Carbon Microphones

[PermO]

PermO's world ...



Found the old 3 pin chassis in a store right around the corner this morning, there was this one weird old one left, got it for half the price of a new one...

 

[Ricardus]

My plan was to make one of these that looked like a bottle mic as well. I have enough elements to do many classic mic styles.

 

[Tubetec]

I have a Grampian MCR from the 30's somewhere in a box , might be a good candidate for a Carbon mic as the original dynamic driver is long gone .

I'd like to try the dual button circuit below , but instead of battery, apply phantom power via pri/sec centre taps , the pot might serve to vary capsule bias , might be a useful way to adjust the output ,depending on source .

 

[Tubetec]

I salvaged a load of Cannon XLR sockets and balanced send return jacks from an old Yamaha pm 3000 today , the handy thing is the connector is nicely terminated with twin screened and plenty of lenght on the cable for any kind of enclosure .
I also pulled out about 16 large VU meters with integral drive electronics and source selectors ,
I'll probably also keep the mic input modules which have quite a nice DOA op amp . I couldnt face just pitching the thing into landfill without reclaiming as much as possible , the planet only has finite resources after all .

 

[Ricardus]

If someone comes up with an alternative phantom power circuit I would be interested.

 

[PermO]

So.. there it is;


Rumour goes The Oracle of Delphi used the same microphone as this one...

 

[abbey road d enfer]

Rumour goes The Oracle of Delphi used the same microphone as this one...

But wasn't it powered with Greek fire?

 

[PermO]

Yes, but I'm in the Netherlands, so I had to get a battery from the super market to make it work...

 

[PermO]

Here's an example of some creative use of this thing...

Just got a phone call from James Brown...



I'm under the impression that my carbon mic sounds a lot better than other examples I've heard on the internet, I don't get that squarewave broken mess unless I overload it, I just put it 25 cm in front of my speaker and played the funky drummer from youtube and recorded it using a BLA B173 pre going into my interface.

I like it, I'll find some use for it sometimes...

 

[Tubetec]

Lo-fi drum mics can be very effective , muting the overhead condensers and introducing the carbon mic to the mix on verse/chorus or middle 8 can make a nice contrast .

I like the way they do the vocals on this track , cutting back and forth between lo-fi and regular mic ,
Mick exagerates the effect with an officer like tone of voice .

 

[Ricardus]

So.. there it is;

View attachment 92346
Rumour goes The Oracle of Delphi used the same microphone as this one...

What transformer are you using?

 

[PermO]

I used a 1:1 600 Ohm line isolator from a broadcast installation, don't know the brand, you can see it on the first picture of my first experiment.

It's just a small audio transformer.

 

[PermO]

So not all these things you find in old plones and military surplus ar carbon elements.

If it's really old, the microphone piece is most likely a carbon element... I took one apart that had a rattling broken plastic membrane;

They look like this, it's hard to see on the photo but I made a new membrane (it's on top of the brass ring) and that actually makes it work again.

If the unit is thicker and heavyer.... it's not a carbon element;

It's two little coils with magnetic polepieces on a thin metal plate... so don't battery or phantom power these... it makes no sense.
They work for a super distorted crap sound only, if you want to make something sort of decent you will need a carbon element and some current so it won't crap out 100% all of the time.

You don't have to take them apart like I did, if it measures about 60 Ohms, it's not a carbon element but a speaker, carbon elements will give no reading at all.

 

[pinchemotherloaf]

So.. there it is;

View attachment 92346
Rumour goes The Oracle of Delphi used the same microphone as this one...

That's terrific! Well done!

 

[abbey road d enfer]

If the unit is thicker and heavyer.... it's not a carbon element;
View attachment 92423
It's two little coils with magnetic polepieces on a thin metal plate... so don't battery or phantom power these... it makes no sense.

Seems to me it's an electromagnetic speaker, the listening device in many old style telephones. They work by superposing the AC field to a permanent magnet's field., which produces the variable force to a magnetic membrane.

They work for a super distorted crap sound only,

They are not designed to be used as microphones, but still manage to produce some sound.

carbon elements will give no reading at all.

That would indicate a non-working carbon mic to me. All those I have used indicated something, quite variable, depending on position.

 

[PermO]

Seems to me it's an electromagnetic speaker, the listening device in many old style telephones. They work by superposing the AC field to a permanent magnet's field., which produces the variable force to a magnetic membrane.

Yup indeed, the membrane is magnetic, it seems to be a "rattling in a sort of controlled way" device.

They are not designed to be used as microphones, but still manage to produce some sound.

Yup, I know, but people hook these up to phantom power, and that makes no sense...

That would indicate a non-working carbon mic to me. All those I have used indicated something, quite variable, depending on position.

Two working units gave me an OL readout on my Fluke when trying to measure resistance, could be that the corroded contacts are hard to probe and get a good reading of.

 

[pinchemotherloaf]

Seems to me it's an electromagnetic speaker, the listening device in many old style telephones. They work by superposing the AC field to a permanent magnet's field., which produces the variable force to a magnetic membrane.

They are not designed to be used as microphones, but still manage to produce some sound.

Known as a 'receiver' in the US. This one looks like a 1950s-1960s era Ericsson maybe? The interesting thing is that diaphragm has to be spaced away from the magnet correctly or it will stick and won't vibrate. Very much like a LDC.

 

[revmonkey]

I built the circuit comprised of 3 10k resistors, and 2 capacitors, and it was really hot and sounded like trash in an especially trashy way that was beyond what I was going for. I just ran across the following link yesterday, that looks like a 48v Phantom power version of the 600:600 transformer circuit.
The maker also put in a flip switch to switch between the carbon button mic and the ear-piece as a dynamic mic.
https://www.pro-tools-expert.com/production-expert-1/use-an-old-phone-handset-to-produce-a-lo-fi-mic
Having a center tap transformer should improve the sound a great deal, just from being able to balance it. The builder has sound comparisons in the video, and I like the old-timey sound of the carbon button that sounds nothing like what I was getting with the transformerless circuit. I just ordered some 600:600 center tap transformers, and I'm going to ditch the flip switch and see if I can fit all the electronics into the handset to use the ear piece as a monitor and carbon mic as a mic and plug into my audio interface for zoom calls.

It's been a while since I've tried building a PCB in KiCAD, but I figure it should be a pretty simple board.

 

[PermO]

Known as a 'receiver' in the US. This one looks like a 1950s-1960s era Ericsson maybe? The interesting thing is that diaphragm has to be spaced away from the magnet correctly or it will stick and won't vibrate. Very much like a LDC.

It's from an old "Dutch phone company " phone, late 50's could be right, bakelite horn, cottonwrapped wire.
Not sure if Ericsson was allready operating here in that period.

@revmonkey ... that seems like a nice way to do it, I've seen some nice 600 Ohm telecom transformers on Farnell for less than 10 dollar.

 

[revmonkey]

@revmonkey ... that seems like a nice way to do it, I've seen some nice 600 Ohm telecom transformers on Farnell for less than 10 dollar.

Nice! I ordered a few of these: AEI24-4410
I'll report back with samples once I have it working.

The Bournes LM-NP-1002L looks like a good choice, and much cheaper.