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The Catch-22 of Negative Feedback aka NFB
Some meterhead clown demands to know:
What's wrong with feedback? and another meterhead clown, on being told by Rudy that: an OPA2604 ... needs a *thousand* times more feedback demands to know: So what? ***** It ain't rocket science. Explanation taken from: http://members.lycos.co.uk/fiultra/T...mp%20INDEX.htm ***** CATCH-22 ***** The customer complained that his new suit didn't fit. 'The sleeves are too short,' he said to the tailor. 'Yes sir,' said the tailor, 'but if you hold your arm just so, at an angle as if you're drinking tea with your auntie, it will show just the right amount of cuff.' The customer tried it. The tailor was right! 'But what about the other sleeve? It is definitely too short.' 'Just lower your shoulder, sir. Yes, yes, a little more. Put your foot out so you can lower your shoulder a little more still. Bend your knee. Yes, that's it. See how beautifully your suitcoat now fits?' The customer had to admit the tailor was right. 'Wow. But now the leg of the pants is all twisted around.' 'That's easily fixed,' said the tailor. 'Just point your other toe westward, sir, and look over your shoulder to where I am holding up the hand mirror. See? Doesn't that fit beautifully?' 'Yes,' the customer said doubtfully, 'but-' 'Now would sir like to wear his brilliant new suit or shall we wrap it?' The customer was too intimidated to argue. He walked out into the street in his new suit, his arm crooked as if he were drinking tea, his other shoulder well down over a bent knee with his foot out to the side, his other foot pointing westwards, his head twisted back between hunched shoulders as if complaining to God about a cruel fate. Behind him he heard a boy say to his father. 'Oh, Daddy, look at the poor twisted cripple.' 'Hush,' the father said. 'Be grateful the poor man found such a brilliant tailor.' **** The Ultrafidelista view of Negative Feedback by Andre Jute Negative feedback is the paradigm of modern electronic design. It is mother's milk to an electronics engineer. He learns to say '100dB of NFB,' in his sleep before he finishes his first week at the most humble polytechnic. At the great institutions the professor of feedback is the most honoured man in the department. In Massachusetts and Minnesota the feedback guru is the most honoured man on the entire campus, equal in stature to the football coach. When a guru of transistor high fidelity (and some in tubes) says, 'I studied under Ron,' one doesn't have to ask which Ron, one just knows it is the holy name of the prophet of feedback from the Midwest. Before I even finished the design of the KISS 300B it was forcefully suggested by a wannabe guru that with only 50dB more gain (about seven times as much as is likely to be in the actual design) I can apply 50dB of negative feedback to linearize my amplifier. Negative feedback, shorthanded as NFB, is the instant response of the audio engineering fraternity to all ills, real, perceived, non-existent. They don't even ask if there is a problem, they swing the club of NFB regardless. NFB has become a reflex axiom of mainstream audio design. An audio engineer with his negative feedback is like a policeman who runs out into the street with his stick and starts beating a confession out of the first housewife he sees. The difference is that the policeman is relieved of duty to await punishment and the audio engineer gets away with it. In the case of the policeman it is unacceptable behaviour, in the case of the audio engineer so much the expected norm that no one except the ultrafidelista notice. I guess that if one in ten million audio amplifiers does not have negative feedback added, it will be a lot... No one asked if my KISS Amp requires linearization. The presumption by all except those already of the ultrafidelista persuasion was that I would welcome suggestions about A Good Thing. In the face of such overwhelming acceptance by qualified engineers, why do we as ultrafidelista not take the same easy path of negative feedback? Especially considering that superficially NFB is easy to understand and apply. How does negative feedback work? Negative feedback is simply a negative voltage fed back from the output to the input amplifying device to offset part of the harmonic distortion which is present as a positive voltage. It costs nothing except a loss of gain and a few side effects such as phase shift and possible instability which are well known in the mathematical literature and more or less easily guarded against depending on the level of NFB. 'Wow!' those meeting NFB for the first time will now say, 'Something for free! I'll grab some of that for my amp.' Hey, I said it, and I am a professional intellectual, by definition an infinite skeptic. NFB is a thing of beauty that will draw you in. It is like an electronic Marxism which admits of no contrary arguments because it has subsumed them all into The Holy Measurements. To question The Measurements is to commit heresy. You need to be of strong mind to resist the blandishments of such a universal panacea and of strong stomach to withstand the hysterical assaults of the lesser engineers defending their holy grail. (And when you do get hold of a superior engineer to explain NFB to you, you need to be high-domed indeed because suddenly NFB can turn very intricate.) Unfortunately NFB doesn't come without a price. It levies a cruel charge on the perceived quality of the sound. Negative feedback is what gives all those 'blameless' transistor and big PP tube amps their chillingly unnatural sound. Then how did NFB come to be such a panacea in amplifier design? Your guess is as good as mine. Hi-fi design is not prestige work for engineers, or highly paid. The most talented and best qualified engineers go into automobiles or military hardware or big construction projects or computer design. The left-overs design amplifiers in the time they have to spare from writing up specs for requesting a CE mark for a new electric kettle. Lemmings storming en masse over a cliff come to mind; such people don't see the necessity of original thought, or have the mental equipment for it. The exceptions to this rule are normally audio enthusiasts in charge of their own small audio manufactories with niche markets; those who grow larger from this base follow the mainstream mantra of "mo' NFB give lowa' THD" because the marketing channels demand it from them if they wish to grow. At this point they usually cease to offer anything different, only the exclusivity of a very high price. (I know, because a sub-board I designed for a supplier to the trade turns up in so many very expensive amps with so many different big names neatly silkscreened on it... it strikes me as the sort of detail a real designer, as distinct from a marketer, would take under his own control.) Those very few makers who will sell you an ultrafi amp without any NFB operate even tinier shops, usually one man and a cat, just hanging on. The mechanism by which NFB wrecks your sound Negative feedback at first acquaintance sounds good enough to take to bed and cuddle. It isn't. It isn't even as simple as a superficial acquaintance may suggest. Follow the steps with me, from the theory as she is received to what arrives at your brain as music: 1. In theory NFB reduces all harmonic distortion equally, without discrimination. Strictly in theory it does not reshape harmonic distortion by reducing the most objectionable third and higher order odd harmonic distortion to a greater extent than the relatively harmless 2nd harmonic. Thus NFB at its theoretically most benign is already useless in terms of psychoacoustics, as will become clear at point 4. If you disregard psychoacoustics, as many audio engineers do, NFB is brilliant in reducing total harmonic distortion to a number as tiny as you want. You just pile on more NFB. 2. In real life, as distinct from simplified theory, NFB adds artifacts of its own. Remember, it is a loop. The signal starts at the input and is amplified by devices until it reaches the output. From the output a part of the signal called the negative feedback is fed back to the input. Here a loop is completed and the combination, less distorted, reaches the output again, a part of the combination is fed back, endlessly. The artifacts we want to consider here are created by the fed-back residue of harmonic distortions adding to both the fundamental and the distortions already created by the amplifier, then some portion of the sum of the original and the feedback distortion is fed back again and added on, until the ooh-ah bird flies up its own fundament. It looks marginally less disgusting as a recursive mathematical formula with lots of nested parenthetical parcels of noise being loaded onto your music. But it is a monkey on the back of your sound, with a smaller monkey on the back of the first monkey, a still smaller monkey on the back of the second monkey, and so on ad infinitum. These additive artifacts are all higher harmonics and the more dominant ones are all odd. Suppose, for the sake of simplicity, a superbly designed ultrafidelista amp with some second harmonic and zero odd harmonics before NFB. Add NFB and the second harmonic will be lowered but the recombinant new loop now contains newly added intermodulation effects between the fundamental and the residual second harmonic, and that is third harmonic. In the next cycle a small but nasty dose of fifth harmonic that wasn't there before is added by interaction between the still residual second harmonic and reduced newly added third harmonic. In short, the artifacts NFB adds to the distortion mix are all of the most harmful kind. But, say the proponents of NFB, so what? Every time the loop cycles the added artifacts are smaller, even if there are more of them... The whole affair starts to smell of trying to argue with a Marxist who simply declares any inconvenient truth 'an anomaly'. (If this sounds like a mess from which you should run a mile, you have come to the right conclusion. Start running now. It gets worse.) 