Class D at low volume

Serge,

You asked:

"Thanks Noble100. Speaking of feedback, what do you think of Benchmark AHB2."

Sorry, I've never listened to the Benchmark AHB2 or read any reviews on it yet. I've read very positive reviews on Benchmark DACs, however, and knew they recently came out with a couple of new class D amp products. If I were to venture a guess, I'd bet that the obvious care and attention to detail they devote to their high quality DACs is likely to also be devoted to their amps.

Mapman,

I've read your comments about class D, on this thread and several others, and typically find your impressions on this evolving technology closely mirror my own. I think that 2 independent users of class D amps having such similar impressions of their high quality performance, among other benefits, speaks volumes about the excellent high-end bargains these amps actually are. It seems like we're both now drinking the class D Kool-Aid.... but only because it's delicious.

Bombaywalla and Kijanki,

Thank you both for explaining things in more detail.

Bombaywalla, you stated:

"The way class-D architecture was invented/designed/formulated, global negative feedback is part of its entity. So, just because you read global negative feedback you shouldn't relate it in the same way as you would to GNFB in a class-A, AB power amp. The class-D architecture needs GNFB while class-A, AB architectures have topologies that can do with little or no GNFB."

I realize that many traditional amp designers have considered the use of GNFB in their amps to be something to be avoided since the early tube designs, believing having zero feedback improved sound quality. Solid-state amp designers seemingly just adopted this low/or no GNFB principle in their next generation class A and A/B amps.

I also realize class D amp designers have taken a radically different approach, embracing GNFB and making it an integral circuit within the amp to lower noise and distortion levels as well as to ensure the input signal faithfully matches the outputted signal.

In my visits to various hi-end audio shops over the years, I've listened to many systems powered by tube, class A and A/B amps (with zero and low GNFB design) that sounded excellent. However, most of these amps exceeded my budget by a large amount.

From my layman's perspective, good class D amps that utilize high levels of GNFB represent a great alternative that is more cost effective and energy efficient means of achieving excellent sound.

I'm not technically savy enough to know what the main reason is for my class D amps' excellent performance. I just know they are lightweight, small in size, sip electricity, run cool and sound great.

But I think Paypet's comment probably drives the point home the best:

"Listening to wyred for the last month , difference between wyred and wytech ruby mono blocks , wyred $2995 , ruby $25000 , can't pick out the differences they are that close ,doing A B tests for 3 weeks nothing stands out in either one , will keep trying."

That is a very honest and powerful testimonial.

I apologize for getting a bit off topic from Serge's original post question and am now stepping off my class D soapbox.

Tim

Tim, GNFB serves the same role in class D as it does in class AB - reduces distortions, widens bandwidth, reduces output impedance. It is not necessary for operation in general but particular circuit can be designed to depend on it.

Since even shallow 20dB of GNFB will improve everything tenfold it is very tempting to use it. It can be done without negative effect if certain rules are obeyed but then GNFB will be shallow. Designer has to start with good design and quality components to start with and not to fix everything with GNFB.

As for new Benchmark amp - it is class AB with SMPS supply.
http://www.enjoythemusic.com/magazine/equipment/1214/Benchmark_Media_AHB2_Stereo_Power_Amplifier_Review.htm

Pavpet, yes TIM distortion can be fatiguing. It is basically an overshoot, in time domain, of the rapidly changing signal. It won't show with any THD or IMD measuring. In freqeuncy domain it translates to very audible odd order harmonics that our hearing is sensitive to. It makes sound bright. In extreme cases (perhaps some SS amps in 70s) overshoot was so big that it could choke output transistors that remained choked for a while (charge trapped at the junction) creating short gaps in music. These gaps were not audible, since our brain compensates for that, but made listeners very tired.

lots of great thought in this forum discussing class d amplifiers, being an audiophile since 1970 , and owning very expensive gear , its nice to purchase a product at a very very resonable price , and make magic , the wyetech ruby monoblocks list for approx $25000, plus ive installed 1942 rca 211 radiotron tubes , ( took out stock tubes as well as input tubes)AT an additional $3300 , plus upgraded power cords , but thats the audiophile journey , this i know , but now and then its nice to get a breath of fresh air ,without getting a second mortgage. guys thanks for all your input very much appreciated

Tim: thanks for getting off topic. This thread is educational for me and hopefully for many others as well.

Thanks for your comments folks! Please keep the discussion going.

Serge,

I was looking at your initial thread posting again and you stated you're considering Lab.gruppen, Powersoft, XTL and others. These are pro amps used for large venues and not intended for home audio.

I apologize that I didn't read your original post closely enough. However, all of my replies, as well as most replies from others, were based on the assumption you were looking for an amp for your home audio system.

I've never used pro amps and have no useful knowledge about them. The info and impressions about class D amps was based solely on my experiences and knowledge of home audio class D amps and may not be applicable to the pro audio class D amps you mentioned.

I think you need to clarify exactly what purpose these amps are intended for.

Audiogon is a site devoted to home audio, not pro audio environments that are used in clubs, concert halls and auditoriums.

If you are interested in pro audio amps, I would suggest posting your question on a forum devoted to such.

All of my replies, and I think most if not all of the other replies on this thread, were given from the perspective of home audio and therefore should be disregarded.

Sorry for the confusion,
Tim