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
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