I am debbuging a preamplifier for two electret mics. I use opa345 which is a dual low power op amp and i use the same topology as in this link for each channel: http://freecircuitdiagram.com/2008/09/02/battery-powered-electret-microphone-pre-amplifier/ . I have a Vcc voltage of 3.3 volt and i use 2.2K bias resistors (which is the output resistor of the electret mic).
While one channel is playing great, the other gives no output. It can't be a design problem, since an identical board i have built has both channels functioning properly. While searching for an error, like a bad soldering joint or a faulty component, I noticed that with no input, voltage at negative input terminal of the "faulty channel" op amp is not the same as voltage at positive input terminal, which is the case for all other functioning channels.
I suspect that this is happening due to a faulty op amp as i cannot explain why the virtual ground is not reflected to the negative terminal input, but again, the ic is a dual op amp and the other channel is working fine. Obviously I am missing something. Any Ideas or any experiences of half faulty op amps are welcome.
P.S: I havent changed the op amp since I dont have an identical readilly availiable. I have ordered a few spare, but while waiting for them to arrive, do you suggest any more common op amp replacement?
Thanks!
Aris
Best Answer
Here's the circuit you linked to with a couple of red circles on it: -
Check that capacitor C1 isn't somehow leaking some DC current through to the -Vin pin.
Also check that R4 is fitted correctly and you can measure the same voltages on each of it's pins as you do on the op-amp pins. R4 could be broken - can you measure it with a meter?
Measure that copper tracks are fine between R4 and the op-amp.
If necessary lift C1 from the circuit to check that -Vin and +Vin become the same level.
The OPA345 is unquestionable a good device for this application if you don't care too much about noise. The AD8606 does the same but will produce less noise.