Re: More On M-Audio MicroTrack 24/96: S/PDIF, Mono, Playback

spud wrote:

> I tested a few production preamps for phantom power when trying to
> make my own supply. They produced in the neighborhood of 24 - 50v
> depending on what mic you plugged in. One mic runs off two 9v
> batteries for many hours so I'm not sure how critical phantom voltage
> really is. Be nice to know.

spud, as Arny Krueger pointed out there are dropping resistors in
series with the phantom voltage supply, so it is normal for the voltage
at the socked to drop a certain amount when a microphone is connected.

Since those resistors are supposed to be 6.8 kOhms and each one
conducts half the current, the voltage drop should be 3.4 Volts for
each mA that the microphone draws. Meanwhile, the overall tolerance
limit is plus or minus 4 Volts. So for example, if a microphone draws 4
mA from the supply, 3.4 times 4 gives a 13.6 Volt drop across the
supply resistors, and 48 minus 13.6 is 34.4 Volts. The voltage at the
XLR socket should be within four Volts of that amount when the
microphone is connected. (When the microphone isn't connected, it
should be between 44 and 52 Volts, of course.)

If a microphone is plugged in and the resulting voltage falls below
that range, the microphone's performance, especially its overload limit
but also other parameters, could well be affected--in some cases quite
severely. A supply that's only 20% below the tolerance range, for
example, could cause some microphones to start clipping 10 dB sooner
than their specs would indicate. That isn't something you want to find
out during a live recording.

Certain types of microphone can tolerate a rather wide range of supply
voltages. But most traditional, externally-polarized condensers can't,
and that includes most of the really good ones.

--best regards

.

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