Class D Technology

So I get the obvious strengths of Class D. Efficiency, power output & running cool which allows for small form factors. I also understand the weaknesses somewhat. 1. Non-linear & lots of distortion that needs to be cleaned up with an output filter. 
So my question is, if it weren't for efficiency & power, would there be any reason to own a Class D amp? Do they beat Class A in any other categories that count for sound quality?  
Listen and decide. My Class D amps are the best I’ve ever had by far. They sound great always with most any speaker and that’s all that matters. Gotta match to right speakers for absolute best results though just like any amp.

Plus they are most efficient. That’s a good thing for amps just as it for speakers. That is really the thing that distinguishes them most especially for use with most any power and current hungry modern speaker that also tend to be smaller and more extended meaning power and current needed for best results. It’s a natural technical match that results in top notch performance in most cases out of a smaller more easily handled package which also means better sound possible for more people.  Of course personal preferences regarding what sounds best really is totally subjective and preferences will vary.
you have to use a low pass filter and some think the attendant problems with that cause SQ issues

you also have a lot more EMF generated

and more...

Class A maps work relly well in the arctic where you can use them to heat your igloo

I have no dog in this fight since I use a Sunfire
The day will come when Class D will compete and very well be better than any linear amps tube,s/s, class A, AB or whatever, BUT!!

 Class D needs much higher switching frequency (needs to be at least 5 x higher) technology isn't there yet but in the future it will come, only then will ClassD become a contender. Till then good for budget hifi and subwoofer/bass amps.

Technics has progress the Class D race and developed 2 x the present switching frequency with their own made components, in this amp but it’s very expensive $20-30k

When the switching frequency is 3mhz or better, then it can be called hi-end, and will own it myself, then my linear heavy boat anchors will be sold or become door stops.

Cheers George
I have only listened to a couple of brands of Class D amps and was totally unimpressed. That pretty much steered me away from them but I'm sure there are some good ones out there. I'll stick with my Class A thank you.
As Mapman said you have to listen to them and decide. Some people are very pleased with certain class D amplifiers. Thus far in my experience a high quality class AB and definitely class A are better sounding. I’m with George on this one. This could change in the future with further class D development, so time will tell. For some listeners such as Mapman the future has arrived and I can appreciate that per their perspective,  truly an individual call.
I'm not going to list them all, but I've owned quite a few. I stopped with the almighty NCore amps. They almost always sound impressive at first listen, clean, quiet, and cool running. For me though, they begin to show their true sound after a month or so. I've given up on them.

About six months ago I was given an old Hafler P225 which I rebuilt with parts from Musical Concepts. The warm, inviting sound has yet to disappoint. Am I hearing "audiophile" sound? Don't know and don't care.
I tried the Nord Hypex NCore NC500 One UP monoblocks in a bid for more power. However even with the switchable opamps, including one that operates in "Class A", they were no comparison to my real Class A amp and were soon removed. Sounded cold and heartless. They weren’t right for me but seem to work for others.
as georgelofi has pointed out I think class D strength lies in sub-woofer applications. They do a remarkable job in my Golden Ear Triton One’s.

What I don’t understand - but am rather certain you guys do - is that in the case of the Triton One, the class D amp is rated at 1600 watts. The speaker is powered with 115V AC. That would mean over 10 amps even if 100 percent efficiency which it isn’t. I have both (2) speakers on the same breaker. Why haven’t I blown a fuse? Seriously, doesn’t ohms law here dictate that the amperage would be (if played loud) greater than 10 amps per speaker? I feel they are drawing very little power and don’t understand how that could be given the 1600 amp rating, unless spec provided by GET is inaccurate.

georgelofi you are correct about switching frequency being teh key to even better Class D amps in the future.

Have you listened to any good quality class D amps or are you judging solely on technical interpretation?

Fact is the good ones sound very good today, as good or better than most. There is plenty of reviews and such published that support that.

I can vouch for it as well FWIW.

