Audiophiles, generally don't like class D amps!

[Kijanki]

You're missing detail. The Benchmark uses both feedforward and feedback as does the Sansui. The Sansui's feedforward is contributing much less than the Benchmark's feedforward correction stage, but the principles are likely similar. Benchmark clearly saw what the FFWD could do and figured they could save a ton of quiescent dissipation and kill Xover distortion at the same time, whereas Sansui attempted to take an existing, already excellent topology of theirs, to the next level.

I haven't seen or read the THX AAA patent and I wonder what prior art mentions there are in it. Nor have I seen the Benchmark schematic (I'll ask for that when one lands on my bench for repair if ever), but there's not much new in this space- it's all been done in the past by much smarter people than me. Standing on the shoulders of giants who rarely get credit is something that really upsets me. I'd much rather buy products from companies that speak to the past, celebrate it, and demonstrate what they've done better, instead of claiming it was 100% their own brilliant idea...

My impression was that Benchmark uses non-recursive feedback. Name would suggest that feedback is not applied back to the same signal amplifier.
I'm sure it is not a simple design and I also wish I could see schematics. Company I worked for never provided them to customers, charging $200/hr for repairs.

 

[Kijanki]

Seriously, you need to relax and chill out.

Thanks, I will try.

 

[restorer-john]

My impression was that Benchmark uses non-recursive feedback. Name would suggest that feedback is not applied back to the same signal amplifier.

Here:

Feed-Forward Error Correction

The Benchmark AHB2 power amplifier and HPA4 headphone amplifier both feature feed-forward error correction. This correction system is an important subset of the patented THX-AAA™ (Achromatic Audio Amplifier) technology. It is one of the systems that keeps these Benchmark amplifiers virtually...

benchmarkmedia.com

Funny, it was Benchmark that used the term "clean up the mess". I used it of my own accord.

 

[antcollinet]

Yes, it got lost, thanks to unnecessary comments like yours. I wrote about TIM distortions because of the claim that since class D measures quite well (THD, IMD) different sound has to be caused by something else, like TIM.

Isn't TIM measureable?

 

[JSmith]

schematics

It seems Benchmark either purchased or paid to use the THX Achromatic patent as part of their further design;

THX Achromatic™ Audio Amplifier Technology

Introducing the Benchmark AHB2 – The world’s first amplifier featuring THX Achromatic™ Audio Amplifier technology. Benchmark Media Systems is one of only a few companies whose products have been enthusiastically received by both audiophiles as well as audio production professionals. Their...

benchmarkmedia.com

After a quick search for that patent I located this which may provide some info, however haven't dug into the detail yet;

US8004355B2 - Low dissipation amplifier - Google Patents

A low dissipation, low distortion amplifier includes a driver amplifier stage and a main output stage, with a plurality of impedance networks providing, among other things, feedback paths from outputs of the driver and main output stages to the input of the driver stage. The impedance networks...

patents.google.com

The driver and main output stages may be operated in Class AB and B modes respectively, and/or in Class G or H modes. An intermediate amplifier driver stage may be added between the driver and main output stages.

JSmith

 

[Galliardist]

Yes, it got lost, thanks to unnecessary comments like yours. I wrote about TIM distortions because of the claim that since class D measures quite well (THD, IMD) different sound has to be caused by something else, like TIM. It was on the subject.

My emphasis, but I suggest that there's your problem...

 

[restorer-john]

It seems Benchmark either purchased or paid to use the THX Achromatic patent as part of their further design;

THX Achromatic™ Audio Amplifier Technology

Introducing the Benchmark AHB2 – The world’s first amplifier featuring THX Achromatic™ Audio Amplifier technology. Benchmark Media Systems is one of only a few companies whose products have been enthusiastically received by both audiophiles as well as audio production professionals. Their...

benchmarkmedia.com

After a quick search for that patent I located this which may provide some info, however haven't dug into the detail yet;

US8004355B2 - Low dissipation amplifier - Google Patents

A low dissipation, low distortion amplifier includes a driver amplifier stage and a main output stage, with a plurality of impedance networks providing, among other things, feedback paths from outputs of the driver and main output stages to the input of the driver stage. The impedance networks...

patents.google.com

JSmith

Yes, I read that 'patent' a while back.

