microphone picks up fm radio signal?!

They work very well on 1.5V. Amazing pieces! Low noise, flat frequency response, makes clear and natural sound. And they are cheap ($5 each), I really recommend them!

A bit tricky to send schematics as text, maybe this helps:

The basic schematics is here:

and the changes are: battery=1.5V, R1=0, R2=10K, and added light bulb in parallell with the battery and switch.

Do you mean the connection cables? I used cheap audio cables for the "radio-mic", two wires with individual shields = two wires + ground.

Hello Martin,

Don't they need 2V?

Post the schematic. It probably comes in on the wires so you could try a small ceramic cap and/or ferrite beads, one over each wire.

Regards, Joerg

Hello Martin,

That worked well. Good old hand drawings.

Beads over the wires should do. There should be only one wire but I guess you are using stereo, so you'd need two beads. In tough cases a small cap directly across the terminals at the mike can help. Start with a few hundred pF, maybe even over 1000pF. But the higher the more this will mess up your high frequency response.

Regards, Joerg

But if it's an FM station, there's not going to be any recovery of the modulation.

If something is rectifying the signal, the result will be a constant DC voltage somewhere, which may affect operation of the unit. But there shouldn't be any recovery of the modulating signal, because it's FM rather than amplitude.

Michael

Yes, I've seen exactly the same interference on mic pre-amps I have prototyped.

I started with simple discrete circuits off the web, on plugblock, then tried more complex ones on soldered protoboard, and asked this group in frustration.

Conclusion was that you really need a balanced input for the tiny signals of a moving coil mic, and that it was far better and simpler to buy a low-noise differential amp IC.

So I used an INA103 and it works without radio pick-up.

Still not perfect: you could not use it to hear a room like 'telepresence' because the s/n is not as good as human ears (even mine). I think it is as good as I can get without a lot more effort, and if used as a microphone in front of someone speaking I expect the s/n is more than adequate.

I don't know how the audio got demodulated, nor care much, I was just glad to get rid of it.

Hello Michael,

Only if the path in front of whatever is demodulating it has a flat frequency response. If it doesn't then you have a weak slope detector.

Regards, Joerg