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nyk 12th December 2011 01:48 PM

Electret microphone Pre-amp
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Hi all!!

I am using this microphone as mentioned in my last thread (

We built a common pre-amp circuit(Audio Preamplifier | Op-Amp Mic Preamp) and it was not working.
We got the output of the OPAMP as 'HIGH' always i.e. around 12V(Vcc) and not as varying ac signal.

We also tried with the other but similar circuit(pics attached) but still no response.

Can anybody please help as to how to make it work or as to how to debug the fault in the circuit.......

Mooly 12th December 2011 06:29 PM

The voltage at pin 3 should be Vcc/2 which is 4.5 volts in your circuit.
Pin 6 should be identical to this voltage, as should pin 2. The setting of the pot has no effect on the DC conditions.
These voltages have to be correct.

If they are not then remove C2 and recheck.
If it is still not right then there is either a construction error or the opamp is duff.

For no audio output you may need much more gain than this depending on what you are feeding it into. If a mic capsule puts out 1mv then this circuit can only increase that to just over 20 millivolts. For a line input you need another gain stage after this to raise that 20mv to around 1 to 2 volts.

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