High end Class D amps?

After probably at least a year of driving my EVS 1200 direct from my LPM moded Oppo 105, and having upgraded all cables and speakers I decided to reinsert my Audio Alchemy DDP- 1 + PS 5.


It took a solid 24 hours to relax, and certainly sounds better than when I first reinserted it. I meant to hit replay on my 105 before I went out last night, so it wasn't running any signal. At first I was disappointed, but then realized I needed to turn the volume all the way up on the 105, which I did just before heading out last night


Now, the 'problem' is: when I want to play SACD/DVD-As, I have to reroute my cables back to direct, which is a RPITA. What I did before was also hooked up my Marantz HDCD-1 (as a transport) to the DDP-1, and run them separate. That may be what I do again "Back to the Future"

@dchang05 I think some people are threatened by Class D for some strange reason. I have a nicer Class A/B amp than your average 'audiophile' in an Ayre VX-5 Twenty. I also have a 7-channel Class D amp (utilizing Pascal modules) for my integrated HT setup. Just for the hell of it, I hooked up the Class D to my mains. And the difference was less than you would think. No harshness, plenty of 'musicality', tight bass and extremely clean midrange. I listened for several hours this past Holiday weekend and my ears weren't bleeding. 

There is a lot of total nonsense posted online regarding many topics, including Class D amps.

Are people threatened? Fearful? Did they just hear a bad setup which no one technology alone can fix? Did they just prefer something else in their case? Who knows.

I have Class D amps ranging from $80 to $6000 retail. They all sound top notch used properly in the right application. Just like anything else. Any new amps I buy in the foreseeable future are likely to be Class D or similar as well.

Does not mean they will be everyone’s cup of tea since personal tastes always vary.

But to diss the technology categorically as not worthy for consideration is just a huge mistake. It is most worthy with a very unique set of advantages and little if any downside, personal preferences aside.

Personally I bless the technology. Just apply it wisely. Nothing new there!

Problems with any amps shouldn’t be fixed by using even more feedback as a fix, it’s a added by the best designers to clean thing up a little and usually just local not global in an already well designed amp.

All the greatest amp designers say it. An amp should be reasonably good spec’d, and to use it just as a clean up tool.
And the preference with the best is to use "local feedback" only by the best designers. Not global as being talked about here, and then even including the Class-D output filter in the global loop, what next throw the speaker wire and the speaker in the loop as well, like Trio/Kenwood tried all those years ago, utter sterilized disaster sound.

The first paragraph is true. So is the 1st sentence in the 2nd paragraph. Right after that things go off a cliff...


The fact of the matter is design-wise you start with a class D circuit with no feedback and get it to sound decent and perform fairly well too. Then you add the feedback, but in this case so much that the amp goes into oscillation. With a normal amp you could never get away with this. But with a class D amp, the oscillation is then used as the switching frequency, so you put a few frequency sensitive components in the feedback network so that when it goes into oscillation, it really can find only one solution- one frequency- at which it oscillates. The switching frequency itself is then used as one of the inputs to the amp (the other being the actual audio) but is converted to a triangle wave for comparison to the audio signal.


Now your already fairly linear amplifier has a mess of feedback (over 35dB) too, and the reason for doing this is to allow the amp to compensate for the distortion that is normally generated by the application of feedback. You need over 35dB for this phenomena to occur. This gets rid of the brightness and harshness that is otherwise associated with **all** amplifiers that have used feedback at insufficient levels (which describes nearly all traditional solid state designs of the last 60 years). So its quite unlike that unsuccessful Kenwood system George mentioned above, and yes, you do include the filter in the loop so that the amp can compensate for phase shift induced by the filter.


In a conventional amplifier you have a thing called Gain Bandwidth Product, which is how much gain you have vs how much bandwidth you have. The idea here is that you are going to blow off the gain when you use feedback to reduce distortion (and output impedance). But there’s a sort of zero crossing where the the amp circuit just does not have the bandwidth to support the amount of feedback used. In a nutshell you could say (inaccurately...) that the GBP is a sort of fuel that you use up when you add feedback to the circuit. When its gone the feedback at that frequency is gone too. Put another way, an amp with insufficient GBP will have distortion low at low frequencies and the distortion will rise as frequency rises. This is very audible- our ears are very sensitive at 3-7KHz and they use higher ordered harmonics to sense sound pressure. We’ve been hearing this problem for 60 years and its literally why there are still vacuum tubes around after all this time.


This is why solid state amps traditionally do bass well as there is plenty of feedback to allow them to be low distortion. But at 7KHz there isn’t enough, so harmonics at that frequency aren’t suppressed, and there are actually more harmonics because the feedback is adding them through a process called ’bifurcation’. And so we wind up with an amp that is bright but measures flat. To get around this problem, manufacturers and the test and measurement industry have resorted to a simple technique called ’lying’. The distortion of the amp and the resulting harmonics are measured at 100Hz, where the amp measures quite well. If you measured it at 2KHz you would see something vastly different, which is why that’s never done.


Because a class D amp can have so much feedback, it can measure the same low distortion at 15KHz as it does at 100Hz. IOW the distortion product can be ruler flat across the board. Prior to this technology, the only other way to do this was to have no feedback at all, and use the most linear circuit with the widest bandwidth you could get (200KHz is required to prevent phase shift at 20KHz on that basis). This is all because it really is important to the sound of the amp that the distortion product be both benign (mostly lower harmonics) and ruler flat across the board.