Square waves are good for a starting point. But not that good. I would go then for impulse instead of squarewaves.

The only reason to use squarewaves IMO is that you really wanted to do a step response but your oscilloscope could not latch to it or something like that. And that's because the step response is the integral of the impulse response.

One could capture a time domain sinusoid for visualization purposes however. It could also help discriminate cross-over distortion on class A/B like Atomic Bomb did. Or perhaps where an amp stops being class A. I think a 1 kHz tone would suffice if this is of interest.

I do like the idea of zooming in on x-over distortion. Although I'm guessing it would be very difficult to resolve in most cases

I'm still fond of the ARTA/STEPs type distortion plot, especially for getting a fix on low-end tube amp distortion. (As long as the D6+ element is included).

Looking at a few single-frequency ffts seems limiting, and that why I like the above style plot: it's a whole picture. The limitation of the above style, compared to fft, is that you can't get a good look at any higher order distortion products.

I've been toying with the idea of combining the above style distortion plot with the fft presentation into a video that decomposes a long sweep (or a series of 'steps') with a tracking window. With a good video player that can 'scrub' to individual frames, it's possible to take a close look anywhere one chooses, bypassing the limitation of just a couple fft plots.


But that's just a future possibility. The above STEPS type distortion graph seems doable, and I think should be considered for a standard, at least for tube/simple-discrete-SS amps. That is, for amps where the basic HD may be audible, I'd like to know what it is and what its character is.

Agree. One could do that and call it THD+Noise. Could do dB or %. I think one can convert between % and dB as long as one has the fundamental (frequency response) as reference for dB. We could start with 0.55Vrms 33/300 ohms as we seem to be converging with the THD and other measurements.

Also, with sample rate going through the roof these days it might be interesting to do something like a 39+40KHz IMD test and see if any garbage shows up in the audible range.

That reminds me of some stuff that I think the Vorbis from Xiph.org dude did. Still, I think must stuff doesn't have much signal above 20 kHz to start with, even if recorded at overkill sampling rates. But I could be wrong...

There shouldn't be much, but since nobody can hear it anyway who knows what kind of weird noise or interference is actually hiding up there.

Considering there are SACDs that are just upsampled redbook I would be surprised if there were plenty of recordings with lots of weird ultrasonic noise that nobody bothered to do anything about.