I re- ran the Output impedance test with these settings:
The FFT size was 128k. Following a bit of research on the damping factor measurement, I set the level such that I had 1w/8ohms at 1khz (-22.2dBV). This time I got the correct plot when I set the scale to go from 20hz-20khz:
It would be nice to be able to save the automated test settings like you do the “normal” settings… I still have no clue why damping factor is off so much from what manufacturers specified decades ago, but this amp has one of the best I have measured.
I think damping factor is usually measured into a complex load (speaker coil), while the QA451 and resistive loads don’t have any inductance or any mass/inertia/mechanical damping. Maybe back in the day the McIntosh folks did their testing into a reference speaker?
My guess is that you’ll get different damping factors as you move from speaker to speaker because the amp and speaker work together as a team.
Moving from speaker to speaker would not make sense for a standard. I’m going to paste the section form the 1978 IHF spec in regards to output impedance and damping factor. It looks like they do an open circuit reading and then with the load…?
Looking at it again I am not sure how to measure this- measure the voltage with 8ohms connected and then remove the 8ohm load and measure the voltage and then it gets confusing. Good thing I have some time to research further…
Simple enough. My question for you is it possible to do this with the QA40x by setting the 4ohm load to say an open, maybe 1Mohm, or even have the routine modified slightly so that it asks for an open and then an 8ohm load… ? I certainly will try it at one frequency once my bench opens up again… Thanks,
Hi @Var, there is the manual PWR Output Impedance Test. There you can specify two loads. Now, the first load is currently required to be <1000. But if you try the test using an open and like the result, then that limit of 1000 can be raised to 1 million or so, effectively allowing you to specify an open.
But my guess is it will give a result very close to the other tests.
I have only been using the manual test as I don’t have a QA451 (maybe one day). You said that the 1st load needs to be less than 1000, which is not a problem since it is going to be the 8ohm load. What about the 2nd load, does it also have to be less than 1000 or can it be set higher, say to 1Mohm. I won’t be able to try the measurement for a bit, but think that you are correct in that the difference will not be great for most “good” amps…
Thanks Matt for including this in the update. Prior to knowing about the update I measured an old receiver that I’m working on using 1000ohms for the 2nd impedance and just not connecting the load. Not really a lot of difference between the two plots. The 1st one was was “normal” using 8/4ohms:
If I use the lowest impedance I get a damping factor of 32. The spec from 1969 on this Marantz receiver for damping factor was 35- the closest to what I have measured yet. I will try the new software revision using 1Mohm (open ckt) as soon as I can and report back.
