Class D = Trash?

georgehifi,

     From the comments of your fellow mates on www.stereonet.au,
it seems you've been posting about the need for an increase in the switching frequency of class D amps to the 3-5mHz range for years to the point of their apparent consternation.  

     I'm not certain, but I'm fairly sure you've been stalking threads concerning class D, and posting similar messages about the dire need for class D amps to raise its switching frequency to 3-5mHz, for years here on Audiogon.  If it hasn't been years, then I apologize, but it sure seems like years to me and I'd hazard to guess to other frequent readers of class D related Audiogon threads.

     Please excuse me while I flash my 'Literary License Badge' and briefly pontificate: Georgehifi is like a stealthy panther, desperately hungry for the definitive answer to the eternal delicious question of the optimal switching frequency for class D amplification that, due to his being a magnificent beast of nature possessing extraordinary feline aural senses,  he alone having the capacity to ever so faintly detect when that frequency is set a few kHz too low.  
     He persistently lurks and stalks through the wild plains of the audio forum hunting grounds until he spots a class D thread and then he suddenly pounces with an uncontrolled fury upon his allegorical prey and maliciously metes out his vengeance by predictably, mundanely, and rather annoyingly, launching into another repetitive attack/statement concerning his bloodthirsty quest for the elusive optimal class D switching frequency.  

     Okay, I think we've all had our fill of whatever that was.  

      I'm apparently to the point of consternation myself.  Unfortunately, I  must admit I lack the complete expertise of  mathematics, electrical engineering and class D amp circuitry and design that I believe may be required to definitively determine whether your claims of the necessity of a higher switching frequency utilization in class D amps is justified.

     Therefore, the only current recourse that came to mind is to directly quote the acknowledged guru of modern class D amplification as well as the inventor of class D UcD and Hypex NCore technology, Bruno Putzeys.  The quote below is a relevant portion of an interview conducted by Peter Roth (PR) of Bruno Putzeys (BP) done on March 1st,2014 and posted on www.sounstage!ultra.com (I'll link the entire interview at the end of my post):

"PR: At Hypex, obviously, you continued development and implementation of the UcD modules for OEM clients -- but tell me about the much newer NCore class-D devices. Is NCore a further extension of what you’ve been doing with UcD, or is it an entirely different class-D scheme?

"BP: I think it would be fair to say that it builds on from UcD. Certainly the fundamental math is the same. The really crucial part of NCore was to figure out how to improve the loop gain even further from what we had -- UcD had substantial loop gain up to 20kHz of 35dB or something -- and I wanted to go beyond that. As I said, there is no such thing as too much feedback, so I was looking for a way to add 20 more dB. That’s actually a very hard problem to crack, because once you start doing that, you have to remember that a class-D amplifier has a limited bandwidth. A reasonable switching frequency for a class-D amplifier is just under 500kHz or so. If you go much above that, you run into efficiency and headroom problems. That, in turn, implies that you have no more than some 200kHz of bandwidth to play with -- actually, less than that. And if you want to cram 50 or 60dB of audioband loop gain into that bandwidth, you have to think completely differently from the way that linear amplifiers are usually designed.

Linear amplifiers typically have what we call single-pole compensation; some of them have two-pole compensation, but nothing much beyond that. UcD has four-pole compensation, and NCore has five. Once you reach into the four- and five-pole compensation, you have this problem that the amplifier can be operating in perfect stability until you clip it, and then it will suddenly start oscillating at a frequency that will immediately damage the amplifier and the loudspeaker -- so you want to avoid it like the plague. And not just that, but you actually want to return the amplifier, once it comes out of clip, to its normal operating regime so quickly that you don’t hear any glitches. The whole NCore patent revolves around the practical solution to that stability problem, the way that it actually catches the feedback loop at the moment that it’s thinking of going unstable, and then lets it go when it is safe to do so.

