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?



  • \$\begingroup\$ Try hear this has lots of info datasheetdir.com/MAX9810+Microphones Re: Jeff \$\endgroup\$
    – user29837
    Oct 8, 2013 at 9:55
  • \$\begingroup\$ I don't think this helps the OP sort his problem out. \$\endgroup\$
    – Andy aka
    Oct 8, 2013 at 10:05
  • 1
    \$\begingroup\$ This does not help the OP understand his problem, much less solve it. If you are just going to suggest a different circuit design you need to justify that recommendation. \$\endgroup\$
    – Joe Hass
    Oct 8, 2013 at 10:49

1 Answer 1


Here's the circuit you linked to with a couple of red circles on it: -

enter image description here

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.

  • \$\begingroup\$ I m pretty sure that R4 and copper tracks are fine, since I have already bouble checked. I ll lift C1 and come back to comment if voltages are the same. But how can i tell if C1(ceramic) is leaking DC Current? Actually apart from the micro current drawn by the opamp input, the output is capacitor coupled(ceramic capacitor too) so i cannot find a dc current path. I am not right now at the lab and i dont have my measurments log with me. Thanks for the response anyway! I guess if by lifting C1 voltage on both terminals get the same, I ll determine if i have either a dead capacitor or a dead ic? \$\endgroup\$
    – Aris
    Oct 8, 2013 at 10:56
  • \$\begingroup\$ Try it for sure \$\endgroup\$
    – Andy aka
    Oct 8, 2013 at 12:00
  • \$\begingroup\$ No luck. Lifted C1 and voltage level at +Vin and -Vin are still different. V+in is set at the virtual ground level(2volts for 4V Vcc) and V-in is at 1.4 volts.Same goes for the output of this channel...Guess the opamp is fried, (which is very strange since the other channel on the same op amp ic works fine)..once i ll get my replacement parts, I ll get back to comment so this post might be helpfull sometime. Thanks again! \$\endgroup\$
    – Aris
    Oct 10, 2013 at 9:55
  • \$\begingroup\$ Good luck Aris - sounds like a fried IC. \$\endgroup\$
    – Andy aka
    Oct 10, 2013 at 10:12
  • \$\begingroup\$ changed the opamp and that was it... so for the record a half fried op amp can happen. I must have supplied with over voltage during my first tests. I changed both ics just to be on the safe side. Thanks for the answers once more! \$\endgroup\$
    – Aris
    Oct 11, 2013 at 10:35

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