Closer look at the curves ...
Now we have some discussion about this you've motivated me to have a look at the curves in more detail.
There's not much we can learn from the basic frequency response curves. These amps all look great in that respect; far flatter than our ears and certainly as good as they need to be. But you're right: the (perhaps) tell-tale distortion plots are taken at different power levels, and as we know that amplifiers (just like speakers) exhibit more distortion the harder they are working (the more power they are generating). As you point out, we are indeed not comparing apples with apples. Not remotely so. Why?
It seems that now I've scanned the specs that amp C is by far the most powerful. Judging solely from the specs, the technical performance is really examplary: it was evidently designed by a master craftsman who had many tricks available to him to drive distortion down. There is a generation or more of technical skill revealed by these figures. It is not a amateur design. It certainly would have involved a very careful, intuitive positioning of all the components on the pcb, mm by mm consideration of the circuit board track layout, the dressing of the cable loom - let alone the actual circuit design and component values. It really is an exceptional piece of engineering. I have not read the review so I do not know how it was subjectively rated but I'd be very happy indeed to own one, based on reading the graphs alone.
Picking amp A (the first letter of the alphabet, I could have picked B) informs us that the distortion traces were, as noted, made at a much lower power of only 10W. I can only imagine that this low level was being kind to the amp, and that had it been worked harder, the distortion would have risen, perhaps dramatically. Perhaps there was some discussion between SP and the makers as to what power level represented a typical in-use level, relative to the maximum power available from the design. That would be an entirely reasonable approch: you wouldn't normally measure a cars 100-120mph acceleration; the 30-60mph figure would be much more relavant for the ordinary user in town.
But the distortion curves are indicative of two very different design approaches, much more than just a comparison of two very different power output capabilities. Let's look at just one type of measurement: the THD + Noise graph. THD is Total Harmonic Distortion, and in the old days using an analogue moving coil meter, some simple circuit would add together the total of all the harmonics and arrive at a number. For example, ignoring the single, precision fundamental test tone, if there was some second harmonic distortion generated in the amp + some third + some fourth + some fifth ...... the final amount would be lumped together into one nice convenient number. Say, "1.5% THD @ 10W into 4 ohms, 50Hz - 10kHz" or something like that. What that didn't tell us at all was how much of each harmonic was present in the 1.5%. And that really matters because odd harmonics like 3rd, 5th, 7th ... sound horrible when excessive to the ear, whereas even order harmonics can actually sound very pleasant. Yes, true: they can warm-up the sound as musical instrument designers have known for centuries.
Then along came the computer based audio spectrum analyser of the type used to make these graphs and we can clearly see all the harmonics laid out from lowest frequency (the fundamental) to the highest across the audio band, left to right. So we can pick-out visually the 2nd, 3rd and so on. Luckily for us, the fundamental tone injected into this amp has been scaled to use the full vertical height of the graph, and peaks at exactly 0dB. That's perfect because we can now easily read-off the individual harmonics and their contribution will be correctly scaled relative to the stimulus. So, looking along the horizontal line, we can see that the 2nd harmonic of the test signal (100Hz) is a little smaller than the 3rd harmonic, which I read as being at about '-70dB relative to the fundamental'. Make sense?
Very important point! The test stimulus was a high precision, technically perfect, absolutely pure, computer generated 50Hz sine wave. It contained no harmonics at all*. So all those harmonics you see spread from left to right at multiples of the 50Hz were generated inside the amplifier alone. They should not be there at all. They will add some character to the music.
*The AP test set used is absolutely state-of-the-art. It's purity of tone generation would produce (a few, tiny, irrelevant) harmonics that are below the bottom of the vertical graph scale. We can safely say that what we see here is the amp alone, not the amp + test set together.
Alan A. Shaw
Harbeth Audio UK