Low phantom power volts on Shure mixer

Archived from groups: rec.audio.pro (More info?)

In article <429309f9_4@news.chariot.net.au> rthomas@chariot.net.au writes:

> Here's a bit of a trap for young players, or those who assume phantom = 48
> volts. The old warhorse Shure FP42 four mics input field mixer (its been
> around a long time !) actually has a published spec of 30 volts phantom
> power, not 48...

The Shure Pro Master mixer provided 12V for phantom power, so they
make the SM81 mic (roughly contemporary) so that it would work on 12V.
The Mackie PPM series of powered mixers provide 15V for phantom power
(it's just tapped off the IC power supply positive rail).

Read the specs!

One spec that's rarely published is the total amount of current
available from the phantom supply. It doesn't seem like it should be a
big deal, and it isn't if there's a real power supply for the mics,
but often this is a place where manufacturers save money. Nowadays
with common and inexpensive condenser mics, it's not unusual for a
mixer to have nearly a "full house" of mic power, but half a dozen
years ago, some mixers derived phantom power using a voltage
multiplier, or a low current winding on the power transformer. They
were betting that only about 1/4 of the channels would need power, so
instead of having enough current for a reasonably greedy mic on every
channel, they limited the current available. Plug in too many
condenser mics and the voltage to all of them drops, sometimes to the
point where a mic no longer works properly.

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Archived from groups: rec.audio.pro (More info?)

In article <1117394915.778955.36880@o13g2000cwo.googlegroups.com> DSatz@msn.com writes:

> The thing is, our ears can only tell us that something is wrong if we
> know what it's supposed to sound like in the first place. When I see
> people posting opinions of certain microphones, I sometimes wonder if
> they've ever checked the phantom powering, the impedance and overload
> point of the preamp or mixer input that they based their opinion on.

The thing that gets me is when someone says "the preamp has very
little headroom." You can make as much headroom as you want by
switching in the pad on the mic if there is one, switching in the pad
on the preamp if there is one, (usually) turning down the preamp gain,
or putting a pad in line between the mic and the preamp.

What they usually mean is that with the gain set up fairly high so
they can tell how "sensitive" the mic is, when someone screams into
the mic, the preamp clips. One of the things that makes a mic preamp
design less than trivial is that it needs to be able to deal
comfortably with a wide dynamic range, but there's a maximum output
voltage for any circuit. If you put a sensitive mic in front of a
loud source and follow it with a lot of gain, you'll exceed the limit.

--
I'm really Mike Rivers (mrivers@d-and-d.com)
However, until the spam goes away or Hell freezes over,
lots of IP addresses are blocked from this system. If
you e-mail me and it bounces, use your secret decoder ring
and reach me here: double-m-eleven-double-zero at yahoo