Hypex N core module

Proof will be in the pudding Ralph, 60db of negative feedback makes the hair stand up on my neck.
As everything I have learnt and read from the masters about feedback, say it's in some cases a necessary evil in small amounts, 15-30db and hopefully just local not global, but 60db global!!!! YIKES!!!!
Maybe that the reason for those that hear it in class-D that it's sound is lifeless in character.

Cheers George   

George, I’m a bit uncomfortable with this statement-

Maybe I should have said in "most"

A little feedback should "only" be used to clean things up a little, not to make design blunders look better on test. And if designed very well, all that needed is "local feedback"

In tube amps that have average output transformers global feedback is used to reduce the transformers output impedance to get better damping factor specs, this is a band-aid fix, the output transformer should have it without the need of feedback, and the amp will sound better for it.

Global feedback compares output to input and try’s to correct any differences, trouble is that the output is already on it’s way to the speakers. And the more global feedback applied the more unstable an amp can become.

If you think about it Ralph, linear solid state amps that are well made but with global feed back and a bit of it, also have the same lifeless sound that many say Class-D has.

We have done an experiment ourselves back in the 70's on a very good amp Pass’s A20 Class-A, which we morphed into a 150w Class-A beast with water cooling (a two man lift), which had global feedback and sounded very good, then we changed it and made it local feedback and it sounded superb so much better it wasn’t funny.
http://www.firstwatt.com/pdf/art_classa_20.pdf

Sorry to go on- I just felt this also needed to be said.

Cheers George

 "Local feedback with a bit of global feedback is, actually, identical to full global feedback -- mathematical fact."  

You can have local without global, this is preferable in linear solid state, if the output stage is well designed you can do without the global and just have local around the input and driving stages, many good hi-end amps do this and it sounds much better for it, Gryphon, ME, Krell  ect.

He also believes there is no such thing as 'too much feedback' as it pertains to class D amps

That's his opinion, not by many noted others. And it looks like Ralph's going minimal FB as well.
Large amounts of global feedback used in linear amps is used in poor designs, to get good specs, the challenge is to get good specs with only a little local feedback.
 
Cheers George

Class D amps are very sensitive to power i.e.

 Yes they have one of the most dirtiest power supplies made, SMP's
That's why you'll see many caring Class-D manufacturers giving them more quieter linear supplies, just to null out one of the problems of Class-D amplification.

Cheers George 

I’ve been reading for decades that negative feedback in traditional linear amps should be avoided since it (sorry, no pun intended) negatively affects its sonic performance, especially in the upper frequencies. I’ve taken this as the gospel truth ever since I first learned about the use of feedback in amps.

First you need a bit more clarification on just what "feedback" are you talking about, "global feedback" or Local feedback"? As both are very different.
"local" is a good thing if the output stage is designed right with minimum problems. "Global" is also ok if used sparingly and not use to correct big problems.

Then there's "feed forward" that I've seen in Rouge M120 monoblocks.

Cheers George

Now the parts we work with are not that expensive, but we found a way to eliminate deadtime. This allow us to switch at much higher frequencies.

Naturally your not going to tell us how it’s done, I’m hope for you that you can do it, but also sceptical at the same time, as minds more attuned to solid state Class-D topology than yours have not succeeded, save for Technics who are half the way there.

Cheers George

FWIW the squarewave response you linked does look pretty bad. Our tube amps can do better than that!

Of course it can, any linear amp can, not just yours!

It’s very good for any Class-D without the need of special external output test filters.
https://forum.audiogon.com/posts/1501266
Cheers George

Here is an example of today’s switching frequency trying to be filtered by Mark Levinson No.53 with the $50k usd Class-D. They tried very hard by as you can see by the 4 massive x filter chokes added together (paralleled or series or both) to give a steeper filtering effect on the switching frequency, it worked as the 10khZ square wave shows, without the use of Stereophiles AUX-0025 passive low-pass low power filter, but adding filters together is well known to create other problems, this amp it seems was not a success for ML.

" However, the more I listened, the more the overall sonic picture seemed flat and uninvolving. An enigma.—John Atkinson"

ML Class-D monoblock x1 showing 4 x filter chokes.
https://i.pinimg.com/originals/f1/c5/3a/f1c53a9e8f03bd1ea3a4ed38f65c0b1e.jpg

ML Class-D 10khz square wave without Stereophiles special testing filter.
As you can see very acceptable for Class-D with no switching frequency ringing getting through those filters.
https://www.stereophile.com/images/1212ML53fig02.jpg

Cheers George

Several MHz isn't a problem. We're working with inexpensive devices that can easily do 5MHz and beyond.

Technics have produced a Class-D amp (SE-R1) that has double today's switching frequency speed so the output filter has less of a chore to filter it all out, but it's $30k usd and special order only if your the God or Emperor of Japan?