3. We thus arrive at a situation where distortion has been lowered by NFB but where the most disturbing odd harmonic distortions are still present to some measure, with the added disadvantage that new and extremely disturbing artifacts of higher harmonic distortions have been created by the very process of using negative feedback to lower distortion. Regardless of the absolute level of THD, or the volume setting, the mix of harmonics has been adversely affected and now includes a higher proportion of third and higher harmonics than before NFB. Let me say that again: after NFB, third and higher harmonics will make up a greater part of the distortion than before. 4. Low volume levels perforce accounts for 99 per cent of audiophile listening because we all have families or neighbours, and we would like to keep our ears. Unfortunately for the lowest common denominator of hi-fi designer, the one who specifies NFB as a conditioned response much like Pavlov's dogs slavered when the bell rang, human physiology and psycho-acoustic response is such that odd harmonics are disproportionately more disturbing at lower than at higher listening levels. This inescapable effect is independent of definition of 'listening level.' At the 110dB in-room SPL (only 14dB louder than an automatic riveter!) advocated by the already deaf Transient Overload Elite known on newsgroups as the Borg, this poisonous concoction of original distortions and NFB recombinant artifacts will be least disturbing (and soon not heard at all!). At any lower level perceived interference of this harmonics cocktail with the music will increase in inverse proportion to the volume level. At low volume levels the artifacts generated by NFB will by their nature as higher harmonic distortions be disproportionately far more disturbing. At these normal listening levels 0.75 per cent of second harmonic distortion may be below the threshhold of perception for sophisticated listeners, whereas tiny amounts of third and higher odd harmonic distortions grate. And they still use Negative Feedback? Are they stupid? No, they are not stupid. Most of them march to the drum of a cost accountant on whom we wouldn't spit if he were alight. NFB is as cheap in money terms as it is expensive in terms of perceived quality of music. We shall come to those who claim to be sympathetic to high-fidelity but insist on devices which do not work without NFB, who have another devious answer. Here, meanwhile, for you to keep in mind, is a single-sentence summary of a complicated interdisciplinary argument: The case against NFB is that for 99 per cent of listening the NFB cure is worse than the disease. But surely we don't have to do anything so stupid? It follows from the argument above that ultrafidelista should choose an intrinsically linear topology and device which does not require added negative feedback to 'linearize' the output. The intrinsically linear device is the thermionic tube in either its triode form or as a pentode hogtied to work as a triode, which can be a most pleasing alternative both economically and sonically. The topology is often single-ended operation, chosen also for several other reasons described elsewhere in these articles, including KISS; if the chosen topology is push-pull operation, which is more difficult but far from impossible to arrange without NFB, operation should be specified as Class A1. Inside the argued case above lies too the overwhelming reason to accept the potential small disadvantage that may accompany the preferred topology in comparison to the discarded alternatives. The disadvantage is of course the potential for a residual second harmonic that measures high by transistor or NFB tube standards. (Note the word potential. With a conservatively designed DHT amp the potential problem should not arise.) The ultrafidelista, who are as keen on silent amps as anyone else, accept this small potential difficulty because it is the lesser evil compared to NFB. Unbelievers (largely unwashed, according to reports) sneer that ultrafidelista like this approach because of the 'added euphonics', which is bow-wow techie talk for the warmth a big chunk of second harmonic lays on a zero negative feedback single-ended amplifier. But competent design can easily reduce the level of second harmonic to below the level of perception without the need for NFB and its deleterious after-effects. In any event, it is your amplifier. You paid for it. You have a right to tune it as you please. The key thing is to get rid of NFB and to understand why you did it. Can we prove any of this scientifically? We have already. All of this is the technical subtext to my longtime contention that what the ultrafidelista hear and love is not a directly heated triode sound as is claimed by many enthusiasts but a Class A1, ZNFB sound. (Admittedly, as we have seen above, the right sound is virtually guaranteed with a ZNFB DHT SE amp of conservative provenance but may have to be developed the hard way with more economical or higher-power contenders.) In comparative ABX tests conducted over a number of years, I found that professional musicians, certified golden ears, choose the triode-linked Class A1 PP ZNFB EL34 whenever it is present in the test over all other contenders including SE 300B and 'blameless' high-NFB silicon. Science also proceeds by pure reason. Ultrafidelista have long doubted whether what engineers insist we measure (the absolute level of distortion, THD) predicts success in audio gear. This is the full circle, because I have just proven by logical, individually uncontested steps that what matters, once a certain modest level of silence is assured to an amplifier, is not the absolute level of disharmonics but their composition. The same proof demonstrates that a more beneficial distribution follows instantly from doing without NFB. But transistor amps won't work at all without NFB! That is not our problem. Those who choose inefficient speakers and consequently are forced to accept monstrous amps made possible only by gigadeciBels of NFB, will receive our sympathy - and the music they deserve. Engineering hangers-on of transistor attempts at high fidelity, where the measure of success is vanishing THD rather than sonic hedonism, pretend to be enthusiasts for NFB. To make it work for them, they have attempted to change the rules so that we won't hear what their treasured NFB does to our sound. They sneer that low level listening, which 99 per cent of us prefer and where NFB does most to wreck the sound, is 'easy listening' and therefore not permissible. According to them we should all be forced to listen at the high volume level which suits NFB amps, which they call 'realistic'. This is a contemptible circular argument, only too characteristic of a fascist mentality in a part of the audiophile spectrum which wants to prescribe their arid vision without regard for our enjoyment. We can recommend a good tailor to them. It hurts every time you wear his suit. No pain, no gain, fellers! In summary Almost everyone listens at low level most of the time. NFB wrecks everybody's sound at all levels but most wretchedly at normal listening levels. We started out with a contemptible circular argument and we have met another along the way. We can now put both in context: An 'engineer' who designs an amplifier which does not work perfectly without negative feedback is like a tailor cutting the suit incompetently and then demanding that you walk like a cripple to make it fit, so that everyone can admire the brilliance of your tailor. Negative feedback is a bodge. That is why it is despicable to the ultrafidelista. ******* More Zero Negative Feedback amplifiers at Jute on Amps http://members.lycos.co.uk/fiultra/JUTE%20ON%20AMPS.htm The KISS Amp project, which explains much more, is here http://members.lycos.co.uk/fiultra/T...mp%20INDEX.htm and its schematics and transfer curves are here http://members.lycos.co.uk/fiultra/KISS%20190.htm ******** THE VOLTAGES IN THIS AMP WILL KILL YOU. GET EXPERIENCED SUPERVISION IF IT IS YOUR FIRST TUBE AMP All text and illustration is Copyright © Andre Jute 2001, 2004, 2006 and may not be reproduced except in the thread KISS xxx on rec.audio.tubes |
