The Xaudia Blog

29-03-2013

MOTM: Siemens-Telefunken M201/1 Ribbon

We have seen an influx of weird and wonderful ribbon microphones this month, including a rash of Bang and Olufsens, half a dozen Film Industries mics, and a cardioid Toshiba BK5 copy, so we were spoilt for choice for a Microphone of the Month…. until this came along…

Telefunken M201/1 cardioid ribbon microphone

The Telefunken M201/1 was one of the very first commercial ribbon microphones. It was made by around the early 1930s and would most likely have been used for radio broadcast. These ribbon mics were a big improvement on the carbon microphones that they replaced, but in Germany they were quickly superseded by the new valve condenser microphone technology.

Telefunken logo on the M201/1 microphone

There are very few of these left in circulation – this one is in very nice condition and after a clean and a new ribbon sounds truly excellent, with a strong output not much below a modern dynamic.* After over 80 years, the magnets are still strong with a field on >5000 Gauss in the narrow 2.5 mm gap between the ribbons.

Telefunken M201/1 rear

The M201/1 is constructed around a huge horseshoe magnet, which surrounds the transformer and (presumably) some kind of acoustic labyrinth or wadding to control the pattern. This dictates the shape of the body and gives it an unusual cylindrical aspect. The ribbon sits behind a fine brass grill at the front of the mic. With the rear of the ribbon being obstructed by the magnet, the mic is almost cardioid in nature,  becoming more figure of 8 towards the very bottom of the frequency range. Two chrome B-shaped vents sit above and below the ribbon to equalise the pressure behind the ribbon. The output connection is made via a pair of screw terminals hidden behind a circular plate on the rear of the mic.

Although we can’t see the transformer, we can make some measurements. The primary and secondary DC resistances are  50 milliohms and 19 ohms respectively, and the inductances are 5.25mH and 2.43H  at 100 Hz, which gives a turns ratio of (about) 1:22. The low DC resistance and relatively high inductance point to a well designed transformer with a good quality core and thick wire for the primary. Excessive resistance in the primary winding leads to noise, so it is especially important to see a low value here.

M201 ribbon 

The ribbon itself is long and thin, and the old broken one inside the mic was fully corrugated (the one shown is a replacement). For ‘ease of service’ the ribbon is held in a frame behind the front grill. However, once in place the assembly hides the ribbon pole pieces from sight, which means that alignment is a case of trial and error, which is possibly the only weak part of the design that I have found. With the new ribbon, the mic has an output impedance of around 300 ohms.

Telefunken M201 under test

I did a test sweep of the mic in our little testing chamber.** The mic shows a strong output with good bass response, a little proximity effect below 100Hz, and a graceful roll-off above 6KHz. The noise above 14KHz are likely to be diffraction from the grill and other parts.

This microphone is thought to have been made originally by Siemens & Halske and supplied under various model numbers including the KVM3, SM3 and others, and this rather wonderful example on Martin Mitchell’s excellent microphone blog, which is just called model ‘R’. In German a ribbon mic is called a Bändchen-mikrofon. However, the name plate on this is in English, and perhaps this is the export model, with the R simply for Ribbon. There is a bit more history at Martin’s blog, which is well worth a read.

Mitchell’s Siemens type R ribbon microphone

* Testing against a Shure Beta57, the preamps were on the same gain setting for a comparable output
**Calibration is with an omni measurement mic, so there can be some bumps due to the differences in pattern picking up more or less reflections from the room.

15-03-2013

Toshiba Model H mic – RCA BK5 copy

Although now more associated with household TVs and video players, Toshiba once made some very nicely engineered studio ribbon microphones. Very often they were close copies of RCA microphones, but with a few added innovations and improvements.
Toshiba type H ribbon mic – looks like a BK5

The ‘Model H’ is no exception, being almost identical to the RCA BK5 cardioid ribbon in both looks and function. The small format ribbon sits between powerful magnets, in front of an acoustic chamber or labyrinth that takes up the middle segment of the microphone

Rotating the base of the mic implements a high pass inductive filter.

In fact it seems slightly better engineered than the BK5, with a 5 position high pass filter switch that is actuated by rotating the dial at the very bottom of the microphone.

Toshiba type H ribbon

There is some additional electrical screening around the ribbon, but less acoustic baffling in front of it, which may be to give a more ‘open’ sound. In practice the ribbon is more prone to being stretched by air blasts than the RCA model.

