Hi @Var, these are good ideas. There’s the output impedance for the QA451 that does its measurement with the 4 and 8 ohm loads, and for class D with the big LC output filters, this does make sense. Plus, class D don’t really like driving into open loads because the output LC assumes 4 or 8 ohms
But you are right, maybe the non-QA451 impedance module should focus more on a manual measurement, with the option to run open load, and also plot damping factor.
PS. The math used to compute the Z given two loads is as follows.
If you substitute in your manual measurements, I think the correct answer comes as expected.
@matt- I did the calculations using your equation with the manual measurements and got an impedance of 0.13ohms, which makes sense looking at the graph before it went negative. I guess the question is why the two measurements are not in agreement- it has only happened twice, so most of the time things are fine. I could try measuring from 20hz-1khz every 20hz and see what happens when using the same FFT and sampling ? maybe try doing the measurement at 5w?
If you measured output impedance using the same loads and connections you also use for power output testing, I can see how it could potentially return a negative value.
Amplifier output impedance should be measured at the loudspeaker terminals of the amplifier itself, and the load value reference keyed in should include the resistance of the connecting cable to the load as well, whereas power output testing is connected over the actual load, not at the speaker terminals.
So two distinct connection methods- one Kelvin-like, the other not.
In my case, I use about 3’ of 12gauge wire for all of my connections, but they would start more at the load side of things than at the amp side of things, so there is a very small amount of resistance that is not counted for. I saw something about this relating to the THD improvement that could be had if your connections were at the amp end of things. I wished I had a 4-wire resistance measurement meter to actually measure the cable resistance, but it is not going to be that significant, particularly with what I do. BTW, I measured the Qx8000 today and did not get a negative impedance- it actually was better than the spec!
I remeasured the M501 amp using the same FFT and sample rate as before, but changed the step size to 20hz, starting at 20hz and going to 1100hz and got the same slope and negative value at 20hz, but it did not start going negative until 40hz…?
That sounds good, I will be returning that amp to its owner probably within the week, though. Most likely it will happen at some point in the future.Thanks for the help!
Any developments or additional wisdom gained on the output impedance automated test since these past posts? Any progress or fixes related to the negative values you mentioned?
Is your method of removing the QA403 red input clips for the open-circuit measurement still considered valid? Any other known good methods at this point?
I’m getting good horizontal lines L and R, but at -8 Ohms. They are easily overlooked down there, so some might think the screen is blank. Zooming the image will reveal the plots, or it did for me.
I have measured a lot of output impedances since that post and have not had anything out of the ordinary. Very few devices meet their posted spec, but most are older gear. The original output impedance IHF spec at least, used the open circuit voltage and with 8ohms attached to the amp. Set the the QA to 100,000ohms (the max setting I believe) for the 2nd measurement. I have been told the next QA release will have Damping factor…
I forget if the max you can enter is 100k or 1Meg- won’t make much of a difference- and yest, just disconnect the + leads going to the loads, but obviously not the + lead from the amp to the QA40x + input. And be sure you reconnect the load up- that has bit me several times when I do the next measurement(s) and realized my load was not connected.
(Chuckling) Thanks for the compliment, but I am learning too and just picked up something from two posts here the other day, and will incorporate them into my testing methods. I like putting together videos and have learned a lot about gear. Not always fun- currently have a pair of JBL speakers that weigh 150lbs each that I am working on a review on- a bit difficult to move around by yourself, but I think it will be a nice video when finished.