Thing is I am over 50 and do not hear to 20khz like I know I could when younger. So I am not the best one to judge what’s happening at the highest frequencies in detail.

In general, its something of a good thing not to be able to hear up to 20khz because there is often more noise and distortion at those frequencies than music.

Check the audio frequency music chart and one can see little music happens at the highest frequencies people can hear. It’s mostly "air". That’s part of music granted but since most audio involves a compromise of some sort, this is a reasonable one.

All I know is I cannot concretely fault anything in the sound I hear from my Class D amps with any speakers up to the task of handling all they can deliver. Most people would be thrilled with them. They are the quietest amps ever, with a totally black noise floor. At least that’s what I hear.

gdhal I think one of the things switching amps excel at is delivering power and current for very short periods of time which makes for excellent detail and transients. Its a very important aspect of music that often distinguishes great sound from just good. Power and current specs alone as published probably do not indicate an amps ability to deliver power and current quickly and efficienctly for very short periods of time when needed.

One way this is refelcted uniquely with the Class D amps I own and use compared to others is detail and articulation of the bass in particular. It is leaps and bounds more articulate and detailed than any other amps I have owned or even heard. When I first heard it, I was shocked thinking my bass was gone when in fact it had leaped to an entirely different level not heard prior.

My impression is tube amps tend to be a bit softer in this regard (which some may actually prefer) and class a/b SS amps often just miss it altogether.

Class A amps, tube or otherwise are historically the ones that probably do this best, but that is changing.....
Lots of great responses. When switching frequencies are 5x higher than today's, what will be improved? Will that prevent the saw tooth waveform challenge? Or prevent the need for a low pass output filter?

What makes today's class D amps sound cold and dry? I have experienced this firsthand with an ICE Module that I once owned.

Mapman alluded to a totally black noise floor. I also experienced this with the ICE module. So far, this is the only benefit other than efficiency & power output that has been discussed and can be attributed directly to the Class D topology. Or maybe this noise floor issue is unnatural and sounds are being cut off prematurely by not allowing for decay?   
" What makes today’s class D amps sound cold and dry?"

I’d attribute it to the high damping factor in combo with the aforementioned transient capabilities.

Mine definitely sound best with speakers that like high damping. Usually larger ones with larger drivers that are inherently harder to control.

They are more towards the cold and dry on my little Triangle Titus monitors.

Not so on my somewhat larger and more extended Dynaudio Contour monitors or any of my other much larger and more extended speakers.

They are the cats meow on my largest the big OHM F5 series 3 floorstanders with much larger Walsh style driver often cited as benefiting from monster amps with high damping. That is very true!

The switching time is an issue. The output devices have to switch on and off at a pretty high speed. The problem is, they can't both be conducting at the same time. That is to say, it takes a certain amount of time for the devices to turn on and to turn off. If one is not quite off while the other is turning on, you get something called 'shoot though current' as the devices are essentially a short across the power supply without doing anything to drive the speaker. Shoot through current causes the output devices to heat up quite a lot. So it has to be avoided!

To solve this, there is something called 'dead time'. Its a circuit that makes the output devices wait just a little so that one is completely off before the other starts conducting.

The longer the dead time the higher the distortion. So if you can switch faster dead time is reduced and so is distortion. Right now the technology is to the point where the amps have become practical for mid fi, but they still have a ways to go before they can challenge a good class A transistor amp or a good tube amp. There is no question that this is a technology to be watched.
" Right now the technology is to the point where the amps have become practical for mid fi, but they still have a ways to go before they can challenge a good class A transistor amp or a good tube amp."

Mid-fi?  That's actually a funny statement!

As for challenging others, people need to listen and decide.   All amps have technical issues and challenges.  Citing these and comparing does not tell you anything about which sounds best.
I've owned several Class D amps and just don't think they have the musicality of very good Class A and AB amps.

That’s certainly a fair assessment. Everyone has their own personal preferences.

