all, please let us know!
UPDATE 22/2/11 This microphone has become a bit of a favourite for guitar cabs – it seems to have just the right amount of proximity boost for that application, giving clear and solid presence to the lower end. It also makes a decent tom mic.
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.
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.)
The ‘Tesla’ looks very much like an imitation of the Neumann / Gefell CMV563 bottle mic. In fact the microphone is smaller in diameter than the CMV, has no output transformer and has an unbalanced output. Like the CMV, the capsule may be swapped, and presumably other polar patterns were available. This one is marked with a red circle, which probably means omnidirectional. (I have yet to test the capsule).
The amplifier is a very simple grounded cathode amplifier, based around a Soviet 6Ж1Л (6Z1P) tube, which is a small signal pentode similar to EF95. These are also found in some Lomo and Oktava microphones, including the Lomo 19a9 and Oktava MKL2500.
Without the original power supply we can only speculate on the operating voltages. However, a B+ supply of 90V would be a good place for experiments to start – this would give a voltage on the capsule of around 60, and a sensible current through the tube circuit.
