How do you measure output impedance?

It is very simple if you can hook up a potentiometer to the output of the preamp. Use a value that has a maximum value at least 10 times the estimated output impedance, 25K ohms should be enough.

Feed the preamp a fixed level signal such as a reference 1,000 Hz tone or pink noise from a test CD.

Turn the resistance to maximum and measure the AC voltage across the pot. Then turn it down slowly until the voltage drops to 1/2 the initial value. Disconnect it from the preamp and measure the resistance, it will be equal to the output impedance of the preamp.

Contact me if you wish and I can fax some drawings if you need them.

Actually, Herman, that all sounds pretty intimidating to me. I'd need a lot of hand-holding, plus a pot. Thanks for posting it, though.

Drubin, may I ask - if you feel intimidated by having to measure some voltages, why do you need it? What are you trying to achieve?

Maybe he's trying to feed a low impedence input power amp, like an Aleph, known to be tricky with unruly pre's?

Ditto, Subaruguru. I had assumed the same thing.

That's exactly right. And I had my preamp modified recently with additional resistance on one set of outputs (to lower gain) and would like to know how, if at all, this has affected the output impedance for that set of outputs.

Zoya, your question strikes me as odd. Just because the process is intimidating, why do you suspect my interest in the result?

I've been curious for some time about this issue of mismatch between pre and power amp. I'd like to have real measurements to compare with what I hear to try to get a better handle on things.

Drubin, You have misunderstood me. I didn't suspect anything, I was just curious because I am new to the hi-end audio and just was trying to learn what can be accomplished by knowing output impedance of the pre.

Sorry, but you really can't simplify it any more than I described. Maybe it could be described better, but that is the process. It really isn't all that complicated. You can get the parts at Radio Shack for a few $$.

The modifications done to your preamp may have lowered or raised the output impedance depending on how it was done. If they put a resistor in series with the ouput it would raise the impedance. If they used a voltage divider it could go either way depending on the values chosen.