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#2
Posted to rec.audio.tubes,uk.rec.audio,rec.audio.opinion
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The Catch-22 of Negative Feedback aka NFB
"Andre Jute" wrote in message ups.com... Some meterhead clown demands to know: What's wrong with feedback? and another meterhead clown, on being told by Rudy that: an OPA2604 ... needs a *thousand* times more feedback demands to know: So what? ***** It ain't rocket science. Explanation taken from: http://members.lycos.co.uk/fiultra/T...mp%20INDEX.htm ***** CATCH-22 ***** The customer complained that his new suit didn't fit. 'The sleeves are too short,' he said to the tailor. 'Yes sir,' said the tailor, 'but if you hold your arm just so, at an angle as if you're drinking tea with your auntie, it will show just the right amount of cuff.' The customer tried it. The tailor was right! 'But what about the other sleeve? It is definitely too short.' 'Just lower your shoulder, sir. Yes, yes, a little more. Put your foot out so you can lower your shoulder a little more still. Bend your knee. Yes, that's it. See how beautifully your suitcoat now fits?' The customer had to admit the tailor was right. 'Wow. But now the leg of the pants is all twisted around.' 'That's easily fixed,' said the tailor. 'Just point your other toe westward, sir, and look over your shoulder to where I am holding up the hand mirror. See? Doesn't that fit beautifully?' 'Yes,' the customer said doubtfully, 'but-' 'Now would sir like to wear his brilliant new suit or shall we wrap it?' The customer was too intimidated to argue. He walked out into the street in his new suit, his arm crooked as if he were drinking tea, his other shoulder well down over a bent knee with his foot out to the side, his other foot pointing westwards, his head twisted back between hunched shoulders as if complaining to God about a cruel fate. Behind him he heard a boy say to his father. 'Oh, Daddy, look at the poor twisted cripple.' 'Hush,' the father said. 'Be grateful the poor man found such a brilliant tailor.' **** The Ultrafidelista view of Negative Feedback by Andre Jute Negative feedback is the paradigm of modern electronic design. It is mother's milk to an electronics engineer. He learns to say '100dB of NFB,' in his sleep before he finishes his first week at the most humble polytechnic. At the great institutions the professor of feedback is the most honoured man in the department. In Massachusetts and Minnesota the feedback guru is the most honoured man on the entire campus, equal in stature to the football coach. When a guru of transistor high fidelity (and some in tubes) says, 'I studied under Ron,' one doesn't have to ask which Ron, one just knows it is the holy name of the prophet of feedback from the Midwest. Before I even finished the design of the KISS 300B it was forcefully suggested by a wannabe guru that with only 50dB more gain (about seven times as much as is likely to be in the actual design) I can apply 50dB of negative feedback to linearize my amplifier. **ALL amplifiers utilise NFB. Every single one. Some do it vai the use of local feedback loops (like in emitter/cathode degeneration). Some use internal NFB (such as what occurs within a triode). Some use so-called 'nested feedback' loops, whilst others (the vast majority) use Global feedback systems. Your KISS 300B uses NFB. Lots of it. Perhaps not as much as a typical SS amp, but it still uses NFB for linearisation. Without it, the sound would suck. Big time. Negative feedback, shorthanded as NFB, is the instant response of the audio engineering fraternity to all ills, real, perceived, non-existent. They don't even ask if there is a problem, they swing the club of NFB regardless. NFB has become a reflex axiom of mainstream audio design. An audio engineer with his negative feedback is like a policeman who runs out into the street with his stick and starts beating a confession out of the first housewife he sees. The difference is that the policeman is relieved of duty to await punishment and the audio engineer gets away with it. In the case of the policeman it is unacceptable behaviour, in the case of the audio engineer so much the expected norm that no one except the ultrafidelista notice. I guess that if one in ten million audio amplifiers does not have negative feedback added, it will be a lot... **ALL amplifiers utilise NFB for linearisation. Every single one. Not one amplifier, anywhere, uses Zero NFB. None, nada, zip, zero. So, if you want to re-state your comment, please do so, but do so ACCURATELY. Define what types of NFB you are speaking of. No one asked if my KISS Amp requires linearization. The presumption by all except those already of the ultrafidelista persuasion was that I would welcome suggestions about A Good Thing. In the face of such overwhelming acceptance by qualified engineers, why do we as ultrafidelista not take the same easy path of negative feedback? Especially considering that superficially NFB is easy to understand and apply. How does negative feedback work? Negative feedback is simply a negative voltage fed back from the output to the input amplifying device to offset part of the harmonic distortion which is present as a positive voltage. It costs nothing except a loss of gain and a few side effects such as phase shift and possible instability which are well known in the mathematical literature and more or less easily guarded against depending on the level of NFB. **A gross oversimplification. And you know it. There are many methods of applying NFB. Some methods have minimal side effects. 'Wow!' those meeting NFB for the first time will now say, 'Something for free! I'll grab some of that for my amp.' Hey, I said it, and I am a professional intellectual, by definition an infinite skeptic. NFB is a thing of beauty that will draw you in. It is like an electronic Marxism which admits of no contrary arguments because it has subsumed them all into The Holy Measurements. To question The Measurements is to commit heresy. You need to be of strong mind to resist the blandishments of such a universal panacea and of strong stomach to withstand the hysterical assaults of the lesser engineers defending their holy grail. (And when you do get hold of a superior engineer to explain NFB to you, you need to be high-domed indeed because suddenly NFB can turn very intricate.) Unfortunately NFB doesn't come without a price. It levies a cruel charge on the perceived quality of the sound. Negative feedback is what gives all those 'blameless' transistor and big PP tube amps their chillingly unnatural sound. **Er, there is another possibility: NFB allows those amplifiers to provide clean, undistorted sound. Lack (or more correctly: INSUFFICIENT) NFB merely alloows an amplifier to possess a distrotion characteristic which is enjoyed by many listeners. ANECDOTE Many years ago, I service musical instrument amplifiers (almost always tube types) on a regular basis. One lead guitarist client was very particular about his favourite amplifier. It had to produce just the right amount of distortion. It was an ancient 20 Watt amp, with an inbuilt speaker. The amp was miked to a large (soild state) PA system. He chose the SS PA system because it did not distort the sound of his little 20 Watt amp. /ANECDOTE Then how did NFB come to be such a panacea in amplifier design? **Er, because it works. Your KISS 300B amp uses NFB. YOU know that NFB works. Your guess is as good as mine. Hi-fi design is not prestige work for engineers, or highly paid. The most talented and best qualified engineers go into automobiles or military hardware or big construction projects or computer design. **And, guess what? All modern automobiles use NFB systems of some complexity in their fuel injection systems. Same deal with military control systems. Like it or not, NFB is pretty much essential to the operation of all electronic systems. Including your KISS 300B amplifier. The left-overs design amplifiers in the time they have to spare from writing up specs for requesting a CE mark for a new electric kettle. Lemmings storming en masse over a cliff come to mind; such people don't see the necessity of original thought, or have the mental equipment for it. The exceptions to this rule are normally audio enthusiasts in charge of