Class D Technology

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.

Tube amps generally require certain speakers that are an easier load and more efficient to shine.

This is true of all amps including class D. You can see it in the specs. If high end audio reproduction is your goal, there is no argument for a speaker that's hard to drive. The last thing you want to do with **any** amplifier is make it work hard!

Ignore the pronouncements of any manufacturer who is committed to any particular topology -- Hello Atma, how doing?

Fine, thank-you. A drunk ran a stop sign and T-boned my car, totaling it, back in July. I had a lower back injury that was really painful (bulging disk into my sciatic nerve); its only now healing to the point that I can dress and move in a normal fashion.

In case you were not seriously asking about my health, it might interest you to know that we've been studying and working on class D for some years now. So you might want to re-think your notion that we are 'committed to a particular topology'. We practice pragmatism, not ideology.

Its not 'dead time'! I would not expect the filter to have anything to do with it either.

Today it's become a surefire tell of sad ignorance.

This statement is false.

Just because you heard a class D amp sound better than a tube amp in a particular situation does not mean that all class D amps sound better than all tube amps, or even that the particular class D amp sounds better than all tube amps. It just means that in that situation you heard something better that you liked between the two involved.

Heck, even in our lineup we've managed to make improvements in the last several years. I encounter people that think that just because they heard our amps 12 years ago on a certain speaker that they have a complete understanding of how our amps perform today on different speakers (or even the same speaker they heard 12 years ago....). They don't.

It just doesn't work that way!!! I regard this comment (taken out of context of course so simply as quoted) as being outrageously inaccurate.

Atmasphere why does 200khz bandwidth matter if there are no sources practically (record or CD res digital) that can even come close to delivering it today?

I did explain this in my post but here it is again:

In order to reproduce phase correctly bandwidth is a requirement. Put another way, if the bandwidth is not there phase shift is the result. You are correct that digital doesn't have bandwidth and that is one of the reasons that the LP sounds better and is still around after all these years of being 'obsolete'. Nevertheless, to prevent phase shift in the amp you need bandwidth even it its not there in the recording. In fact the recording doesn't matter.

To put this another way: phase shift components can be heard to 1/10th of the cutoff frequency. So if you are cutting off at 20KHz there will be phase shift to 2 Khz. 100KHz cutoff means that phase shift will exist down to 10KHz. We older adults don't hear so well at 10KHz so you can sort of get by with 100KHz bandwidth but if you really want to do it right you better have 200KHz so your kids will like the stereo (unless you're the type that just wants them off your lawn...).

What prominence does the heft of the power supply play in the sonic performance of Class D amps relative to SS or tube amps?

The supply should be clean else IMD components related to the noise in the supply and the scan frequency will show up in the output. IMD is pretty audible to the human ear (shows up as brightness and grit or 'grundge'). Since the scan frequency is pretty high the power supply had better be properly bypassed to be effective at the scan frequency! But it can't have any sawtooth going on either. Otherwise the supply has to be able to support the operation of the amp at full power, even though most of the time the supply will see a fraction of the current draw that a traditional amplifier supply would see, so most of the time the power draw from the wall is minimal.

Traditional power supply design or SMPS is irrelevant so long as the supply is quiet (SMPSs have the advantage of being easier to regulate...).

but nobody talks about phase shifts.

I do and all the time. The ear can't hear phase shift in single notes, but over a spectrum it can hear phase shift in a variety of ways- as a tonality or as a change in the soundstage. The engineering rule of thumb to prevent phase shift is to have bandwidth exceeding the highest frequency to be amplified by about 10 times.

Now that is generally interpreted to be a bandwidth of 200KHz (but might be higher- for example nearly all LPs have bandwidth well past 30KHz in record and in playback; usually this is limited by the source tape or file or the microphones themselves...). However you need the bandwidth in the playback electronics even if the source lacks the bandwidth if you expect to reproduce all the phases correctly! We get that out of our amplifiers but to do that with class D is still a bit of a trick. We are barely getting switching speeds much past 250KHz which means true 200KHz bandwidth isn't there yet.