Overall the type H is a decent alternative to a ‘real’ BK5, and is a little less peaky than most of the BK5s that we have measured.

Thanks to Kore Studios.

13-03-2013

Grampian Amplifier Speaker Set

As well as microphones, Grampian made amplifiers and other PA equipment. Here’s an old Grampian amplifier and speaker set.

I found the amp a few years ago in Dave Dee’s junk shop in York. The circuit is actually very similar to a Mullard Twenty, with the addition of a transformer balanced mic input, and several taps on the output transformer for 8 ohm loudspeaker or 100V connection. I have even used this in the past as a mic preamp for recording, for some warm and slightly dirty vocal tracks.

The speaker is a recent purchase found on GroupDIY.com. It has a rather nice and unusual tapered shape, with (no longer very) white vinyl covering. The whole set has a really cool vintage look to it – very different from today’s ubiquitous square black boxes.

I am now using this as a guitar rig – it has a great vintage valve tone with just the right amount of breakup coming from the speakers. There is plenty of volume for recording, but I would hesitate to use it for rehearsals with a modern loud drum kit!

I am unsure of the age of these but I would guess 1950s or early 60s – they don’t appear in Grampian’s 1968 product line, and presumably pre-date that. If anybody knows more, please get in touch.

07-03-2013

Hexapup switching

Here’s an easy way to switch between the ‘split’ and ‘spread’ modes on our hexaphonic pickups, using a DPDT switch.
The E&D strings are wired in series, as are the G&e strings, observing polarity. The negative sides of these pairs are wired to the sleeve(s), and the positive ends of these are connected to a double pole, double throw switch as shown. 
The B and A strings are wired between the switch and the two outputs.  Flicking the switch then routes the E and D to either the B or the A string, and so on.
This is ideal for a tip-ring-sleeve stereo output jack. If using two mono jacks then the sleeves of these should be connected together, and to ground.
03-03-2013

GEC Ribbon Microphone Evolution

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.
The mics look very similar to the drawing in a patent by GEC and Thomas Julian, from 1947.
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!
01-03-2013

Grampian GR1 & GR2 ribbon mic manuals

Grampian made some fine ribbon microphones, and the GR1 and GR2 were our Microphones of the Month in May of last year. Here’s a scan of the original Grampian GR1/2 manual.

Grampian GR1 or GR2 ribbon mic

The mic also came with a word of caution….

Don’t test a ribbon mic with a multi-meter or else…

I am particularly pleased to see the warning not to use a DC meter on these. Quite often I come across ebay auctions where the seller measures ribbon mics with a multimeter, unaware that the carrier current can bump the ribbon out of the magnetic field and stretch it. 🙁

Grampian also made matching transformers for their mics – we have a few available for sale. Here is the factory drawing for the transformer.

Grampian matching transformer wiring.

I posted some other Grampian documents last November.

Many thanks to Pete Guppy for sending this in.

24-02-2013

Melodium documents from 1970s

Melodium 78A cardioid dynamic microphone

Here is a scan of an English language Melodium product catalog for their line of microphones, which (probably) dates from the 1970s. It includes the RM6 ribbon microphone, 76A & 78A cardioid dynamics, and 77A, 79A & 88 omnidirectional mics, along with a range of accessories.

French language data sheet for the RM6.

Melodium C163T Data sheet 

Many thanks to Santiago Ramos for sharing these.

21-02-2013

Thiele Microphones schematics and documents

Thiele, of Liepzig, were one of several manufacturers of tube condenser microphones in post-war Germany, although they are less well known than Neumann, Gefell and Schoeps. Thiele microphones also appeared under the Elektro-Medizin and Wetzel brands.

Thiele M4 microphone

The Thiele M1 (cardioid) and M4 (cardioid and omni) are great looking microphones but feature some frankly bizarre design decisions, most notably the placing of the tube directly behind the capsule, which interferes with the rear pickup whilst simultaneously cooking the PVC on the rear capsule. Of course the manufacturers would never have expected that we would be picking up these mics sixty years on and trying to record with them.

Inside a Thiele M4

In addition, the power supply was built into the base of the mic which can lead to hum issues despite the on-board potentiometer that acts as a ‘null’. This also means that the mic has two cables running from it – one for power and one for audio. Both mics use two-stage head amplifiers based around an ECC83 tube, and both have unbalanced outputs. Here are the official schematics for the M1 and M4 mics.

