How to measure impedance on stuff like microphones

[reel_to_real]

OK, here goes...

I've been trying to work out how i can test/measure the impedance of some microphones i have, and maybe the inputs/outputs of some tape machines i have, and also if this is even possible.

I can't seem to find an answer though. From what i've read so far you may be able to do it by using a volt-ohm meter/multimeter. I think it's something like... taking voltage and current readings and then plugging them into a formula, Z (impedance) = V/A

Am i on the right lines? It would be very helpful if someone could give me some advice on how to do this

Thanks

[The Scum]

First, I'd start with internet sleuth work, looking for spec sheets that list impedances.

Measuring microphone output impedance is a little painful, because you have to get the mic to produce an output of continuous level & frequency while the test is performed. An anechoic chamber might be called for.

Z=V/I is correct, but only at a specific frequency. It can help to use a load that's hefty enough to cause easily measurable losses...loaded with 1M-Ohm, you 'll have trouble drawing enough current to get a measurement you can see on a modest multimeter...10K or 100 Ohms might make it easier. A little Kirchoff loop analysis, and off you go.

A different way to measure output impedance: measure the output voltage unloaded. Then add a variable load (pot, decade box, fistful of resistors), and adjust it downward until the output voltage has dropped by half. The load will now be equal to the source impedance.

You can do a similar measurement on inputs - drive the input with a signal generator, with a very low impedance in series to the input under test. measure the voltage at the generator output, and at the input. When the voltage at the input has dripped by half, the series resistance is equal to the input resistance.

It gets a little more complicated with balanced inputs, and your thought of measuring current can be a nice way around that.

Second, impedance can vary with frequency, so you'll get better result is you measure at more than one frequency. You could do spot checks at 100, 1K and 10KHz, or do frequency sweeps for a more detailed picture.

Third, anything with an opamp-based output circuit will have a very low output impedance. Theoretically close to zero, but often built out to 50 or 100 Ohms with a series resistor. Additionally, some active outputs are constrained internally as to how much current they'll supply - they'll limit the delivered current if their abilities are exceeded. If you're drawing much current, it can help to verify the purity of the test signal, making sure nothing intermediate is current limiting.

The ol' standard book on audio measurements is Norm Crowhurst's "Audio Measurements." But a little cleverness and Ohm's law can get you started.
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