The Future of Audio Amplification

Tubes are way cool and if not for deciding to try the latest and greatest new amp technology (Class D) first I might well have headed down the tube amp path several years back but hasn’t happened and I have little interest these days. Technology does in fact keep advancing and you don’t know what is possible until you try it so older technologies face more challenges these days than before perhaps.
Looks do count though and tube amps still look way the coolest by far.

George , Have you researched Exogal or Lyngdorf ? What's your opinion on these 100% digital amplifiers ÷

The Technics is the "blue" uses the GaN transistors, very fast quickly settling and reduced ringing, much closer to resembling the ideal grey square wave.
https://ibb.co/87Kh2mV

And now bench tests of Stereophile, use a special very low power high order -100db line filter between amps output and test gear input.
(Audio Precision’s auxiliary AUX-0025 passive low-pass filter),
Which eliminates noise between amp’s output and test gear input so reader can’t see it anymore.

1khz square wave with AUX-0025 filter in place:
https://www.stereophile.com/images/1212AM1fig03.jpg

1khz square wave without AUX-0025 filter in place:
https://www.stereophile.com/images/1212AM1fig02.jpg

Hm. Seems to make a difference about what you might know about what you are looking at.

In the first link above, no frequency is specified! Its clearly marketing and nothing else.
In the second set of links, what we are seeing appears to be the residual imposed on the square wave. Nothing unexpected. Since the filter is low power, so is the square wave depicted. These links IOW really don’t show anything other than that at the output of a class D amp, there is a residual sine wave as I mentioned before.

Really then no one one yet has been able to rid of this, and they are mightier minds than you. Only Technics has so far been able to reduce it greatly, with double the switching frequency speed and then the normal output low order filtering of it.

:) That’s funny! The reason the Technics has lower residual is because for a given filter turnover frequency (probably about 80KHz), the higher the switching frequency the lower the residual. No mystery- just faster switching times. IOW, what you referred to in the Stereophile links as the ’noise’ is a red herring- its not at all what you **seem** to think you’re talking about.

600khz v 1.5mhz switching frequency
The higher the switching frequency the higher the low order output filter can be set, so by the time the residual reaches 20khz what’s left with the 1.5mhz is down nearly 3 lower in amplitude and has less phase shift in the audio band than that of normal 600khz switching frequency. Blind Freddy can see that.

^^ This is partly true as written.

George, the residual **is** the switching frequency. So this bit

so by the time the residual reaches 20khz what’s left with the 1.5mhz is down nearly 3 lower in amplitude and has less phase shift in the audio band than that of normal 600khz switching frequency.

doesn't make sense as written. I think what you are trying to say is that with a switching frequency 3x higher, the residual will be reduced (given that the filter in question is the same whether the amp is switching at 600KHz or 1.5MHz). Its helpful to express this in db; assuming a 6db slope the residual will be down an additional 9 or so db. Which is nice!

The 20KHz part has nothing to do with it. Whether you measure at 20KHz as opposed to 20Hz (if you change the timebase on the oscilloscope if you are observing either frequency as a test signal) the residual will be the same amplitude.