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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
atmasphere's avatar
atmasphere
11,859 posts
 
Kijanki, not to detract from your comments of the review of the review, but just so you know the numbers that are quoted are not speaker current. If it were:

First, reality:
For 100w into 8 ohms, the current would be about 3.54 Amps.
For 100w into 4 ohms I = 5Amps, for 2 ohms I = 7.1Amps

then the 'claim':
working it backwards, 50A into 4 ohms is 10,000 watts
into 2 ohms its 5000 watts.

So- 20Amps into 4 ohms is 1600 watts. But your amp does not make that kind of power!

I agree these numbers are common in a lot of amplifiers, the 'FWIW' part is that the numbers are *not* current in the speaker. I suspect they are power supply numbers, that number being the current output of the supply when shorted for 10ms. Our MA-2 amplifier which is an all-tube amplifier measures about 80 Amps using this technique...
kijanki's avatar
kijanki
4,428 posts
 
Atmasphere

Quoted current - as I said before is the peak current and has nothing to do with average power. It is used to show how well amplifier can respond to impulse.

Internally my 200ASC Icepower can deliver 48V to speaker what would make at 3 Ohm 16A current. Limiting circuitry is set to 11A. Power delivered to speaker will be 363W.

I don't know how long this pulse can be or what would be crest factor with multiple pulses but would like to point that Bel Canto rates the same module at 300W (10% thd)
muralman1
1,527 posts
 
Kijanki, On my 1 ohm speakers, there is very very little resistance in the way of the signal. The signal is beautiful and natural when it is left alone. Oversampling, as elegant a circuit as is, cannot operate without introducing some timing error. That flaw is painfully obvious on my Scintillas.

These amps are so good at being faithful to the signal, minute distortions, usually deaf on listeners, work their way through. I have found speaker cables must be bereft of insulation, DACs must be simple.
kijanki's avatar
kijanki
4,428 posts
 
Muralman - I have no experience with high end DAC circuits but my Benchmark has surprisingly clean and transparaent sound. Part of this is due to anti-jitter circuitry. Jitter produces sidebands of very low amplitude (typicaly -80dB) but very audible since not harmonically related to root signal. It sounds less clean (hash) but often people prefare sound with some amount of hash or thd. They often call clean systems "analytical". Sombody even said, after listening to hi-end system, that everything (instruments) sound separate and he preferes all mixed together. I suspect I have similar hearing limitations with my audio experience.

I will search for more info on oversampling. I just don't know how they can filter out carrier of 44.kHz. With the ratio of 2:1 Bessel filters just start attenuating - no matter how many poles. Usage of other filters will affect the sound.
muralman1
1,527 posts
 
Kijanki, I hear you. I know someone who told me they feel music should be blended because that is how he hears it in the audience. He went out and bought the same speakers as I, and put a Krell on them. Talk about opposing forces.....

My DAC has no anti jitter circuitry. I believe that is the clock, the same thing that is scalping the crud off the signal supposedly. There is no clock that can get the signal perfectly aligned. There are too many complexities. It's sort of like electron studies. It can't be done, because that which is observing unavoidably effects the outcome of the measurement. It is all explained under Quantum Theory.

Like I said before, I can get right to the ribbons and hear nothing but pure music.

By the way, try a naked wire for your speaker cable, then try a wide ultra thin cable. This is a very interesting experiment.

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