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Carbon microphone amplitude

Discussion in 'Electronic Basics' started by Rune D. Jørgensen, Feb 26, 2006.

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  1. Hi.

    I'm building a microphone amplifier and filter for a communication system
    for my motorcycle. I'm filtering away frequencies below 1KHz to suppress
    wind noise.

    I have a microphone that has a resistance of 1.8K ohm, so I guess it's a
    carbon. I have read somewhere that I should have approximately 1mA running
    through it. Is that correct and what kind of amplitude should I expect out
    of the microphone?
  2. ehsjr

    ehsjr Guest

    If you have 1 mA current running through 1.8K ohms
    resistance, you will have a voltage across the resistance
    of .001 * 1800 or 1.8 volts.

    As to what you read (the 1 mA) and what you guess about
    the mike, who can say? We'd be guessing, based on your
    guess. Facts would be helpful!


  3. Ed, I think he's asking about the AC amplitude.

    What AC amplitude is typical, for speech into a carbon mic element?
  4. Det var smuk og solrig dag da ehsjr skrev
    i sci.electronics.basics:
    I know ohm's law...
    Facts would be helpful, indeed. But I don't have the facts. If I did I
    wouldn't be asking here.
    If I had an oscilloscope I could just measure it, but I haven't got one.

    There must be some rule of thumb, as to what current should run through the
    microphone, and what signal amplitude to expect. Is it 10mV, a 100mV or a
  5. Det var smuk og solrig dag da Walter Harley skrev
    You're spot on :) It's of course the signal amplitude, I need to know.
  6. Don Bowey

    Don Bowey Guest

    I don't believe that is a carbon mike. I may not be up-to-date on them, but
    in my experience carbon mikes tend to have a resistance of 200 Ohms or so.
    In practice the current could be 10 milliamps, more or less, and the
    resulting signal from a good quality unit might be as much as 0.5V with a
    circuit voltage of about 4 to 6V.

  7. Dan Akers

    Dan Akers Guest

    Rune D. Jørgensen wrote;
    "I'm building a microphone amplifier and filter for a communication
    system for my motorcycle. I'm filtering away frequencies below 1KHz to
    suppress wind noise.
    I have a microphone that has a resistance of 1.8K ohm, so I guess it's a
    carbon. I have read somewhere that I should have approximately 1mA
    running through it. Is that correct and what kind of amplitude should I
    expect out of the microphone?"
    Assuming it is a carbon microphone and you don't have the make and model
    #. There is no current/sound pressure thumb-rule, that I know of, that
    would cover all carbon mikes. Although the characteristic resistance for
    yours may be 1.8kohm, the voltage amplitude produced for a given sound
    pressure with 1 mA excitation is highly dependent on the surface area
    and frequency response of the diaphragm, the particular density response
    of the carbon pack, and the characteristic input impedance of the
    electrical load on the microphone and it's power supply. Without these
    particulars you are just guessing, as are we, and you will probably need
    an oscilloscope to determine the electrical response to voice in your
    planned circuit.

    Dan Akers
  8. ehsjr

    ehsjr Guest

    You guys may not be aware of how a carbon mike works.
    You seem to think that the mike generates a voltage.
    It doesn't. A voltage must be supplied to it. Sound
    varies the pressure on the carbon, which causes its
    resistance to vary. That causes the current through
    the mike to vary.

    What is the typical signal amplitude of a carbon mike?
    Meaningless question. A carbon mike is not a source.
    Without knowing the source voltage one cannot say what
    the amplitude will be. Likewise, without knowing the load
    impedance, one cannot say what the amplitude will be.
    The signal (and by that I assume you mean the voltage
    drop across the mike) is a result of the current through
    the resistance of the carbon. That current depends on
    source voltage and load impedance, as well as mike
    impedance at a given audio level & frequency.

  9. Ed, the OP asked about how much AC voltage is generated across the mic if he
    runs a DC current through it and presents it with an acoustic signal. Since
    a carbon mic behaves like a variable resistor, that is an entirely
    meaningful question: if a DC current is applied to a variable resistor, the
    voltage across the resistor will vary. The unanswered part is "how variable
    is the resistor"? Does it vary by 1% before clippping, or 10%, or what?
    Does the resistance increase, or decrease, or both, compared to nominal?

    Or, you could answer a different question, that still could reasonably be
    presumed to be helpful: what sort of circuit might one typically and
    usefully employ, to develop an electrical signal from a carbon mic?

    Dan Akers suggested that there's just no way to know. But I suspect that
    knowing the answer for "some" specific carbon mic would be more helpful than
    knowing nothing at all - even if it's an order of magnitude away from this
    one, it's a starting point.
  10. Ban

    Ban Guest

    A carbon capsule is likely to be big and went after the advent of electret
    mikes into the garbage bin and no sane person will use this technology for
    other then close voice pickup and drving a horn speaker directly though a
    12V battery, a megaphone without amp. When the OP measured some resistance,
    it doesn't mean it is only resistive, but might be an Electret mike which
    have about 0.5mA bias current and might on a multimeter show up as some
    obscure value.
    If the OP would also measure with the test leads crossed the same resistance
    it is certainly not an electret mike, exept those with 3 pins where there is
    a resistor between supply and output.
  11. Yes.
    Hoping someone would mention they came out of the Ark.
    Measured one of the Electret types at 1.0Kohm in one direction and 1.4Kohm
    in the other direction.
    Also hooked up a couple of mic's I've kicking around.
    A cheap Electret, with a 5V supply and 4k7 load resistor gives about 3mV
    rms, normal voice, 50mm away.
    Another cheap mic' (dynamic?) measured 560ohms and gave about 200uV rms
    under same speaking distance.
  12. ehsjr

    ehsjr Guest

    You don't even know if he has a carbon mike! Then you post
    the above and call it an "entirely meaningful" question?
    Balderdash. The OP is *guessing* he has a carbon mike.
    If he does not then what good is it posting "the answer"
    (if one existed) concerning a specific carbon mike?