their own small audio manufactories with niche markets; those who grow larger from this base follow the mainstream mantra of "mo' NFB give lowa' THD" because the marketing channels demand it from them if they wish to grow. At this point they usually cease to offer anything different, only the exclusivity of a very high price. (I know, because a sub-board I designed for a supplier to the trade turns up in so many very expensive amps with so many different big names neatly silkscreened on it... it strikes me as the sort of detail a real designer, as distinct from a marketer, would take under his own control.) Those very few makers who will sell you an ultrafi amp without any NFB operate even tinier shops, usually one man and a cat, just hanging on. **They lie. ALL amplifiers utilise NFB for linearisation. Every single one. Including your KISS 300B amp. The mechanism by which NFB wrecks your sound Negative feedback at first acquaintance sounds good enough to take to bed and cuddle. It isn't. It isn't even as simple as a superficial acquaintance may suggest. Follow the steps with me, from the theory as she is received to what arrives at your brain as music: 1. In theory NFB reduces all harmonic distortion equally, without discrimination. Strictly in theory it does not reshape harmonic distortion by reducing the most objectionable third and higher order odd harmonic distortion to a greater extent than the relatively harmless 2nd harmonic. **No distortion is harmless. 2nd harmonic *may* be less objectionable to SOME listeners. It is not harmless. Moreover, NFB reduces odd order distortion. And even order distortion. Thus NFB at its theoretically most benign is already useless in terms of psychoacoustics, as will become clear at point 4. If you disregard psychoacoustics, as many audio engineers do, NFB is brilliant in reducing total harmonic distortion to a number as tiny as you want. You just pile on more NFB. 2. In real life, as distinct from simplified theory, NFB adds artifacts of its own. Remember, it is a loop. The signal starts at the input and is amplified by devices until it reaches the output. **That would depend on the size of the loop. The NFB loop contained with a triode is small. As is the loop contained with an emitter degeneration system. As a consequence, the problems tend to be reduced. From the output a part of the signal called the negative feedback is fed back to the input. Here a loop is completed and the combination, less distorted, reaches the output again, a part of the combination is fed back, endlessly. The artifacts we want to consider here are created by the fed-back residue of harmonic distortions adding to both the fundamental and the distortions already created by the amplifier, then some portion of the sum of the original and the feedback distortion is fed back again and added on, until the ooh-ah bird flies up its own fundament. It looks marginally less disgusting as a recursive mathematical formula with lots of nested parenthetical parcels of noise being loaded onto your music. But it is a monkey on the back of your sound, with a smaller monkey on the back of the first monkey, a still smaller monkey on the back of the second monkey, and so on ad infinitum. These additive artifacts are all higher harmonics and the more dominant ones are all odd. **Perhaps. What if these artefacts are above (say) 30kHz and below (say) 0.01% of the total? Are they audible? Can you hear less than 0.01% THD, regardless of the harmonic structure? Suppose, for the sake of simplicity, a superbly designed ultrafidelista amp with some second harmonic and zero odd harmonics before NFB. **ALL amplifiers utilise NFB. Every single one. Therre is no such thing as an amplifier which has no NFB. Add NFB and the second harmonic will be lowered but the recombinant new loop now contains newly added intermodulation effects between the fundamental and the residual second harmonic, and that is third harmonic. In the next cycle a small but nasty dose of fifth harmonic that wasn't there before is added by interaction between the still residual second harmonic and reduced newly added third harmonic. In short, the artifacts NFB adds to the distortion mix are all of the most harmful kind. But, say the proponents of NFB, so what? Every time the loop cycles the added artifacts are smaller, even if there are more of them... The whole affair starts to smell of trying to argue with a Marxist who simply declares any inconvenient truth 'an anomaly'. (If this sounds like a mess from which you should run a mile, you have come to the right conclusion. Start running now. It gets worse.) **You're making a whole bunch of bad assumptions. Starting with your most basic, incorrect assumption: That an amplifier exists which does not use NFB. It ain't so. ALL amplifiers use NFB. It is tthe TYPE and amount of NFB which varies between amplifiers. 3. We thus arrive at a situation where distortion has been lowered by NFB but where the most disturbing odd harmonic distortions are still present to some measure, **What if those artefacts are below (say) 0.01%? with the added disadvantage that new and extremely disturbing artifacts of higher harmonic distortions have been created by the very process of using negative feedback to lower distortion. Regardless of the absolute level of THD, or the volume setting, the mix of harmonics has been adversely affected and now includes a higher proportion of third and higher harmonics than before NFB. Let me say that again: after NFB, third and higher harmonics will make up a greater part of the distortion than before. **What if those artefacts are below (say) 0.01%? 4. Low volume levels perforce accounts for 99 per cent of audiophile listening because we all have families or neighbours, and we would like to keep our ears. **Cite you evidence to prove this. Additionally, cite the levels that you are referring to. Is a "low volume level" equivalent to 0.1 Watt, 1 Watt, 10 Watts, or 100 Watts? Please be specific. What is the room size? Speaker efficiency? The dynamic range of the recordings used? Unfortunately for the lowest common denominator of hi-fi designer, the one who specifies NFB as a conditioned response much like Pavlov's dogs slavered when the bell rang, human physiology and psycho-acoustic response is such that odd harmonics are disproportionately more disturbing at lower than at higher listening levels. This inescapable effect is independent of definition of 'listening level.' At the 110dB in-room SPL (only 14dB louder than an automatic riveter!) advocated by the already deaf Transient Overload Elite known on newsgroups as the Borg, this poisonous concoction of original distortions and NFB recombinant artifacts will be least disturbing (and soon not heard at all!). At any lower level perceived interference of this harmonics cocktail with the music will increase in inverse proportion to the volume level. At low volume levels the artifacts generated by NFB will by their nature as higher harmonic distortions be disproportionately far more disturbing. At these normal listening levels 0.75 per cent of second harmonic distortion may be below the threshhold of perception for sophisticated listeners, whereas tiny amounts of third and higher odd harmonic distortions grate. **How much odd harmonic distortion? 0.01%? More? Less? And they still use Negative Feedback? **All amplifiers use NFB. Every single one. Are they stupid? **Your own KISS 300B amplifier uses NFB. Does that make you stupid? No, they are not stupid. Most of them march to the drum of a cost accountant on whom we wouldn't spit if he were alight. NFB is as cheap in money terms as it is expensive in terms of perceived quality of music. We shall come to those who claim to be sympathetic to high-fidelity but insist on devices which do not work without NFB, who have another devious answer. Here, meanwhile, for you to keep in mind, is a single-sentence summary of a complicated interdisciplinary argument: The case against NFB is that for 99 per cent of listening the NFB cure is worse than the disease. **ALL amplifiers use NFB. Every single one. If you imagine that one exists which does not use NFB, then present your evidence. But surely we don't have to do anything so stupid? It follows from the argument above that ultrafidelista should choose an intrinsically linear topology and device which does not require added negative feedback to 'linearize' the output. **No such device, nor system exists. The intrinsically linear device is the thermionic tube in either its triode form or as a pentode hogtied to work as a triode, which can be a most pleasing alternative both economically and sonically. **Incorrect. On several counts. Triodes utilise an internal NFB system to linearise their operation. Modern BJTs are more linear than triodes. Yes, MORE linear. Even quite high power BJTs are very, very linear