I like this bit:

Not just a basic technique described previously, Hitachi had a Class G on the market over a year before in 1977, the HMA-8300. I have one in my storeoom. 200wpc monster with big meters.

 

[SIY]

Some authors believe that global feedback is bad and should be replaced with local feedbacks

Well, let me grab the standard texts on linear audio amp design... Cordell, Self, Baxandall.

Nope, they say no such thing. And there's a reason for that, which is explained in Bruno's article.

I repeat my earlier advice.

 

[fpitas]

If you disagree with technical merit of my post just say so and explain why, instead of silly remarks like that. Frankly, I expected more from "Technical Expert" and "Master Contributor"

Silly remark? Really? He is suggesting you read and educate yourself.

 

[fpitas]

Isn't TIM measureable?

Yes. A long long time ago. It has now become internet audiophile tosh.

AES E-Library » A Method for Measuring Transient Intermodulation Distortion

A description of the square-sine intermodulation measuring method is given. The practical measuring procedure is described, and measurement results of commercial audio power amplifiers and integrated operational amplifiers are presented, in many cases showing appreciable transient...

www.aes.org

 

[Galliardist]

Some authors believe that global feedback is bad and should be replaced with local feedbacks while Douglas Self likes global feedback.

I know some manufacturers prefer not to use global feedback, and that it's become a selling point with a few. I have a Simaudio integrated amp here and they say that they use no global feedback, only local. That became a selling point because you can't stop marketers saying any point of difference is an advantage: but I'm pretty certain it just started out as a preference of the designers. It's probably still equivalent to global feedback and a non-argument.

It's also a talking point in parts of subjectiveland. Of course there are no blind tests of the alternative topologies to guide us. I've not chased objective measurements of such amps but I'd guess some are good and some are bad, as usual.

 

[BDWoody]

I do not "argue" with anybody, but rather receive unpleasant patronizing comments from so called "experts"

You're done in this thread. We are fortunate to have some world class engineers, designers and general electronic experts here, and they aren't here to have their time wasted by people who have no idea what they are talking about.

 

[SoNic]

Nothing of this exists on class D, there are no linear power amplification stages and no triangulation is possible (at least not unless the designer did something really wrong, like using a too big output capacitance with a too low current limit, so unlikely).

I guess you know nothing about class D topography and how the PW modulation is actually produced. Nothing is instantaneous.
Noise shaping integration takes time to happen. Output integrator takes time - even if the speaker coil is used as inductance, it is still a low-pass filter at modulating frequency.

https://www.analog.com/en/technical-articles/fundamentals-of-class-d-amplifiers.html

 

[DonH56]

Isn't the AHB2 hybrid AB & H class, i.e. tracking the input signal and modulating the voltage, rather than switching between discrete voltage steps?


JSmith

John Siau has contradicted himself in several posts and on the benchmark site. He's probably just being deliberately vague. I'd buy one and pull it apart except it's several thousand dollars too much just for curiosities' sake. And there's too much locked up in the FGPA for my liking.

In the post I found in a quick search here, and am too busy/lazy to find again, he clearly said it switched between two rails (class G in the USA, class H in Europe, IIRC) and was not a tracking supply. He cited the same concerns I had and have hit when trying to design them myself (not for audio, again my day job has been at higher frequencies), primarily the difficulty in controlling the tracking rails while making them fast enough and low enough in impedance to provide good performance. Switching between two rails is hard enough in my limited experience. The class H RF amps were for applications that had relatively easy distortion specs, more like -40 to -60 dBc and not the -100 dB and under audio amps provide, and relative to the carrier frequency (>10 GHz) the envelope was fairly low in frequency (<100 MHz).

