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I have done some measurements to find the output impedance of my Sound Blaster Z sound card, mostly just for curiosity and fun. Here is what I did:

 

Create a 100Hz sin wave file in matlab and play it on repeat in foobar.

Attach a multimeter measuring AC voltage to the left channel output with no load (ie. measuring open circuit voltage).

Set the volume such that the AC open circuit voltage is 1V.

Attach a 33Ohm resistor load to the left channel and measure AC voltage drop across the resistor.

 

And here is the result I got:

 

Headphone output: 0.545V

LR channel line out: 0.648V

Logitech Z-2300 sub-woofer's headphone output: 0.472V.

 

That means the output impedance is about 28 Ohms for the headphone output, 18 Ohms for the line out, and 36 Ohms for my sub-woofer.

 

I also measured that the maximum open circuit voltage was 3.33V and 2.2V for the headphone and line out respectively.

 

I think it's interesting that the headphone amp has higher impedance than the line out.