Mid-fi is not a fair assessment though IMHO. No amp is perfect. In that sense they are all "mid-fi" maybe. But I tend to think not. Each can only shine best when set up well and properly matched to speakers, etc. They are all the same in that way.

I've owned several Class D amps and just don't think they have the musicality of very good Class A and AB amps.
Same experience here.
I have two Class D amps. Bel Canto ref1000m monoblocs in my main system (with ARC tube pre-amplifier) and true digital Bel Canto c5i integrated in my other smaller setup.

They each sound way different. Both are "musical" but not in the same way. You should hear the c5i with headphones. It is quite spectacular. Plus it does all the rest extremely well to boot!

Mid--fi? I think not. I’ve heard a lot of mid and hi fi over the years. Both play in the bigger league for sure.  Kinda like the Cubs and Indians.  Maybe even the Yankees and Cardinals!
Also keep in mind if one is keeping score, Class D amps have the advantages of small size, efficiency and ability to drive most any modern speaker well, including smaller more extended models that are very popular these days.

Other SS amps can do it but are larger heavier and probably more expensive especially when power consumption per watt delivered is considered.

Class D amps are very "green".  Class A amps are "gas guzzlers".   That part is undeniable.

Tube amps generally require certain speakers that are an easier load and more efficient to shine. Otherwise we can be talking mid-fi results there as well. I’ve heard that on many occasions where speaker choice for tube amp is poor for various reasons.

So the only way its really a fair competition is to discount the sound quality of Class D, which anyone can do.

Then there is the fact that the Class D amps continue to improve as Glofi mentioned.

So Class D is definitely a player but the game is different from tube amps in that the speakers one ends up with for teh best hifi results will be much different.
Mapman, you said earlier in the thread that the high dampening factor of Class D makes them a better fit for larger speakers that like high dampening...with your smaller speakers sounding towards the cold and dry side with Class D. So would it be safe to say that Class D should be carefully matched to speakers that like high dampening?    
sean I think that is a particularly strong application for them that may be hard to match as cost effectively elsewhere.

I think they are also a fairly safe bet to be able to technically drive most any speaker well up to a certain volume depending on specific amp used. Meaning the ability to get teh most possible out of most any particualr speaker.

That’s not to say I would even think they always will sound best.

For example, I anticipate my Triangle Titus monitors, which are small but known to be very tube amp friendly, would sound better (within their limits) with a good SET OTL or push pull tube amp. I plan to try taht someday soon and see.

My Bel Canto amps do a great job of driving the Titus on paper technically, and still sound quite good together, but the sound is somewhat lean in comparison to the others. I have found that corner placement with teh bass reinforcement that offers does help a lot.

Also I would say that in general the best Class D amp demos I have heard over the years also tend to be with bigger and better more full range speakers that have no problem delivering flat bass to the lowest octaves when driven well.   That's one of the hardest things for any amp/speaker combo to do.  Just like Class D amps excel for use in Subwoofers, they also excel for use with speakers that require no separate powered sub to sound great when driven well.  
Thanks mapman. That is consistent with the new KEF LS50 active monitor. It uses Class D for the bass and Class AB for the highs. 
Non-linear & lots of distortion that needs to be cleaned up with an output filter.
I don't understand what you mean by that.  Class D is extremely linear.  Class D modulator, converting voltage to duty cycle, resembles Delta-Sigma D/A converters (principle of operation is the same) that are extremely linear.  Output filter, Zobel network,  is only used to obtain average value and reduce amount of noise on the speaker wires (that is inaudible).  Like with any amp - don't guess and just listen.  I like sound of my small class D Rowland amp.  I like imediacy, transients, liquid midrange, low noise floor and composure under loud passages (regulated power supply).  The only problem I had initially was lower midrange.  It was a little thin sounding.  I replaced speaker cables with Acoustic Zen Satoris and lower midrange came back.  Cello sounds like cello and I can hear "chestiness" in male voices.  Perhaps the question is not whether class D is worse or better than class A (Jeff Rowland makes some wonderfull class D amps), but rather if it is better for you, for the money you can spend.  Switching technology is very promising - just look at SACD - pretty much the same as class D.  I agree with George that switching frequency should be increased, since 50-60kHz bandwidth limitation introduces around 20deg phase shift at 20kHz (causing wrong harmonics summing), but I'm not even sure I can hear such nuances.  There are many highly praised amps that have similar bandwidth, I'm sure.  
georgelofi you are correct about switching frequency being teh key to even better Class D amps in the future.