Now the AES says that if you can get about 2 octaves above the limit of human hearing (80KHz IOW) that you are doing OK for a monitor amplifier. Audiophiles routinely operate gear that has performance in excess of that. And one of the things I don't like about many of the class D amps I've heard has been the lack of speed and spaciousness on the top end. Like mapman, there's no way my hearing is as good now as when I was in my 20s but I still notice this stuff nevertheless.

So if you're going to get that 200KHz response you need a switching speed of around 1 MHz and even then its going to be a bit spotty (2Mhz would be much better). There's no problem getting the converters to run that high. A prototype we're building here in the shop can do conversion at 3MHz easily no worries (the chips we're using have a bandwidth product of about 50MHz). The real problem is the switching speed of the output devices (and whatever junk occurs between the output of the converter and the inputs of the output devices...). If one were to spend the extra cash to get the really high speed stuff then one of the primary advantages of class D goes away real fast: low cost/high profit margin. Really fast, powerful switching transistors aren't cheap and you don't see them in 99% of all class D amps! So to compete against traditional solid state and tube amplifiers in a nutshell Class D has to improve bandwidth, and you don't have to be a bat to appreciate the difference.

This is one reason why class D is still a rising star. Its also why some class D amps are quite diss-worthy and others are not too bad. 

Assuming nothing is there to start with isn't a phase shift of 0 = 0?

Nope! The point is that if you don't have the bandwidth in the amp it will cause phase shift at lower frequencies- down to about 1/10th the cutoff frequency is then its considered negligible (of course, 'negligible' by whom is a different story; obviously some designers don't care about that so much). IOW the ear might hear 20-20KHz but to reproduce that in an amplifier without phase shift you need 2Hz to 200KHz if you really want to do it right (the same rule applies on the bottom end; otherwise the bass loses impact due to phase shift). For this reason Stuart Hegman, who designed the h/k Citation 1 and Citation 2 was a big fan of wide bandwidth. 

I did not know vinyl has frequency bandwidth so high.   Most home hifi always talked of 20-20K frequency response.     Is this in practice or theoretical?   I'd agree theoretically vinyl could do more but practically its news to me.   The best digital (higher res) sounds as good as most vinyl to me these days.   RTR is better but look where that got us practically.

Its not just theoretical.

Its true that most LP systems don't spec past 20KHz but you would be very much mistaken if you think it stops there. Most phono cartridges and phono sections made since the mid 1960s go much higher than that (although it does not show in the specs as at the time there was no thought that it made any difference). Our cutter head is an early Westerex 3D, made about 1959 and it has no worries cutting 30KHz which can then be played back by a 1970s Technics on a 1970s h/k receiver (the Westerex mastering system has a filter that cuts it off at about 42KHz). By contrast RTR does not have this sort of bandwidth; about 25KHz or so is the best you're going to get at 15 i.p.s. IOW LPs have wider bandwidth than tape and its been that way since the inception of reel to reel. 

Every time a new system has come into the scene the previous knowledge of the prior engineering often goes by the wayside for a while until the new technology gets its pants on. This happened with digital; its only been recently that its begun pressing bandwidth past 20KHz and we're seeing the same thing with class D right now.  In time this will all get sorted out as the technology improves to the point that such bandwidths are routine; until then essentially what you will see is the industry collectively placing its head in the sand as if these facts don't exist. But this is not rocket science and we've known that bandwidth is essential going on half a century now. But people have short memories when new technology comes in... Look how long people put up with unruly fuel injection while carburetion was pretty figured out and actually performed better. But fuel injection was 'new' so people put up with it. Now days its sorted and no-one would consider a carburetor. But literally it took 3 decades to get there!

BTW has anyone ever noticed a reviewer complain about phase shift when reviewing gear at a show? They can find all kinds of faults but seldom if ever that. Are they not listening right or is it just not there? Gotta wonder.....

