Bang & Olufsen BeoMic 3 Ribbon Microphone

In 1995 I bought a second hand Beocord 2000 Reel-to-reel tape recorder, as I always wanted one, and I have a lot of old reels with music that I need to at least be able to transfer. At the time I was also working on recording some new music, that would become the album Fin de Siecle, so I needed a microphone. I went out and found an old and, at the time cheap, Bang and Olufsen-made microphone that I expected would be compatible with the Beocord 2000. Also the microphone looked kind of stylish in a 30'ies/Metropolis kind of way. It even had its own foot and cable, so it could be placed upright on a plane surface. Very practical for guitar recording.

Indeed it worked with the Beocord, and actually sounded quite nice, so I ended up recording a fair bit of instruments on Fin de Siecle using this microphone. Little did I know about it, other than it was a "ribbon microphone", and must be from the early 60'ies or maybe older. The microphone is a BM-3 ribbon microphone, and was built from the mid-50'ies to the mid-60'ies. This is it:

Specifications

The BeoWorld website reports the following specs for the microphone:

Manfactured:	1955 - 1966
Designer:	Erik Rørbæk Madsen
Features:	No resonance peaks. Extremely smooth response. Bi-Directional - no side pickup. No frequency discrimination, front to back. High efficiency Nu-Metal transformer placed in microphone housing. Switch for muting and voice/music conditions. The miniature non-corrosive duraluminium ribbon was only 0.0001 inch thick and its weight only 1.3 mg. This negligible mass prevented any resonance within the audio range. Shock and blast proof. Ball swivel for tilting to any position.
Specifications:	Dimensions 195mm high, 30mm diameter Impedance 50 ohms balanced Response M position: 20-11000 c/s +/- 2,5 dB T position: 10 dB lower at 100 c/s Sensitivity M position, 50 ohms: 88 dB below 1V/µbar T position: 4 dB lower
Response Frequency Characteristic:	M (Music) Position: From 20 till 11,000 c/s +/- 3 dB T (Close Talk) Position (8-16 inches) Rolled off below 100 c/s: 12 dB RTMA Microphone Rating GM (sensitivity): -158dB; Hum Sensitivity (RTMA) GH: -145 dB. RTMA Standard SE - 105. BM 3. Open Circuit Voltage Level: -88 dB, OdB = 1 Volt per Microbar
Output Impedance:	Nominal 80 ohms
Rating Impedance:	150 ohms
Bang & Olufsen Beomic 3 (150 ohms) and Beomic 4 (multi- impedance) models were pressure-gradient full-bass microphones with a perfect figure-eight directional pattern.

Repairing a damaged BeoMic 3

To my great surprise, it now seems to have become a bit of a collectors item. These microphones are relatively common here, but very often they do not work anymore. The ribbon construction is pretty fragile, and the ribbon itself is a strip of ultra-thin aluminum, that breaks at too much air pressure. Lucky for me, mine worked, and still does. Common wisdom is that once the ribbon is gone, the microphone is irrepairable, but other reports on the internet seem to suggest that there are people out there who can do it.

I prefer to belive the last statement. I found this website (http://www.makarateyp.com/RZHP/RMG/Beomic/beomic1.htm) where someone has detailed a homemade repair of a BeoMic 3. He is actually using thin aluminum chocolate wrapping to replace the ribbon, and incredibly it works!!! But beware, the site is in german!

If the microphone is to be restored to original specs, it is probably nessecary to trace some aluminum of the correct thin quality - possibly some ribbons made for other similar microphones. But even so, the details of the replacement process described on the german site will still be valid.

Pre-amplification for the BeoMic 3

The fact that the microphone contains a transformer and that the output is balanced, means that the three pins may be connected to a standard XLR socket. The DIN pinout is pin 1 +, pin 2 ground and pin 3 -. Now, the BeoMic 3 microphone is a low impedance microphone, with an output impedance of only 50 ohms, and it requires a very high gain to operate satisfactory. A high-gain microphone pre-amp is necessary. Selecting microphone pre-amps seems to be an art in its own right, and you can spend a lot of time researching schematics and specs to find the right type and load.

My approach was much simpler. As I mentioned, I bought the microphone to work with my old late-60'ies Beocord 2000 de Luxe reel-to-reel, that has a built in microphone pre-amp. These are built with a switch for 50 ohm and 200 ohm impedance microphones. I figured that since the BeoMic was the standard microphone for Bang&Olufsen equipment in the mid-60'ies, then other Bang&Olufsen equipment would most likely be built to match the impedance and characteristics of their own microphones. And indeed this is the case. The Beocord 2000 is a classic in its own right, and can be found at bargain prices.

Here are two quickly put together examples of recording the BeoMic 3 through the Beocord 2000 used as preamp. The instrument played is a portuguese 12-string, which sounds a bit like a mandolin. I may have recorded it a bit too close to the microphone, but the point is that the Beocord preamp is quite fine for recording:

Battle of Evermore, (and here slightly postproduced)
Fat Man, (here also slightly postproduced)

The mic passes a number of other amp stages internally in the Beocord before it reaches my mixer. These stages add to the noise. but I think that with new transistors it may have a much lower noise floor. Alternatively a direct line out from the preamp could be installed.

Without a Beocord at hand, you could still build your own period-correct copy pre-amp, based on the Beocord microphone amp circuit. The circuit looks like this (borrowed from Beocentral.com pages):

It does not seem too complex, with only two transistors, ten resistors and eleven capacitors per channel. However the schematics are from the late-60'ies, so some components must be replaced with other types. Most notably, the 2N2613 transistor is a germanium type and is probably hard or impossible to find, but could be replaced by AC125, AC126 or AC151 for instance. The UW 0029 should be equivalent to 2N2303, BC160, BC161 or BC239 for instance. Also, on original boards pulled from Beocord machines, the transistors are likely to be either broken or very noisy.

Another important component that affects the sound is the input transformer, designated "8012003"? Who knows the correct turn ratio of that thing? I don't. I know there are methods to measure transformers, but I don't know how to, so constructing a complete copy will have to wait.

Meanwhile I transplanted the preamp PCB to a stand-alone box, and this is the result:

Currently, I have replaced all components, except the transformer, on the right side of the PCB, while I left the left side original, to compare the two. For the germanium transistor I used an AC152, which seems to work fine. I also added an input for a -12 V supply. It really should be -17V, but seems to work fine at -12V. I cannot hear any significant difference between the two channels. The only thing missing now would be to add a switch to each side, to switch between the 50 ohm and 200 ohm modes of the two inputs.