I hate to encourage these really OT threads but having machined and built some
condensor mic's and
designed some condensor amplifier electronics, I feel I can comment...
Joe and others commented
>>
These used a 300-volt power supply, to polarize the capacitor microphone
element, and to power the
built-in miniature tube preamplifier.
>>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.
<<
JFET preamps became common as soon as the JFET availability became good. I was
using them for
building my own homemade units in the late 60's and I would guess they were
already common before
that in commercial products. The performance of JFET's is so much better than
vacum devices due
to not being microphonic themselves,having much lower noise figures and not
suffering from heater
hum.
Very early on a popular method was to use an RF capacitance bridge circuit
rather than DC HV
polarization. This has a number of advantages,for example no 1/f noise, closer
diaphragm spacing
(higher sensitivity) being another, along with some disadvantages (RF intermod
etc)and circuit
complexity.
>>
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.)
<<
Early professional mikes were always metal diaphragms, not metalized Mylar.
This is because the
plastic diaphragm sensitivity can be strongly dependent on humidity and
temperature. The highest
quality industrial reference microphones (B&K) are still metal for this reason.
As the electronics
improved and went solid state the voltages dropped. My microphones ran on 90V
(with amplifier NF
of <0.1dB). With a plastic diaphragm there is more of a stability issue since
the DC polarization
attracts the diaphragm in a non-linear manner for large displacements, so DC
voltages dropped for
that reason as well. The rear plate is actualy usually NOT perforated (except
for a pressure
equalization hole) but contains a special pattern of small blind holes that act
as a lossy spring
for damping diaphragm movement. Dual diaphragm cardiod mikes I believe, may
have perforations that
go through to back both diaphragms.
>>
Electret microphones have now pretty much displaced all other kinds.
<<
I believe dynamic microphones still have a following with bands because they
are very robust and
"they like the trad sound". National Semi-conductor believes it can replace a
large percentage of
the enormous consumer electret market with micromachined integrated
amplifier-sensors and
including replacing the discrete JFET amplifier. (The National part is not as
good ! but they are
cheaper.) See their website for information. I recently bought 40 electret
capsules for an
experimental directional microphone I am building, it is amazing how
inexpensive they are. (Jameco
on the web here in the US has good volume prices.)
Regards,
Tim Hughes
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