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Phantom Power and Microphones

Discussion in 'Electronic Design' started by Tom, Oct 16, 2003.

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  1. Tom

    Tom Guest

    I have two professional electret microphones and a separate phantom
    power unit giving me +48V. I also have a home built pre-amp with two
    channels. Trouble is I have no idea how to interface such a microphone
    to my pre-amp. The phantom power unit I have is a Proel ppb2. It has
    three output pins for each microphone (it is a dual unit). How do I
    connect this to my pre-amp which has only two inputs per channel ie
    signal + ground?
    From the pre-amp I am going to connect the signals into my sound card
    - the line in which is stereo. I already do this successfully with two
    dynamic microphones but I am switching over to electret.

    Thanks

    Tom
     
  2. Todd H.

    Todd H. Guest

    If you don't want to redesign your preamp to accept a balanced
    input... you need a balun -- an XLR to 1/4 transformer
    dohicky. Transforms the balanced, low impedance 3 conductor line of
    the microphone to an unbalanced high impedance line.

    Such as....
    http://www.zzounds.com/item--WHRLITTLEIMP
     
  3. Tom

    Tom Guest

    If it is just a differential output could I not just use an op-amp in
    differential mode?

    Tom
     
  4. Todd H.

    Todd H. Guest

    Yes--I believe so. Then your preamp would be what they call
    "electronically balanced inputs."

    Using a transformer like the device I mentioned would give you
    "transformer balanced inputs." Jensen makes widely revered audio
    transformers touted in much pro gear, if you'd like to add this
    feature to your preamp:
    http://www.jensen-transformers.com/mic_in.html

    Best Regards,
     
  5. Todd H.

    Todd H. Guest

    Oh, also be sure to be careful that your design handles the presence
    of absence of phantom power on the line. You didn't post any details
    on that unit...one of the benefits of the transformer design is that
    transformers blissfully ignore DC and only pass AC signals like audio
    to the secondary winding.
     
  6. Tom

    Tom Guest

    I just need to AC couple it then I suppose.

    Tom
     
  7. CJT

    CJT Guest

    That is only true up to a point. You can saturate a core with DC
    to the detriment of AC characteristics.
     
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