Hi. Can I just measure the input resistance of the laptop's line-in? Is it
likely to be purely resistive? I need to know it for some work on a measurement tool using the sound-card as part of the instrument.
I can't find it in the specs on-line but if someone knows better, then it would be great to get some published figures.

 

I did a little research on the subject concerning a creative sound card, which more or less set the standard (think sound blaster). It should be 600Ω with a max limit around 200mv P-P, capacitively coupled. It is possible yours is different, but it gives you something to start with.

 

I found a bit more: Generally, sound cards can range from 600 Ω to 50 kΩ input.
The hardware chip codec in this particular laptop is Crystal 4297A-JQ EP and the datasheet for that
says 10kΩ / 5pF.

So: problem solved! Thanks.

 

For future reference,

You can't just measure input/ouput impedance with an ohmeter.

What you can do is place a test resistor or resistance box in series with the desired impedance, supply a signal and measure the voltage across the test resistor.

Direct calculation will yield the desired impedance. If your test resistor can be varied, vary it until the signal voltage across the test resistor is equal to half the signal voltage.
Then the resistance of the test resistor equals the desired impedance.

 

600 ohms is the input impedance of a dynamic mic input. It is like a dead short to most line level sources that are designed to feed 10k ohms.

 

For future reference,

You can't just measure input/ouput impedance with an ohmeter.

What you can do is place a test resistor or resistance box in series with the desired impedance, supply a 1kHz signal and measure the voltage across the test resistor.

Direct calculation will yield the desired impedance. If your test resistor can be varied, vary it until the signal voltage across the test resistor is equal to half the signal voltage.
Then the resistance of the test resistor equals the desired impedance.

Just added the item in bold. Audio impedances are typically rated using a frequency of 1kHz.

 

All excellent advice. Thank you very much.