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[on:]

Ok, this is weird. So I built a microphone amplifier using the circuit from here:

http://www.rason.org/Projects/hbmic/hbmic.pdf

It works perfectly, except it picks up police radio? WTH is up with that? If you don't believe me, I've recorded a conversation here:

http://iluvtocnc.googlepages.com/test.mp3

Can anyone explain what's going on? It seems like I only hear the radio if the microphone is attached. And the microphone wire is acting like an antenna, since the audio quality changes if I move it around.

-Z-

[Reply #1 on:]

well you actually built a radio.. not weird at all.  

Look at this circuit... look familiar?

A radio only needs three things:
- a tuned circuit ( the length of wire on your mic input and its capacitance makes a tuned circuit)
- a detector ( the diodes in the op-amp are the detector) and
- an amplifier with gain... that's the op-amp itself.


You'll find, sadly, that if you put a 1nf or 500pf capacitor from pin 2 to ground, your radio will die, but it'll work as a mic amplifer!

D

PS: try putting an aligator clip on pin 2 as an antenna, and you should get all kinds of stuff.

[Reply #2 on:]

Ah I see...

It must be some coincidence that it was tuned to the police frequency... I thought the police used more sophisticated radios, I guess not.

Thanks for the info!

-Z-