John, Raimund and all...
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.
As the input impedance of the LS-11 is only 2k, it means that less
than 1/4 of the signal level remains.
Raimund:
1/ What do we know about the LS-11 input impedance with and without
plug in power?
2/ Do we know that it is a resistor?
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.
Klas.
At 22:41 2012-12-29, you wrote:
> > I see. Interesting. I probably shouldn't worry about this, but I
> can't help myself! If the input and output impedances are nearly
> equal, the input voltage is being cut in half, correct?
>
>Right.
>
> > And that would mean that the output noise is getting very close
> to the LS-11 EIN limit.
>
>Yes, but it is probably still no problem with the low EIN of the LS-11.
>
> > Is it difficult to modify the battery box circuit for the mic
> capsules to lower the output impedance? I am using 6.8k resistors
> with a 9v battery to power two EM172 capsules.
> >
> > circuit diagram:
> >
> > http://www.megalithia.com/elect/BATTBOX.pdf
>
>I think the only way to improve the circuit would be to insert a
>preamplifier that acts as an impedance converter. Unfortunately,
>such an additional active part might in turn also increase the noise
>floor, if not selected properly. In other words, one would probably
>end up with a complete preamp stage with some gain (let's say 20
>dB). But I'm not sure whether that can be really justified.
>
>Regards,
>Raimund
>
>
>
>
>------------------------------------
>
>"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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>
>
>
Telinga Microphones, Botarbo,
S-748 96 Tobo, Sweden.
Phone & fax int + 295 310 01
email:
website: www.telinga.com
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