> I'm now thinking that resistors any lower than 1.8k with a 4.8v
> supply might raise the current too high. It depends on how the
> apparent resistance of the capsule changes with the voltage, which I
> do not understand. All I can tell is that the apparent resistance
> goes up as the voltage goes up.
>
> That's about as far as my limited knowledge can take me. I'm going to =
> try 4.8v and 1.8 kohms and see what happens.
John,
There is a field effect transistor (FET) in the capsule that converts the v=
ery high impedance of the electret microphone element (it's in the range of=
Mega or Giga Ohms) to the much lower output impedance.
The operating point (the source-drain resistance) of that transistor is det=
ermined by the DC current that flows through the capsule (there is also ano=
ther internal source resistor inside that creates a bias voltage that is ap=
plied to the gate of the FET), which is the reason why you get different ca=
psule resistance values for different load resistors and supply voltages. T=
his is indeed a bit complicated and it is probably not the right place here=
to discuss all these details.
To make it short: You cannot further improve that circuit for a low-impedan=
ce preamp input by reducing the resistors and the supply voltage - I guess =
it would make it only worse.
Regards,
Raimund
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