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Impedance Matching

Discussion in 'Electronic Basics' started by Greg Esres, Feb 16, 2004.

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  1. Greg Esres

    Greg Esres Guest

    I have a low impedance output (aircraft intercom system) that needs to
    go into a medium impedance input (digital voice recorder microphone
    jack). Right now, the recording is distorted and low volume.

    The only impedance matchers I can find work in reverse....high
    impedance output, low impedance input.

    What do I need to do what I want?

    Thank you.
     
  2. Bob Masta

    Bob Masta Guest

    You don't specify what "low" and "medium" impedance
    values are, but in general this is not the sort of area
    where impedance matching is done anymore. Modern
    designs usually have low impedance outputs similar
    to an ideal voltage source, and they feed high impedance
    inputs that don't load it enough to matter. Impedance
    matching for best power transfer was done in the good
    old days, and still in certain PA *speaker* applications, but it isn't
    an issue with mic recording.

    So the real question is what is causing the distortion
    and low output levels? I'm assuming the intercom
    output can easily drive any mic input, and since most
    mic inputs are *much* more sensitive than typical line
    outputs, you will probably need to attenuate the signal
    quite a bit before it goes to the mic input. (I'm assuming
    there's no line input.) Also, mic inputs often supply
    phantom power on the ring connector, so depending
    on how you've wired things that could be causing a
    level shift. You can probably check that with a DVM.
    Also, are you sure the intercom output is good?




    Bob Masta
    dqatechATdaqartaDOTcom

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
     
  3. Greg Esres

    Greg Esres Guest

    <<you will probably need to attenuate the signal quite a bit before it
    goes to the mic input. (I'm assuming there's no line input.) Also,
    mic inputs often supply phantom power on the ring connector, so
    depending on how you've wired things that could be causing a
    level shift. You can probably check that with a DVM.
    Also, are you sure the intercom output is good?>>

    Bob, thanks for your excellent response. I think you're on target and
    that makes a lot of things click. The lack of devices to adjust the
    impedance in the way I described suggests that no one wants to do
    this, because, I suppose, it doesn't need to be done.

    I purchased a 50db attenuating cable at Radio Shack yesterday and am
    looking forward to trying it out today. I did use it on my
    transceiver, and the voice communication was very intelligible.
    However, there was a strong background hiss, which I read is due to
    recording at low volume levels. The transceiver probably doesn't put
    out as much signal as the actual aircraft intercom system, so that may
    improve when I use it in the aircraft. Regardless, I can filter that
    out with my software.

    Not exactly sure what you mean by phantom power, but the aircraft
    intercom system does provide a DC bias in order to power the
    microphone.

    Thanks for your help!
     
  4. Bob Masta

    Bob Masta Guest

    Greg, you might want to jury-rig a simple pot or
    2-resistor voltage divider to determine the best
    attenuation to use. The resistance of the pot
    or resistor chain should be around 10-50K,
    but anything in the 1K to 100K range is fine.
    You can easily rig up the pot by cutting a
    cable in two and soldering it in-line. There
    will most likely be some hum pickup since
    the pot and your connections won't be
    shielded, but unless it is swamping the
    mic input you can probably get by without
    a metal can around things. This is just a
    test to determine the proper level. If it's
    too high you get distortion, too low and
    you get hiss, which might be what you are
    getting now with your 50 dB cable.
    Once you have the right range, you can
    wire up a cable with fixed resistors under
    the shield. (Use shrink tubing around the
    resistors, or even electrical tape.)

    As for phantom power, that's used to
    supply power to electret mics. Your
    sound card probably uses 5V or something
    like that. This is typically on a separate
    connector of the mic jack, which is the little
    ring on the plug. Avoid that, since the added
    DC bias onto the intercom output may
    cause problems.

    I think some sound cards may supply
    the phantom power directly to the
    mic signal connector (the tip), in which
    case you may need a coupling capacitor
    if the DC is messing up the intercom
    output. (It shouldn't, but then again I don't
    know what sorts of outputs aircraft
    intercoms use!)

    Hope this helps!



    Bob Masta
    dqatechATdaqartaDOTcom

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
     
  5. Greg Esres

    Greg Esres Guest

    <<Hope this helps!>>

    That helps enormously, Bob. Understanding the problem makes it half
    solved. You're the first one I've communicated with on this subject
    that had enough understanding to tell me that my original analysis of
    the problem was incorrect.

    Thank you!
     
  6. That's an idea, but please note that the mic input preamp can get messed up
    by a decoupling capacitor unless a resistor is added to ensure that it has a
    (minimal) load attached. Since some mic preamps require a current to flow
    through the electret mic's FET, providing an 'infinite resistance' may cause
    problems with the preamp's operation. A 1K resistor will probably be fine.
    Than a decoupling cap can be connected to get rid of the DC and a resistive
    voltage divider to attenuate the signal as you suggested. Impedance matching
    will still be necessary, but only to a limited extent (just to make sure the
    mic input is loaded and the whole thing doesn't become a high pass filer).

    Hope, this wasn't too wrong.

    Dimitrij
     
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