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Discussion in 'Electronic Basics' started by P L U M !, Feb 15, 2005.

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  1. P L U M !

    P L U M ! Guest

    I need to build a preamplifier for a mic to connect it to a PC
    soundcard. The mic I am using is a proffesional mic which uses an
    electret mic capsule inside. I dont need to provide phantom power also
    coz it operates on a 1.5V battery. Now, the plan is to build a preamp
    with a female jack into which I can connect the mic's output jack. I
    tried an instrumentation amplifier circuit which has three 741s in
    it.The circuit worked fine for a gain of 10.
    Although I could see a gain of 10 only when I put the mic real close to
    the speaker.As I moved it farther the mic output(amp input) and also
    the gain dropped.When I increased gain further, that is to 11 it worked
    fine too. Then I increased it further to 20. Now the output of the
    amplifier was zero! Then I tried to give a gain of 2 thru an additional
    stage(a noninverting ampliflier using 741). So that would be 2*10=20
    gain...But again the output was zero. Can anybody tell me where I am
    going wrong???

    The mic without any amplification showed an output range of 5-20 mV.
    The link to the schematic of the insturmentation amplifier circuit is
    given below:
  2. tempus fugit

    tempus fugit Guest

    If you are plugging that mic into a line in on your soundcard, you should be
    looking at a pre with a gain of more like 1000 if you are micing quiet
    sources. You don't really need an instrumentation amp for this purpose; a
    single dual opamp would do the job nicely. Also, 741s suck for audio. Get a
    decent opamp with wider bandwidth and a higher slew rate. Even something
    like a TL072 or NE5532 (both dual opamps) will sound way better than a 741.
    You could build the differential amp on the page you have linked to, or get
    2 of the opamps I suggested and use them to build the instrumentation amp.
    Or do a search on Google for mic preamp schematics and see what you get.
    Your mic is more sensitive to sounds that are close to it, and as you mnove
    it farther away, the sound fades. This is to be expected.

    When I increased gain further, that is to 11 it worked
    What are you using for the supply voltage? I assume you adjusted the gain
    resistor in the circuit to increase the gain. What values did you use for
    the resistors?
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