3

Hi all,

lately I discovered an old pickup mic in my attic whose impedance I don't know... is there a cheap and easy way to estimate it? I'm aware that a precise measurement would require an oscilloscope and/or some kind of AC voltage source, but I don't want to kill it in the first place :) I just want to know if and what kind of preamp I need...

best, Julian

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When you say "pickup mic" which is it? Does it look like the sort of thing that would be stuck on the body of a musical instrument IE a pickup. They are hi impedance, often very high impedance that would be best plugged into an active DI box that would have an input impedance in the Meg Ohm region. Or is it a mic? If so the kind of connector on it can give a good clue as to the impedance. If it were a 3 pin XLR, then low Z (impedance).

BTW to actually test the output Z you do NOT need a voltage source. The basic method is:- 1)put the mic in front of a loudspeaker with a constant tone or noise source coming out of it. 2) connect mic to something that has a very hi Z input, yes an oscilloscope would do fine, or a DI box connected to a mixer, computer soundcard, anything that can show you level. 3) What this will show you is the so called "open circuit" output, or what come out the mic with no appreciable or significant load. 4) you then put a variable resistor across the 2 wires coming out the mic. You are now adding a load, adjust the resistor till the level goes to half what you started with. If you are looking at a meter calibrated in dB's this is when the level has dropped by 6dB. If a scope, the waveform is 1/2 as high, if a millivolt meter 1/2 the voltage. 5) measure what resistance value you had to use to do this. This is the source Impedance.

  • Now place a chewing gum wrapping paper across the wires, place the cone-shaped nozzle of a fire extinguisher on top of it, fill the cone half-way up with vinegar, add caustic soda, get a mullet haircut and you should start hearing the song... youtube.com/watch?v=HaQvBbzslJE @John no offense ;) – Justin Huss Feb 19 '11 at 22:42

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