I have an old Braun Cosmophon BMF2020 microphone at home, which I have long been wanting to use for good ol' fun.

I have virtually no technical knowledge, so I am completely uncertain how to power the device, and if it would be possible to connect the microphone to my PC.

The mic holds a slot for a gigantic "torch" battery of 4.5V.
There are four additional ports, two on each side of the stand:

  • The two on the left are marked with "4V"
  • The two on the right are marked with "V"

I am thinking these two pairs are for a plus and minus pole, but I wouldn't know why the mic features both a 4V and what I assume is a 1V inlet.

Any help would be greatly appreciated

Useful information

  • \$\begingroup\$ I really would not try to connect this thing to my computer. It has a really good chance to fry it or some part of it. \$\endgroup\$
    – Eugene Sh.
    Commented May 1, 2019 at 14:44
  • \$\begingroup\$ @EugeneSh. well then, I will see if it has a 6.35 plug for my headset. The power question remains! \$\endgroup\$
    – Tim Stack
    Commented May 1, 2019 at 14:48
  • 10
    \$\begingroup\$ Optimistic characters back then. "Carbon microphone" and "low noise" aren't usually things you see together in one sentence. \$\endgroup\$
    – JRE
    Commented May 1, 2019 at 14:51
  • 1
    \$\begingroup\$ OT, but I would try to sell it to some antiquarian :) \$\endgroup\$
    – Eugene Sh.
    Commented May 1, 2019 at 14:53
  • 1
    \$\begingroup\$ I think you could safely try to use this device. Carbon microphones used a transformer for isolation; the primary circuit had the carbon cell in series with the battery. The carbon cell changed resistance (slightly!) in response to sound pressure, and the transformer amplified these changes. The output should therefore be isolated from everything else - whether you can get a useful signal can only be found by experiment. \$\endgroup\$
    – user131342
    Commented May 1, 2019 at 16:06

3 Answers 3


A quick look at the Radio Museum pictures and descriptions makes it look like it ought to be fairly easy to hook up.

Here is the picture of the connections from the Radio Museum:

enter image description here

You provide it with 4V through the jacks on the left side.

You could use a battery pack with 3 normal AA or C cells in series. That's the easiest way, and will probably sound best.

A 5V powersupply like the one you charge your phone with would work as well, though it would be noisy (whines and squeals) since switching power supplies are that way.

It won't need much current - the old battery was small and supplied 4.5V, so it wouldn't be capable of supplying much current. Tens of milliamps, maybe a hundred or so.

[Edit] Just noticed the power on lamp. That'll probably consume more current than the microphone itself. It may also be in series with the microphone. If so, then it won't add to the current drawn from the battery. It would serve to limit the current through the microphone, in that case.

Carbon microphones are basically a squishy, noisy resistor.

Internally, the carbon button and the transformer primary are simply in series. Noise squishes the carbon button and varies the current through the primary of the transformer.

It is basically just a resistor connected across a battery. The current it draws will depend mostly on the resting resistance of the carbon button. Probably some tens to hundreds of ohms, so the current isn't very high.

The polarity of the power supply pretty much doesn't matter. There might be some slight advantage in reducing hum from power lines if you connect ground one way or another, but probably not much, and only if you are using a grounded powersupply.

On batteries, it won't matter at all. That side is very low impedance, so hum hasn't got much of a chance anyway.

Now the output.

The microphone uses a transformer to convert the low voltage and (relatively) high current from the button to a higher voltage, lower current signal.

Given that the output is high impedance (think of it as having a big resistor in series with it,) I don't think you can damage a normal amplifier or sound card with it.

The output might be too high, and cause clipping, but there shouldn't be any damage.

The output is symmetrical - it comes out of a transformer. It therefore doesn't really matter which you connect to the ground of your amplifier.

If you have an amplifier with a symmetric input (like most XLR inputs) then you've got it made. A symmetrical XLR cable with two banana plugs on the other end.