Have you listened to any good quality class D amps or are you judging solely on technical interpretation?

Yes the new Belcanto 600 monoblocks, they were the best of the Class D's I've heard so far, but still didn't convince me to go to the dark side yet.

In the future when the switching frequency is up around 3-5mhz then the filtering can do it's job properly without it's effects influencing the audio band.
The Belcanto 600's did a series of output filters to counter this, but it has it's own set of problems.

The Technics are up to 1.5mhz but you pay for this $$$$$

Cheers George

I agree with George that switching frequency should be increased, since 50-60kHz bandwidth limitation introduces around 20deg phase shift at 20kHz (causing wrong harmonics summing), but I’m not even sure I can hear such nuances. There are many highly praised amps that have similar bandwidth, I’m sure.

Switching frequencies are up around 600-800khz at the moment, except for that Technics above.
The problem is the switching noise output filter has to handle the full power of the amp and can only be low order, otherwise it will burn out. And being low order it has an effect down into the audio band. If the switching frequency is 3-5MHz then it can do it’s job well away from the audio band without having phase, fr, ect limitations effects .

Just look at the ringing artifacts in Stereophiles 1kzH square waves of Class D, they now test with a inline heavy filter to make this (ringing) disappear, that Audio Precission’s filter they use makes the test wave look clean, but it’s a con job as it’s done at very low power so it doesn’t burn out, in real use it would last a microsecond.
I have asked JA to include both unfiltered (normal) and filtered square waves, just so readers can see the real deal.
Read comments at the bottom of the page, no answer from him.

Cheers George
I don’t buy the "gas guzzler" A/B vs. "green" class D arguments for the latter. Kilowatt hours are cheap and relatively clean until you pile up a WHOLE lot of ’em. With typical audiophile usage patterns, and the fact that we’re a VERY small segment of the population, there’s no significant ecological impact here.

The extreme case would be a 100+ Watts pure Class A amp user, who leaves amps on 24/7, switching to a class D amp. Then yes, you’re going from a $1,000+ yearly power bill to like 100 bucks. But that’s an outlier. With more sane usage patterns -- e.g. a class A/B tube amp that you turn on/off as needed -- the difference is close to negligible. And as far as ecological impact, class D probably makes things worse overall, due to the fact that audiophiles ship ’em over, decide they don’t like the sound, and then ship ’em right off again.
If you want to be eco friendly, and be a little compromised at the moment, then sure get a ClassD.
Or pay the bucks and get "maybe the new Technics SE-R1 Class D".
Cheers George
I have two integrateds, an Audio Research VSi55 running KT120s and a Peachtree Grand X-1 (Class D).  Through both pairs of speakers I own, Wilson Benesch Arc and BMC PureVox (both retail for $6500, whether these qualify as low-fi, mid-fi or otherwise is a matter of perspective and budget), I'd choose the Peachtree every day.     It sounds every bit as nice as the Audio Research, but runs cool, keeps the electric bill low, and takes up very little space while putting out huge power.  I guess admittedly there are a few cold days each winter where it's nice to use the Audio Research to both play music and heat the room, but otherwise it collects dust.   So I guess in my case, the future arrived 3 years ago when I bought the Peachtree. 

