I am a new QA403 user moving from older tools like the HP8903B, and some soundcard apps. Are there any guidelines/instructions on how to perform this test. I am testing a Heathkit AA-71 (W-4B) Williamson Type amp and getting good results on other tests or from the main UI. For this test there appears to be 2 scans for 2 different or same output impedances? No matter how I set it up the resulting graph is totally blank. I must be missing something. Any insights? Thanks!
Hi CRL! The output impedance for just about all the tube amps (older ones) I have measured is high so the damping factor is low, just for an fyi. TO do the test I set up the output power up to give 1w at 8ohms (based on the method for measuring output impedance). THe 1st measurement is done at 8ohms and for the send measurement, I disconnect the loads (+ lead) so you are measuring an open circuit (don’t forget to hook the load back up later 
) You need to tell the automated test that the second load is 1000000 ohms, or whatever the max is that you can enter- basically an open circuit for us. I think I take measurements every 200hz. Look up my video on the Paco Integrated amp to see what that tube amp’s output impedance was. Good luck
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VAR Thanks for your reply - I tried this and the graph is still blank. I have to say that I am scratching my head at the rationale of measuring “open circuits”. I have seen several of your videos where you have carried out this test so it must work. When I have a chance I will try it with a solid-state amp. Thanks again!
The only thing I can think you may doing wrong is not setting the scales properly namely the Y1 max and min. You can search for IHF-A-202 which is an older document defining tests and in para. 3.10.3.1 it talks about output impedance. I found the 1watt for the output power in another document.
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