It was just more detailed with subtleties that increased realism beyond
anything else. That's what I'm asking about in a possible Class D
replacement.
Speaking from experience, the right class D amp can do that. Class D has as many variations as any other type of amp does, so just because one class D might not bring home the bacon says nothing about another.
If your used to your SET tube sound, no class D will ever match the tone.
Again, speaking from experience, I've not found this to be the case. The main differences we hear in amplifiers are two-fold: what is the distortion signature, and how flat is the frequency response.
SETs generate a lot of distortion. At full power its typically 10%. If you're really serious about using one, your loudspeakers should be efficient enough that the SET is never asked to make more than about 20-25% of full power (this will limit the higher ordered harmonic distortion that causes them to sound 'dynamic'). It should be noted that the dynamic nature of music should come from the recording, not the amplification!
Now the advantage of SET is that as power is decreased to zero, the distortion drops to unmeasurable. This is the source of that 'magical inner detail' that so many talk about. Put simply, its that First Watt that is actually very good (unless you have a type 45-based amp, in which case it will be that first 100th of a watt).
But SETs by no means rule the roost in this regard. Our amps have the same property of linearly decreasing distortion. Our amps are push-pull; that sort of quality in a push-pull amp is rare! But there isn't some sort of pixie dust that allows for it; you simply have to be aware of how amps make distortion, and what distortions are going to be heard by the ear. Any zero feedback tube amplifier that is fully differential from input to output will have this quality.
Let's talk about the lower ordered harmonics for a moment. Its the 2nd order that makes SETs have that ever-lovin' tube sound- it give the amp a rich, warm quality and assists the ear/brain system in winnowing out detail and soundstage (that latter bit is IMO something that could do with more research as to why this is so). To this end, the 2nd harmonic is fairly innocuous. Other than making the amp a bit more euphonic, you don't hear it.
The 3rd harmonic has this same quality- the ear treats it the same way. The presence of the 2nd and 3rd in sufficient quantity can mask the presence of higher ordered harmonics to a certain extent. This is part of why tube amps sound smoother than a lot of solid state. But that isn't the entire picture.
The ear/brain system assigns a tonality to all forms of distortion. The lower orders mostly richness, the higher orders cause harshness and brightness even in tiny amounts. This is because the ear uses them to sense sound pressure and so has to be keenly sensitive to their presence.
This is why many solid state amps have sounded bright- its entirely due to the higher ordered harmonics making themselves known.
The problem has been feedback in amplifiers. If used in insufficient quantity, it will add distortion of its own while suppressing the innate distortion of the circuit in which its used. But- if you use **enough** feedback this will allow the amplifier to compensate for the distortion caused by feedback itself. Nelson Pass comments on this in an article he wrote about distortion. In it he speculates that you'll need more than 70dB of open loop gain (this is how much gain the amp has if there is no feedback). This is a very difficult number to achieve in traditional solid state amps since oscillation is a clear and present danger due to a phenomena known as phase shift. How that works is that at some frequency above the audio band, the phase shift of the amplifier starts to go up. When it passes about 90 degrees, the amp can use the feedback as positive rather than negative- and so oscillation can result. So many designers don't push their luck.
But Luck has nothing to do with it. Sound engineering does, and some designers have succeeded in making amps of this type. The Benchmark is one example. There is another way to get around this problem. That way is to use a class D amplifier and use so much feedback that the amp goes into oscillation. But in this case, the oscillation is used as the switching frequency of the amplifier, so we're OK with that. Such an amp can have north of 40dB of feedback, allowing it to compensate not only for distortion caused by feedback but also phase shift caused by the output filter of the amp.
Once either of these approaches are achieved, its then up to the designer to understand how the ear senses distortion and how the ear's masking principle works. If he does this right, then he will see to it that despite the resulting low distortion of the amp, the primary distortion components are still the lower orders.
If he does this, the resulting amp will be smooth like tubes and will be extremely detailed at the same time. So you can get that 'magic'. The only downside is that any amplifier that does this will have a hard clipping character, so it will be important to have more than enough power to do the job- IOW you *never* clip the amp!
The flip side of this is to avoid using any feedback at all, relying on other techniques to control distortion. Do there you have it. You can have an SET which has no feedback, or a class D which uses a ton of feedback, and the two can sound surprisingly similar (although IME you'll hear more detail from the class D, and of course it will have more power)!
