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Carbon microphone adapted to condenser mic input?

Discussion in 'General Electronics' started by Vince Bafetti, Jul 14, 2004.

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  1. I'm working on a project that involves switching an amplifier input
    between a condenser mic and a carbon mic (telephone handset). The
    amplifier is designed for the condenser mic: it has a 5vdc bias, etc.

    I think it shouldn't be too hard to adapt this amp to work with a
    carbon mic...but are we talking about a purely passive mod, or do I
    need to make a preamp?

    Interstingly, I've found just about nothing on the web regarding
    carbon microphone amplifiers. Any help in this area will be much

    Vince Bafetti
  2. Rich Grise

    Rich Grise Guest

    Do a little searching on their specs - I can't quote carbon mic
    specs off the top of my head, but remember they used to use
    them in telephones, with no amplifiers. You'll probably need
    an attenuator. :)
  3. Ban

    Ban Guest

    A carbon mike has pellets of graphite being squeezed by a diaphragm. This is
    equivalent to a variable resistor. The value should be in the k-ohm range.
    you can measure it with the ohmmeter. You can connect it exactly as a
    two-terminal electret capsule with bias voltage.. Probably the amplitude
    will be bigger, so you can just use a series resistor with the capsule to
    get the same level as the other mike. There will be a lot of distortion and
    granular noise since the capsules are non linear and the compression of the
    carbon will cause little steps and discontinueties in the transfer function,
    but for telephone the quality was adequate.
  4. Thanks for the info, guys.

    I measured the resistance of the mic in the 150K-ohm range. Does this
    make sense?

    Then, I connected it to the input...and got nothin. Tried with and
    without series resistors, still nothing.

    The mic casing has + and - markings--these things aren't polarized,

    anyway, I'll keep you posted on further developments.

  5. Jim Thompson

    Jim Thompson Guest

    A carbon microphone is just that, carbon particles in a cannister,
    arranged so that it's effectively a variable resistor between two

    So your amplifier needs to run some current thru the variable resistor
    so that you get a variable voltage (or conversely, apply a voltage and
    see a varying current).

    It's been so long since I've used one in a design (try 300 baud modem
    :), so someone else will need to provide specifics.

    ...Jim Thompson
  6. Tim Wescott

    Tim Wescott Guest

    150K sounds way too high.

    As far as I know they're not polarized -- but it wouldn't hurt to check.

    The carbon granules can get packed -- on a telephone receiver you whack
    the receiver _hard_ against a wood surface and it shakes them loose. On
    just about anything else this would probably leave you with little bits
    of microphone, so maybe you don't want to try.
  7. Jim Thompson

    Jim Thompson Guest

    I remember that trick!

    They're also good for **serious** harmonic distortion. That's why, as
    you may recall, my mention of driving signal into the earpiece
    **inductively**, instead of acoustically into the carbon mike, of a
    phone set; 300 baud "muff" modems ;-)

    ...Jim Thompson
  8. Externet

    Externet Guest

    Hi Vince.
    I would bet a six pack you took the first step wrong assumming the
    Telephones before the sixties may have a carbon microphone. I doubt
    yours has it. Shaking it you will confirm this as you would hear the
    carbon granules.
    What you see marked with a + - is a condenser electret microphone,
    from a modern handset. The - is connected to its body, about 10mm Ø.
    Then, try the conections on the RJ22 connector for the handset this
    Outer two terminals are microphone. Reverse connection if the
    microphone does not work... Inner two are the earphone.
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