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Old 11th January 2012, 07:12 PM   #1501
gmarsh is offline gmarsh  Canada
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Quote:
Originally Posted by Bruno Putzeys View Post
Re ADC latency, one of the other linked papers actually shows a working amp with ADC feedback. Theoretically you can live with latency of up to a whole switching cycle, in practice you need a lot less because of the extra phase shift incurred in antialias and noise filtering. But still, some of the new tricks I've worked out over the past two years have staggering implications as to how much delay and phase shift you work around without affecting loop dynamics at all.
Just gave that a read... congrats on successfully making the ADC-feedback system work, I dismissed the idea as too much trouble to implement. Well done. I'll switch to stating that it's way too complicated for an audio amp.

The "DSD DAC" paper was a really interesting read also, especially the DSD/shift register/scrambler trick which is beautifully simple.
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Old 11th January 2012, 07:18 PM   #1502
Julf is offline Julf  Europe
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Originally Posted by Bruno Putzeys View Post
There are probably exceedingly creative ways of boiling water, but that doesn't make all of them worthy of pursuit, bar as an art project perhaps.
But is it Art?
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Old 11th January 2012, 07:29 PM   #1503
bbggg is offline bbggg  United States
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Originally Posted by Bruno Putzeys View Post
it didn't mention Hypex by name so I suppose that's how they want it.
Help us out here, Bruno, if you cannot reveal the name of the company, can you probably reveal the approximate meridian of its location?
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Old 11th January 2012, 07:33 PM   #1504
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yes indeed, is it by chance a reputable British manufacturer of some of the most well regarded digital players that has been known to use Hypex amps before? starts with M but won't give more details

PS: Dynaudio just released their active "wireless" consumer speakers, guess they missed the opportunity.
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Old 11th January 2012, 07:44 PM   #1505
NicMac is offline NicMac  Italy
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Any news on a possible chassis for the ncore?
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Old 11th January 2012, 07:54 PM   #1506
iand is offline iand  United Kingdom
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Quote:
Originally Posted by Bruno Putzeys View Post
Oh dear. I was a bit disappointed to find that after my posting a link to the "all amplifiers" paper (which makes the argument in quite some detail) there was still some uncertainty left in some readers' eyes but that could be due to the fact that a lengthy paper isn't necessarily a clear answer. Presuming of course that people actually read it before piping up...
But it's not necessary that people get miffed about it on my behalf

Re ADC latency, one of the other linked papers actually shows a working amp with ADC feedback. Theoretically you can live with latency of up to a whole switching cycle, in practice you need a lot less because of the extra phase shift incurred in antialias and noise filtering. But still, some of the new tricks I've worked out over the past two years have staggering implications as to how much delay and phase shift you work around without affecting loop dynamics at all.

Re Zetex: Zetex is essentially the "PEDEC" system, in that it starts off with a "perfect PWM" signal generated in the small-signal domain (i.e. without the nonidealities of the power stage) which is then subtracted from the actual power stage output to form the error signal which is subsequently integrated and used to add variable delay to the switch transitions. The only thing Zetex does that makes it look "more digital" is to convert the integrated error signal to digital and do the variable delay digitally (block diagram pages 51-52 of http://www.hypex.nl/docs/papers/AES124BP.pdf). I've not yet worked out the reasoning behind this added complexity. Performance, though not bad, is not in line with the overhead.
I don't think Zetex's circuit is as simple as you're saying -- the integrating error feedback and subsequent PWM ADC forms part of a noise shaping feedback loop back into a digital noise shaper running at >100MHz (not just moving the edge delays), with the same equivalent ADC sampling rate so the feedback delay is very small (equivalent to very high oversampling ratio on the PWM waveforms). There is no front-end DAC as such to add its own noise and distortion, though as you say the "perfect reference DAC" sets the performance -- this is pretty good though, certainly better than most standalone DACs (including many that would be used to drive an nCore amp :-)

The key is to think of it not just as a power amp but a complete system with digital signal processing, including preamp/filter/EQ/dynamic limiting/speaker monitoring -- the real reason for going digital is so that all this can be done starting from a digital input signal with negligible added noise and distortion, at a low cost. Once you've decided to do a chip the "overhead" is essentially nil, adding other features is pretty much free.

If you just want a power amp with an analogue input and nothing else then the complexity doesn't make sense, as you say.

(disclaimer -- I was involved with the low-jitter driver/clock circuit used as the reference DAC, which has in the region of 1ps jitter :-)
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Old 11th January 2012, 07:55 PM   #1507
Omholt is offline Omholt  Norway
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Quote:
Originally Posted by Bruno Putzeys View Post
The trouble with purely subjectivist reviews is that you're essentially at the mercy of the reviewer's taste.
Not to mention that the reviewer's speakers and room/acoustics come into the consideration of which amp he will label as 'musical'. He might find a coloration/distortion that fits his frequency response or his problem with high gain reflections better and that has in reality very little meaning for others.

