Hypex N core module

Feedback, while suppressing distortion, adds some of its own.

In addition, it is a destabilizing factor in any amplifier design. 'Stable' means that it is resistant to oscillation.

The distortion added by feedback contributes to brightness (a coloration) and hardness (which is unpleasant); both due to the ear's quality of converting all forms of distortion into tonality.

So far we've been running our class D zero feedback as well. Output impedance does not suffer, so driving a load really isn't the problem. Controlling distortion is, but if you are careful with the design that can be controlled too.

Hi Ralph,

     Thank you for your informative response. 
      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. 
      Since I've become interested in class D amps in the last few years and began reading about how it works, however,  several articles and white papers I've read have caused me to rethink my view of negative feedback in class D amps.  For example, here's a quote on this subject from an interview with Bruno Putzeys, inventor of UcD and Hypex NCore technology:

"If you take a simple amplifier which has acceptable distortion (just a second harmonic is what I use as an example) and you start applying feedback, harmonics will appear that were not there originally. Higher-order harmonics, even and odd, turn up out of the blue. So if you apply a little bit of feedback, the second harmonic that you wanted to reduce drops by a little, but out of the blue you get this whole smattering of high harmonics. It is quite understandable that this doesn’t sound good. That observation has been made and published by various people over the years, but the most important conclusion was never drawn: If you keep increasing feedback, if you turn the feedback knob up and up and up, you quickly hit a point where those distortion products all start coming down again and the signal does start getting cleaner. And if you get to very large amounts of feedback, the result is just supersmooth. So that is why I say that it is normal for an experimenter to experience that if you take a good-sounding zero-feedback amplifier and add 6dB of feedback, the result sounds worse. They heard that right. But had they been in a position to add 60dB, well then, suddenly they would have been confronted with a sound that is little short of magical."  

     I've always respected your audio knowledge and found your posts very informative.  I have no desire to start an argument with you and I'm fairly sure you've forgotten more about good amp design than I've ever known.  
     I'm just very interested on your thoughts about negative feedback in class D amps.  Here's the link to the entire Bruno Putzeys interview I quoted from above:
www.soundstageultra.com/index.php/features-menu/general-interest-interviews-menu/455-searching-for-t...
Thank you,
   Tim

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

They heard that right. But had they been in a position to add 60dB, well then, suddenly they would have been confronted with a sound that is little short of magical."  

Bruno is spot on with this. The usual rule of thumb is that very low amounts (under 4 db or so) is not harmful, but more than that is a problem; after about 20 db or so things start to settle down. 60db hasn't been practical until the introduction of class D (since gain is developed in an entirely different way); so I have no argument with this; my prior comments should be limited to traditional amps where making the kind of gain is impractical.

I use the Acoustic Imaginery Atsah monos and also do not experience any high frequency issues or soundstaging issues. The Ncore 1200, imo, is extremely revealing, neutral and transparent and will take on the character of the source equipment, cables and material being played.

As such, careful attention needs to be paid for the Ncores to sing and for its potential to be maximised (just like any high end amp). It has been quite an interesting journey and I have spent considerable efforts matching the appropriate cables with the Ncores and cannot be happier with how they sound at this point. Class D amps are very sensitive to power i.e. power conditioning and power cords and so I will suggest these be properly looked into and matched before any critical evaluation of Class D is made.