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Mic circuit problem

  1. Aug 12, 2009 #1
    I need a way to limit the maximum voltage that goes to mic input, for e.g. if the amplitude of the signal is 100mV and fluctuates between 100mV and 100.1mV I need the signal going to the mic to be 1mV fluctuating between 1 and 1.1mV. So there is a minimum signal amplitude(about 100mV) that needs to be ignored completely(or reduced) and the fluctuation at the peak of the signal that needs to be recorded.
  2. jcsd
  3. Aug 12, 2009 #2


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    If I'm reading you right, you have a signal with a 100 mV DC bias. You can easily block this by including a series capacitor on the input to the amplifier.
  4. Aug 12, 2009 #3
    the 100mV is part of the signal its AC.
  5. Aug 12, 2009 #4


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    Ah. Sometimes information is important. Perhaps you could give us some.
  6. Aug 12, 2009 #5
    the signal going to the mic is from a preamp circuit thats picking up alot of 60hz mains(the 100mV)if I hook it up as it is it might burn my soundcard.
  7. Aug 12, 2009 #6


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    Much better. Also, a tough problem--60 Hz hum is notoriously difficult to filter out, particularly in audio applications. If the noise is common mode, you can try a common-mode choke--this will attenuate any common-mode noise (that is, noise which is present on both input lines in phase) but allow differential signal to pass unimpeded.

    If your signal is much higher than 60 Hz (say a few hundred Hz or more), you might be able to filter it out with a high-pass filter, the design of which depends on the precise nature of the signal you wish to capture.

    It is also advisable to go over your setup and look for potential connection issues, such as damaged, frayed or poorly-constructed interconnect cables, ground loops and proper shielding. Try to narrow down the source of the hum as much as possible; this will allow you deal with the problem at the sources, rather than try to band-aid it with a filter.
  8. Aug 13, 2009 #7


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    Just a small nit (negitron has this thread well in hand). There is no signal that goes from the preamp to the mic. The mic is the source of the signal that the preamp picks up and amplifies. There might be a bootstrap voltage/signal sent to the mic, but that would appear to be beyond the scope of this thread.
  9. Aug 13, 2009 #8
    I meant mic input.
  10. Aug 18, 2009 #9
    So where is the 60Hz being picked up? At the mic or the preamp.
    If you use a differential amplifier you should be able to get rid of the 60Hz. There are lots of differential op amps which have high common mode rejection ration at 60Hz.
    Try differential or instrumentation op amps. Should work without much headache.
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