connecting multiple 48v phamon power

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

 

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.

 

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

 

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.

 

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