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connecting multiple 48v phamon power

Discussion in 'Electronic Basics' started by tempus fugit, Jun 16, 2005.

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  1. tempus fugit

    tempus fugit Guest

    Hey all;

    I'm goin to add a 2nd channel to my existing micpre (it's homebrew). The
    existing channel has 48v for phantom power, connected in the standard (via
    6.81k resistors with blocking caps at input) fashion. I'd like to include
    phantom power to the 2nd channel as well, but can I just feed it off the
    existing phantom power circuit? Looking at the schematic, it seems to me
    that I would get bleed into the existing channel, but I cant imagine that
    every $200 mixing board with phantom power has 8 or so individual circuits
    for this purpose.

    How should I do this?

    Thanks
     
  2. Ban

    Ban Guest

    If you understood an instrumentation amplifier, you would see that the same
    current gets superimposed on the balanced lines. And since everything is
    symmetric the preamp cancels this influence (supply current x line
    resistance).
    Of course the second and every other channel have the 6k8 feeding resistors
    as well. They connect to the common +48V point where there is a big
    electrolytic capacitor shunting any AC to ground.
    Both the Common Mode Rejection Ratio and the lowpass effect add up and
    suppress the crosstalk (what you call bleeding) to less than -100dB. So even
    very expensive desks have and need only one common 48V supply.
     
  3. tempus fugit

    tempus fugit Guest

    OK, I don't really understand what you mean, but I'll take your word for it.

    For my own knowledge, though, how does CMRR apply here, when there are 2
    different signals coming into the preamp? IOW, if I have one mic on a guitar
    and another on someone's voice, these are not common mode signals. Since the
    2nd channel will also be connected to the common 48v point (which is
    connected to the inputs of the 1st preamp), how will the signal not arrive
    at the inputs of the 2nd preamp (there are 2 separate opamps for the 2
    channels)?

    Thanks
     
  4. tlbs

    tlbs Guest

    You can just feed the phantom power to your 2nd microphone from the
    existing power supply.

    Here's another way to look at your "concern". From an AC signal
    point-of-view, the 48 VDC phantom power supply is a "short" to ground.
    Microphone 1 sees a 6.8k Ohm load to AC ground on each sides of the
    (balanced) line. Microphone 2 sees a 6.8k Ohm load to AC ground on
    each sides of the (balanced) line. No AC signal can "move" from
    channel 1 to channel 2 because the common point is AC ground.

    Whenever you perform AC signal analysis on a circuit, one of the first
    things you do is eliminate the independant voltage sources (i.e.
    batteries and set-voltage power supplies) by shorting them out.

    When I first read the start of your question, I thought you would be
    more concerned about the amount of current the phantom power supply
    could put out. I would assume that the existing phantom supply could
    reasonably handle a few more milliAmps of current to a 2nd mic. If the
    2nd mic has some kind of major circuitry or tube circuitry -- then you
    might have to take a closer look at the current capacity of the
    existing phantom power supply.
     
  5. tempus fugit

    tempus fugit Guest

    OK, that makes more sense now. Also, my existing power supply will be able
    to handle the extra current draw from a 2nd mic.
     
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