> 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
I agree
> 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.
Hmmm, the output impedance of the complete circuit it created by both that =
Message: 6.
Subject: 8 k ohms resistor AND the impedance between the "GND" and "HOT" pads
of t=
he capsule. That impedance is the sum of the internal source resistor and t=
he source-drain impedance of the internal transistor, which is deterimed by=
the operating point of the FET (which in turn is determined by many differ=
ent variables such as the supply voltage and both the source and drain resi=
stors. In any case, the output impedance of the entire electret mic circuit=
should always be significantly lower than the drain resistor (less than 6.=
8k in this example).
> As the input impedance of the LS-11 is only 2k, it means that less
> than 1/4 of the signal level remains.
I would guess it would be a bit more because the mic output impedance is le=
ss than 6.8k (maybe half of that).
> Raimund:
> 1/ What do we know about the LS-11 input impedance with and without
> plug in power?
No, I never cared out that.
> 2/ Do we know that it is a resistor?
Yes, I think that it is a simple resistor. If I remeber correctly, someone =
already created an Excel spreadsheet on that for a number of different reco=
rders, but I can't find it at the moement.
> Also: When reading the specs of electret microphones, you have to
> have a look at the "test-circuit" diagram. If Primo says that a
> EM-172 has an "output impedance" of 2.4k, it is because they have
> used 2.4k in the test circuit.
The EM172 specification (http://www.frogloggers.com/Documents/EM172.pdf?cat=
=3D13&en=3D1) provides this information:
"Impedance : 2.4 k=83=B6 =81}30 =81" at 1 kHz (RL=81=813.9 k=83=B6)"
As far as I understand it, it just means that they used a 3.9 k resistor (a=
nd a supply voltage of 5 V) in their test circuit that results in a output =
impedance of the test circuit of only 2.4k.
Regards,
Raimund
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