You see them writing about it all the time. Don't look for them to use the expression 'phase shift' though. Look for them to talk about the soundstage, how deep and expansive it is. That's one indication (but not the only one).

The thing is, you can build an amplifier that only goes to 20KHz or so that can still sound pretty good (lots of SETs only go that high). The effects of phase shift are more subtle but they do exist. So you might be happy with what you have (which is good) but ultimately its all in comparison to what, and if you've not heard that 'what' then you just don't know how much better it can get. That's why many people listen to boom boxes. They just don't get how much better it can be.

So based on a combo of facts and experience I still see it as one of theoretical things that exist but do not add up to much if anything in practice for most.

Are you going to be producing an amplifier anytime soon?

If an amp cuts off at 20KHz then expect phase shift artifacts down to 2KHz. If an amp cuts off at 60KHz then its 6KHz. If the cutoff has a steep slope the artifacts can be more severe. The trick, if you can't get bandwidth the way you want, is to not have a severe rolloff.

You'd think this stuff is inaudible but it isn't. Many factors add up to how a given amp sounds and its plain foolish to attempt to focus on any one factor by holding it above others **or** below. They **all** affect the result.

So you are right in a sense. But I'll give you an example of how phase shift can manifest. Years ago a dealer brought an MFA Magus preamp to me with the complaint that it was really bright in the phono section. In fact I had heard this problem in the preamp myself. I put it on the bench and  found that in the RIAA equalization there was a circuit that caused the equalization to go to flat at 50KHz. This is a ways above human hearing. Normally the RIAA curve would be rolling off at 6db per octave.

So I took the circuit out, thus restoring the RIAA curve (FWIW the RIAA  did not spec the curve past 20KHz but it usually designers assume that it will continue with its 6db per octave rolloff as frequency goes up). The **very audible** brightness was eliminated! The dealer was thrilled, and MFA changed their production so that this circuit was omitted. Apparently they didn't like the brightness either, but had not made the phase shift connection. They said it was there to "improve square wave response". I'm sure it did that!

If your amp does not have lot of bandwidth then you are in the same boat as a lot of SET owners and they seem to like their amps just fine. The fact is that its not the most important thing in the world. But don't confuse that with it not being audible at all! In a class D amp, most of the artifact that its going to have is coming from the input circuit and in that regard whatever amplification it has (probably an opamp) which imposes its own signature. That's a lot better than the signature that many transistor amps impose; opamps if treated right can be pretty musical. If the bandwidth is limited then the amp might not seem very bright and a lot of audiophiles (myself amongst them) cringe when things get too bright. You like your amp a lot so if your hearing isn't too off than I have to assume that the designer chose a reasonable set of compromises in the design. I guarantee though that the designer would prefer to work with greater bandwidth if possible. When they come out with the latest greatest replacement for your amp (which will happen sooner or later), take a look at the bandwidth spec and see if they didn't improve on it. 

^^ Or not. You can't put them all in one basket. That's why this thread is as long as it is.

Class D amps vary in sound due to the fact that some have higher scan frequencies than others, some have more 'dead time' than others, some have more distortion in the input circuit than others, some have cheesy power supplies and some don't, some have feedback and others don't!

Just my opinion, but all you guys out there claiming audible switching noise are a little misguided.

Speak for yourself Kimosabe.

You probably missed the part where I mentioned we have a class D amp in the works, right?

1. They are extremely quiet, having THE HIGHEST S/N Ratio and THE WIDEST DYNAMIC RANGE of any amp type, with almost no audible background noise and highly detailed.  I can turn my volume control to maximum without any source playing, put my ear an inch from the tweeter panel of either speaker, and there is absolutely no sound. Is this possible with a class A amp?

Yes, and with tube amps too.

The reason? Your volume control setting is saying something about your preamp and sources but nothing about the amp. We make tube amps that have to be quiet on speakers that are 107 db... try putting your amps and preamp on a set of speakers like that and see how much noise you have then. Low noise is not a weakness nor advantage of class D amps.