indeed. More linear than triodes and MUCH more linear than the accused Pentodes. Get with the programme and look at the spec sheets (run the curves yourself, if you wish) of (say) the Toshiba 2SC5200. From less than 0.01A all the way past 3 Amps, it demonstates bugger all variation in gain. Let's see a triode (any triode) come close to that feat. Modern BJTs are phenomenally linear. Something which most tube nuts seem blissfully unaware of of, since they lost interestin transistors back in 1970, when single diffused, low frequency and highly non-linear devices were the only things available. Move into the 21st Century and you will se a whole new world of very impressive devices available to the designer. The topology is often single-ended operation, chosen also for several other reasons described elsewhere in these articles, including KISS; **Of course. Some listeners enjoy distortion. PP reduces distortion. No downsides. Just less distortion. if the chosen topology is push-pull operation, which is more difficult but far from impossible to arrange without NFB, operation should be specified as Class A1. **Why? With suitably matched devices (impossible with tubes, I admit), one need only organise enough bias current to keep operation in the linear part of the amplification curve. Any more is wasteful. Inside the argued case above lies too the overwhelming reason to accept the potential small disadvantage that may accompany the preferred topology in comparison to the discarded alternatives. The disadvantage is of course the potential for a residual second harmonic that measures high by transistor or NFB tube standards. (Note the word potential. With a conservatively designed DHT amp the potential problem should not arise.) The ultrafidelista, who are as keen on silent amps as anyone else, accept this small potential difficulty because it is the lesser evil compared to NFB. **ALL amplifiers use NFB. Every single one. Unbelievers (largely unwashed, according to reports) sneer that ultrafidelista like this approach because of the 'added euphonics', which is bow-wow techie talk for the warmth a big chunk of second harmonic lays on a zero negative feedback single-ended amplifier. **There is nothing that a SE design can do that a PP design cannot do cheaper, better and more efficiently. But competent design can easily reduce the level of second harmonic to below the level of perception without the need for NFB and its deleterious after-effects. **All amplifier use NFB. Every single one. In any event, it is your amplifier. You paid for it. You have a right to tune it as you please. The key thing is to get rid of NFB and to understand why you did it. **You can't "get rid" of NFB. Particularly in triode amplifiers. It is built right into the triode. If you "get rid" of the feedback in a triode, you end up with a pentode. I prefer to call that an abomination. Can we prove any of this scientifically? We have already. **You first need to prove that an amplifier eixists which can operate without NFB. All of this is the technical subtext to my longtime contention that what the ultrafidelista hear and love is not a directly heated triode sound as is claimed by many enthusiasts but a Class A1, ZNFB sound. (Admittedly, as we have seen above, the right sound is virtually guaranteed with a ZNFB DHT SE amp of conservative provenance but may have to be developed the hard way with more economical or higher-power contenders.) In comparative ABX tests conducted over a number of years, I found that professional musicians, certified golden ears, choose the triode-linked Class A1 PP ZNFB EL34 whenever it is present in the test over all other contenders including SE 300B and 'blameless' high-NFB silicon. **Cite please. Cite SPECIFIC amplifiers used. Cite operating conditions. Science also proceeds by pure reason. Ultrafidelista have long doubted whether what engineers insist we measure (the absolute level of distortion, THD) predicts success in audio gear. **Duh. It is reasonable assumed by most that levels of THD below (say) 0.1% are pretty much inaudible. Reductions below this level are pretty much inconsequential. Conversely, levels ABOVE (say) 0.5% are not inconsequential. They are audible to many listeners. This is the full circle, because I have just proven by logical, individually uncontested steps that what matters, once a certain modest level of silence is assured to an amplifier, is not the absolute level of disharmonics but their composition. The same proof demonstrates that a more beneficial distribution follows instantly from doing without NFB. **Nope. Your train of logic is utterly and completely flawed. In several ways: * ALL amplifiers use NFB. Every single one. * Amplifiers of different types, can use widely different types of NFB application. You have been spectacularly vague about the different types of NFB available to designers. * Modern transistors (specifically output transistors) are MUCH more linear than any tube. Including triodes. But transistor amps won't work at all without NFB! **NO AMPLIFIER WORKS WITHOUT NFB. Not one. That is not our problem. Those who choose inefficient speakers and consequently are forced to accept monstrous amps made possible only by gigadeciBels of NFB, will receive our sympathy - and the music they deserve. **It is extremely costly and difficult to build high efficiency speakers, which also possess a wide bandwidth and good dispersion characteristics. If you know of any, then cite them. Engineering hangers-on of transistor attempts at high fidelity, where the measure of success is vanishing THD rather than sonic hedonism, pretend to be enthusiasts for NFB. To make it work for them, they have attempted to change the rules so that we won't hear what their treasured NFB does to our sound. They sneer that low level listening, which 99 per cent of us prefer and where NFB does most to wreck the sound, is 'easy listening' and therefore not permissible. According to them we should all be forced to listen at the high volume level which suits NFB amps, which they call 'realistic'. **I know of no listeners who are "forced" to do anything. This is a contemptible circular argument, only too characteristic of a fascist mentality in a part of the audiophile spectrum which wants to prescribe their arid vision without regard for our enjoyment. We can recommend a good tailor to them. It hurts every time you wear his suit. No pain, no gain, fellers! In summary Almost everyone listens at low level most of the time. **Prove it. NFB wrecks everybody's sound at all levels but most wretchedly at normal listening levels. **Even your own KISS 300B amp, which uses NFB? How curious. We started out with a contemptible circular argument and we have met another along the way. We can now put both in context: An 'engineer' who designs an amplifier which does not work perfectly without negative feedback is like a tailor cutting the suit incompetently and then demanding that you walk like a cripple to make it fit, so that everyone can admire the brilliance of your tailor. Negative feedback is a bodge. That is why it is despicable to the ultrafidelista. **ALL amplifiers use NFB for linearisation. Every single one. -- Trevor Wilson www.rageaudio.com.au |
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Posted to rec.audio.tubes,uk.rec.audio,rec.audio.opinion
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The Catch-22 of Negative Feedback aka NFB
"Trevor Wilson" wrote in message ... : : "Andre Jute" wrote in message : ups.com... : Some meterhead clown demands to know: : What's wrong with feedback? : and another meterhead clown, on being told by Rudy that: : an OPA2604 ... needs a *thousand* times more feedback : demands to know: : So what? snipped lotsastuff youcanreadupwardsinthethread : -- : Trevor Wilson : www.rageaudio.com.au we've been through this 'linearity contest' before, here. whereas you define it in terms of current amplification, i define it in terms of voltage amplification. as real-world interfacing in audio is with *voltages*, doesn't seem unreasonable... excusez for not having included you on the ss shortlist. so, why not build flipper's frontend , completed with that Toshiba 2SC5200 for power & report back ? Rudy |
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#4
Posted to rec.audio.tubes,uk.rec.audio,rec.audio.opinion
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The Catch-22 of Negative Feedback aka NFB
Trevor Wilson wrote:
