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remove DC voltage

Discussion in 'Electronic Basics' started by Quinn Valente, Jan 24, 2006.

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  1. Hello,

    I came across the following problem which I would like to solve with your
    help. My brand new iRiver iFP899 provides a stereo line-in jack for
    connection to any kind of source providing line-in level for direct
    encoding. It can also be used as external mic input via settings in the menu
    of the player.
    I guess the jack input features so-called "plug-in power" which means that a
    few volts of bias on the mic input is injected, which can power some small
    consumer-type electret mics that don't have their own battery (although most
    do have batteries anyway). I measured the input and it provides about 3.15V
    (DC unloaded) for bias voltage. Funny enough, the bias voltage does not
    dissappear when I switch to "line-in" mode from "external mic" mode.
    Now I want to connect my hifi mixer to the device to record old vinyl
    records, CDs, whole real-time mixes etc. As soon as I plug the device into
    the mixer line-out output the VU meters overflow, i.e. they show full
    amplitude. I guess that is due to the "wrong direction" current travelling
    INTO but not OUT OF the mixer because of the bias voltage on the wire. What
    tricks can I use to couple the device to my mixer so the iRiver line-input
    DC bias does not reach the external source and does not affect it?

    Many thanks in advance and best regards

    Quinn
     
  2. Robert Baer

    Robert Baer Guest

    A capacitor will block the DC and allow the AC to flow.
     
  3. Thank you. What size for the capacitor should I chose? Depends on the
    frequencies that have to pass, right? They are in the audio frequency
    spectrum, i.e. max. 20 kHz. When I try to claculate the right capacitor I
    have to consider the time constant (T=RC). What can I assume for the R
    value?
     
  4. Antonio

    Antonio Guest

    <reorganized previous top-posted article>

    Since a series capacitor is basically a High Pass Filter, you should
    consider the lowest frequency in the audio range to make your design.
    20 Hz is a common value.

    As for the resistance that you should use in your design, you should
    use the nominal input impedance of your device.
    Given that your only porpouse is decoupling the DC component, your
    design criteria should be to make the corner frequency much lower (a
    decade at least) than the lower audio frequncy, in order to avoid
    atenuation and phase distortion in the frequencies of interest.

    HTH
     
  5. What Antonio said was correct. But to follow it to the conclusion: the
    input impedance of the input is probably on the order of 1k. The corner
    frequency of an RC highpass filter is f = 1/(2*pi*RC). Rearranging that,
    you get C = 1/(2*pi*Rf) = about 10uF. So, use a 10uF or larger capacitor.

    At that value, you'll need to use an electrolytic capacitor. They come in
    different voltage ratings; anything bigger than the voltage you're trying to
    block will be fine. Make sure you orient the polarity correctly: the + pin
    on the capacitor needs to be facing the positive voltage, which probably
    means it needs to be connected to the iRiver side.
     
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