Excellent post by the way.
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Old 11th January 2012, 08:34 PM   #1508
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Originally Posted by Bruno Putzeys View Post
I presume that by "other inputs" you mean additional measurements. After all, nobody would seriously argue that an on-axis magnitude response fully describes what a speaker does. How a speaker sounds is fully determined by the sound field that speaker produces in your listening room in response to an electrical signal (I hope that isn't controversial). If a speaker does something sonically that you couldn't guess from the measurements it just means there's something you forgot to measure.

Frequency response measurements tell most of the story provided that you don't just measure on axis. Of course a large collection of responses taken at various angles becomes difficult to read but luckily you can get a good indication by averaging the response over all angles (horizontal and vertical) to get an idea of what sort of energy it's putting into the room (called the power response). If that doesn't look too funny you know that the room response has some chance of integrating well with the direct sound. Also interesting are responses taken at the angles that bounce back at the listener as first reflections.

If the on-axis response is flat, if the power response is smooth (it may have a slope or a gentle shelf) and if distortion across frequency doesn't do any ridiculous things you can be pretty much sure the speaker is going to give you years of very convincing playback. That doesn't mean that speakers who don't tick all the boxes will sound bad. Frequency response deviations and nonlinear distortion can sound really nice. That is what most sound engineering (mixing and mastering) is about: adding distortion, EQ'ing, compressing and whatnot. It is sometimes thought that audio objectivists presume that all distortions sound ugly. And since that's clearly not true, audio objectivism is therefore supposedly false. Well, no objectivist has ever claimed distortion (linear or nonlinear) necessarily sounds ugly.

They can sound really really nice. But that doesn't mean correct. Just like some amplifiers with loads of distortion add a lot of musicality that wasn't originally there. The trouble with purely subjectivist reviews is that you're essentially at the mercy of the reviewer's taste. If he just runs after that thing which sounds just that bit nicer than the previous thing he liked, he's locking himself into a positive feedback loop searching for the ultimate ear-candy. It's reasonable to expect that the occasional visit to unamplified live sound should keep this in check, but that presumes that the people who record music don't go out and buy the same high-end loudspeakers that the reviewers have just praised all the way to the heavens. But that's precisely what they do, particularly classical music studios and mastering engineers. Unless measurements get given their proper place as a design and evaluation tool, just the way they are in any engineering endeavour, there is no end in sight. Probably the only reason why people think not relying on measurements is somehow acceptable is because it's easy to confuse audio with music. I hate to break it but music is art, and audio is engineering. I don't like engineering to get in the way of art, which is why I think it should be undetectable.

From an audio quality point of view the ideal situation would be where reviews simply established to what degree the product fulfils its requirements. If it changes the sound it's bad, if it doesn't it's good. The longer I'm in this business the clearer it's become to me that measurements and sound go hand in hand provided you strive for the least colouration. If magazines actually reviewed along those lines you would be surprised how rapidly the audio industry would converge on seriously good equipment, both technically and sonically. Really good audio would just turn into a commodity.

But there lies the rub. Audio magazines are not in the business of evaluating kit. They're in the business of selling paper, which they do by expedient of printing things on that paper in order to make it more attractive. Reasonably objective magazines, so far, have either failed or moved into lighter prose and more sparse data. What all of this means is that it isn't in audio journalists' interest to tow the sonically neutral line. You just can't keep filling magazines with reviews of amplifiers that become ever and ever more transparent (and hence identical).

This is not an indictment. It's just logical.


I read their press release (which embarrassingly implied the amplifier is digital) and it didn't mention Hypex by name so I suppose that's how they want it. Still, when you bump into it there'll be no mistaking.


I'm not sure what you're trying to say. There are probably exceedingly creative ways of boiling water, but that doesn't make all of them worthy of pursuit, bar as an art project perhaps. The "first watt" philosophy is predicated on an outright lie and a fantastically illogical one at that: that products with good performance all the way up to clipping automatically perform worse at 1W than those designed to be good only up to 1W. The minimalist mode of thinking is only partly valid when one refuses to use global feedback. Indeed, in those conditions the fewer stages, the better. In spite of this, even the most arduous minimalist will know where adding a cascode does serious miracles. They may feel vindicated knowing that even "aircraft hangar" amp designers these days have succumbed to minimalism.
Bruno thanks for your thorough and very logically argued response.

I donīt think at all that you are controversial in any way when stating that the sound of a speaker is determined by the in room sound field it creates in response to an electrical signal. I fully agree, and that was also greatly what I had in mind writing my short comment.

Where I donīt follow you all the way, is in how I read you to suggest we should recognize measurements. It is not that I would like to argue for audio subjectivism as opposed to audio-objectivism, but there may be more into it than that dichotomy.

Let me try to explain what I mean by this.
I am currently doing an ethnography on science. The particular science is in applied physics on an engineering university, so they shouldnīt be too far off regarding your expressed position on audio engineering as applying natural scientific representations of the world in design and thereby the role of measurements, no?