(a long commentary on my original post, all of it reproduced in full below) I never said an amp can work without NFB; that's your desperate spin on the matter. In fact, I took part in a long thread which determined that a 300B has about 12-14dB of internal or natural NFB. You, Trevor Wilson, know that what I actually believe is that most amps work better without *added* negative feedback. And I told you so again: It follows from the argument above that ultrafidelista should choose an intrinsically linear topology and device which does not require added negative feedback to 'linearize' the output. See the "added"? It makes clear to even the rawest newbie what I intend. No one who has been in high level tube amps for more than a semester needs it spelled out. Everyone knows the convention is that an amp without added NFB is described as having Zero Negative Feedback. You, Trevor Wilson, cannot fail to know it, therefore you are picking desperate nits -- and giving away your desperation by hysterically screeching your misleading lie "no such thing as a ZNFB amp" over and over again, probably more than twenty times (those who care can count below). Your problem is that you know I'm right but that you have thirty years invested in commercially telling people tubes are obsolete. Andre Jute Here is Trevor Wilson's full commentary: "Andre Jute" wrote in message ups.com... Some meterhead clown demands to know: What's wrong with feedback? and another meterhead clown, on being told by Rudy that: an OPA2604 ... needs a *thousand* times more feedback demands to know: So what? ***** It ain't rocket science. Explanation taken from: http://members.lycos.co.uk/fiultra/T...mp%20INDEX.htm ***** CATCH-22 ***** The customer complained that his new suit didn't fit. 'The sleeves are too short,' he said to the tailor. 'Yes sir,' said the tailor, 'but if you hold your arm just so, at an angle as if you're drinking tea with your auntie, it will show just the right amount of cuff.' The customer tried it. The tailor was right! 'But what about the other sleeve? It is definitely too short.' 'Just lower your shoulder, sir. Yes, yes, a little more. Put your foot out so you can lower your shoulder a little more still. Bend your knee. Yes, that's it. See how beautifully your suitcoat now fits?' The customer had to admit the tailor was right. 'Wow. But now the leg of the pants is all twisted around.' 'That's easily fixed,' said the tailor. 'Just point your other toe westward, sir, and look over your shoulder to where I am holding up the hand mirror. See? Doesn't that fit beautifully?' 'Yes,' the customer said doubtfully, 'but-' 'Now would sir like to wear his brilliant new suit or shall we wrap it?' The customer was too intimidated to argue. He walked out into the street in his new suit, his arm crooked as if he were drinking tea, his other shoulder well down over a bent knee with his foot out to the side, his other foot pointing westwards, his head twisted back between hunched shoulders as if complaining to God about a cruel fate. Behind him he heard a boy say to his father. 'Oh, Daddy, look at the poor twisted cripple.' 'Hush,' the father said. 'Be grateful the poor man found such a brilliant tailor.' **** The Ultrafidelista view of Negative Feedback by Andre Jute Negative feedback is the paradigm of modern electronic design. It is mother's milk to an electronics engineer. He learns to say '100dB of NFB,' in his sleep before he finishes his first week at the most humble polytechnic. At the great institutions the professor of feedback is the most honoured man in the department. In Massachusetts and Minnesota the feedback guru is the most honoured man on the entire campus, equal in stature to the football coach. When a guru of transistor high fidelity (and some in tubes) says, 'I studied under Ron,' one doesn't have to ask which Ron, one just knows it is the holy name of the prophet of feedback from the Midwest. Before I even finished the design of the KISS 300B it was forcefully suggested by a wannabe guru that with only 50dB more gain (about seven times as much as is likely to be in the actual design) I can apply 50dB of negative feedback to linearize my amplifier. **ALL amplifiers utilise NFB. Every single one. Some do it vai the use of local feedback loops (like in emitter/cathode degeneration). Some use internal NFB (such as what occurs within a triode). Some use so-called 'nested feedback' loops, whilst others (the vast majority) use Global feedback systems. Your KISS 300B uses NFB. Lots of it. Perhaps not as much as a typical SS amp, but it still uses NFB for linearisation. Without it, the sound would suck. Big time. Negative feedback, shorthanded as NFB, is the instant response of the audio engineering fraternity to all ills, real, perceived, non-existent. They don't even ask if there is a problem, they swing the club of NFB regardless. NFB has become a reflex axiom of mainstream audio design. An audio engineer with his negative feedback is like a policeman who runs out into the street with his stick and starts beating a confession out of the first housewife he sees. The difference is that the policeman is relieved of duty to await punishment and the audio engineer gets away with it. In the case of the policeman it is unacceptable behaviour, in the case of the audio engineer so much the expected norm that no one except the ultrafidelista notice. I guess that if one in ten million audio amplifiers does not have negative feedback added, it will be a lot... **ALL amplifiers utilise NFB for linearisation. Every single one. Not one amplifier, anywhere, uses Zero NFB. None, nada, zip, zero. So, if you want to re-state your comment, please do so, but do so ACCURATELY. Define what types of NFB you are speaking of. No one asked if my KISS Amp requires linearization. The presumption by all except those already of the ultrafidelista persuasion was that I would welcome suggestions about A Good Thing. In the face of such overwhelming acceptance by qualified engineers, why do we as ultrafidelista not take the same easy path of negative feedback? Especially considering that superficially NFB is easy to understand and apply. How does negative feedback work? Negative feedback is simply a negative voltage fed back from the output to the input amplifying device to offset part of the harmonic distortion which is present as a positive voltage. It costs nothing except a loss of gain and a few side effects such as phase shift and possible instability which are well known in the mathematical literature and more or less easily guarded against depending on the level of NFB. **A gross oversimplification. And you know it. There are many methods of applying NFB. Some methods have minimal side effects. 'Wow!' those meeting NFB for the first time will now say, 'Something for free! I'll grab some of that for my amp.' Hey, I said it, and I am a professional intellectual, by definition an infinite skeptic. NFB is a thing of beauty that will draw you in. It is like an electronic Marxism which admits of no contrary arguments because it has subsumed them all into The Holy Measurements. To question The Measurements is to commit heresy. You need to be of strong mind to resist the blandishments of such a universal panacea and of strong stomach to withstand the hysterical assaults of the lesser engineers defending their holy grail. (And when you do get hold of a superior engineer to explain NFB to you, you need to be high-domed indeed because suddenly NFB can turn very intricate.) Unfortunately NFB doesn't come without a price. It levies a cruel charge on the perceived quality of the sound. Negative feedback is what gives all those 'blameless' transistor and big PP tube amps their chillingly unnatural sound. **Er, there is another possibility: NFB allows those amplifiers to provide clean, undistorted sound. Lack (or more correctly: INSUFFICIENT) NFB merely alloows an amplifier to possess a distrotion characteristic which is enjoyed by many listeners. ANECDOTE Many years ago, I service musical instrument amplifiers (almost always tube types) on a regular basis. One lead guitarist client was very particular about his favourite amplifier. It had to produce just the right amount of distortion. It was an ancient 20 Watt amp, with an inbuilt speaker. The amp was miked to a large (soild state) PA system. He chose the SS PA system because it did not distort the sound of his little 20 Watt amp. /ANECDOTE Then how did NFB come to be such a panacea in amplifier design? **Er, because it works. Your KISS 300B amp uses NFB. YOU know that NFB works. Your guess is as good as mine. Hi-fi design is not prestige work for engineers, or highly paid. The most talented and best qualified engineers go into automobiles or military hardware or big construction projects or computer design. **And, guess what? All modern automobiles use NFB systems of some complexity in their fuel injection systems. Same deal with military control systems. Like it or not, NFB is pretty much essential to the operation of all electronic systems. Including your KISS 300B amplifier. The left-overs design amplifiers in the time they have to spare from writing up specs for requesting a CE mark for a new electric kettle. Lemmings storming en masse over a cliff come to mind; such people