Dynamic range isn't a particular strength or weakness either- it is the same as any good amp.

The real strengths of Class D are low cost, high efficiency (about 85%) and because of the latter, smaller size as large heatsinks are not as important. Class D was first proposed in the late 1940s and can be done with tubes. Its a fairly simple technology which is why its also inexpensive to build. **Those** are the strengths; what an individual designer brings to the table and how well its executed is going to have a big effect on how the amp sounds and performs. There is nothing inherent in the technology otherwise that make it inherently better. The reason its showing up in spades is the semiconductor industry is really wanting to make switching transistors right now instead of linear transistors, so audio manufacturers hare having a much harder time getting linear devices as easily as they did in the past and can read the writing on the wall.

If you want to demystify what class D is all about, I recommend reading this article, which really demonstrates how simple class D really is:
 https://hackaday.io/project/4369-class-d-subwoofer-amplifier-out-of-scrap

Yet you, Mr. Designer, dismiss Class D as nothing more than midfi.......

Apparently you've not read this thread or you would not have said that; IOW this statement is false.

Lower cost for manufactures, which seems to result in higher margins instead of reduced retail prices. Manufactures have to of course be careful of cannibalization.

This is true- and is a time-honored technique in the world of audio going back a good 60-70 years. First I can recall is when they went from electro-magnet speakers to permanent magnets. Permanent magnet speakers are cheaper to make but we didn't see the price go down way back then. After that it was the transition from high efficiency loudspeakers to lower efficiency types as transistors came in; lower efficiency speakers are cheaper to make but we didn't see the price go down all that much. Then there was the transition from tubes to solid state. Solid state amps are a lot cheaper to build- no filament circuit and no output transformer but the price really didn't go down.  Then we went from LP to CD and CDs are lot cheaper to build but we didn't see the price go down (instead we saw the price of LPs go up after the record industry sorted out that there was still a demand...). Now we're seeing class D supplant traditional solid state in many ways but for some reason they aren't particularly cheaper even though they are cheaper to make.  

Yes, I have read this thread in its entirety. You could not have made it anymore clear. Go back to the beginning of this thread and read your own post. Not a false statement.

Obviously I could have been more clear or this conversation would not be happening!

Al’s got it right. Note where I say ’good’ twice in the statement Al is quoting.

As I have mentioned before, we’ve been playing with Class D for about the last 15 years. While I think they have gotten much better (15 years ago they were a joke) I’ve yet to hear one that keeps up with a ***good*** amplifier of conventional design (note emphasis; FWIW I’m used to listening to some pretty ’good’ amplifiers...). High end audio is pretty variable and I know of amps that are considered high end that I personally don’t think are all that good. I know of one such amp that has balanced inputs yet its CMRR (Common Mode Rejection Ratio) is about 18 which is pretty terrible if not outright criminal.

So I can see the interpretation issue. Most class D amps have specs and sound that are similar to many midfi amps (take a look at a Kenwood car stereo’s specs and compare to many ’high end’ amps and you will see that there’s not a lot of difference). Some don’t; they’re better. And they will continue to improve.

For example the scan frequency is continuing to rise. George makes a pretty big point about this. You can’t scan at 100KHz and get good high end resolution. 250KHz is in fact marginal if you really want to do it right.

As I mentioned before, you can get high power high speed output devices that switch at the speeds needed, but they cost so much right now that the resulting amplifier would be a lot more expensive than a tube amp of the same power would be (tubes are usually the most expensive form of power). So as a result no-one uses devices like that. In fact many designers simply use chip sets so they’re locked into the performance of whatever the semiconductor house provides (who rarely have audiophile interest at heart).

Whatever Ralph meant the mid-fi comment did make me laugh knowing that the sound I hear from practical Class D amps today (like the ones I own which are neither the least or most expensive) is not mid-fi by any reasonable listening standard.

