[ProAudio] Microphones question

Scott Dorsey kludge at panix.com
Sat Jun 12 14:42:35 PDT 2021

On Sat, June 12, 2021 11:15 am, Dan Lavry via ProAudio wrote:
> I was hoping that someone would talk about how to measure the electronic
> part of the mic, without contributions from the membrane. I would assume
> that one could replace a variable capacitor membrane with a fixed cap.
> The old carbon mic could be replaced by a resistor. That or similar
> methods to remove the acoustic contributions can help characterize the
> circuit (flatness, noise, impedance...).

If the microphone has an electronic output stage, you don't even need
to measure it, you can just plug and chug.  You know it's a voltage
follower with a FET into a transformer, for instance, so you get the
equation for the output impedance of a follower out of the Radiotron
Handbook section 7.5, put in the numbers for the fet and then multiply 
by the impedance ratio of the transformer and voila, the effective 
output Z of the microphone.  If it's a bipolar transistor or a 
drain-coupled amplifier the equations are different but readily available.
You don't need to do a full analysis of the circuit.  You can if you
want, and all the kids love SPICE these days, but there is no reason to.

If the microphone has no electronics, it's probably easier to measure it.
Put a scope and a pot on the output and put it in front of a speaker 
generating a 1kc square wave.  Turn the pot until the voltage drops to half
the voltage as measured with an open, and voila, now you know the effective
output Z of the microphone.  (Unfortunately the effective output Z of the
microphone is -different- than the load impedance that gives you the flattest
frequency response with such microphones, since the frequency response
varies with loading, but oh, well.)

In a perfect world the lowest noise would be when the input impedance of the
preamp matches the output impedance of the microphone, but this seldom
happens and it is problematic to sell a preamp "optimized for condenser
microphones" or "optimized for ribbon microphones" although Wes Dooley
somehow manages it.

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