don't see the necessity of original thought, or have the mental equipment for it. The exceptions to this rule are normally audio enthusiasts in charge of their own small audio manufactories with niche markets; those who grow larger from this base follow the mainstream mantra of "mo' NFB give lowa' THD" because the marketing channels demand it from them if they wish to grow. At this point they usually cease to offer anything different, only the exclusivity of a very high price. (I know, because a sub-board I designed for a supplier to the trade turns up in so many very expensive amps with so many different big names neatly silkscreened on it... it strikes me as the sort of detail a real designer, as distinct from a marketer, would take under his own control.) Those very few makers who will sell you an ultrafi amp without any NFB operate even tinier shops, usually one man and a cat, just hanging on. **They lie. ALL amplifiers utilise NFB for linearisation. Every single one. Including your KISS 300B amp. The mechanism by which NFB wrecks your sound Negative feedback at first acquaintance sounds good enough to take to bed and cuddle. It isn't. It isn't even as simple as a superficial acquaintance may suggest. Follow the steps with me, from the theory as she is received to what arrives at your brain as music: 1. In theory NFB reduces all harmonic distortion equally, without discrimination. Strictly in theory it does not reshape harmonic distortion by reducing the most objectionable third and higher order odd harmonic distortion to a greater extent than the relatively harmless 2nd harmonic. **No distortion is harmless. 2nd harmonic *may* be less objectionable to SOME listeners. It is not harmless. Moreover, NFB reduces odd order distortion. And even order distortion. Thus NFB at its theoretically most benign is already useless in terms of psychoacoustics, as will become clear at point 4. If you disregard psychoacoustics, as many audio engineers do, NFB is brilliant in reducing total harmonic distortion to a number as tiny as you want. You just pile on more NFB. 2. In real life, as distinct from simplified theory, NFB adds artifacts of its own. Remember, it is a loop. The signal starts at the input and is amplified by devices until it reaches the output. **That would depend on the size of the loop. The NFB loop contained with a triode is small. As is the loop contained with an emitter degeneration system. As a consequence, the problems tend to be reduced. From the output a part of the signal called the negative feedback is fed back to the input. Here a loop is completed and the combination, less distorted, reaches the output again, a part of the combination is fed back, endlessly. The artifacts we want to consider here are created by the fed-back residue of harmonic distortions adding to both the fundamental and the distortions already created by the amplifier, then some portion of the sum of the original and the feedback distortion is fed back again and added on, until the ooh-ah bird flies up its own fundament. It looks marginally less disgusting as a recursive mathematical formula with lots of nested parenthetical parcels of noise being loaded onto your music. But it is a monkey on the back of your sound, with a smaller monkey on the back of the first monkey, a still smaller monkey on the back of the second monkey, and so on ad infinitum. These additive artifacts are all higher harmonics and the more dominant ones are all odd. **Perhaps. What if these artefacts are above (say) 30kHz and below (say) 0.01% of the total? Are they audible? Can you hear less than 0.01% THD, regardless of the harmonic structure? Suppose, for the sake of simplicity, a superbly designed ultrafidelista amp with some second harmonic and zero odd harmonics before NFB. **ALL amplifiers utilise NFB. Every single one. Therre is no such thing as an amplifier which has no NFB. Add NFB and the second harmonic will be lowered but the recombinant new loop now contains newly added intermodulation effects between the fundamental and the residual second harmonic, and that is third harmonic. In the next cycle a small but nasty dose of fifth harmonic that wasn't there before is added by interaction between the still residual second harmonic and reduced newly added third harmonic. In short, the artifacts NFB adds to the distortion mix are all of the most harmful kind. But, say the proponents of NFB, so what? Every time the loop cycles the added artifacts are smaller, even if there are more of them... The whole affair starts to smell of trying to argue with a Marxist who simply declares any inconvenient truth 'an anomaly'. (If this sounds like a mess from which you should run a mile, you have come to the right conclusion. Start running now. It gets worse.) **You're making a whole bunch of bad assumptions. Starting with your most basic, incorrect assumption: That an amplifier exists which does not use NFB. It ain't so. ALL amplifiers use NFB. It is tthe TYPE and amount of NFB which varies between amplifiers. 3. We thus arrive at a situation where distortion has been lowered by NFB but where the most disturbing odd harmonic distortions are still present to some measure, **What if those artefacts are below (say) 0.01%? with the added disadvantage that new and extremely disturbing artifacts of higher harmonic distortions have been created by the very process of using negative feedback to lower distortion. Regardless of the absolute level of THD, or the volume setting, the mix of harmonics has been adversely affected and now includes a higher proportion of third and higher harmonics than before NFB. Let me say that again: after NFB, third and higher harmonics will make up a greater part of the distortion than before. **What if those artefacts are below (say) 0.01%? 4. Low volume levels perforce accounts for 99 per cent of audiophile listening because we all have families or neighbours, and we would like to keep our ears. **Cite you evidence to prove this. Additionally, cite the levels that you are referring to. Is a "low volume level" equivalent to 0.1 Watt, 1 Watt, 10 Watts, or 100 Watts? Please be specific. What is the room size? Speaker efficiency? The dynamic range of the recordings used? Unfortunately for the lowest common denominator of hi-fi designer, the one who specifies NFB as a conditioned response much like Pavlov's dogs slavered when the bell rang, human physiology and psycho-acoustic response is such that odd harmonics are disproportionately more disturbing at lower than at higher listening levels. This inescapable effect is independent of definition of 'listening level.' At the 110dB in-room SPL (only 14dB louder than an automatic riveter!) advocated by the already deaf Transient Overload Elite known on newsgroups as the Borg, this poisonous concoction of original distortions and NFB recombinant artifacts will be least disturbing (and soon not heard at all!). At any lower level perceived interference of this harmonics cocktail with the music will increase in inverse proportion to the volume level. At low volume levels the artifacts generated by NFB will by their nature as higher harmonic distortions be disproportionately far more disturbing. At these normal listening levels 0.75 per cent of second harmonic distortion may be below the threshhold of perception for sophisticated listeners, whereas tiny amounts of third and higher odd harmonic distortions grate. **How much odd harmonic distortion? 0.01%? More? Less? And they still use Negative Feedback? **All amplifiers use NFB. Every single one. Are they stupid? **Your own KISS 300B amplifier uses NFB. Does that make you stupid? No, they are not stupid. Most of them march to the drum of a cost accountant on whom we wouldn't spit if he were alight. NFB is as cheap in money terms as it is expensive in terms of perceived quality of music. We shall come to those who claim to be sympathetic to high-fidelity but insist on devices which do not work without NFB, who have another devious answer. Here, meanwhile, for you to keep in mind, is a single-sentence summary of a complicated interdisciplinary argument: The case against NFB is that for 99 per cent of listening the NFB cure is worse than the disease. **ALL amplifiers use NFB. Every single one. If you imagine that one exists which does not use NFB, then present your evidence. But surely we don't have to do anything so stupid? It follows from the argument above that ultrafidelista should choose an intrinsically linear topology and device which does not require added negative feedback to 'linearize' the output. **No such device, nor system exists. The intrinsically linear device is the thermionic tube in either its triode form or as a pentode hogtied to work as a triode, which can be a most pleasing alternative both economically and sonically. **Incorrect. On several counts. Triodes utilise an internal