I have only read a few papers on applying class G/H topology to class D amplifier cores, and am unaware of any audio amps (the ones I saw were for low-power RF amps, not sure any made it to market). All the audio class G (switching output supply rails) or class H (tracking rails) amplifiers I have seen use a class AB core, including Benchmark, Emotiva, etc.

Trust John (@restorer-john) to find a great example of "old school" tech like that Sansui amp that used feedforward (and feedback) correction from the heyday of Japanese designs.

Edit: Found the Benchmark post: https://www.audiosciencereview.com/...benchmark-ahb2-amp.7628/page-153#post-1306761

 

[atmasphere]

Ralph, of course I agree. It appears that people mix two different types of distortion IMD and TIM (Transient Intermodulation Distortion). TIM is an effect of phase shift (time delay) thru the amp causing negative feedback to feed back late, causing overshoot with fast changing input signals (unpleasant higher order odd harmonics). Negative feedback improves everything THD, IMD, output impedance, bandwidth etc. but 40dB of negative feedback means that amp with open feedback has gain of 3000 instead of 30. Late summing of the fast changing input signal has the same effect as amplifier having higher gain for a moment, causing overshoot. Before it was discovered in 70s amps had no distortions (very deep feedback) but unpleasant bright sound. There are remedies for that. The first obvious one is to use quality design and components instead of very deep feedback. Second is to avoid deep long global feedback and use more of the local feedbacks. The third is to limit slew rate at the input, before feedback summing point. CD should have limited bandwidth, but filters are not perfect and even tiny amount of high slew rate will get amplified 100 times more (at 40dB feedback). I believe that TIM is related to phase shift (time delay) thru the amp related again to number of amplification stages (each stage causing delay) and if anything class AB with many stages should be more prone to TIM than class D amp that has basically only modulator and output Mosfets. I don't design amps, but fortunately Atmasphere does. Ralph, Is it the right assumption? - you designed both types.

@SIY gave good advice. Bruno's article is excellent; When I finally did read it I realized I had been ignoring things that had been right in front of me for a good portion of my career- I never thought about the implications of part of the circuit being outside the feedback loop. Norman Crowhurst did mention over 60 years ago what happens when feedback is applied to a non-linear node (such as a transistor which is part of a differential input circuit).

Read Bruno's article.

 

[fpitas]

@SIY gave good advice. Bruno's article is excellent; When I finally did read it I realized I had been ignoring things that had been right in front of me for a good portion of my career- I never thought about the implications of part of the circuit being outside the feedback loop. Norman Crowhurst did mention over 60 years ago what happens when feedback is applied to a non-linear node (such as a transistor which is part of a differential input circuit).

Read Bruno's article.

He's been cast into the outer darkness already. But I hope he does read and learn.

 

[roog]

We don’t do that do we, ‘cast out’ surely not?
I would hope that we are able to patiently and politely help others to demystify and understand the wonderful world of electronics and to help to clear the years of obtuse marketing nonsense from their minds.

The relief from endless trial and error failures should and could be a given.

 

[roog]

@SIY gave good advice. Bruno's article is excellent; When I finally did read it I realized I had been ignoring things that had been right in front of me for a good portion of my career- I never thought about the implications of part of the circuit being outside the feedback loop. Norman Crowhurst did mention over 60 years ago what happens when feedback is applied to a non-linear node (such as a transistor which is part of a differential input circuit).

Read Bruno's article.

agreed Bruno’s article is very informative, now all I need is a series of lectures and tutorials on what he says so that I understand it more fully, I would love to be able to put this into practice Myself.

 

[Dismayed]

My Martin Logan 11A speakers contain class D amps to drive the woofer. Sounds great! But I use my old reliable Aragon amp to drive the panels.

 

[Vive la dissonance!]

...some measurebators like the ASR cohort.

No one with pretentions of being a respected contributor should be using language like this.