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phone line level to microphone input

Discussion in 'Electronic Design' started by D. G., Apr 3, 2006.

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  1. D. G.

    D. G. Guest

    I would like to adapt audio level on a phone line to a computer microphone
    input. I assume the microphone sensitivity is
    roughly the same among computer sound cards.
    In North America (correct me if I'm wrong), the phone line level is -10dBm
    (or dBu, I'm a bit confused here...), this
    should translate to 77.5mVrms into 600 ohms.

    How much attenuation should I use to connect to a microphone input?

  2. Deefoo

    Deefoo Guest

  3. <snippety>

    You would do well to consider that most phone companies are still
    picky about what gets hooked up to their lines, and that ringing voltage
    is on the order of 90-100VAC at 20Hz, and get yourself a proper voice
    coupler. Such a device will protect you and your equipment.

    Have a look at this page:

    Happy hunting.
  4. D. G.

    D. G. Guest

    It seems the full signal or may be half of it is fed into the microphone
    input in this design. Strange! I would fear it can overload the input or be
    more compatible with a LINE level input...
    Coincidence, what I'm building is for the same purpose (for VoIP) but I use
    a 600 ohm transfo for the hybrid circuit. The design at www.vital.pri cannot
    work on my system as the mic and the speaker share the same ground :(

  5. Deefoo

    Deefoo Guest

    BTW -10dBm translates to 245 mVrms into 600 ohms: 10*log10(PmW/1mW)

  6. D. G.

    D. G. Guest

    I was using 20*log() equation.

    Thanks for the link at www.vital.pri. I quickly built the first circuit
    showed on that web page but I replaced the mic input by a small traff
    (primary) to isolate ground. Then I connected the mic input at the traff
    (secondary) through a 22K/220 divider. Perfect!

  7. Ross Herbert

    Ross Herbert Guest

    Another simple solution
  8. mires

    mires Guest

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