@mapman  Do you think that because we've taken this technology very seriously that at the same time we don't?

If I can point out some things without people trying to put words in my mouth, first- tube amps are rarely mid-fi. The term 'Mid-fi' refers to equipment generally sold in box stores like Best Buy (not including their in-store Magnolia dealerships); stamped out to make money. Tube amps are usually too expensive for that (tube power is generally expensive, which is why there were many high efficiency speakers on the market when tubes were the only game in town decades ago).

The thing about most solid state is that the specs really haven't changed a whole lot since the late 70s or early 80s. Like I said, compare a Kenwood amplifier a Krell and you will see what I mean. The big difference is power (on paper). Bandwidth and distortion are almost the same. Of course a Krell is built better, has bigger heatsinks and bigger power supplies. And its not sold in box stores.

Class D has almost entirely taken over at the bottom end of the audio market. And the middle part too. You can't buy a boom box now days that isn't class D. For the number of class D amps in total on the market, only a tiny minority (the ones that generally have our attention) are considered high end. Many of them use the same modules found in mid fi gear.

Think about that for a moment before railing on me? I'm only working with the facts here so don't shoot the messenger. If you look at a high end tube amp or a traditional solid state amp, **NONE** of them use parts/circuitry/assemblies also found in mid fi gear.  This is something different about class D and has entirely to do with how inexpensive the technology actually is.

(To give you an idea of that, I have a module on my desk that was sent to me by a customer (who happens to be the vice president of Phillips Semiconductor). This was one of their best modules built about 12-13 years ago. It makes 100 watts per channel and is a stereo module that is complete except for the power supply. It even has connectors on the board for easy hookup. The specs are quite good (Phillips TDA8920B, scans at 317KHz!; not bad for decade-old technology; faster than some amps made today and features no 'dead time') and it sounds alright too. In quantities of 1 it cost $25.00. I know of at least one 'high end' amp that uses this module that cost $2500.00. This module is the size of a pack of cigarettes complete with connectors and heatsink.)

So when I talk about mid fi and class D, you **should** know why now. I think a lot of posters here are attaching a negative meaning to the 'mid fi' term that does not exist; Just because its mid fi does not mean it has to sound bad and I can point to plenty of examples (we could start with the Radio Shack Lineaum speakers). What I'm seeing in class D is something I've not seen in audio before- amplifiers that use the same parts as a mid fi amp as in a high end amp. In fact its safe to say that class D has lifted mid fi performance to what many would have called 'high end' only a decade earlier.

So you might like your amp and I'm not disputing that! OK, I'll say it again, you might like your amp and I'm not disputing that. But if you think that the amp does not share a lot with mid fi gear you are ***really*** mistaken.

There is one other thing that keeps getting left out of this discussion. Its a simple fact that no matter what the amp is, if you make it work hard for a living its not going to sound as good because its distortion will be higher. This is easy to hear and easy to measure!! You can see it in the specs of any amp. Tube amps in general are best experienced when driving higher impedances 8 ohms or more. All output transformers are more efficient driving higher impedances and also will have wider bandwidth (its not unusual for an output transformer to loose an octave of low frequency bandwidth going from the 4 ohm tap as opposed to the 8 ohm tap). For this simple reason, Magnaplanars and Ohm Walsh loudspeakers are not the best candidates to really tell how good a tube amp actually is (the tube amp will sound better on a load that's easier to drive, and so will any solid state amp). My speakers at home and here at the shop are both 16 ohms and very different designs; both extremely transparent. When both amps are on a level playing field their differences are more easily assessed.

At any rate, if you are happy with what you hear than that's really what's important. If you have audiophile nervosa, which is a deadly disease (I've had it for decades and its no picnic) you are always left wondering what's around the next bend, how you can arrange things to get better sound. I've outlined already how class D has to improve (FWIW, some of the complaints I have apply to regular amps as well), which we hope to address in our design.

