Thanks for doing that, and obviously for understanding WHY it was important. I couldn't find where the power level was given for the CCIF, but if that is at full power, it's impressively clean, and would to an extent alleviate some of the concerns that the bad THD20 creates. If it was at 5W like the THD+N, it suggests it is impressively clean at lower power levels.
For those who still doubt why any of this 20kHz THD stuff matters, here you go:
from Doug Self.
And a Q&A with Bruno Putzeys (designer of the Purifi/Hypex modules):
https://audiophilestyle.com/ca/bits...eaders-qa-with-lars-risbo-bruno-putzeys-r815/
Bruno: I like the way you put it, “unmeasured”. Because that’s really the point when measurements and ears diverge. Measurements are scientific experiments: to test a hypothesis. Remember that you can’t ever prove a hypothesis, the best thing you can do is try very, very hard to disprove it. Every time you fail to prove your hypothesis false, it becomes more solid. So if your hypothesis is that “this is a good amplifier” you try to make it do things you don’t want it to do. It’s not enough to run a handful of standardised tests, you have to invent all sorts of tests that you target specifically at weaknesses you expect.
Lars: Like a 20kHz THD test in its own right is not very useful because the harmonics are inaudible. So you do an IMD test with 20kHz and 19kHz tones and all of a sudden the whole audible spectrum clutters up with distortion. The silly thing is that the standard CCIF test that this signal came from then ignores all of that and only looks at the lone second order product at 1kHz… You have to look at the whole spectrum and that’s really enlightening. It’s one of the major tests that really tell you which is the better amp. Probably a reason why this test isn’t commonly included in amplifier data sheets.
Bruno: The nice thing is that this high frequency IMD test is about the worst thing you do to an amp with an input that’s still technically an audio signal. Of course you can make an amplifier go completely mad by feeding it radio frequency signals but that’s not going to tell you anything about the sound. But to come back to your question, I’m always looking for test methods that are within the remit of audio and that somehow make amplifiers do unexpected things. Admittedly that well has dried up a little. Even a class D amplifier is simple enough that with two sine waves you can pretty much probe all there is to probe. The only real surprise we had recently was to do with the output choke. Magnetic materials have something called hysteresis, but there is precious little information about what this really does. If you test a magnetic core with a sinewave the distortion looks a little like soft clipping, perfectly benign. But what came out of tests on iron parts in loudspeakers was that hysteresis has a long term memory so you can get intermodulation between things that happen now and things that happened 10 minutes ago. With music this distortion sounds like half correlated noise.
Yet, the closest thing ASR generally will test which comes even close to telling us whether "this is a good amplifier" is a 15kHz THD+N sweep. All the standard tests are doing is telling us whether this
might be a good amplifier. As Putzeys says above, though, it just is not enough to do a "handful of standardized tests". Yeah, there's a multitone. Which is good for, um,
frequency response.
https://www.ap.com/technical-library/multitone-analysis-with-apx500/. Now, that's a little uncharitable, since it can show more than that, but why this isn't a terribly difficult test was pointed on here more than three years ago:
https://www.audiosciencereview.com/...odulation-distortion-testing.4769/post-105899. Still, it persists. Two minutes more effort doing a high power and a low power two tone up front and there would be far, far fewer doubts, and frankly, more than double the information currently being presented about whether an amplifier is really good, from a perspective of technical superiority. The theory, of course, being that if an amplifier can pass this worst-case test with flying colors, it's unlikely to perform worse when less stress is placed on it. Whether any of this is all that audible unless the results are really bad is highly debatable.
Case studies:
https://www.stereophile.com/content/mytek-hifi-brooklyn-amp-power-amplifier-measurements. Now go down to Fig. 7. Recognize that wild rise in distortion at high frequencies? It's what the Topping does, except that Topping is an even more comical rise since the THD+N at 1kHz is maybe an order of magnitude lower. Now go here:
https://www.stereophile.com/content/classé-delta-mono-monoblock-power-amplifier-measurements. No rise in high frequencies, and a very clean CCIF. And here's a Parasound which is more typical:
https://www.stereophile.com/content/parasound-halo-jc-1-monoblock-power-amplifier-measurements-0. Same THD20 rise, bad CCIF. And a McIntosh:
https://www.stereophile.com/content/mcintosh-laboratory-mc462-power-amplifier-measurements. Some rise, but much less, and 20dB lower on the sideband and the 1kHz difference component is quite low (and this is at 100W). Is it necessarily guaranteed that a poor showing on a frequency sweep will lead to bad results on the CCIF IMD test? No. There are some contrary examples, particularly with Purifi or Hypex modules where people have done stupid implementations. But that's why it matters to do the test, and why that 20kHz distortion rise is a big, flashing red WARNING sign.
Like it or not, Stereophile is still doing one of the better jobs out there of doing the proper tests to characterize an amplifier's performance. ASR still does not do them, which leads to a litany of people who care about technical performance thinking they do not matter, and refusing to believe they might. It's depressing on a site dedicated to audio
science. It's like testing a Bugatti Veyron and running it through a 0-60 and a top speed test, and refusing to go around a corner because you think corners just don't matter. I suppose if you just drag race and flying mile your Veyron or just listen to 1kHz tones and no cymbals on your amplifier, both would be enough. Anyway, thanks to pkane for filling in some gaps.
If this was at a higher power than just 5W, Topping deserves a lot of credit for making a silk purse out of a sow's ear. The nasty THD20 might not be causing the problems it usually does.
Like I said, read Cordell and Self. If you want to buy things based on specifications (which you should, in part), at least know enough not to say something like this. It's why "specmanship" got a bad name in audio. It's easy to bamboozle people who don't understand what they're looking at or why a particular specification matters. 
