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.
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Muralman - people are often confused and becuse they like guitar sound on 30W Vox (remeber The Beatles)with tubes and no feedback (high thd) and want their home system to be like that - close to real performance. Home system of course should not change sound - even if clean jazz guitar does not sound as lively as distorted one. Issue of real life performance is very interesting because real life performance is often very poor (poor acoustics, bad seat etc) - not the studio level what becomes clear after you compare sound from the studio and the concert records.

Jitter is resolved by means of PLL - single loop in cheaper and double in more expensive CD players. DAC spits out with the rate from the disc (to be exact from internal clock phase lock looped to CD and going thru FIFO buffer at this rate). Good transport means lower jitter but is costly. Another method is to completely reclock signal to DAC's clock, upsample it and send to DAC in corrected intervals. Benchmark is trying to do it (correct)acurrate to 5ps. Both methods produce great results - reclocking gives better supression of the jitter and allows to use cheap DVD player as a transport.

I checked audio DACs at Analog Devices website - almost all of them are sigma-Delta (oversampled). There is still few with traditional architecture carrently being discontinued. All Dacs shown as 24-bit are sigma-delta. Traditional Dacs go only to 18-bit (maybe 20-bit).

Dielectric in the speaker wire is evil (even teflon) - I agree. Latest trick is to use foams (lower dielectric constant than solid material) and wide tubes so that wire has only contact with one wall. Another problem is to compromise between low resistance and low inductance (thick wire) and skineffect free (thin wire). Skin effect starts at gauge 20 at 20kHz and splitting wire into many helps only a little (more surface area but still in the sum of magnetic fields). Tapes are wonderful but not practicall solution. I replaced coils in my cheap speakers with the "tape coils". It sounds better - at least I'm telling myself. Capacitor in series with the tweeter is the reson for brigtness in many speakers. Mylar caps are horrible (high dielectric constant) but Teflon are too expensive. Polypropylene paralleled by small teflon is the ticked. I also changed resitors to non-inductive. Every bit helps.
Kijanki, just reading your description of the path the signal has to go through to be cleansed leads me to imagine just what I hear from those DACs, forced. My music is free breathing, unrestricted. There is absolutely no audible distortion. I just can't listen to SACD, and I don't care to.

The two of us are discovering new rules of engagement when dealing with class D. I am thrilled with my results. It seems you are very happy with yours as well.

"The truth of sonics" Now theres a phrase that only a tube amplifier designer could thow out.
I recently heard the Onkyo A 9555 integrated amplifier. It is a Class D amplifier, and sounded excellent, and infact better than my SimAudio i-3. Its a steal for its price. I feel that it rightfully got good reviews from Stereophile and TAS.
Onkyo makes profits by volume and selling it at a profit low margin, unlike most hi-end audio manufacturers who try to make profits by selling less volume at high profit margins. Thats the reason many hi-end audio manufacturers go out of business.
Kijanki:

Your post mentioning jitter and PLL seems to be off-base, and probably ill-suited to this thread. Perhaps you would like to clarify it, but may I suggest in a different thread.

RF gumby:

Yes, I think I am in the right place. I am pretty sure I did not hit the bookmark for the Insane Asylum.

Or it might have something to do with my being an RF engineer, partially out-of-place in the role of front man for a minor high-end company. (It fell upon me by default. Long story.)

The "talking points" that were mentioned as proof that Class D amps are by design inferior seem to be a mix of technical points and sophistry. I could make a similar list for any number of other technologies. Equally convincing, equally misleading. If I didn't know better, I would come to the conclusion that someone had an agenda.

However...........some points do have a bit truth to them. Some Class D amp modules have an output frequency that does vary with speaker load. The maker(s) of these modules clearly show this in their data sheets. I would think that anyone who truly has a background in electronics design could understand how changing the impedance a second-order LPF operates into could have an effect on its damping ratio.

I believe the voltage on the output that he refers to is the Vcc/2 in the ICEpower-based units. They operate with only one polarity of power supply, and being a full bridge design, will have 1/2 of the rail voltage on BOTH output terminals.

Translation:

Don't grab your speaker cable in one hand, and ground in the other! Not a good idea. (Maybe that is why so many have shook-proof terminals.)

Also, there will be a certain amount of the carrier frequency on the output. Yes, it may be a few volts. Somehow, I don't know of many tweeters that will generate tons of IMD with that sort of voltage present.

As I said, some technical facts, seemingly massaged to possibly scare off the uninformed.