Russell McMahon <apptech@paradise.net.nz> wrote:
> > > You can get the opposite polarity signal by just swapping the
> > > leads.
> >
> > Not with condenser microphones.
> 
> Olin and Russell are both correct - and both wrong :-)
> 
> The comment is correct as intended, but ambiguous.
> ELECTRICALLY it is correct in the pure sense, even for condenser 
> microphones.

No, it isn't. For positive pressure on the diaphram, the plate spacing
goes down and the capacitance goes up. This is independent of whatever
electrical connection you use to measure the capacitance.

The only way to reverse the polarity of a condensor microphone is
to physically reverse it so that the pressure impinges on the other
side of the diaphram, relative to the backing plate. It's easier
to reconfigure one of the preamplifier stages from noninverting to
inverting, or vice versa. Or, in your case, swapping the leads to
an amplifier with balanced inputs, relative to the bias supply.

If you have a bare electret capsule (no preamplifier built-in), then
swapping the leads works because the bias supply is an integral part
of the condensor. But most such capsules include the preamplifier,
which only works with one polarity of external power supply.

-- Dave Tweed
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