What is very apparent to the theoretician scientists whoīs work I currently follow, is that the data their work rely on is in no way purely "objective" as it is the outcome of a measuring process which means it is both technically and intellectually constructed. By this I do not mean to imply that the data is false, but elevating it above criticism is not good practice for them. Then again they have to use data, so what else could they do than to rely on the data they can get, although they know it isnīt perfect or anywhere near being objective.

What I learn from these observations is to be cautious about how we rely on data and measurements as they in fact are constructed and therefore not only a purification of a greater whole, but also an interpretation of what they are designed to represent.

If referring to measurements as "objective" I argue that we are missing the greater picture of how technology (herein also our measurement tools and methods) have developed and matured to be replaced by new methods and technology over and over again throughout history. Recognizing the limitations of our technology and measurement tools is how we strive for developing not just better interpretations of the measurements we produce and rely on in design and science, but also better technology. If we emphasize the role of our current measurement technology to be elevated above criticism, we are in fact driving a technocratic process blindfolded. Thatīs why I think its important to open our eyes and ears too, to listen and value (also subjectively) what we make of our technology

Do you really believe that what we currently can technically measure is the last there will ever be to say about how well a loudspeaker or any other technical artifact performs? No of course not, I guess that you are pragmatical about your tools too, and that is probably greatly contributing to how you continue to perfect your designs over and over again. At least I would be surprised if we will not see some great advances in measuring technology that will influence how we understand and objectify what we work with!

Other positions than the dichotomy you put forth (audio- objectivism and subjectivism) could be audio-constructivism and relativism, which are putcomes of the philosophy of science debates that my above explanation has been greatly inspired from. Personally I consider myself as all, and none of these positions at once (probably mostly the last one if I have to choose). I think it is important to acknowledge and be reflexive about that all positions have their respective positive and negative aspects -just to be symmetrical in the argumentation at least

Your analysis on the audio review magazine industry is spot on BTW. Of course they sell glossy papers and that is what they either become good at or stop doing. That said I donīt think that purely "objective" reviews would capture all of what audio gear is about. Some subjective input is probably necessary as after all, we all have different tastes and products with no sound of their own is utopia (I havenīt heard the ncore yet of course ) as I see it though we should strive for it as I read you to suggest. I think that anyone who reads a review has to be able to interpret the reviewer. That of course implies that one should know the reviewer and know how to relate to his or her taste (not trivial of course!).

kind regards,
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Old 11th January 2012, 08:55 PM   #1509
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Quote:
Originally Posted by Bruno Putzeys View Post
......Just like some amplifiers with loads of distortion add a lot of musicality that wasn't originally there.....
Essentially agreed on what Bruno puts up, but I can't stand the term "musicality" in relation with playback equipment.
For me "musicality" only occurs at the other side of the microphones; a performance can be, or can be percepted being "musical"; the performer(s) can subjectively being judged "musical".
Playback equipment can therefore only strive for a reproduction which does the least harm of what happened at the performance side. When the playback sound is found to add a lot of "musicality" there actually is a lot of coloration coming in which should not be there; I guess that's what Bruno means.
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Old 11th January 2012, 08:59 PM   #1510
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@iand, I know how the Zetex works in much better detail than I showed in that post, but I don't think the detailed operation detracts from the basic idea that it's comparing the power stage output with the output of what is essentially a small-signal 1-bit DAC and feeding back the integrated error into the digital domain through an ADC.

Of course that needs to add further loop gain to shape the quantisation noise of the correction (which is determined by the system clock as well). But still the amplifier essentially ends up reproducing the spectrum found at the 1-bit DAC "reference". All of it. Just try adding an extra, externally generated analogue signal. It gets amplified. The circuit cannot differentiate between the intended signal and any you add externally. So the "reference" is truly a DAC. That's why it has to be this good.

This arrangement does have its merits, mind you. Since the reference contains roughly the same HF as the output, the error signal is essentially ripple free. This eliminates "ripple aliasing distortion" (i.e. the gain change the ripple would otherwise cause in the modulation process). But you get the same in an ordinary PEDEC controller.

@juhleren, I think I'll be in trouble (wife waiting in bed) if I stay up late enough to give your reply the consideration it deserves tonight. So tomorrow

Quote:
Originally Posted by pieter t View Post
Essentially agreed on what Bruno puts up, but I can't stand the term "musicality" in relation with playback equipment.
For me "musicality" only occurs at the other side of the microphones; a performance can be, or can be percepted being "musical"; the performer(s) can subjectively being judged "musical".
Playback equipment can therefore only strive for a reproduction which does the least harm of what happened at the performance side. When the playback sound is found to add a lot of "musicality" there actually is a lot of coloration coming in which should not be there; I guess that's what Bruno means.
Let's call it "pleasantness" then. Indeed, to call euphonic distortion "musicality" may be giving it too much credit.
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Last edited by Bruno Putzeys; 11th January 2012 at 09:02 PM.
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