Strange mic behavior

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msr
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2010/11/18 08:48:27 (permalink)

Strange mic behavior

I am working with a pair of non-matched AKG SC-1 condenser mics in an ORTF configuration.  I noticed something kind of weird.  When I apply phantom power to both mics I get this weird high pitched whine being produced.  Sometimes it actually changes pitch as well and sounds a bit like a police siren.  If I turn the phantom power off to one of the two mics, the sound stops.  If I reverse the phase on one of the two mics, the sound sometimes stops or might stop for a short time, but then starts back up.  The only thing I can think of is that this is some weird field coil effect due to the powered mics being in close proximity of each other.  Any ideas and particular, ideas of what I can to do stop this?
 
THanks,
 
msr

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    bitflipper
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    Re:Strange mic behavior 2010/11/18 09:51:23 (permalink)
    Does the sound appear to emanate from within the microphones themselves, and does it disappear when you move the mics apart from one another? Or could the sound possibly be emanating from your preamp?


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    The Maillard Reaction
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    Re:Strange mic behavior 2010/11/18 22:32:01 (permalink)
    Is it possible you mean an ADK SC-1?

    FWIW, there are no coils in a condenser mic.

    Squealing condensers are not unheard of... some old models actually had a reputation for squealing in high humidity conditions. There are a variety of reasons why it happens... all having to do with something in the circuit drifting from the intended spec when you have it in operation. It could be a capacitor leaking the 48vDC. It could be a bad solder joint. It could be the result of many things.

    To follow Bit's line of thinking..  Does the whining sound show up on one or the other other Channel? (or Left or Right). Can you isolate it to one mic?

    Once you do... send the mic in for repair.

    Good Luck.

    best regards,
    mike


    edit spelling
    post edited by mike_mccue - 2010/11/19 06:19:23


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    msr
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    Re:Strange mic behavior 2010/11/19 10:45:40 (permalink)
    Thanks for the replies.  Mike, you are correct, I was a little mic dyslexic.  THey are ADKs not AKGs.  I guess I can say I don't think it's not a preamp issue, but I can't say for sure.  I'll do some more experimenting.  The issue is more pronounced when the gain is turned up and less pronounced when I move the mics apart.  But again, I'll do some more playing around and see if I can figure out any trends.

    msr

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    Guitarhacker
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    Re:Strange mic behavior 2010/11/19 11:08:21 (permalink)
    if it occurs when the gain is turned up.... it's likely feedback.... but the question is...coming from where or what.

    I have seen amps feedback with nothing plugged in to their input  due to the internal leakage in the circuitry.

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    #5
    The Maillard Reaction
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    Re:Strange mic behavior 2010/11/19 11:12:11 (permalink)
    They seem quieter when you move them apart?

    This has got me really curious.

    Which device is providing the phantom power?

    Keep us posted on what you find.

    I was going to also suggest you use a third mic to split the pair and see if you can isolate it to one mic or the other... but it seems like checking each channel (or side) is the easiest place to start.

    Good Luck,
    mike




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    feedback50
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    Re:Strange mic behavior 2010/11/21 17:55:29 (permalink)
    A guess here... Perhaps the phantom supply is a bit marginal, or poorly isolated. (Are both mic's going through two channels of the same preamp or interface?). The thought being that with both mic's loading the same phantom supply, the supply sags a bit causing a slight offset to the audio returned by one (or both) of the mic's. The returned offset noise, in turn, loads the phantom supply somewhat differently causing the offset to shift again. Repeating the cycle rapidly creates an oscillation. The concept being that the load of the mic's is unintentionally modulating the phantom power, which in turn is heard through the audio output of the mics..

    If you have a different preamp you could try on one of the mic's it might prove or disprove this theory.
    #7
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