From: JOSEPHSON@AppleLink.Apple.COM (Josephson Engineering,VCA)
Newsgroups: rec.audio.pro
Subject: Microphone circuits and noise
Date: 15 Dec 92 03:06:00 GMT

Tom Matthews writes:
 
    I have a question about the amplifier inside of a condenser microphone.
    Is it simply a source follower, or is it another topology that provides
    voltage gain?
 
    I have heard the term "self noise" with regard to a condenser microphone
    (but only once).  How is this defined?
 
Typically, a condenser mic has some sort of impedance converter stage first
that deals with the signal from the capsule, and then a line driver stage. Very
high impedance (>1G ohm) is required in order that the low frequency response
doesn't roll off too badly (the -3dB point is 1/2*pi*r*c where c is the capsule
capacitance, typically 10 to 100 pF, and r is the input impedance of the active
circuitry).
 
Early condenser mics used a vacuum tube or FET with a gain of 5 to 20,
typically feeding a transformer with a voltage step-down ratio of a similar
amount, to yield a "microphone level" signal which is normally between 1 and 10
millivolts rms for a 1 Pascal (94 dB SPL) input.
 
Cheap electret mic capsules have the FET built in to the housing, with the gate
lead spot-welded to the backplate. Some of these FETs (particularly in the
Panasonic WM-060 and WM-063 capsules) are good and quiet, but there is a
drawback. They can't afford a >1G ohm resistor in there, so they use the
leakage current of a diode fabricated on the same chip as the FET to provide
gate bias. Electret mics are often credited with a "harsh" sound, especially at
high SPLs, and this may be due to the diode's leakage current and/or
capacitance changing with applied voltage (conjecture, not supported by test
yet - any takers?). The source of the FET is typically wired to the shell of
the capsule and thus to ground, the drain is supplied with a few hundred uA
through a load resistor across which the output signal appears. Some capsules
are available with the source brought out on a separate pin so it can be
operated in source-follower mode, or you can cut away the PC board trace that
ties this pin to the capsule shell.
 
Professional mics now typically use a FET as a source follower driving some
sort of output stage, either bipolars driving a transformer, or a bipolar
balanced line driver. Some transformerless types, including Schoeps, Audio
Technica and mine (Josephson) use the input FET as an impedance converter/
phase splitter (same value R on the source and the drain), with each output of
the phase splitter driving a separate emitter follower to drive the line:
 
 
                 I---------------------I
                 > R1                  I
                 < 2K        /---------)------------------- (mic conn pin 3)
                 I C1 .1  eI<          I
 signal          I---I(--b-I       R3  I
 from         I--I        cI\      5K  I  /
    ----)(--->I             I----/\/\/-I--I<------I-- (mic conn pin 1/case/gnd)
 capsule      I--I C2 .1  cI/           /(zener)  I
                 I---I(--b-I              6V      I
                 I        eI<                     I
                 > R2        \--------------------)-------- (mic conn pin 2)
                 < 2K                             I
                 I--------------------------------I
 
(biasing and bypass components omitted for sanity in tty mode)
 
Microphone self-noise is defined as the noise output of the device with no
sound input. It's normally quoted in equivalent sound pressure level, dB SPL.
Usually A-weighting --dBA-- is used which makes the mic specification look
nice, but the German DIN spec calls for spectral weighting which is much more
accurate (i.e. is intended for measuring the objectionability of circuit noise
under a signal rather than dominant environmental noise) and also much less
flattering.  This is an area where most of the audio textbooks are simply
wrong. First, the noise is the sum of noise power from both electrical and
acoustic sources. The main acoustic source is thermal agitation or Brownian
motion noise of the air molecules around the diaphragm (it can be thought of as
thermal noise in the resultant acoustical resistance). This noise varies with
diaphragm area and compliance. The main noise source in a dynamic microphone is
the thermal noise in the resistance of the ribbon or voice coil and the
transformer, if there is one. Most texts go on to calculate this noise based on
the nominal 200 ohm _impedance_ of the typical microphone. Impedance doesn't
make noise, resistance does, and the resistance of the transformer winding is
typically much lower than 200 ohms. In condenser mics the primary source of
electrical noise is the set of noises from the mic's active circuitry. Very low
noise FETs are available, but they have high gate capacitance which (a)
contributes to the parasitic capacitance and thus distortion, and (b) loads
down the output of the capsule resulting in lower signal. A good compromise is
to have the FET input capacitance no more than half the capsule capacitance.
 
Good references on microphone noise include: "Microphone Thermal Agitation
Noise" by Harry Olson, Journal of the Acoustic Soc of America, Vol 51 No 2, "A
Low-Noise High-Output Capacitor Microphone System" by Richard Burwen, Journal
of the AES, 1977 May, and "Circuits of Transistorized rf Condenser Microphones"
by H G Griese of Sennheiser, JAES, 1965 January.
 
Now the ugly reality is that nearly all microphone makers report only the
electrical noise, because the acoustical noise of the capsule is too ugly! In
most cases you will see a spec "equivalent loudness level of noise from
internal sources" or some such weasel. As both the Burwen and Griese papers
point out, a cardioid is typically 8 to 10 dB noisier than an omni. So... how
come the equivalent noise level specs in all the catalogs are the same, eh?