[Boatanchors] Mic's / RED FLAG

[Richard Knoppow]

----- Original Message ----- 
From: <bcarling at cfl.rr.com>
To: "Richard Knoppow" <1oldlens1 at ix.netcom.com>; "Rob 
Atkinson" <ranchorobbo at gmail.com>; 
<boatanchors at mailman.qth.net>
Sent: Tuesday, July 19, 2011 12:42 PM
Subject: Re: [Boatanchors] Mic's / RED FLAG


> For maximum transfer of energy, the source impedance and 
> the load impedance need to be identical.
> The worst situation is when you have a very high impedance 
> microphone going into a 600 ohm transmitter input.
> So a 600 ohm microphone into a low impedance transmitter 
> input is best for that situation, but for the high 
> impedance situations most of them are around 50 k ohms.
> Older tube riggs may have an input impedance of around 125 
> megohms also. I have never tried it, but I would imagine 
> that a 50 k ohms microphone might work just fine in that 
> situation.
>
     Well, this is the main source of confusion. Microphones 
are NOT power sources and _should not be matched_. That 
reduces the voltage and increases the noise. Microphones, 
other than some special cases as I will note below, are 
_voltage_ sources and all, regardless of their nominal 
impedance, are intended to work into essentially an open 
circuit, or at leas many times the nominal impedance. The 
impedance given is a average _source_ impedance. Now, some 
types of elements look like resistors and have a constant 
source impedance over their frequency range. Nearly all 
non-directional moving coil microphones fall into this 
category. Crystal and ceramic microphones look like 
capacitors and any resistance in parallel with them acts 
like a high pass filter. Capacitance in parallel with them, 
like cable capacitance, becomes a low pass filter.
     Some microphones have resonant elements, these are 
mostly directional microphones, with the special exception 
of Electro-Voice "Variable-D" microphones which have a 
similar mechanism to non-directional mics. Nonetheless, they 
work best into the highest impedance possible because the 
voltage output is greatest there.  Microphones with a 
resonant mechanism do not have an impedance that is constant 
with frequency. They are like loudspeakers which have a peak 
in impedance value at the resonant frequency. However, 
unlike a speaker they are not intended to have efficient 
power transfer. In fact, if operated into a matched load the 
low frequency response will be reduced.
     The exceptions I mentioned are microphones that are not 
passive. That includes carbon, condenser, and electret 
types. The carbon microphone, once the most common of all 
types because it was found in every telephone, is an 
_amplifier_ and must work with an external source of power. 
Nonetheless, the output of the microphone, usually taken 
from the secondary of a transformer, should not be matched 
because, again, the voltage will be reduced. Condenser and 
electret microphones are also control elements. The electret 
is similar to a condenser microphone but has a permanent 
charge. The internal impedance is extremely high so its 
operated into an active amplifier so that the external 
circuits won't load it. A condenser mica requires an 
external source of charging voltage and also needs an 
isolating amplifier, in other respects the two are quite 
similar. The requirements for terminating the active element 
depends on its design. Many are designed to work into 
conventional microphone amplifiers but, because they are 
electronic circuits can be made so that they are pretty much 
independent of the load impedance, although again, the 
voltage out will be reduced as a matching impedance is 
approached.
      There is a great deal more to this story but its 
certainly beyond this quick note. Microphones and 
loudspeakers are very complex because they are triple 
transducers. The microphone must translate acoustic power 
into mechanical power and then into electrical power and the 
loudspeaker the reverse. None of these steps is done 
perfectly and all the parasitic resonances and other 
imperfections of each stage is reflected into the others. 
Furthermore calculating an equivalent circuit for the 
purpose of predicting performance is made difficult because 
the sign convention changes from acoustical to mechanical 
and again from mechanical to electrical. This problem was 
addressed by Harry Olson of RCA, in his book on Dynamical 
Analogies,  but was done more thoroughly by  Frederick V, 
Hunt of Harvard University.  His approach is disused in his 
book _Electroacoustics_ .  Olson's book is available free on 
line from:  http://www.pmillett.com/   Hunt's book has been 
re-issued by the Acoustical Society of America and is 
available via Amazon.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com