Microphone To Speaker Schematic

Discussion in 'General Electronics Chat' started by crazyengineer, Jan 7, 2012.

  1. crazyengineer

    Thread Starter Member

    Dec 29, 2010
    156
    2
    Hello! One project I want to work on is to simply transmit sound from a microphone to a speaker. I know I have to add decoupling capacitors at the VCC pins of both Op amps. The microphone is modeled as vac in the schematic.Also the speaker is modeled as a 8 ohm resistor at end of the right op amp.

    [​IMG]

    I just want to know whether there are a coupling things I should fix in this schematic? Can anyone recommend a good audio op amp to use? I have zero intention of using the LF324 that's in this schematic.
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,052
    3,244
    Your output amp won't work since there is no dc bias path for pin 5. Move C3 from its location and connect it between pin 7 and R6.

    The resistors loading the mic seem rather low. The equivalent impedance the mic sees is only 1/3 of 1k or 333 ohms. I suspect that will significantly reduce the signal voltage from the mic.

    Audioguru has some suggestions for good audio op amps but I don't recall what they are. If you search for some of his posts, you should find it.
     
    crazyengineer likes this.
  3. crazyengineer

    Thread Starter Member

    Dec 29, 2010
    156
    2
    Do you know an equation that will tell me the signal voltage?Do you think having a 10k resistor for the microphone will be fine?
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    896
    1) An electret mic uses only 0.5mA. Then R5 should be 18k (18 times higher) and R2 and R3 should be 180k each (180 times higher).
    2) A lousy old LM324 is NEVER used as a mic preamp because its noise (hiss) level is too high, it has class-B crossover distortion and it performs poorly above only 2kHz. Most people can hear to 20kHz except gun shooters and acid rock "musicians".
    3) I agree that the pin 5 input has no bias nor reference voltage then the opamp will not work.
    4) An opamp does not have enough output current to drive a speaker. Use a power amp IC instead.
     
    Last edited: Jan 8, 2012
  5. Audioguru

    New Member

    Dec 20, 2007
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    Do you understand the problems and solutions I discussed with your defective design?
     
    Last edited: Jan 8, 2012
  6. crazyengineer

    Thread Starter Member

    Dec 29, 2010
    156
    2
    Yes. I understand completely. I just need to find a power op amplifier to use.
     
  7. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    In North America, semiconductor manufacturers make at least 100 audio amp ICs. Take your pick.
     
    Last edited: Jan 8, 2012
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