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Scott Dorsey
 
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Default How to measure a dynamic mic's output impedance?

Andy wrote:
I'm a total beginner. How do I measure a balanced or unbalanced dynamic
mic's impedance? Can I do it with a multimeter and, if so, how?


No. It's not an easy thing to do.

The QUICK AND DIRTY APPROXIMATION is to measure across the coil with an
ohmmeter, and assume the mike impedance is from two to ten times the DC
resistance. You can just check between pins 2 and 3 on the fly. This
gets you within an order of magnitude. This may not work on mikes with
transformer coupling inside, though (like the SM-57).

The CLOSE APPROXIMATION is to use reciprocal law and measure the _load_
impedance of the mike when used as a speaker. Apply 1 KHz to the mike
with a signal generator through a 100 ohm resistor. Measure the voltage
across the resistor and across the mike, and use ohm's law and the resistive
divider law to figure the impedance at 1 KHz. Note that the impedance at
other frequencies may be different. You don't need to use a fancy RMS
voltmeter here, because any errors in the meter will be the same on both
measurements and they null one another out. This gets you within a factor
of about two, I think.

The RIGHT WAY to do it is to apply a reference sound source to the
meter with a pistonphone, then measure the voltage developed open-circuit
off the mike, then through a 100 ohm resistor. The math is basically
the same, but now you have the problem of the a signal source. You could
probably build one with a speaker in a tube if you didn't really care about
knowing how the impedance changes with frequency.

For the most part, it's a lot easier just to look it up on the data sheet.

Note that the actual microphone output impedance is not the same as the
load impedance that the mike should be terminated with. It's usually a
good bit lower.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."