The Future of Audio Amplification

Ralph, you are more capable than anyone on this thread to explain why class D is the future from a technical standpoint.  Lay it on us.  Also, have you (as George is implying) had a religious conversion to the world of class D to the point of implementing it in your products?

My apologies for a long answer to a short question.

No religious conversion. Its just engineering. We've been watching the development of class D for 20 years; the early amps were a joke, like the one under test that is represented in the little graphics George likes to post.


About 3-5 years ago they started to really come around. I've been using one for about ten years for my keyboard rig in my band, mostly because its light and powerful, and I have to move that stuff myself when we do a show. Its a Crown, and doesn't sound that bad. But for high end it falls short, although it is an older amp.

About two years ago I realized that Atma-Sphere had something to bring to the table so we investigated and it turned out that was the case. As a result we have a patent pending.


So my experience is by listening to our class D side by side against our regular production OTL amplifiers. They are remarkably similar. I have to assume that if Merrill has their ducks in a row, their amp should be very nice also.

As Kosst has pointed out, with any amplifier its all about distortion- what the amp has and what it doesn't have. Traditional solid state has always had a bit of low level higher ordered harmonic distortion, which is the source of the brightness and hardness for which solid state is known (and the basis of the tubes/transistors debate). Class D does not have the mechanism to create that sort of distortion. The higher-ordered harmonic structure of traditional solid state is partially the result of non-linear capacitive aspects imbued in the junctions of most transistors *and* also the feedback needed to linearize many transistor circuits.


Class D relies on switching and so eliminates that capacitive problem- and they can be linear enough that its nearly as easy as using tubes to build one that runs zero feedback.


So think about that last paragraph- the two main reasons transistors sound bright and harsh (which, make no mistake, is a coloration) can be eliminated with this technology. Of course, in solving that problem other problems are created; distortion in class D amps is increased by low switching speeds, deadtime circuits and the precision of the encoding scheme.


That is why we've seen the steady march to higher switching speeds- to reduce distortion and increase bandwidth. The problem has been shoot-through current (this is where both output devices are partially on at the same time, allowing current from the power supply to shoot through both devices at the same time, heating them up quite quickly). We can get the encoding schemes to work pretty well, but some modules employ opamps or other amplification at their inputs which can color the sound. But they are not mandatory and some circuits have the audio proceed directly into the encoding circuits without any processing whatsoever.

Oddly, the thing that most class D amps get attacked for, switching noise, isn't a thing because you can't have a noisy amp channel sharing any kind of circuitry with another noisy amp channel. They will find a way to talk to each other and it gets ugly (oscillations, hetrodynes, intermodulations, etc.). So any commercial design has to have that problem licked and they do.  This is actually a requirement to meet international radiation standards.


So there are a **bunch** of variables! But if the following criteria are met, the amp will have low distortion and in particular will be lacking the higher ordered harmonics and IMD, which means they can sound very 'tube-like', 'organic', 'musical' and so on:

1) High switching speed with low residual
2) no deadtime introduced (deadtime increases distortion)
3) accurate encoding
4) no feedback

Regarding feedback, unlike regular amps in which there are things like phase margin and the like that can cause phase shift issues with feedback (and possibly result in oscillation), class D amps employ switching, which introduces *propagation delay*. This means that the output of the amp is occurring time-wise always slightly behind that of the input. Its a tiny bit, and so gets treated by many designers as a phase shift issue, but in essence the feedback **over the entire bandwidth of the amplifier** is going to always be slightly late. This means it will make distortion; such amps IME will have amusical properties. IOW I don't think applying feedback in a class D amp to be a good idea.

Zero-feedback Class D would seem to be a unique twist.

hello brotw 
Sorry for the late response, this thread has come to large and techincal. I find it hard to follow.
Well, my Nord NC500 One UP is just burning, but even out of the cage it sounded better than the LM508, to my ears and in my system.
I also have to say that the Nord only has about 30 hs of use and it has a long way to go before reaching its best. Nevertheless, during this period I know it will sound sometimes better and sometimes worst. For instance, I found it yestarday a "little bit" thin and harsh, more than at the beginning. I'm sure that it will get warmer and more delicate with time, as it happened with other owners.
I also have to say that I really like my LM508 and I would like to compare both amps again, once the Nord has settled down, in a few weeks. But I also have to mention that I was expecting subtle differences, if any, but I found out noticeable ones, mostly in texture, noise and 3D image.
FInally, I don't think Digital Room Correction has something to do with the amp and the speakers. For me it makes the sound much transparent and plus I have the option to set it to my taste, rising some frequencies and lowering others.

Zero-feedback Class D would seem to be a unique twist.

Bit late to the party Merrill already did it using the GaN transistors, I believe the SE-R1 does also.

Merrill Quote:
 Merrill Audio claims that this is by far their most ambitious design mostly because they are using open loop, zero feedback, and zero deadtime with GaN  gallium nitride transistors, which are better than most transistors used in other solid-state amplifiers because that have close to zero capacitance, which allows fast switching.
Plus, the gallium nitride transistors can operate in the GHz range. Gallium nitride transistors offer a fast, clean, low distortion power capability", and so Merrill Audio claims that are "natural and smooth in their musical capability".  And after doing this, since the distortion was eliminated, there was no reason to use any feedback at all. He went on to tell me that he is very proud of not having to use feedback.  Eliminating feedback has two distinct advantages. The speed capability is increased significantly and a remarkable gain in the clarity of musical information is achieved. Since there is no signal loop-back, all the feedback loop distortions as well as the attendant time delays are eliminated.  

Cheers George

There are two reasons I will be keeping my Class D amps for a long time:


1) they work and sound great

2) the longer I wait the better (and more affordable) this still evolving technology will continue to get in years to come for many many reasons.

Same reasons I tend to hold on to most good quality digital gear (and cars) I buy for as long as I can as well.

BTW I believe it was Merrill Class D amps I heard driving German Physiks speakers at CAF 2017 and loved what I heard, not unlike what I hear with my Class D amps and OHMs at home.