Class D

Of course, I checked every channel of every amp that I sold and looked at the residual of each channel. I just did not do an exact measurement of the before and after. From what I remember, it was very much the same, about a half a volt or so of pure 500K with no added high frequency noise (but since I did not do an exact measurement then that is what I tell you I did). However, it sounded way better without the extra filter (most class D amps just use one coil and one cap per phase). if I had seen any kind of added noise on the 500K wave or a noticeable increase of value I would of taken note......there was basically no difference (that I remember). The secondary coil is 15 uh and the cap to ground is 1000pf.......so it looks like a 1.3 meg filter......that is not going to make any difference in a 500K sine wave. If you add another 1000 pf for speaker wire than maybe a hair difference. By the way, I first removed the 1000 pf caps to ground on the output and heard no difference.  But when I remove the inductor.....OMG

When Nuforce started making class D amps I was a dealer. That first amp had all kinds of noise coming out of it. One if the fixes they used was to add a secondary coil right on the speaker output terminal. When I look at the output stage of the latest class D Nuprime amps (Nuforce morphed into Nuprime) then I notice a bunch of coils near the output.....so I believe they are still doing this today. Most everyone just uses one coil and cap. More ferrite coils on the output is not merrier. How many coils (hopefully air coil) are you using on your output? Coils are not good......but you have to have one in a class D amp or you fry your speakers......so make that one the best sounding one in the universe!

It does not matter that Levinson used a bunch of air core coils (one thing does not make a whole).......what matters is the sound.

I did not say that it was the idea of  series'd air core coil output filters that ML used to get a steeper roll off as not to give phase shift down into the audio bad that made it sound bad.
The fact that a big company like Mark Levinson knew that it is a problem with Class-D, as this phase shift graph from output filters shows (red) https://ibb.co/vvwzGV5

That's why it's seems only fixable by moving the switching frequency and  filters way up higher so there's no effect of phase integrity down into the audio band, and the only way to do this is move the switching frequency up much higher to 1.5mhz and the filter as Technics did with the SE-R1 and hopefully with the SU-R1000 also.

And you can't fix it but throwing "more global feedback" at it, as that is a sound destroyer in itself.  The only way is the > 1.5mhz switching frequency, so the filter can do it's job without introducing phase problems down into the audio band.

The Levinson amp switches at 2Meg and it sounds bad.  For what reason?........who cares.  You buy it, you lose.  The AGD amp switches at 500K.....it sounds good.   For what reasons?......who cares.  You buy it, you grin.  End of story.....Everything matters.

That's why it's seems only fixable by moving the switching frequency and filters way up higher so there's no effect of phase integrity down into the audio band, and the only way to do this is move the switching frequency up much higher to 1.5mhz and the filter as Technics did with the SE-R1 and hopefully with the SU-R1000 also.

And you can't fix it but throwing "more global feedback" at it, as that is a sound destroyer in itself. The only way is the > 1.5mhz switching frequency, so the filter can do it's job without introducing phase problems down into the audio band.

Rubbish. Bruno Putzey's UcD module, which is quite old at this point (and predates NCore and Hypex), has less than 1 degree of phase shift at 20KHz. It switches at 400KHz and is self-oscillating, with about 40dB of feedback.

Rubbish. Bruno Putzey’s UcD module, which is quite old at this point (and predates NCore and Hypex), has less than 1 degree of phase shift at 20KHz. It switches at 400KHz and is self-oscillating, with about 40dB of feedback.

Serious!!! your rubbish, those things have always sounded crap in the upper-mid/highs, most probably because of all that global feedback, that’s why it’s considered an ok bass/sub amp.