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Re: Hi-MD Input Clipping (was Sony HiMD questions

Subject: Re: Hi-MD Input Clipping (was Sony HiMD questions
From: "stoatwizard" stoatwizard
Date: Wed Jul 12, 2006 2:32 pm (PDT)
> The Hi-MD mic pre in Hi sens should be able to handle this level
> without distortion (although ECM mics, in general, have lower SPL
> maximums).

It may not be the HiMD mic pre clipping. Something that is pretty
sick about my HiMD NH700 is the gutless plug-in-power capability. The
PIP measures an acceptable 2.3V open circuit, but when I plug my
OKMII binaurals the votage available to the mic drops to a lousy 0.7V
on one side and 0.4V on the other. Which is a bit crap IMHO, so I got
to poking around to find out what is going down. The short-circuit
current is 340uA, which isn't enough to get many electrets going. I
have two of these MD recorders, and both measure the same. This
implies the tail resistor is 6k8 which is waaaay too big for such a
low voltage. The result is the electret mic is being starved for
drive current, and there is not enough voltage across the FET for an
output of much above 0.2Vp-p. Which is why the signal disappears in
wind blast for example.

Starving the mic for current and FET voltage isn't going to do
wonders for its sensitivity, headroom, linearity or noise
performance. The solution is cheap and easy, though a right pain in
the field. Use a battery box - it's a 9V PP3 battery, two stereo
sockets, a couple of 6k8 resistors and two 100uF caps. That way
you'll get about 4V across you mic, and gain headroom and some
sensitivity (electrets seem to go low gain at FET voltages < 1V)

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