Could Class D really be that good?

@atmasphere 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.

Agreed. But I want to emphasize, with the low 2nd HD found in these Purifi / Hypex class D, you won’t be able to have distortion signature comparable to a tube amp and possibly produce the tube-like smooth sound. Let me give you an example of sound color signatures built in the Smsl Su-8. The 2nd, 3rd and other higher order spikes in the standard setting (upper left) are insignificant (-120dB, similar to Purifi / Hypex. Users could elect the "Tube 1-3" sound signatures to experience the emulated tube sound, where you could find the 2nd-order HD boosted to -90dB in Tube 1 (light) setting and -70dB in Tube 3 (strong) setting. That tells me the 2nd order HD needs to have high enough ’dominant’ spike (relative to 3rd & higher-order) to exhibit the tube-like smooth sound.

Here is the FFT plot for a real good tube amp, McIntosh 40, which confirms with the above. But, interestingly, Mc40 actually has slightly higher non-dominant 3rd hd.

Most agree that the 2nd harmonic is innocuous to the human ear; the real issue here is are the 2nd and 3rd able to mask higher ordered harmonics. I've heard the argument before that if they are 90dB down you aren't going to here them, which I think is false due to the fact that the ear uses higher ordered harmonics to sense sound pressure and it has over a 120dB range.

But if the 2nd or 3rd is masking them, then you have a 'tube amp' signature. The lower the distortion overall though, the more transparent the amp becomes.

@mattw73

I’m impressed with both the Nilai Mono and Apollon St in terms of their specifications. However, I lean slightly towards the Apollon for a couple of reasons: (1) it offers 25% more reserved maximum power in both 4 and 8 ohms (despite a slight sacrifice in dB noise in 4 ohms), and (2) it has the lowest measured power on/off noise, which is inaudible. Additionally, while the Nilai Mono has the advantage of being a stable 2-ohm load amplifier, the Apollon Stereo is half the cost of a pair of Nilai Monos. If you or anyone else has experience with these two amplifiers especially how they sound, I would greatly appreciate any advice or insights you could offer. Thank you!

Yes, the amount of distortion and its spectrum has some correlation to sound. However, it is not the major or only thing that makes and amp SOUND the way it does. You can give me 3 pairs of Atmasphere amps and I will change some of the parts in one to really bad sounding parts.....leave one of them stock and in the 3rd one I will change a bunch of parts and execution that I know will make the sound better. They will still all measure the same. Ralph or anyone would be able to pick them apart..........even on a double blind test.

All Hypex amps sound different from each orther. All Purifi amps sound different from each other. Please read the review on 10 Audio on the NAD Purifi based amp and then the review he did a little later on my modified VTV Purifi amp......way different and better sound. All parts and execution change the sound. I mod all these amps so I know that they all sound good stock.....but way, way better with mods. Yes, some do not believe it....they only believe in measurements. My mods do not change measurements.....they change the way an amp sounds.

I have been modding and manufacturing gear since the 1970s.....Everyone likes what I do......everyone (I mean, those that listen). If you change the coupling cap in a tubed piece of gear.....or brand of tube....or brand of resistor.....you change the sound.....but you do not change how it measures (obviously, the tubes must have the same gain). All op amps have a sound. Many amps out there right now (all the Fosi and Ayima amps,etc.) with sockets in them so you can "opamp roll". All the various op amps measure such low distortion that they do not change the measurement of the amp......but everyone can hear the difference in sound between the different op amps. This is why the best versions of Purifi and Hypex use discrete op amps and super low noise regulators (because they sound better). All the discrete opamps have a sound, as well. VTV sells various op amps for those that like different flavors of sound. I modify the Sparko labs Pro opamps to give better sound (one of my favorites, so far, along with the LKS discrete op amps). Warren at VTV sells various amps and he slightly prefers the Purifi (and especially the high powered Purifi) modules to the Nilai and NCX500 that he sells. He is a tube guy so he really likes the tube buffer in front of the Purifi module. Listening to a very modded VTV Purifi amp as I type.....Wow, what a great sound.

I am probably going to have a tour with a stereo dual mono high powered modded VTV Purifi amp soon. This way, this less than $3K (delivered including mods) amp can be compared to Atmasphere, Orchard, Hypex, Nilai and others. This dual mono stereo amp will do 222 watts into 8 ohms, 436 watts into 4 ohms and 750 watts into 2 ohms. I will warrantee the modded amp for 2 years. Fun stuff. Low distortion class D modules with serious tweaks equals serious goosebump factor.

@ricevs

Thank you for the write up / advices. I pretty much agree with what you said there regarding the sound signature heavily depending on how the modules are implemented. The reason I put so much effort there investigaing measurements is because there are still significant variations in the performance specification among the diy or commercialized products based on the same Hypex / Purifi modules. A few notes / questions here if you don’t mind:

- I am interested in VTV Purifi with tube buffer as well but I still want to see the detailed measurements there like ASR; I won’t bet / invest 2-3 grands just for experimentation. Hopefully ASR could have a chance to evaluate the product.

- Discrete op-amp like Sparko S3602 is certainly top notch but, according to FFT freq. spectrum analysis, the TI LM4562 is no slouch either and only costs 1/6. I know you might throw in the similar comment like they will sound differently even measurement is alike BUT I wish to hear your opinion in case if you had some experience with it.