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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
kijanki's avatar
kijanki
4,428 posts
 
Ar_t

Icepower uses full H-Bridge of 4 Mosfets switching direction of the speaker between +VCC and GND. In spite of VCC/2 on the speaker terminals, voltage between terminals is zero. The only disadvantage of this scheme I know is that you cannot bridge amplifiers. My amp has 23.5V on speaker terminals - it won't zap you (voltage up to 60V is considered safe)

As to effect on the damping ratio - filter is within negative feedback and DF is very high. At higher frequencies it drops as I stated before to about 30 at 10kHz. What about DF of a tube amp at 10kHz?
What about zobel networks on traditional transistor amps.

It has about 1% of the carrier - not important (cannot radiate and the tweeter produces IMD only if it's membrane moves - not possible at 500kHz).

Talking about "scare" - the scarriest part of Mr. Colloms presentation is the fact that he believes that amplifiers like Icepower or Hypex (he tested Channel Island D100 - didn't he) are sampling systems and have AD/DA inside with limited resolution.

Sure anyone can find something wrong with any type of amps. The wrong think to do is to pick possible deficiencies of different amplifiers and make general statement about whole class of them. At the same time he did not mention any positives like very low THD, extremely low IMD+TIM etc.

I would advise everybody to listen to Icepower, Hypex, NuForce, Flying Mole and make up your own mind.
simontju
257 posts
 
Kijanki:

"Simontju, Amp is close to 95% efficient and does not have to be large to do so."

1 - Even if efficiency would be 99.99% some heat would be generated and...dissipated somehow

2 - No heatsinks! I am talking NOT about chasses of the amp or whatever but inside the very small power module packed DENSLY with microscopic size electronics

3 - I can re-assure you that each responsible designer and manufacturer of class D module like Spectron, Bang & Olafsen, Philips did themselves or invited outside consultant(s) to make most comprehensive thermodynamic analyses.

4 - All The Best

Simon
ar_t
165 posts
 
Kijanki:

If you have a 1000ASP, then it will have much more than 22 V on the output terminals, so I would not advise touching them. (Rowlan uses that module in some of their amps.)

As for the output network, I was talking about the damping factor, or Q, of the filter. Not the damping factor of the amp.

If Colloms believes that they have sampling stages inside, then he is ignorant as to their operation, or else one can conclude that he has an agenda. Modern Class D amps of the self-oscillating style (like ICEpower and Hypex/UcD) are basically an amplifier that is unstable, and use that oscillation to generate a carrier wave. (Unlike clocked designs, which no one seemed to like.) They use some simple circuits, in some rather clever manners, to modulate that carrier with the audio signal. Over-simplified story on how they work.
mrtennis
3,295 posts
 
what is the reason this thread has generated so many responses ?
muralman1
1,527 posts
 
Increased ownership of class D amps. As little as a year ago, there may have been maybe a dozen posts. Some few of us were championing class D four years ago. We were laughed at then. Now there is strength in numbers. This is great!
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