The reason I asked is that I looked at the specs for the Parasound A21+ and shows that TIM is unmeasurable while THD is higher than I would have expected. I remember when amplifiers tried to get THD as low as possible and it often appeared in the <.001% range. OTOH, it wasn't long before, that we had people coming into the store to ask "How many amps does this put out" and, because Sansui harped about it, "What's the Slew Rate?".
Many of these measurements require some interpretation before their significance can be fully appreciated.
A spec of 0.001% THD means less than were I to say that the distortion of an amp lies at -100dB. Things come into even sharper focus when I compare this with human hearing, which (IIRC) performs no better than -120dB detection in ideal circumstances (virgin ears, good health, high concentration, low ambient noise, lab environment).
To that we need to add the type of harmonics. Even order harmonics are part of everyday sound and are often interpreted as a normal part of the fundamental frequency being heard by the human ear. Odd order harmonics are not normal and are interpreted differently.
Now if a competing manufacturer were to say that *their* amp's THD was 0.0001%, it doesn't mean that the noise is ten times lower than the competing unit. It means that the noise is less by a very larger factor because this equates to a THD of -120dB. Not only is this -20dB better, but the nature of the decibel scale means that it has at least "half as half" as much noise.
So the difference between 0.001 and 0.0001 is like the difference between going to the moon and going to Mars. Big difference.
Now think about what this means when the THD is advertised as 0.01 or 0.1 in a tube amp. The sound might be pleasing because of the nature of the noise (even harmonics) but it is still quite noisy and well within the hearing range of retired deep sea divers and artillery officers. And that, my friends, IS noisy (though not quite as bad as the phase distortion of many loudspeakers at their crossover points).
In an earlier post, I provided a link to an EE Times article on noise measurement in Class D amps. I need to read more on the subject, but the thrust of the article is that audible non linearity is a pernicious problem inherent to Class D topologies. The author of the piece designs Class D amps, so this is not an advocacy article written by a Class A or AB designer using misdirection to avoid discussion on gain or switching noise. It's an electrical engineer talking about (what he sees as) an engineering problem.
Much gets buried (or worse, goes unmentioned) by Class D audio amp manufacturers who, with some but not unassailable legitimacy, drop a really impressive THD figures in a spec sheet.
Manufacturers throw figures around in the manner that best suits them. For a variety of reasons, few take the time to explain what those measurements mean or why they may be relevant (or irrelevant) to the consumer.
This is not a disease unique to Class D amp manufacturers, it is an industry wide problem.
