> An electret like EM172 is not a complete circuit. It has a very high
> open circuit impedance, on the other hand it doesn't work, so it is
> not relevant to talk about "impedance".
> To make it work, a resistor is needed between the drain of the FET
> and + of the battery and then the circuit gets the same (approx.)
> output impedance as the resistor! So John's circuit will have a
> output impedance of approx. 6.8k.
To start off I have to confess that I haven't experiemnted with an
EM172, but I have used electret capsules. Essentially they are current
source devices in other words high impedance. To get the maximum
energy out of the mic circuit you need to maximise the output voltage.
To match this into a low impedance circuit I have used a transformer
and in the UK, Maplin do one, code BV52G:
http://mobile.maplin.co.uk/module.aspx?moduleno=12151
If the current drain is not too high to saturate the transformer,
you can put the high impedance winding in place of the series resistor
and that should give the optimum match into the next stage. It's
probably worth the experiment.
David
David Brinicombe
North Devon, UK
Cogito cogito ergo cogito sum - Ambrose Bierce
"While a picture is worth a thousand words, a
sound is worth a thousand pictures." R. Murray Schafer via Bernie Krause.
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