Could Class D really be that good?

@lanx0003 Sure! Just look at the harmonic spectra of the Purifi module. You'll see a dominant second.

Keep in mind that in a class D amp, the things that cause traditional solid state amps to sound harsh and bright might not exist. For example, in our class D the primary non-linearity is caused by the deadtime used in the output section (something that all class D amps have). In our circuit this results in lower ordered harmonics.

When feedback is applied to most tube and solid state A or AB designs, it is traditionally applied to a non-linear input to the amp (the cathode of an input tube or base/gate of a differential pair at the input of a solid state amp). The feedback signal is thus distorted and so does not do its job properly. This causes higher ordered harmonics and IMD. This has been documented by both Norman Crowhurst (technical writer for Sams publications) and 20 years later by Peter Baxandall.

In most self-oscillating class D amps this technique is not used. So they often lack the higher ordered harmonic generation common to 99% of traditional solid state designs.

Since the ear uses harmonics to tell the difference between sounds, you can see that class D amps are capable of very different distortion as opposed to traditional solid state. So I don't see it as counter-intuitive that a class D amp can have a lot more in common with the sound of a tube amp than regular solid state amps. 

Of course, class D designs vary a lot in their sound so YMMV. IME they can vary more than the sound of tube amps, which can very quite a lot! The point here is don't write off class D because you may have heard one that sucked. As with class A or AB, the sound quality is a function of design and execution rather than the class of operation. 

@atmasphere Just look at the harmonic spectra of the Purifi module. You’ll see a dominant second.

These are freq. spretrum FFT plots for THD and IMD from the published Purifi 1ET400A data sheet. For IMD, all spikes are beyond the audible freq. range so not a concern there. But, in the THD plot, where is the ’dominant’ 2nd HD? All odd- and even- order HD are almost inaudible.

@lanx0003 

I was looking at a different article. In this one we see a dominant 3rd (my memory playing up; I still seem to recall seeing a dominant 2nd) which is treated by the ear in much the same way as the second and is the only odd ordered harmonic for which this is so.

A dominant 3rd suggests cubic non-linearity; if so the circuit is inherently lower distortion than when a 2nd is dominant (which suggests a quadratic non-linearity). Our OTLs, being fully differential and balanced, have a dominant 3rd.

@atmasphere  I was there initially but just like asr said "... The third harmonic hovers around -130 dB which again, is almost at the limit of what we can measure..."   It is inaudible either at that noise floor.

I am sure that, with such a low noise floor the Purifi / Hypex class D modules provides, the sound from it is transparent bar none.  I also concur with you that the resultant sound traits hinges on how the modules is implemented.  But I don't think the sound will be sweetened approaching to tube amp unless specifically with a filter or other component.  Maybe that is why VTV adds the input tube buffer.  Just my two cents.

@lanx0003 The differences you hear in the sound of amps, if FR isn't an issue (which most of the time it isn't) is the difference in distortion of whatever amps are being compared.

There are three things that define those differences. What frequency, if any, that distortion rises (IMO/IME this aspect of amplifier distortion is the most important), the distortion spectra and finally how much distortion. There's no filter that would make a solid state amp sound like tubes; just the distortion signature. So if a class D has the same distortion signature as a good tube amp, it will sound like a  good tube amp simply on that account. 

This is because our ears use harmonics to identify any sound. So you can see that if an amp is to sound musical, its distortion signature must be as innocuous as possible. To that end, the 2nd or 3rd must be significantly higher amplitude than succeeding orders, so as to mask them. That is literally what has kept tubes in business these last 60 years.