   I'm no scientist but I understand the difference between theoretical noise and actual verifiable noise and that, empirically, only the actual noise can actually be heard. by us humans.  Noise needs to be heard to exist.  

Class D amps are nearly immune to any noise they (or their SMPS) might make. The switching occurs at the output of the amp to which speaker cables are attached, which might make a suitable antenna.

Its the other gear in the system that is really the concern (not to mention pacemakers and the like in the area)- they might pick up the radiated RF noise and might react poorly to it.

However, neither of these are really issues with any competently designed amp or power supply, and while I get the concern, its mostly a red herring. You would never be able to ship an amplifier (or power supply) out of the country if it made any appreciable noise!

So I don't think this is anything to be worried about.

Speaker cables would be a very poor antenna.  Since wavelength of 500kHz switching frequency is 600m a typical 2m cables would be 1/300 wavelength antenna.  Usually antenna becomes rapidly ineffective below 1/10 of the wavelength.

That's true, but in order to meet EU directives, UL and a host of others, the speaker cables are effective enough that the issue has to be addressed by any classD amp producer. In fact it is speaker cables that are mentioned when the issue of HF radiation is brought up in the design papers.

However, its so easy to address that I regard it as a non-issue. As I do the switching noise of SMPSs. The only reason we don't use the latter in our OTLs is the cost of custom SMPSs- its a lot cheaper to use traditional supplies!

There is about 1% of switching voltage noise on the speaker cable since filter is not perfect but, as I said before, 1/300 wavelength antenna won't radiate.  In addition, electricity in speaker cable flows in both directions canceling the most of RF (if any).  When you place radio directly on the wire you might get some capacitive coupling, but I doubt it.  Many speaker cables are twisted making it completely inefective (better than shield) for electromagnetic radiation or capacitive coupling (both ways - from cable or to cable).

You may find this of interest:

https://www.maximintegrated.com/en/app-notes/index.mvp/id/3977
scroll down to:
Minimizing EMI with Spread-Spectrum Modulation

The snag class D still deals with is the control loop (feedback).

Its easy enough to build a class D amp that does not use loop feedback.

call me stupid but what do you mean by a "Output Filter" ? A L/C for you amp? Thanks, I am thinking about a Bel Canto but have never heard them.

Class D amps usually have an L/C filter at their outputs to prevent radiation of their switching frequency.

In learning about Class D, it appears that after the linear signal is destroyed and turned into a saw tooth wave form, the output filter? tries to put the signal back together...doing an incredible job considering the complexity of this task...but ultimately is unable to restore micro details to recreate the original signal...which people such as myself notice very quickly, while others either don’t notice or enjoy the new coloration.

The opening statement here is false. Here's how class D works:

There are essentially 3 building blocks:
1) the triangle wave generator (sets the 'scan' frequency)
2) A comparitor, the compares the incoming audio signal to the triangle wave
3) the output section. This is the power transistors that switch on and off, and any circuitry needed to drive them

The sawtooth or triangle wave generator is running all the time. The comparitor is too- taking the audio signal and turning either on or off depending on the state of the triangle wave and the audio signal at any given time- the output is a series of pulses of varying width, which are used to switch the output transistors either on or off. This technique is called 'Sigma Delta' and is a means creating the output pulses of varying width so is also known as 'pulse width modulation'

The scan frequency is a function of the triangle wave generator. Some ICs can't make a good triangle wave at higher frequencies so quite often this can be the reason the scan frequency is limited. The other reason might be the output transistor's ability to switch (these days its pretty easy to find inexpensive parts that can switch at well over 1 or 2 MHz...).

There is usually a filter at the output of the amp to filter out the scan frequency. Once that is filtered out, all that is left is a much higher current version of the input signal.

