This chat on the EM172's is dead on time; I just yesterday received a pair!
Cheers all
Max
--- In "Avocet" <> wrote:
>
> > 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=3D12151
>
> 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
>
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