But the gold standard sonically remains A. Where heat/weight is not a consideration (eg., preamps) class A is the norm.
This statement is false. What is the 'gold standard' is a benign distortion product, one that allows the amplifier to be smooth, fast and detailed all at the same time. That may or may not be a class A amplifier; any amplifier is easy to mess up if feedback is poorly applied and this is often the case.
Very interesting statement; would you please elaborate on this? What kind of distortion does a class-A's output section produce that the class-D is less vulnerable to?
I cannot speak for all class D amplifiers; in our class D amp, the two sources of distortion are the encoding scheme and the use of deadtime (to prevent the output section from overheating due to finite turn-on and turn-off times of the output devices; if both are on at the same time you get 'shoot-through current' which can heat them up quite rapidly to eventually fail) in the output section. In our circuit, these non-linearities produce lower ordered harmonics rather than higher orders. Because the lower orders are benign and innocuous to the human ear, the result is a smoother sounding amp that sounds a lot like a tube amplifier- the distortion signature is really similar.
A class A amplifier, because its output section is not perfectly linear, will generate not just the lower orders but the higher orders as well. The class A operation is used to put the output device or devices in the most linear operating region, but that isn't the same as saying its actually linear and no class A output section is, so it generates distortion.
Again:
The ear is keenly sensitive to the higher orders since it uses them to sense sound pressure. This is why an amp with THD of 0.01% can still sound harsh, if the higher orders are not masked by lower ordered harmonics. The assigns tonality to all forms of distortion and higher ordered harmonics are sensed as harshness and brightness.
