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Circuit to connect "Soundblaster" electret microphone to line-input

Discussion in 'Electronic Basics' started by Robert Ham, Oct 5, 2007.

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  1. Robert Ham

    Robert Ham Guest

    Hi there,

    I have three cheap PC "Soundblaster" electret microphones[0] that I
    need connect to a multi-channel sound card. The problem is that the
    microphones expect a +5V DC bias from their connection with the sound
    card[1]. The multi-channel sound card only has line-level inputs and
    doesn't provide power for electret microphones[2]. I need to build
    three circuits to a) provide the necessary voltage to the microphones,
    and b) amplify the microphone signal.

    I have found a circuit that appears to cover both issues[3]. However,
    I need some clarification on a couple of things. Firstly, I will not
    need the dynamic microphone input. Can I simply remove the dynamic
    mic terminals and capacitor C3 from the circuit, or is there more to
    it than that?

    What I'd like to do is build a single box with three circuits on a
    single piece of copper-track board and have it supplied by a single
    wall-wart power supply. I'm a little unsure how to adjust the circuit
    to accommodate this. How can I supply three such circuits from a
    single power supply? From what I recall of my physics lessons, I can
    either run them in parallel or serial, the consequences of which are
    either a tripling of the power supply voltage (from 9V to 18V) or a
    tripling, or possibly 1/3ing of.. something else.. amps? I forget.
    Any pointers would be appreciated.


    Robert Ham

  2. Bob Masta

    Bob Masta Guest

    Correct, you just remove C3 and the dynamic input.
    Build your 3 circuits such that they all share the same
    common ground. If you want 3 separate power switches,
    connect all 3 circuits together at the 9V point above their
    switches. If you want a single switch for all, you can
    connect them just below a single switch. You should
    keep all 3 LEDs unless you want to redesign the bias
    circuit. You don't need both C1 and C2. This series
    connection is a method to make a non-polarized capacitor
    out of two polarized electrolytic types, but here the input
    on the mic side should always be more positive, thanks
    to the electret bias through R1. You can thus reduce the
    single-cap value from 10 uFd to 5 (actually 4.7) uFd
    to keep the same effective capacitance.

    Note that '4k7' means '4.7k' and '120R' = 120 ohms.
    You don't need this exact transistor part number;
    try any small-signal NPN like a 2n3904.

    As the circuit author notes, this is not a low-distortion
    circuit... you could do better with an op-amp design,
    or even a better single-transistor design.
    But if it is for conferences instead of concerts, it
    should be fine.

    Best regards,

    Bob Masta

    DAQARTA v3.50
    Data AcQuisition And Real-Time Analysis
    Scope, Spectrum, Spectrogram, Signal Generator
    Science with your sound card!
  3. tempus fugit

    tempus fugit Guest

    I've got that soundcard too. The software allows you to use faders to select
    the input level over a fairly wide range. I hooked up an electret mic to the
    line in just to see what kind of signal level I could get, and it was
    typically in the -30 to 25 db range. If that'll do the job for you, you
    might consider dropping the preamp idea altogether, particularly since the
    one you linked to has a distortion figre of 2-3% (that's pretty damn high).
    You could also make one based around an opamp, which wouldn't really be any
    more difficult and would sound WAY better. You can get opamps 4 to a
    package, so 1 16 pin chip would do the trick, plus a few resistors and

    Here's just 1 of many (and there are better out there too):
    Hash: SHA1

    Take a look at
    if you want some more reading material

    - --
    Brendan Gillatt
    brendan {at} brendangillatt {dot} co {dot} uk
    PGP Key:
    Version: GnuPG v1.4.3 (MingW32)

    -----END PGP SIGNATURE-----
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