[sdiy] General Mic supply question

[Magnus Danielson]

From: Peter Blackett <dragon.servicing at virgin.net>
Subject: Re: [sdiy] General Mic supply question
Date: Fri, 29 Aug 2003 22:23:57 +0100

> Dear Paul,

Dear Paul and Peter,

> I think the accepted standard is 48 volts ,but it is possible that some
> mics may work ok on a lower voltage, some won't work correctly.I think
> if you use a lower voltage then you may get less headroom if the mic
> includes a preamp inside it.I think a lower headroom will mean that the
> mic will distort/clip if the input signal ( in sound presure terms ) is
> too great .
> I know that current consumption of some older mics ( Neumann ? ) can be
> quite large , so you do get problems with some makes of mic
> preamps/mixers not being able to supply enough current.
> there is probably an AES standard for all of this but I don't know for
> sure 

There isn't a AES standard for it (please check for yourself at
http://ftp.aessc.org/) but AES made a recommendation (AES X93) to the IEC for
the update of IEC 61938 which should be the correct standard.

The AES document (http://ftp.aessc.org/pub/aes-x93-991028.pdf) gives the
details of how things should be marked, what voltages it should be, what
resistances it should be and what is operational and fault currents.

Naturally you already is following the AES14 standard for the pinning of the
XLR connector and AES26 for the preservation of polarity in the signal. ;O)

I recommend you look at the X93 and AES14 documents.

For my U87A - with a P48 marking, making it a 48V Phantom power mic - I built
a quick-but working phantom power setup. It is basically a transformer, a
diode bridge, a electrolytic capacitor, a resistor (several in parallel for the
power-handling) and two 24V Zener-diodes in serial (capable of handling the
power) to generate the 48V. Then, the two 6,8k resistors down to the XLR
contact and two 10uF caps for DC blockers before hitting the outgoing XLR.
It's not a neat solution (excess heat, no great DC-supply) but it actually
works sufficiently well for my uses.

I have planned to make a new phantom power thing, but without DC-blocker caps
and with some additional gain - basically a input channel.

Cheers,
Magnus