I measured the input impedance of the mic input on a Dell laptop:

AC input impedance: 4.3 kOhms (-6dB with 4k3 resistor in series)
Mic bias voltage: +3.8V (open circuit)
Output current: 809uA (shorted to GND)

This suggest that there is a 4k7 pull-up resistor to +3.8V for mic DC bias.

It may vary somewhat between different brands and models of course but all
soundcard mic inputs that I have seen so far puts out some DC bias for electret
microphones.

73
Johan SM6LKM

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Ken wrote:
> Hello all.
> Does anyone know the approximate impedance of the mic input jack on a pc 
> sound card.
>  
> 73.
> Ken
> M0KHW