All that stuff about switching frequencies and flux capacitors and quantum math, I don't know anything about that.   I just press the power button, hit play, and sweet music comes out.  For all I know there's a team of hamsters inside the Peachtree playing little tiny violins.  I leave all that stuff to you engineers...if it sounds good, that's all I need to know. 
George, I looked at the link you provided.  Yes they use filters for the purpose of measurements to eliminate residue of switching frequency that is otherwise completely benign. This small residue (about 1%) has no effect on the sound.  Speaker membrane will react (move) to its average value only.

I don’t buy the "gas guzzler" A/B vs. "green" class D arguments for the latter. Kilowatt hours are cheap and relatively clean until you pile up a WHOLE lot of ’em. With typical audiophile usage patterns, and the fact that we’re a VERY small segment of the population, there’s no significant ecological impact here. 
According to this pouring used car engine oil to kitchen sink is OK (does not have environmental impact) since very few people do it.

And as far as ecological impact, class D probably makes things worse overall, due to the fact that audiophiles ship ’em over, decide they don’t like the sound, and then ship ’em right off again
That's funny.
This small residue (about 1%) has no effect on the sound
Put your ear to a wide bandwidth tweeter, you will hear it’s poluting artifacts. If not the filter effects is set very low to cut way down to 10khz or even less, creating all sorts of other problems.

Cheers George
Let me add that while I do concede that my engineering knowledge falls short of what many of you possess, there's still a lot of bat guano peddled on these forums.   I like it here, and there are some very smart, very cool, very interesting people that I've learned a lot from, but oftentimes this forum is to audio knowledge what Fox News is to global news coverage.  Sometimes after I visit I have to remind myself that I didn't just spend 10 minutes on Shutter Island.
Here is just one of many that Stereophile tested, not using their test filter. It’s more than 1%.
And this Class D had a linear power supply as well, most use smd now which creates even more other noises as well.

Cheers George

Just a couple of quotes from other Stereophile measurements personel.

I dislike measuring amplifiers with a class-D output stage. This is because such designs can throw out enough ultrasonic noise that you can never be quite sure that what you’re actually measuring is the input stage of your analyzer being driven into slew-rate limiting. What I do, therefore, is to insert, when appropriate, a sixth-order low-pass filter set to 20kHz or 30kHz between the output of the amplifier and the analyzer.
But you can see from this graph how the amplifier’s ultrasonic output rolls off rapidly, presumably due to the low-pass filter necessary to minimize the HF noise produced by the switching output stage
Cheers George
Great thread. I'll add that Class D amps are extremely sensitive to power source and power supply. To hear the best the technology can offer, you would have to pair it carefully with a matching quality power cord and possibly a power conditioner. This took my NCores to a whole new level (refinement, bass, micro detail, etc) and brought out the musicality that had been lacking. 
Ok George we get it.  You aren't sold on class d yet.  Others are. Mine sound great and no faults I could point to when listening.  They just make sweet music as said. 
I am going to chime in here in favor of current class D amps, with a big thumbs up! I have been in the tube amp camp (both SET and others) for 20+ years, and now I am happy as can be with my Red Dragon S500 amp. No, it's not like a tube amp, but it's not SS sounding either.  It does so many things right, and hardly anything wrong, that I can easily live with it. All topologies have their pros and cons.  Throw a tubed preamp in front of a Class D, and there you go! 

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George , I did put my ear to the tweeter and I can hear almost complete silence - much better than class AB amp I had before.  I'm not surprised since tweeter membrane has no chance to move at 500kHz.

Modern SMPS are much quieter than linear power supplies that, in reality, are very primitive noisy switchers operating at 120Hz.  Modern SMPS switches at zero voltage/zero current while much ripple is much easier to filter out than 120Hz, that requires huge amount of capacitors (that produce unwanted inductance that is in series with the speakers).  In spite of all capacitors linear power supply in power amps is unregulated.  Because of that Jeff Rowland does not use linear power supplies at all.  He uses SMPS even in preamps to lower noise floor.  Benchmark replaced linear power supply in their latest DAC with SMPS lowering noise floor by 10dB.