This amp is half the way there because of the special hi speed semiconductors used to get double the switching frequency of 1.3mhZ

It's getting there, but needs to be double d again.

Here is one reviewers take on the Technics Reference SE-R1 Class-D amp:

" No other audio system that’s graced our listening room here at DT has captivated and mesmerised our staff quite like Technics Reference system. Seriously, some of us are having trouble getting our jobs done because we can’t peel ourselves away. This is the sort of audio system that you must hear to understand. Listening to tracks that we’ve heard 100s of times — and on excellent systems at that — is now a revelation of once hidden nuance and detail. Not only are we hearing things we’d never heard before, we’re hearing it in a way we’ve never heard it before."  

Cheers George   

Now the parts we work with are not that expensive, but we found a way to eliminate deadtime. This allow us to switch at much higher frequencies.

Ralph, what is the approx ringing of the square wave I linked to in the 1st pic, it is the switching frequency noise

Yes dead time is another problem.
But you will still have the problem, of eradicating the switching noise completely with the output filter without ANY effect on the audio band from 20hz to 20khz. So say a 10k square wave will look like the ML’s one I linked to above, without any Stereophile external bench filters used.

BTW I wish you luck on your Class-D quest, but Technics has shown the way with limited supply of the transistors they use. In the future there maybe a good supply to everyone with hopefully 3 x the speed of switching frequency. But i suggest not to jump on the gravy train just yet, wait for the technology to catch up, or the end maybe like the ML53.

Cheers George

Here are the 10khz waves of Class-D both done by Stereophile tests, one with and one without the Stereophiles special AUX-0025 passive low-pass low power filter.  This filter can't be used while listening as it would burn out almost instantly.

With the special bench test filter:
https://www.stereophile.com/images/916BC600fig2.jpg

Without the special bench test filter:
https://www.stereophile.com/images/1212AM1fig02.jpg

Now I ask ANY linear amp manufacturer, would they be happy if their amp gave a 10khz square wave that looked like the second pic to sell to the public.

Cheers George   

The filter can do its job properly at current frequencies. There is an advantage to going to faster switching frequencies- lower distortion.

Sorry this is not correct, that is why when Stereophile do their bench tests they have to use a very special low power external steep filter on the output to get some good looking measurements from them.

Stereophiles: Audio Precision’s auxiliary AUX-0025 passive low-pass low power filter.

Trouble is that filter can’t be used at listening levels, as it would burn up in an instant.

Cheers George

mitch2
I have no idea whether my impressions are related to the switching frequency George describes, but they could be. I owned and used in my main system a pair of the Acoustic Imagery Atsah monoblocks made from Bruno Putzeys' highly rated Ncore NC1200 amplifier and power modules. In summary, I sold them after a prolonged, direct comparison with three other Class A and AB amplifiers.
IMO, they just cannot compete with the better amplifiers I have owned.     

statman
I will try and explain my experience. I was happy with my Theta Prometheus monoblocks. I must add that I am much happier with my D’Agostino Progression monoblocks

It's not the Class-D switching frequency itself so much, but the low order filter that's needed that has to get rid of it.
  
If that filter could be much steeper then it's filtering, then it would work and not have any affects into the audio band, and maybe compete with good linear amps. But it can't be a steep filter, because it has to handle the full power of the amp and would burn out very easily.

So the only thing is to make the switching frequency at least three times higher, then that filter can do it's job properly and not have or leave any effects down into the audio band.

The designer of Soulution amps and many other hi-end amps are of the same opinion.

 Cyrill Hammer (Souloution) 
"if you want to have your product performing at the cutting edge it is not possible with today’s known switching technologies. In order to come close to the performance of the best linear design we would need high-current semiconductors that provide switching frequencies of several MHz or even GHz."

Lew Johnson (Conrad Johnson)
"I tend to think that Class D circuit design is an approach best relegated to producing low-cost, physically manageable multichannel amplifiers—where one might accept some compromise in sound quality for the sake of squeezing five, six, or seven 100 watt channels into one moderate-sized package for a budget home-theater installation."

And there are many others if you care to search.
You'll see many "old school" linear amp manufacturers starting to do Class-D just to stay afloat, as they are having an economic effect with those businesses, that can't weather the storm and wait for Class-D to get better with future higher switching frequencies. 

Cheers George

    

I would consider almost any Pass class A/B amp to be beyond 'good' and suggest you were comparing a possibly good class D amp or amps to 'one of the best' class A/B amps available.
   I think a more worthwhile comparison for you would be between your Pass amp and Mola Mola Kartuga or Merrill Audio Veritas mono-blocks that are in a more similar quality and price range.

All Class-D's today have the same problem. The switching frequency is too low to be filtered out effectively completely with the Class-D's output filter, without effecting the upper mids and highs.

In the future when technology allows the switching can then be much higher to allow the output filter to do it's job properly and cut out all the switching frequency noise without effecting the audio band.

Cheers George