Thiele M4 microphone circuit

Thiele M1c tube microphone circuit

The M5 seems to have been the ‘Studio’ version, with an external power supply and output transformer for balanced low impedance operation. I haven’t seen a factory schematic for the M5, but this is a drawing that I traced out from a specimen on the bench.

Thiele M5 microphone, inside and out.

Here are some original sales and technical documents from Thiele, in  German.

Thiele sales brochure for M1 and M4

Elektro-Medizin M4 technical document

Elektro-Medizin M4 & M5 product sheet

Many thanks to the Microphone Online Museum for kindly sharing the documents and schematics.

17-02-2013

Massive Old Dynamic Microphone – GEC?

We’ve had some website issues this weekend, but everything has been sorted out, so let’s get back on track with another vintage microphone curiosity.

This gigantic bronze dynamic mic has no badge or makers name on it, but arrived in a box of GEC microphones. Connection to the outside world is made via two terminal posts at the rear. From the size and style it probably dates to the 1930s or even earlier. 
It is around 11.5 cm across and 8.5 cm deep, weighs nearly 2 kg, and would originally have been mounted on springs within a hoop. Two of the suspension mounts are missing – it will be a quick job on the lathe to turn new ones from a bit of brass. 
Inside the mic is very much like a speaker in reverse, with a paper and fabric cone driving a coil into the field of an enormous magnet.
On initial testing, the mic wasn’t picking up well as the cylinder in the centre of the diaphragm was scraping against something as it moved. Some careful cleaning to remove the dirt eased the movement, and the mic sounds OK, if rather peaky. The addition of some foam inside the body helped to dampen some of the ringing from the cavity.
Even with the foam, the mic is far from flat in response with an enormous bump at 200Hz. So if you want to give something an EQ boost at 200Hz, this is your mic!
Frequency plot for big bronze dynamic microphone!
Other uses include door stop, paperweight, shot-put and burglar deterrent (ouch!).
If you recognise this one or have information on any of our other unidentified mics, please get in touch.
16-02-2013

MOTM: Bruno – Webster Velotron Microphone

The Bruno Velotron (made by and sometimes badged as Webster) was an oddball microphone that took inspiration from both ribbon and condenser technology. Strips of aluminium foil were attached to an insulated metal backplate, and a voltage applied between the two. As the foil vibrates, the capacitance of the device varies, and a signal is generated. They are notoriously fragile, and there are very few around in working condition.

Bruno Velotron microphone

I found this one on ebay a few months ago and could not resist grabbing it for dissection and possible resurrection. The specimen is in very good cosmetic condition so will look nice in the collection even if it it is beyond repair. But it would be even nicer if we could hear it in action!

Looking inside, the element is marked ‘4½M’ and 2/4/38, which in American would be 4th Feb 1938. The ‘4½M’ may refer to the impedance of the device.

Velotron element

The Velotron requires a polarisation supply and a buffer amplifier to operate – I am not sure if these came with the mic, but the tube amps of the era could easily have been modified to provide this from the high voltage supply. The excellent Coutant.org shows a wiring diagram for a suitable polarisation supply and buffer amplifier…

Supply for Bruno microphone – from Coutant.org

Based on this diagram, I wired a 7205 subminiature tube into one of our tube microphone power supplies, to act as a little amplifier circuit for the mic.

Hacking together a power supply and amplifier.
Miraculously, the microphone worked on the first attempt, even after 74 years! The sound is very warm and dark with an obvious roll off at the top end, and it was very exciting to hear one of these for the first time.
Bruno Velotron – frequency plot
I wondered if some of the roll-off may be due to cable capacitance – if the element really does have an impedance of the order of 4.5 MΩ then even a few tens of pF will become significant. However, swapping cables and using a triaxial arrangement didn’t make any different, so it may well just be the natural response of the beast, and certainly how it would have sounded at the time.
The mic is unfortunately prone to noise and pops. They are notorious for breakdown of the insulation on the backplate and that seems to be the case with this one. So the next job would be to drill out the rivets that hold the element together, strip off the ‘ribbon’ foil and re-lacquer the back plate. 
I’ll save that for one day when I am feeling brave, but for now the Bruno shall be our microphone of the month!
Update 27/5/13: It seems that these were sold in Europe under the Radio Marelli brand. As seen on ebay….