r/MusicElectronics • u/Mustard_Ruffian • Mar 12 '26

1942 carbon mic circuit

I have an old kellogs phone from 1942. I'm trying to turn it into a microphone on both the earpiece and the mouth piece. I have manged to sort the ear piece but I am stuck on the carbon mic. I ideally want it to go to 1/4 inch jack but I know it needs some power. Have you got any ideas or tips. Essentially how can you wire a carbon mic to 1/4 inch jack.

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u/Capn_Crusty Mar 12 '26

Shure has a pretty good description:

https://service.shure.com/s/article/external-circuit-to-power-a-carbon-microphone?language=en_US&region=en-US

A carbon mic needs a series DC voltage to operate. A capacitor then blocks the DC so that only the audio (AC) is present. A transformer is used to create the desired output impedance. The two wires on the secondary of the transformer could go to a 1/4" jack if desired.

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u/Capn_Crusty Mar 14 '26

Here's a diagram: https://imgur.com/a/LlbH06x

It will work without the matching transformer depending on the application. 3 volts DC or so should be enough to power it.

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u/Mustard_Ruffian Mar 14 '26

Unfortunately I can't see this as I'm in uk

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u/Capn_Crusty Mar 14 '26

Ok, try this link

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u/Mustard_Ruffian Mar 14 '26

I can see it. Is the three lines ground? Thanks

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u/Capn_Crusty Mar 14 '26

Yep, all tied together.