The scan must be kept constant to maintain fidelity. The output devices can sometimes stay on longer than they are supposed to (typically they take longer to turn off than to turn on) so they can both be conducting at the same time. If this happens, you get a phenomenal called 'shoot-through current'  which can heat up the outputs really fast, so sometimes additional circuitry is used to make one transistor wait until the other is off before it turns on. This wait time is called 'dead time'. The longer the dead time the higher the distortion. Dead time is usually needed at higher switching frequencies, so you can see that the need to go to higher frequencies to reduce distortion (and increase resolution) is hampered by the fact that more dead time might be required, which increases distortion. So you can see that the designer has to weigh options!

This is it in a nutshell. Some things are glossed over and others omitted entirely, but if you know what is written here then you have a pretty good idea of how they work.

I am not sure where obsession with the switching frequency comes from.  It is inaudible

This statement needs clarification! It is very easy to demonstrate that switching frequency is audible. Once it gets above a certain minimum it becomes progressively harder however.

Can someone tell me what can be observed when measuring a class a/b amp that indicates its inferiority to pure class A?

In a class A amp as mentioned the output devices never turn off. Unless its a single-ended amp that makes little power, one advantage is that even ordered harmonics are canceled in the speaker load due to the opposing operation of the output devices. In a class A amp this happens at all power levels but not so with class AB.

Execution is a major portion of the differences between various amplifiers. For this reason it is possible to find inferior and superior amps in nearly all categories. I personally think D is going to win out over AB and A simply because of the cost. If done right, the output section is pretty benign and most of the sonic artifact seems to come from the input section and the converters.

Now this has nothing to do with tubes and both traditional solid state and class D still have inroads to make there. I think part of the advantage of D is that its likely that due to its simplicity and the input section being the lion's share of the artifact, that it will be able to eventually challenge tubes in a way that traditional solid state has failed to do (for those that doubt this last statement, if traditional solid state had really been able to challenge tubes, tubes would be a thing of the past. Gone. Nada. They aren't and that's really all anyone needs to know, and there are good sound technical reasons why this is so).

Because there is less to color a D amplifier, it should be easier to create the neutrality and smoothness that tubes have had all along. Not seen it yet, but they get closer every year.

What they call "Analog Cell" is just class A Mosfet input stage very common in many class A or AB amps.  It sounds like a lot of hot air (for class D).

Some sort of input buffer is almost always required to eliminate input offsets in the comparator and that sort of thing (which is also where most of the class D artifact actually arises). But this does sound like marketing.

@timlub What is the 'switching distortion' to which you refer?

Obviously No Switching distortion.

@noble100 

OK- I just wanted to be clear on what was meant by 'switching distortion' since that's not an engineering term at all!

Artifacts from dead time show up as harmonic and intermodulation distortions, so a class D amplifier can easily be compared to other amps on that basis. IOW, on its own 'switching distortion' has no meaning. 


With regards to the switching frequency, with the current state of affairs with switching devices, its actually easier to make a low distortion amplifier when the switching frequency is less than 1MHz. This is because the amplifier has to meet certain standards for RF radiation- various countries have specs for how much RF noise can be tolerated. The faster you go, the difficulty in getting the amp to behave in this regard goes up on an exponential curve! Its quite a feat that Technics was able to switch so fast, but in reading their ads for the amp, I suspect a bit of marketing hype is also involved.


The problem with a low switching frequency is probably that of bandwidth, but class D amps can have a way of getting around that, since it is possible to add enough feedback to allow the amp to compensate for phase shift that would otherwise plague an amplifier with bandwidth from only 20Hz-20KHz.


In a nutshell its really dangerous to make blanket statements about 'all class D amplifiers' just as it is about 'all tube amplifiers' or the like- there are often many exceptions.

Bottom line is until we can get rid of the nasty 3rd harmonic distortion in class d our ears will always prefer the cozy 2nd harmonics that tubes generate

@kw6 Actually the human ear/brain system treats the 3rd harmonic the same as it does the 2nd, in that its relatively inaudible and adds to a bit of what audiophiles call 'bloom', 'warmth' and the like. The other odd ordered harmonics are not so benign!