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Old 11th January 2012, 10:40 AM   #1471
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Originally Posted by suntechnik View Post
[COLOR=black][FONT=Verdana]all energy "pops" should be precisely equal to each other and could be very well measured.
In the late 1990's there was a whole horde of people who thought likewise: that power FETs switched with infinite speed and had zero resistance, that power supplies are infinitely clean and low-impedance and that the output filter could be ignored. None of these actually hold in reality. But even if they did, it would only mean that the conversion from digital to analogue could be done perfectly using power FETs. It would not imply that the power fets were doing anything digital. It would only mean they'd be performing their analogue function very well. Since then, people have realised that they don't and they have put on their engineering hat "ok these are the physical limitations of real life circuitry, what do we do?" Engineering is first understanding the science of the real universe, and then trying to get something done based on that understanding.

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Since class D output devices either fully open or closed (two discreet states actually and it seems like very few readers of an audio related resources understands what "digital" means really)
Indeed, many people think that 2 states means digital. Hahaha, funny, isn't it (poke poke). Since you evidently consider yourself one of the few who do, you know of course how ridiculous that is. Digital means that whoever receives the signal next will interpret the signal as a string of discrete symbols. An ADSL signal is a continuous time warble but it is digital in that the end result after reception is the same bytes that were put in at the transmission end. A PWM signal is analogue because the recipient just takes the physical waveform. Since you're not one of the many who think of digital as something that happens magically as soon as a square wave turns up, you know all of this.

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But thanks for the links to the articles that were kindly provided in the thread it looks like class D amps not very matured enough to operate without global feedback yet.
Could you supply a reason why a car would be more mature if it could operate without feedback (i.e. a driver or cruise control)? Could you explain why an airplane autopilot would be more mature if it kept the thing flying straight without feedback? Or is it only amplifiers whose maturity depends on their ability to operate without feedback? It just so happens that cars with no mechanism to detect whether it's still on the road are obviously impossible, as is an airplane that stays aloft with the controls stuck in one position, and amplifiers can be made that do produce a recognizeable output signal without feedback (well apart from emitter followers in the output stage of course, which are just single-component transconductance amps with feedback around them).

Is feedbacklessness an engineering concept at all? Suppose I gave you a black box containing an amplifier, and I charged you to discover whether it used feedback or not? How would you do it? Would you measure it? Would you hear it? If you can only determine if an amplifier is "mature" by opening it and looking at the schematic, doesn't that mean that by necessity you've failed to do so by testing how well the amplifier amplifies signals?

I would think that the mature amplifier is the one who manages to replicate the input signal with the greatest accuracy. Ncore does so 30 times better than the best zero-feedback amplifiers ever made. In addition I would think that it is a sign of maturity for the audio engineer not to cling on to culturally transmitted childhood fears of the feedback bogeyman. To avoid feedback just because it's the done thing to speak badly of it is not very mature at all.

At the peril of quoting myself too much, have a look at http://www.linearaudio.net/userfiles...olume_1_BP.pdf. If after reading this you still think feedback is inherently bad, you've gone religious. In that case you might have accidentally stumbled onto the wrong thread. There are enough feedback thrashing threads to go and find fellowship in.
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Last edited by Bruno Putzeys; 11th January 2012 at 10:53 AM.
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Old 11th January 2012, 11:18 AM   #1472
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Could you supply a reason why a car would be more mature if it could operate without feedback (i.e. a driver or cruise control)?
I know a fair bit of drivers I would like to remove from the feedback loop...

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At the peril of quoting myself too much, have a look at http://www.linearaudio.net/userfiles...olume_1_BP.pdf.
Great paper! Guess audio people should read more Norbert Wiener instead of The Absurdist Sound... And it is not often that you see people who can put Otala in a rational perspective.
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Old 11th January 2012, 11:31 AM   #1473
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I actually forgot to trot out the really good argument. Noise shapers. You can't make "digital" class D amplifiers without a noise shaper. As it happens, these are essentially control loops with lots of feedback around a quantizer. The most extreme case being a 1-bit sigma-delta modulator. It continues to amaze me how zero pheedback phools can enjoy listening to SACD's which by definition require an extremely heavy-handed feedback loop in order to get the audio band free from rubbish, whilst at the same time avoiding it elsewhere in their system. I can only surmise they have never seriously looked at noise shaping or sigma-delta modulation. For instance just today someone was clumsily describing an amplifier that used "pops" (pulses) that cancel eachother out in the long run. So how exactly does one create that signal in the first place if not with an integrating feedback loop that keeps the tally? A "digital" class D amp that first makes a noise shaped PWM signal and then recklessly drives a pair of FETs with it is still a feedback amp like just any other, except that the feedback signal is taken entirely at the wrong circuit node.

Oh well. It's all the better when people decide that they are henceforth audio experts. After all, imagine them declaring themselves to be expert surgeons instead.
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Old 11th January 2012, 11:38 AM   #1474
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Oh well. It's all the better when people decide that they are henceforth audio experts. After all, imagine them declaring themselves to be expert surgeons instead.
So are you saying that you with all your stuffy textbooks somehow know better than I do what my ears hear? That's almost like the people who claim that Feng Shui is "unscientific" despite the fact that I *know* it works!

[ok, what's the emoticon for irony here...?]
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Old 11th January 2012, 11:40 AM   #1475
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Bruno the BEST way to do feedback is to use the proper amount of it then HIDE it from the end user's knowledge
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Old 11th January 2012, 12:03 PM   #1476
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So are you saying that you with all your stuffy textbooks somehow know better than I do what my ears hear? That's almost like the people who claim that Feng Shui is "unscientific" despite the fact that I *know* it works!

[ok, what's the emoticon for irony here...?]
Hmmm, I'm not sure if you're serious here!
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Old 11th January 2012, 12:06 PM   #1477
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Imagine Jimi Hendrix without feedback. Would not quite the same....
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Old 11th January 2012, 12:07 PM   #1478
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[ok, what's the emoticon for irony here...?]
perhaps? I tried looking for emoticons saying "somewhat resembling iron" but that got me nowhere.
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Bruno the BEST way to do feedback is to use the proper amount of it then HIDE it from the end user's knowledge
I tried that once, saying that "constant loop gain across the audio band makes the amplifier sound like it has no feedback". But in the long run my embarrassment at such shpiel got the better of me.

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Julf, I would suggest that YOU don't know what your ears hear. Unless you've conducted proper double-blind AB or ABX testing, I suspect you don't.
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Hmmm, I'm not sure if you're serious here!
MODS! We need an Irony emoticon!
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Old 11th January 2012, 12:15 PM   #1479
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Default iron-y

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Julf, I would suggest that YOU don't know what your ears hear. Unless you've conducted proper double-blind AB or ABX testing, I suspect you don't.


Ah, but *everybody* knows double-blind testing is worthless. Nobody listens to music like that, so it is not a realistic test. And in a double-blind situation there is so much performance anxiety and negative vibrations around that there is no way you will be able to listen properly. Not to mention the people sneering at my crystals and destroying the aura.
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Old 11th January 2012, 12:18 PM   #1480
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Default chassis contact

OK, to get back on topic... Bruno, is the cooling plate on the nc400 isolated from the circuitry so that it is safe to attach two nc400's to the same chassis, with galvanic contact between the cooling plates? And how about the cooling fins on the smps600's?
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