Same goes for most amplifiers with the 1/4 inch phono jacks (guitar amps.) Those are symmetric as well.

If you want to connect it to your PC, then you might have to do a little more work.

On a PC, you have two choices:

Line in and microphone in.

Line in would be my first bet.

The output level of the microphone will more likely match that than it will the microphone input level.

Connection is simple for line in. One of the V connections to ground on line in, the other to the left or right wire of the line in cable.

If that isn't loud enough, you might have to use the microphone input.

This is the one that's tricky.

Microphone inputs actually provide low current DC to the electret microphones you are expected to use.

Your transformer output microphone doesn't need that DC, and it could cause problems.

All you need is a capacitor between the microphone output and the microphone input on the PC. Something like 1 microfarad ought to do as a start.

Here's what it would look like:

enter image description here

Pretty simple. Note that line in uses a stereo plug while microphone in uses a mono plug.

Give it a try. You can't break anything, and it really ought to work. If it is too loud on line in, let me know. There's a way around that, too.

This of course assumes that the microphone isn't broken. As Tim Wescott says, the carbon button may have gone bad.

  • \$\begingroup\$ A guitar amp would be the best choice for an easily available amplifier for this -- they're designed for high-impedance inputs, whereas most modern microphone inputs are designed for low-impedance sources. An active DI would probably work to convert it to a low-impedance, modern microphone connection. \$\endgroup\$
    – Nate S.
    Commented May 1, 2019 at 16:45
  • \$\begingroup\$ Yeah, probably true about the guitar amplifier. \$\endgroup\$
    – JRE
    Commented May 1, 2019 at 16:50
  • 2
    \$\begingroup\$ For a first test you can "just stick bare wire in the ports." Seriously. You'll get scratchy noises if they wiggle, but so what? You just want to see if it works. \$\endgroup\$
    – JRE
    Commented May 1, 2019 at 20:16
  • 1
    \$\begingroup\$ If the voltage is the same, it doesn't matter. Just because a battery can deliver 2A (for example) doesn't mean it must deliver 2A. The microphone was made for 4.5V. If you give it only 4.5V, then it will only draw as much current as it needs. \$\endgroup\$
    – JRE
    Commented Jun 6, 2019 at 15:39
  • 1
    \$\begingroup\$ @JRE I know it's been a little under four years, but I got it to work! Some additional smacking might have fixed it. The quality is surprisingly good; it holds up to modern-day cheap microphones. Almost a hundred years later and this mic got hooked up to a piece of engineering it couldn't have fathomed \$\endgroup\$
    – Tim Stack
    Commented Apr 23, 2023 at 9:50

It's a carbon microphone, apparently designed to work off of it's own or a separate 4V power source, and with an isolation transformer.

Even back in the day they were known for mediocre sound, but saved from uselessness by their high output. You can actually make an audio-frequency amplifier by driving a carbon microphone with a speaker coil, and get gain. The fidelity is terrible, but it was either used or experimented with by the phone companies before vacuum-tube amplification came along.

They work by having a button filled with carbon particles that's attached to a cone like a speaker that picks up sound. Speak into it, and the alternate squeezing and rarification of the button modulates it's resistance; by working that against some impedance (like the transformer primary) it makes a signal voltage.

They can die of old age -- in addition to all the things that happen to old electronics, the carbon granules can get stuck together and then they don't change resistance any more. The one "fix" for this that I know applies to old-style telephone receivers, and I'm not even going to suggest that you apply it to that fine vintage thing!

You need to talk to someone with deep knowledge of this. I would find a vintage radio forum that has a high content to BS ratio, and ask there.

  • \$\begingroup\$ "high content to BS ratio" hah. We haven't had those since before Eternal September \$\endgroup\$ Commented May 2, 2019 at 12:19

Just plug it into a mic input with 48v phantom power to test it. I've tested many carbon microphones that way and they don't seem to get damaged. If the carbon is packed together, hit the microphone to loosen it up. These methods may not be recommended by purists but if you just want to test your microphone then it is a fast way to do so.


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