    You mention 'the answer for "some" specific carbon mic'
    There is no "the answer ...", for crying out loud. It depends
    on external (to the mike) circuit elements: the load and the
    source. "The answer" does not exist.

    What you are suggesting is to give the op something that
    doesn't exist, for an unknown circuit for an unknown mike.
    Misleading the op that way is not helpful.


  13. Here, let me try one more time. I'll ask a hypothetical question that is
    different than the OP's, but that might be informative nonetheless. I know
    it's hard to answer hypothetical questions, but some people manage okay.

    Let's say I have a friend who thinks he has a carbon mic element - maybe he
    got it out of a 1950s-era telephone operator's headset. He thinks it's
    carbon because it's old and when he shakes it, it rattles. He offers to
    give it to me.

    Now, I don't have it yet. But I want to think a little about what sort of
    circuit might get some audio out of it. So, on the hypothesis that it is
    indeed a carbon mic, I imagine I will hook it up to a 1mA constant DC
    current source. (Note that the AC impedance of a DC current source is
    infinite. The load and source are thus specified, just as they were in my
    earlier question.)

    1. Roughly what AC voltage level might I reasonably expect to see developed
    across it, when I speak into it (at, say, 75dB SPL)? Are we talking tens of
    uV, or tens of mV, or ...?

    2. Does the performance of this microphone technology depend on a particular
    current being passed through it? Would, for instance, 10mA be better than
    1mA, in terms of the linearity and dynamic range of the mic?

    3. Would it be better (in terms of linearity and dynamic range) to apply a
    constant DC voltage and measure AC current variation, or to apply a constant
    DC current and measure AC voltage variation?

    4. What circuit conditions would this element have typically been used in,
    and in those conditions, what sort of AC signal would it typically develop?
  14. Rich Grise

    Rich Grise Guest

    That's not the kind of facts we're talking about. We need to know all of
    the facts that _you_ have, now - otherwise, we're guessing in the dark.
    The first thing is, how sure are you that it's a carbon mic? If, as you
    say, the mic has a DC resistance of 1.8K and a spec'd current of 1 mA,
    then you could take a 3V battery (like 2x D cells), a 1.2K fixed resistor,
    and put them in series with the mic:

    +----[1K2]-----+ A
    + | |
    [3V batt] [MIC]
    | |
    +--------------+ gnd

    Then, you should get a signal at point "A" that more or less represents
    the audio. You can measure this with a multimeter - put a capacitor at
    point A, and an AC voltmeter from the other end of the cap to ground:

    + | | |
    [3V batt] [MIC] [DVM]
    | | |

    I have no idea what kind of voltage you'll get out of it, but it's a
    start. Put your meter on, say, 1V full-scale, and shout into the mic.
    If the meter doesn't show much, start turning its range down (.1V,
    10 mV, etc) until you see something.

    If you use an electrolytic cap, be sure and get it the right way around. :)

    Good Luck!
  15. Don Bowey

    Don Bowey Guest

    First, figure out if the thing *IS* a carbon mike, then post your questions,
    or better yet search google first.

    I posted a reasonably straightforward reply to the original post, with
    simple circuit information, and giving an estimated output signal amplitude
    for a good grade of carbon mike. What's left to discuss before you do some

  16. Det var smuk og solrig dag da Don Bowey skrev
    i sci.electronics.basics:
    You're right. It isn't a carbon microphone. I have just cut it open(it was
    incapsulated in foam) and a google search on "CZ034D" yielded a datacheet

    It seems that it is an electret microphone.

    The reason is believed that it was a carbon microphone was the following. I
    could measure a DC resistance of 1.8K ohm, and I thought a condenser
    microphone would have infinite DC resistance, due to the capacitor.
    Electret microphones is "dielectric material that has been permanently
    electrically charged or polarised" according to wikipedia, which also
    should have infinite DC resistance to the best of my knowledge.
    Unfortunately my knowledge was wrong :)
    In a dynamic microphone a small movable induction coil, induces a current.
    I believe that the DC resistance measured would be the wire resistance, and
    I found the 1.8K ohm too high for that.
    This lead me to the carbon microphone, and not knowing that they weren't
    used anymore, it seemed obvious.
  17. True. But inside an electret mic element there is more than just the mic
    itself. There is also a small JFET buffer amplifier. The electret itself
    has a very high impedance, so it can't produce a useful amount of current;
    without the buffer, it wouldn't be able to drive the capacitance of the

    1.8k would be unusually high, but not impossible. Inside a dynamic mic,
    there is usually a transformer to change the output impedance; some mics,
    intended for high-impedance loads, have rather high output impedance. High
    output impedance from a transformer means lots and lots of turns of very
    fine wire; the DC resistance can get pretty high.

    The DC resistance of a guitar pickup, which is nothing more than a bunch of
    wire wound around a magnet, can easily be more than 5k.

    The take-home lesson here is that most modern mics have some additional
    circuitry inside them; when you measure across the leads you're not looking
    at the actual mic element, so you can't draw the sort of inference that
    you're trying to.
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