Mapman is right - you're not sold on class d yet.  I enjoy my class D amp immensely.  Efficiency is not that important to me (but it should), but is always welcomed as added benefit.

You aren’t sold on class d yet.
Correct, and Technics are showing the way with the SE-R1, why strive for higher switching frequencies by deleloping their own components, if what we have off the shelf is "good enough". Technics knows and are doing something about it, they can see the forest through the trees.

And like I said when they do get the technology to make it much higher I will be the first to change over, till then I’ll stick with linear for hiend amplification.

Cheers George
Nuforce/Nuprime appoaches Class D differently and is most appropriately called an analogue switching amplifier. I have tried many class D and the Nuforce/Nuprime designs beat them all plus all Class A and AB that I have listened to. They are the most musical in my opinion and that is with a rig with 60K speakers and other fine equipment - give an audition. MY Nuforce Reference 20's are liquid, fast, controlled and natural in the highs
I'll stick with tubes for now, but the Mola Mola/ Vivid G3 Giya system I heard definitely wasn't midfi.
George not much buzz on the new technics gear yet. Maybe to come. I was in their room at Capital Audifest but they kept talking about the gear and did not play it while I was there. I wanted to hear and was disappointed. Nobody had it in their best of show list that I’ve seen.

No doubt higher switching frequencies done well is better. How much better practically in regards to what can be heard is TBD. I will say that each newer generation of Class D gear does seem to only improve so not at the end of the road yet.

I try to hear these high frequency artifacts in my Class D amps that I’m supposed to hear and have not heard it yet. So if it exists it is not blatant. At least to these 57 year old ears.

Again there is often more noise at the highest frequencies human ears can hear than music. So perhaps its an error of omission mostly to the extent present at frequencies that really do not matter much for listening to music.

Take a look at the audio frequency chart and you’ll see what I’m talking about in regards to at what frequencies music mostly all occurs.

Mapman, high frequencies are different for sure, but not for the worse.  Cymbals, for instance, sound more "brassy" and less "splashy".  To me it sounds more natural, but it might create false impression of limited extension.
Cymbals, for instance, sound more "brassy" and less "splashy".  To me it sounds more natural, but it might create false impression of limited extension.
If the switching noise output filter is set at a higher cutoff so to make the amp look to be flat to 20khz on test this will sound hard.
If it's set lower to try to rid most of the switching noise, this will already be rolling of at 10hkz and therefore rid any cymbal harmonics, this will sound more natural but not very extended.
So your compromised either way, the only solution is a much higher switching noise so it can be cut out totally with the output filter without effecting the audio band at all.

Cheers George  
I'll cast a vote with mapman re a D with a Walsh driver.  Although it's just a 'dinky' driving two pairs of my smaller DIY versions, it's a good fit.  As long as I don't get exuberant and crank it to clip, it's amazing to me how well it 'fits' them.

They'll improve as time goes on.  Pretty soon the electronics will shrink to near invisibility and we'll be left with just the interfaces and the radiators.  I'm working on the latter, myself. *S*
Again, switching noise is inaudible, unless anybody can hear 500kHz. Even bats won't hear it.  I'm not sure why is it so difficult to understand.   I addition tweeter's membrane won't move at all.  Personally, I have problem hearing 20kHz.  As for the bandwidth -  in my amplifier it is set to 60kHz -3dB.  Different sound, more tube like, of class D amp can be explained by low TIM distortion, often responsible for the sharp splashy sound (added higher order odd harmonics).  Class D is in fact so clean sounding that sibilants, for the first time, still strong are very clean and wonderful sounding on every record.

Somehow people cannot comprehend that class D amp is purely analog and switching itself is inaudible.  Switching is present all the time - even without input signal.  Place ear against the tweeter and you will hear very low typical amp's hiss - likely much less than with any class AB amp.
From the sitting position nobody will be even able to detect that amp is ON.