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From: Stefan Schäfer <[email protected]>
To: [email protected]
Sent: Thu, December 16, 2010 2:46:22 PM
Subject: Re: LF: Sound Card Impedance

Johan,

Am 16.12.2010 13:26, schrieb Johan H. Bodin:
> 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
>

Interesting! Thank you for this information.

Maybe one should think about this when designing a sensitive preamp
output stage. I will keep it in mind.

73, Stefan/DK7FC