microphone picks up fm radio signal?!

M

Martin vB

Guest
Hi!

I've built a couple of electret condenser microphones using WM61-A and
the design from www.linkwitzlab.com. They work really well, also with
only 1.5 V supply.

But one of them has a really strong disturbance from a fm radio station
(106.1 MHz). The circuit is simple so I guess that it can't work as a
fm reciever. Wrapping the microphone in aluminium foil only slightly
increased(!) the disturbance. The level varies when moving the
microphone in the room.

Does anyone have an idea about how to get rid of this disturbance?
 
Hello Martin,

I've built a couple of electret condenser microphones using WM61-A and
the design from www.linkwitzlab.com. They work really well, also with
only 1.5 V supply.
Click to expand...
Don't they need 2V?

But one of them has a really strong disturbance from a fm radio station
(106.1 MHz). The circuit is simple so I guess that it can't work as a
fm reciever. Wrapping the microphone in aluminium foil only slightly
increased(!) the disturbance. The level varies when moving the
microphone in the room.

Does anyone have an idea about how to get rid of this disturbance?
Click to expand...
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

http://www.analogconsultants.com
 
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:
http://www.linkwitzlab.com/images/graphics/microph2.gif

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,

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

The basic schematics is here:
http://www.linkwitzlab.com/images/graphics/microph2.gif
Click to expand...
That worked well. Good old hand drawings.

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

http://www.analogconsultants.com
 
But one of them has a really strong disturbance from a FM radio station
(106.1 MHz). The circuit is simple so I guess that it can't work as a
fm receiver. Wrapping the microphone in aluminium foil only slightly
increased(!) the disturbance. The level varies when moving the
microphone in the room.
Click to expand...
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,

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.
Click to expand...
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

http://www.analogconsultants.com
 
Joerg (notthisjoergsch@removethispacbell.net) writes:
Hello Martin,

I've built a couple of electret condenser microphones using WM61-A and
the design from www.linkwitzlab.com. They work really well, also with
only 1.5 V supply.

Don't they need 2V?

But one of them has a really strong disturbance from a fm radio station
(106.1 MHz). The circuit is simple so I guess that it can't work as a
fm reciever. Wrapping the microphone in aluminium foil only slightly
increased(!) the disturbance. The level varies when moving the
microphone in the room.

Does anyone have an idea about how to get rid of this disturbance?

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.
Click to expand...
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
 
You must log in or register to reply here.

Welcome to EDABoard.com

Sponsor

Back
Top