Class-D amps - a different re view


Martin Colloms, the editor of HiFi Critic (ad-free mag from the UK) have recently published the review of several different Class-D amps, together with an in depth technical analysys and measurments.

His conclusions were not favourable, to say at least:

"I regret that not a single model merits unqualified recommendation. Price is not the issue; the poor listening tests speak for themselves. (...)
At present we have to take the prudent view that good sound might be possible from switching amps, but we haven't heard it yet."

BelCanto REF1000 (ICEpower) - score 10.5 pooints
"The ICE power module used has a dependable reputation, and the design is well built and finished as a whole. While I would not suggest that you shouldn't try this amp, on sound quality grounds alone I cannot recommend it for audiophile use."

Channel Islands D100 (UcD) - score 13 pooints
"While I have reservations about a number of aspects of sound quality, and advise personal audition, given the solid lab results (...) the overall performance and the moderate price, these CA Audio monos do make it to the 'worth considering' cathegory."

NuForce 8.5V2 (proprietary technology) - score 9 pooints
"Yes, the price is good for the power output. Yes it's pretty, light, small and runs cool. However, the sound quality simply does not justify recommendation." (on top of that the NuForce amp measured very poorly - Elb)

Pro-Ject Amp Box (Flying Mole) - score 5 points
"I'm sorry to say that Project (...) was a real disappointment in the listening tests, and can't be recommended."

Just as a point of reference, recently reviewed Krell 700CX scored 100 points, CJ Premier 350 - 110 points and ARC Ref 110 - 135 points.

At least someone have had the balls to say it. This is why HiFi Critic is THE mag to subscribe.
128x128elberoth2
hi audioperv:

the objections i raised are reasons not to want to listen to solid state amps. what ever faults lie in the design of tube amps do not deter me from wanting to listen to them.

subtractive coloration of some tube designs does not bother me. additive colorations of solid state amps is annoying.

i have heard class d amps at ces many times. i have no desire to listen to them on panel speakers.

ralph, what say you ?
That's interesting MRT. . . roaming the halls and suites at RMAF I was actually dismaied by how so many tube designs at all price points were trying to out-solid-state solid state designers. . . and succeeding admirably, tizziness (additive flaw presumably) and vanishing harmonic structure (subtractive flaw I presume) included. The difference apparently is that. . . if such flaws are heard on classic solid state or switching technology--which I did hear in spades as well--the problem is with the technology or the design, but if any apparent 'solidstatification' of tubes is perceived, the culprit is supposed to be only the wild and utterly uncontrollable acoustics of the suite. . . go figure!
Oh yes, let's not forget that flabby bass, or conversely that magic vanishing bass, or that sudden hard clipping: these are apparently more artifacts that plague room acoustics where some tube designs are featured. . . Yet, if switching amps or solid state amps are present instead, 'tis always safe to point an accusing finger at the designer.
Mrtennis, Class D should not be confused with solid state. They don't even sound similar.

You would not like to hear class D on panel speakers? You obviously have not heard my panel speakers.
If a room sounds good at a show, I pay attention, but if it does not sound good, I don't let that deter me. I've had my own room sound lousy plenty of times... its really tricky to come into a room, set up a high end stereo and have it sounding convincing in a day or a day and a half (which is the timing that the shows give you).

If people keep coming back and wanting to hear more tracks I take that as a good thing, but I've never had a room sound as good as my system at home, nor have I heard *any* system at a show sound as good as what I have at home. I'm pretty sure I have a lot of company!

So we're back to the same deal- you have to audition this stuff. I've had the opportunity to do just that, and tweak the circuits as well. From that, and the feedback of customers, come my comments. My conclusion has been that while class D lacks many of the traditional solid state artifacts, that they do have artifacts of their own, but not ones that are common to class D in general (IOW very unlike traditional transistors in that regard; every transistor amp I've ever heard has identified itself as a transistor amp, class D does not identify itself as 'class D'), instead the artifacts seem to associate themselves with the specific product. That, by itself is a very good sign- it suggests that as the technology evolves, a truly neutral implementation could emerge.

Our auditions and customer input show that its not there yet, that we (as Atma-Sphere) have some breathing room for now. By no means has our own line of work been exhausted- the circuit is always suggesting new ways for it to be improved. So it will be interesting to see where things are in say, five years.

One thing that I have been intrigued with is that it is possible to build class D amps that lack the traditional style of feedback loop- a deadly sin as far as the human ear is concerned (and part of the 'traditional' transistor sound). That suggests that the amplifier can be designed around the rules of human hearing rather than the arbitrary standard of 'constant voltage' output. From what I have seen though, the open loop distortion is going to have to go down a bit yet...