Wht is the impedance output of condenser capsule ?

[mudevilo]

hello there.
Excuse my ignorance!
I have a technical question that I couldn't find the answer to on the forum.
What is the output impedance (more or less) of a condenser capsule?
The input stages of our Fet or tube microphones have very high impedances, but what is the minimum impedance that I need in this first stage so that the capsule is loaded correctly?
Is this capsule impedance dependent on its bias voltage?

 

[gyraf]

The condenser has a certain capacitance - e.g. CK12 is ca. 80pF

So think of the capsule's impedance as a 80pF capacitor in series with the signal

Now, if you want to transfer down to say 5Hz (-3dB-point), you will need a load of no lower than 397884999.99 Ohms - or ca. 400MOhms

http://www.calculatoredge.com/enggcalc/Resistance-Frequency-Capacitance.htm
/Jakob E.

 

[mudevilo]

The condenser has a certain capacitance - e.g. CK12 is ca. 80pF

So think of the capsule's impedance as a 80pF capacitor in series with the signal

Now, if you want to transfer down to say 5Hz (-3dB-point), you will need a load of no lower than 397884999.99 Ohms - or ca. 400MOhms

http://www.calculatoredge.com/enggcalc/Resistance-Frequency-Capacitance.htm
/Jakob E.

I did not expect that!
but it's very clear.
Thank you very much for your response...

 

[mhelin]

what is the minimum impedance that I need in this first stage so that the capsule is loaded correctly?

Zero ohms if that first stage is a charge amplifier...

 

[MicUlli]

That is why typically JFET-circuits are used in the 1st stage of a condenser mic. The polarisation resistor (as already mentioned) dictates the low corner frequency AND produces current noise. Therefore GOhm values are used...