Hypex N core module

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

    

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

If you look at the specs for a Merrill tarranis which uses an ncore module for class d output they show bandwidth to 50 kHz. If you look at the ps audio stellar 300 which uses an ice module and bandwidth to 50khz. The benchmark ahb2 is shown as a class ab/h not class d but class d efficiency and bandwidth shown to 200 kHz. The first watt f7 class a with bandwidth shown to 100k. All of these Amps are well regarded and carry the designers name and therefore affect his reputation. Is it possible that the difference is the difference in the voicing.... sonic signature that each designer intends based on their hearing and memory of music??

"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."

Several MHz isn't a problem. We're working with inexpensive devices that can easily do 5MHz and beyond. The problem is deadtime. That has so far proven to be the switching limit; that's why they don't switch much higher than maybe 2-5MHz tops. The deadtime increases distortion, and the faster you switch, the more you need it- so there is a minimum distortion that you can hit, sort of like Whack-A-Mole. I did state this in my previous post.

When you are switching that fast though, the speaker itself is part of the filter; there's no problem getting a filter to work and it won't burn up.

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