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Attenuating "line out" into a mic socket

Discussion in 'Electronic Equipment' started by Alex Coleman, May 3, 2006.

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  1. Alex Coleman

    Alex Coleman Guest

    I have a handheld recorder which does not have a "line in" socket. The
    only input it has is a "mic" socket.

    The manufacturers sell a lead which is used to attenuate the "line out"
    from another device into my mic socket.

    I would rather make my own lead. Obviously I don't want to alter the
    frequency response.

    What components would I need and in what configuration would they need
    to be?
     
  2. GregS

    GregS Guest

    Put a 470 ohm resistor across the recorder input.
    Put a 10K ohm in series feeding the hot side of the input
    jack fed from the line out. Other side to common
    side of input jack. If the levels need changed, the ratio needs to be adjusted
    between the two resistors. 10k to 27K to reduce, 10K to 5K to increase.

    greg
     
  3. Specifications of mic inputs and line in- and outputs depends of the
    manufacturer. One I ever found says:
    mic input sensitivity 12uV max, input impedance 300-2k4
    line input sensitivity 12uV min, input impedance 10k
    but there are much more possibilities.
    If you the specs of your recorder, you can calculate the resistor network
    you need. If you can't, you can experiment with the network below.

    +-----+-----+
    | | |
    | | R2 .-.
    | |220k| |
    | | | |
    | | '-'
    | | |
    | | |
    | | |
    | R1 .-. .-.
    -+ 12k| | | |<-----
    | | | |
    line out '-' '-'1k mic in
    | |
    -------+-----+-------
    created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

    When the line out signal is too high, so when it is difficult to adjust the
    pot in the lower part, you can rise R2 up to 1M (try some values) and lower
    R1 to 10k. I don't expect the signal to be too week, but if it is you can
    make both resistors 22k.

    petrus bitbyter
     
  4. Beachcomber

    Beachcomber Guest


    A simple resistive pad is all you need. Here is a site that descibes
    several options in good detail.

    http://www.epanorama.net/circuits/line_to_mic.html

    Frequency response should not be a problem, but noise can be. The
    idea for good audio is to have as few amplifiers in the line as
    possible, and with the minimum amplification necessary.

    When you bring down a line level source with a resistive pad, you are
    essentially bringing the original signal closer to the noise level.
    Then, because you have a mic level input, you amplify the signal again
    (including the noise). It's not a problem for most applications, but
    then again, for a critical studio signal, it could be.

    Beachcomber
     
  5. GregS spake thus:
    I don't see why you can't just put a single resistor between the line
    out and mike in. Since the mike input is lower impedance than the line
    output to start with, why would you need a load resistor; why not just
    something to reduce the voltage?


    --
    Pierre, mon ami. Jetez encore un Scientologiste
    dans le baquet d'acide.

    - from a posting in alt.religion.scientology titled
    "France recommends dissolving Scientologists"
     
  6. Mr.T

    Mr.T Guest

    Yes you could if you knew the input impedance of the mic input. Or simply
    use trial and error.
    How do you know that? In fact it is not usually the case. Line *outputs* are
    often around 200 ohms or less, and mic inputs around 1k ohm or more.
    That's just what a voltage divider does no matter how many resitors you use.

    In any case the performance of the mic amp on these units is usually very
    poor. Be prepared to be disapponted whatever lead you use.

    MrT.
     
  7. Arny Krueger

    Arny Krueger Guest

    If you go the series resistor route, also you can easily end up with a
    voltage divider with a high output impedance, which then exposes you to
    losses at high frequencies. Not a problem when the input impedance of of the
    equipment being driven is 5 K ohms, but a recipie for disaster when the
    input impedance of of the equipment being driven is 500 K ohms.
     
  8. GregS

    GregS Guest

     
  9. Arny Krueger spake thus:
    Yes, but--2 things:

    1. Isn't the input impedance of the little recorder likely to be
    significantly less than 500 KΩ?

    2. In any case, how would a mismatched voltage divider cause high
    frequency losses? (This is a request due to ignorance.) I don't see how
    a resistive network, with no L or C components, could affect frequency
    response at all.


    --
    Pierre, mon ami. Jetez encore un Scientologiste
    dans le baquet d'acide.

    - from a posting in alt.religion.scientology titled
    "France recommends dissolving Scientologists"
     
  10. Arny Krueger

    Arny Krueger Guest

    Good design is about not taking unecessary risks.
    The cable and gear downstream of the divider contributes the parallel C.
     
  11. Arny Krueger spake thus:
    For a company designing a product for sale, that would be a
    consideration. For a guy trying to get a signal into an el cheapo
    handheld recorder, I don't think it makes any difference.
    What if the single resistor was right at the microphone plug? No
    significant C then, right?


    --
    Pierre, mon ami. Jetez encore un Scientologiste
    dans le baquet d'acide.

    - from a posting in alt.religion.scientology titled
    "France recommends dissolving Scientologists"
     
  12. Mr.T

    Mr.T Guest

    As Arny said, there is already some shunt capacitance in the box which you
    have no control over.

    MrT.
     
  13. Arny Krueger

    Arny Krueger Guest

    I can't believe that so much air has been warmed (figuratively speaking)
    over a ten cent resistor.
    Not so, even equipment that is specifically designed to have low input
    capacitance like oscilliscopes and other test equipment, has input
    capacitances in the range of 15-35 pF.

    Equipment with high input impedance did not die with the days of the tube.
    Direct boxes are very common and by definition have very high input
    impedances.
     

  14. You can find example circuits for this at
    http://www.epanorama.net/circuits/line_to_mic.html
     
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