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Large DC voltage at opamp inputs?

Discussion in 'General Electronics Discussion' started by ramshackles, Feb 16, 2014.

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  1. ramshackles

    ramshackles

    1
    0
    Feb 16, 2014
    Hi
    I'm building a microphone preamplifier. It basically consists of 2 opamp stages (5534) which are directly coupled:

    [​IMG]

    From the -ve input of the first stage I measure -1.6V to ground and -0.8V to ground from the +ve input. I can't figure out why this is so high?

    This is amplified in the preceding stage, so the voltage at the output of the second stage is about -2.9V

    Any tips on what I'm doing wrong? (PS, the 22pF compenstation cap is no shown in the schematic, but is attached between pins 5 and 8 of each opamp. The gain setting potentiomenter is a 1K dual pot, wired as a variable resistor)
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    What happens if you ermove RSh1 and RSH2? Both OpAmps should be in unity gain mode and the voltage on both inputs should be equal. How is your "gain switch" constructed? Any chance it injects stray current or voltage into the circuit?
     
  3. Arouse1973

    Arouse1973 Adam

    5,165
    1,087
    Dec 18, 2013
    These devices appear to have a balance pin. Have you terminated this properly. And as Harald says you could have stray currents somewhere. You have either current being pushed into the inverting terminal or current being pulled out of the non-inverting terminal. Re-check you wiring on the inputs. Could your input transformer be picking up noise? Have you checked the output with a scope?

    Adam
     
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