Apart from that, of course, I did learn some more tricks as concerns driving the output stage. If you have 50dB of feedback, and you are aiming for -100dB of distortion, you’ll still need to manage to get -50dB of open-loop distortion. In that respect, you are right in your previous question: You have to start out with something that’s good, because there is always a trade-off. If you’ve got yourself 50 or 60dB of negative feedback, but if you can also get 10dB improvement in open-loop distortion, why not do so? So the actual power circuit has also changed between UcD and NCore, and obviously, then, the later UcD modules actually use an improved power stage that was borrowed from NCore. The core of NCore is the feedback circuit, but the actual product contains some more improvements that are now trickling back into the UcD range."

     georgehifi, Putzeys states that a switching frequency of 500kHz or so is reasonable and you're claiming 3-5mHz is necessary for optimal class D performance.
      I'm not actually claiming that he's right and you're wrong.  For all I know, he may believe 500kHz or so is reasonable but may agree with you that 3-5mHz is optimal.  I just don't know if my class D monos would sound better with a higher switching frequency since there's no method to currently compare unless I compare my amps to the $30K Technics amp with the new eGaN FETS.  It's honestly hard for me to imagine how my amps could sound better but I'll be keeping an open mind until I can try a more reasonably priced alternative to the Technics.
      I'm thoroughly pleased with my experiences with class D amps so far and likely a permanent member of the class D Fan Club;I've already got my official club card, big club ring with the diamond studded 'D', club jacket, beanie, "Class D Stands for Delicious" t-shirt, "Class D or Die" bumper stickers , matching underpants and, due to years of practice, now have the unbelievably elaborate secret member handshake down pat.  

     Anyway, my main point is that I know Bruno is a friendly and accessible guy via email and will answer your email inquiries on class D switching frequencies.  Wouldn't you rather receive answers straight from the horse's mouth than constantly raise the subject on Audiogon Forums?  Speaking for myself, I know I'd sincerely enjoy not hearing about this subject again until you, hopefully, post on what you discovered in your discussion with Bruno.  


Complete Putzey Soundstage interview:
http://www.soundstageultra.com/index.php/features-menu/general-interest-interviews-menu/455-searchin...


Thank you,
   Tim  

 Here's a link to what I was referring to in my last post about reading of faster switching mosfets:

http://audiophilereview.com/cd-dac-digital/why-well-soon-be-living-in-a-class-d-world.html

Hmm. I'm a fan of class D amps (really expect them to bring home the bacon someday and am working on one myself) but not the 'digital' amps to which this article refers, because they don't exist. I expect the author really meant 'class D' or 'switching' when he must have accidentally used 'digital amps'... ya think?

Maybe we have to get use to the class D amp, as it has a very black background, which makes the instrument and voices sound very clear.
Interestingly Primare i32 intergrated amp,that is a Class D design, and got very good reviews here in Europe since its release 5 or 6 years ago, will probably be replaced with a new model later this month.
Primare have a long product cycle, so it will be interesting to see if they have advanced the Class D design, or gone back to a/b solid state.

" Interestingly Primare i32 intergrated amp,that is a Class D design, and got very good reviews here in Europe since its release 5 or 6 years ago, will probably be replaced with a new model later this month."

No wonder why my humble bid was accepted. I wish the damn thing would arrive, already, so I could quit reading and get back to listening.

A high-definition eGaN FET-based system with higher PWM switching frequency, reduced feedback, and higher bandwidth produces the sound that has the warmth and sonic quality that audiophiles demand;

For a quality Class D audio solution, it is possible to avoid these transitions all the way through the system to the final Output Filter

The key to great Class-d, is to get it’s LOW ORDER output filter to do it’s job of getting rid of ALL switching noise from the audio band before it get sent to the speakers, and without creating any phase shift.
The only way to do this successfully is to raise the switching frequency noise much higher so this low order output filter (without masses of feedback) can do it’s job properly without any residue effects or switching noise remnants being left within the audio band.

This is why Technics have led the way with the new SE-R1, have striven to DOUBLED the switching frequency with these new eGan Fet transistors, to get closer to the ideal above results.

Credit to Mark Levinson a couple of years ago tried to solve this problem with todays switching frequency, by using much lager (to take the amps power) higher order output filters and less feedback, but it was a bit of a flop, with their very expensive. (see the size of the 4 x chokes for the higher order output filter for each monoblock).

http://www.stereophile.com/content/mark-levinson-no53-reference-monoblock-power-
amplifier#rJEmkELOsfXsME4b.97
It’s 10khz!! (not 1khz) square wave without!!! the AP filter, is the best ever seen for a class-d amp (almost as good as a linear amp) because of it’s massive higher order output filter and less feeback, but this high order filter obviously created other problems, and was not received well, that’s why low order with low feedback is the way to go, but it must do it’s job properly, and the only way to do that is to raise the switching frequency noise much much higher.

Cheers George