At 7:49 PM +0000 8/25/03, olympus-digest wrote:
>Date: Mon, 25 Aug 2003 08:27:53 -0500
>From: "Bill Pearce" <bspearce@xxxxxxxxxxxxx>
>Subject: [OM] [Exceptionally OT] Condensor mikes
>
>Moose,
>
>Actually, battery powered condensor mikes are a relatively new thing. Those
>of us that were fortunate to have worked with older large diaphragm
>condensors will remember the large boxes and heavy cords that were required.
>These mikes (I used a Newmannin the sixties, and yes, we were still using
>mono) were, as our friends across the pond say, mains powered. The box had a
>cannon connector on the output end, and on the other, a larger connector for
>the cable to the mike. If memory serves, a quite high voltage was sent from
>the power supply. All the great recordings from the fifties and sixties used
>at least some of these.
These used a 300-volt power supply, to polarize the capacitor microphone
element, and to power the built-in miniature tube preamplifier. The tube also
needed filament power, which came in the cable as well.
>Later, I was able to use some phantom powered Newmanns that were quite good,
>as well.
This reduces the number of cables, but the principle is the same.
>Battery powered mikes were the result of semiconductor advances, that
>allowed lower PS voltages. I think the earliest ones were FET, but there is
>probably someone more electronically aware that can explain this.
The keys were the invention of the electret microphone, eliminating the need
for high voltage to polarize the microphone element, and the invention of the
junction field-effect transistor (JFET). The tube preamp is replaced by a JFET.
The most basic condensor microphone is a piece of metalized mylar stretched
over a grounded metal ring, with the metallization side against the ring metal.
A nearby perforated metal plate is charged to about 300 volts DC through a
large value resistor. (The plate is perforated to allow air to move through
it, so the film can move freely.) Sound waves impinging on the mylar film
cause it to vibrate, incresing and decreasing the capacitance between
metallization and perforated plate. Because the large resistor won't allow the
average charge on the plate to vary very quickly, the voltage on the plate
varies around 300 volts. A coupling capacitor feeds this voltage variation to
the plate of the nearby preamp tube, and the output of this tube is fed via the
cable back to the main amplifier.
An electret microphone is built like the condensor microphone, except that
there is a sheet of polytetrafluoroethylene (PTFE, also known as teflon) or
poly vinylidene flouride (PVDF, also known as Kynar) also in the gap between
metallization and the perforated metal plate, which is now at ground potential.
During manufacture, the film is heated and a kilovolt-level polarizing voltage
is maintained for some time, causing molecular migration in the film.
Afterwards, the film contains a trapped electric field sufficient to replace
the 300-volt polarization of the condenser mic.
Electret microphones have now pretty much displaced all other kinds.
Joe Gwinn
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