Modification Help

Discussion in 'General Electronics Chat' started by haloway13, Jan 28, 2014.

  1. haloway13

    Thread Starter Member

    Oct 24, 2011
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  2. alfacliff

    Well-Known Member

    Dec 13, 2013
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    you will have to modify the input circuit, the electret micorphone does draw a little current, and that biases the + input of the amp.
     
  3. haloway13

    Thread Starter Member

    Oct 24, 2011
    37
    0
    I imagine that driving a transistor at its base to bias it in the same way as the electret mic would. Any suggestions?
     
  4. alfacliff

    Well-Known Member

    Dec 13, 2013
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    electrets use fet's for the internal amp. adjuxgt bias accordingly.
     
  5. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,988
    745
    If your using an headphone output, i would omit the first stage of the op amp, and input directly into C3, that should give you a dc voltage across C2 depending on the level of the headphone volume.
     
  6. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    784
    Replace the mic with an identical resistor to R8 - that biases the op-amp input to 1/2 Vcc, you need a DC blocking capacitor say about 0.1 to 0.47uF. Its probably a good idea to include a load resistor on the phone side of the blocking cap, probably about 32 or 64R - but the DC winding resistance of a headset as measured should be close enough.
     
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