Small Microphone to Sound Card

Discussion in 'The Projects Forum' started by 314159265, May 29, 2009.

  1. 314159265

    Thread Starter Member

    May 29, 2009
    13
    0
    I'm trying to make a small microphone that will go inside a larger microphone in a VHF radio to run audio to multiple sources. This microphone will run to a computer sound card, where it will be processed on the computer. However, I'm having trouble getting the computer to receive audio from this microphone. Other microphones work fine, even old microphones with no batteries or power source.

    I've built kits before, but my experience with circuits is minimal. I'm not sure how to make a schematic on the computer, so I'll just describe the circuit as best as I can.

    Components Used:
    9 v Battery
    22Ω resistor (This might be the problem, from what I've found online)
    4.7 μF capacitor
    Radioshack #270-101 Microphone element
    1/8" mono audio jack

    Setup:
    The grounds are all connected: the shield on the audio jack, the negative lead from the microphone, and the negative lead from the battery.

    On the positive side of things, the positive lead from the battery is run through the resistor. The other end of the resistor, along with the positive lead from the microphone are connected to one lead of the capacitor.

    The other lead of the capacitor is connected to the center lead for the audio jack. The shield is connected to the negative leads.

    Results:
    Nothing. Not a bit of audio makes it in to the computer.

    I'd really appreciate any advice this forum has to offer.
     
  2. flat5

    Active Member

    Nov 13, 2008
    403
    17
    Are you powering an electerec mic through the 22 ohm resistor?
    If so, change the resistor to 10k ohms.
     
  3. 314159265

    Thread Starter Member

    May 29, 2009
    13
    0
    The diagram that came with the microphone said to use a resistor up to 1 kΩ. However, most circuits that I've found online use a 2.2 kΩ or a 10 kΩ resistor.

    I'm not sure that I really understand what this resistor is doing. My guess is that it is supposed to keep the audio from traveling down the positive wire to the battery. The audio would take the path of least resistance through the capacitor and to the computer. Is this somewhat correct? Would a 5 kΩ resistor result in louder audio than a 10 kΩ?
     
  4. 314159265

    Thread Starter Member

    May 29, 2009
    13
    0
    UPDATE: I bought the 2.2 kΩ resistor and it's working great now. I'd still appreciate an explanation of what this resistor is doing in the circuit.
     
  5. hwy101

    Active Member

    May 23, 2009
    91
    28
    Actually a computer sound card uses a stereo jack for the mic input and puts out a small voltage to power a condenser mic, thats why you need the resistor, the pc mic's are also stereo plugs but the ring and the tip are physically joined together inside the mic.

    so..you maybe shorting out your audio input or reducing it substantionally.
     
  6. flat5

    Active Member

    Nov 13, 2008
    403
    17
    "I'm not sure that I really understand what this resistor is doing. My guess is that it is supposed to keep the audio from traveling down the positive wire to the battery."

    Yes. The mic has a preamp in it. It requires some power. Very little power, so a high resistance can be used even with a low voltage to isolate the power circuit from the audio circuit.
    If you lower the resistance to much, the audio output from the mic will suffer as you figured out.

    more info.
    http://www.epanorama.net/circuits/microphone_powering.html
     
  7. Mike Mandaville

    Active Member

    May 27, 2009
    81
    1
    Again, good answers so far, and I will just add my two copper-clad zinc ingot's worth. The inexpensive and abundant "computer microphones" which are sold at places like Comp USA for around ten dollars and up are all electret microphones which are configured to draw "phantom power" from the computer, and the standard schematic for hooking one up is available in a number of places on the internet.
     
  8. radiohead

    Active Member

    May 28, 2009
    474
    31
    The electret microphone has an FET in it. Therefore it needs a bias voltage in order for it to work. The resistor limits the current so the FET inside the microphone doesn't burn out. You will also need a coupling capacitor that will let the audio sine wave pass but not the DC voltage.
     
  9. radiohead

    Active Member

    May 28, 2009
    474
    31
    You will notice that some electret microphones have three leads instead of two. In that case, one is positive, one is negative and the other is audio.
     
  10. 314159265

    Thread Starter Member

    May 29, 2009
    13
    0
    Thanks for all the replies. This is making more sense now.
     
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