NFB system to linearise their operation. Modern BJTs are more linear than triodes. Yes, MORE linear. Even quite high power BJTs are very, very linear indeed. More linear than triodes and MUCH more linear than the accused Pentodes. Get with the programme and look at the spec sheets (run the curves yourself, if you wish) of (say) the Toshiba 2SC5200. From less than 0.01A all the way past 3 Amps, it demonstates bugger all variation in gain. Let's see a triode (any triode) come close to that feat. Modern BJTs are phenomenally linear. Something which most tube nuts seem blissfully unaware of of, since they lost interestin transistors back in 1970, when single diffused, low frequency and highly non-linear devices were the only things available. Move into the 21st Century and you will se a whole new world of very impressive devices available to the designer. The topology is often single-ended operation, chosen also for several other reasons described elsewhere in these articles, including KISS; **Of course. Some listeners enjoy distortion. PP reduces distortion. No downsides. Just less distortion. if the chosen topology is push-pull operation, which is more difficult but far from impossible to arrange without NFB, operation should be specified as Class A1. **Why? With suitably matched devices (impossible with tubes, I admit), one need only organise enough bias current to keep operation in the linear part of the amplification curve. Any more is wasteful. Inside the argued case above lies too the overwhelming reason to accept the potential small disadvantage that may accompany the preferred topology in comparison to the discarded alternatives. The disadvantage is of course the potential for a residual second harmonic that measures high by transistor or NFB tube standards. (Note the word potential. With a conservatively designed DHT amp the potential problem should not arise.) The ultrafidelista, who are as keen on silent amps as anyone else, accept this small potential difficulty because it is the lesser evil compared to NFB. **ALL amplifiers use NFB. Every single one. Unbelievers (largely unwashed, according to reports) sneer that ultrafidelista like this approach because of the 'added euphonics', which is bow-wow techie talk for the warmth a big chunk of second harmonic lays on a zero negative feedback single-ended amplifier. **There is nothing that a SE design can do that a PP design cannot do cheaper, better and more efficiently. But competent design can easily reduce the level of second harmonic to below the level of perception without the need for NFB and its deleterious after-effects. **All amplifier use NFB. Every single one. In any event, it is your amplifier. You paid for it. You have a right to tune it as you please. The key thing is to get rid of NFB and to understand why you did it. **You can't "get rid" of NFB. Particularly in triode amplifiers. It is built right into the triode. If you "get rid" of the feedback in a triode, you end up with a pentode. I prefer to call that an abomination. Can we prove any of this scientifically? We have already. **You first need to prove that an amplifier eixists which can operate without NFB. All of this is the technical subtext to my longtime contention that what the ultrafidelista hear and love is not a directly heated triode sound as is claimed by many enthusiasts but a Class A1, ZNFB sound. (Admittedly, as we have seen above, the right sound is virtually guaranteed with a ZNFB DHT SE amp of conservative provenance but may have to be developed the hard way with more economical or higher-power contenders.) In comparative ABX tests conducted over a number of years, I found that professional musicians, certified golden ears, choose the triode-linked Class A1 PP ZNFB EL34 whenever it is present in the test over all other contenders including SE 300B and 'blameless' high-NFB silicon. **Cite please. Cite SPECIFIC amplifiers used. Cite operating conditions. Science also proceeds by pure reason. Ultrafidelista have long doubted whether what engineers insist we measure (the absolute level of distortion, THD) predicts success in audio gear. **Duh. It is reasonable assumed by most that levels of THD below (say) 0.1% are pretty much inaudible. Reductions below this level are pretty much inconsequential. Conversely, levels ABOVE (say) 0.5% are not inconsequential. They are audible to many listeners. This is the full circle, because I have just proven by logical, individually uncontested steps that what matters, once a certain modest level of silence is assured to an amplifier, is not the absolute level of disharmonics but their composition. The same proof demonstrates that a more beneficial distribution follows instantly from doing without NFB. **Nope. Your train of logic is utterly and completely flawed. In several ways: * ALL amplifiers use NFB. Every single one. * Amplifiers of different types, can use widely different types of NFB application. You have been spectacularly vague about the different types of NFB available to designers. * Modern transistors (specifically output transistors) are MUCH more linear than any tube. Including triodes. But transistor amps won't work at all without NFB! **NO AMPLIFIER WORKS WITHOUT NFB. Not one. That is not our problem. Those who choose inefficient speakers and consequently are forced to accept monstrous amps made possible only by gigadeciBels of NFB, will receive our sympathy - and the music they deserve. **It is extremely costly and difficult to build high efficiency speakers, which also possess a wide bandwidth and good dispersion characteristics. If you know of any, then cite them. Engineering hangers-on of transistor attempts at high fidelity, where the measure of success is vanishing THD rather than sonic hedonism, pretend to be enthusiasts for NFB. To make it work for them, they have attempted to change the rules so that we won't hear what their treasured NFB does to our sound. They sneer that low level listening, which 99 per cent of us prefer and where NFB does most to wreck the sound, is 'easy listening' and therefore not permissible. According to them we should all be forced to listen at the high volume level which suits NFB amps, which they call 'realistic'. **I know of no listeners who are "forced" to do anything. This is a contemptible circular argument, only too characteristic of a fascist mentality in a part of the audiophile spectrum which wants to prescribe their arid vision without regard for our enjoyment. We can recommend a good tailor to them. It hurts every time you wear his suit. No pain, no gain, fellers! In summary Almost everyone listens at low level most of the time. **Prove it. NFB wrecks everybody's sound at all levels but most wretchedly at normal listening levels. **Even your own KISS 300B amp, which uses NFB? How curious. We started out with a contemptible circular argument and we have met another along the way. We can now put both in context: An 'engineer' who designs an amplifier which does not work perfectly without negative feedback is like a tailor cutting the suit incompetently and then demanding that you walk like a cripple to make it fit, so that everyone can admire the brilliance of your tailor. Negative feedback is a bodge. That is why it is despicable to the ultrafidelista. **ALL amplifiers use NFB for linearisation. Every single one. |
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Posted to rec.audio.tubes,uk.rec.audio,rec.audio.opinion
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The Catch-22 of Negative Feedback aka NFB
"Trevor Wilson" wrote in message ... : : "Ruud Broens" wrote in message : ... : : : we've been through this 'linearity contest' before, here. : whereas you define it in terms of current amplification, : i define it in terms of voltage amplification. : : **In practice, there is no difference. A curent flowing through a resistance : allows a Voltage to be developed = Voltage gain. : : as real-world interfacing in audio is with *voltages*, : doesn't seem unreasonable... : : **The difference is academic. : : : excusez for not having included you on the ss : shortlist. : so, why not build flipper's frontend , completed with that : Toshiba 2SC5200 for power : & report back ? : : **Who's "flipper"? : he, are you playing inspector ClueSo? or something ? get a google clue :-) on rat R. : -- : Trevor Wilson : www.rageaudio.com.au : : |
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#6
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The Catch-22 of Negative Feedback aka NFB
"Andre Jute" wrote in message ups.com... Trevor Wilson wrote: (a long commentary on my original post, all of it reproduced in full below) I never said an amp can work without NFB; **Yes, you did. Several times, in fact. that's your desperate spin on the matter. In fact, I took part in a long thread which determined that a 300B has about 12-14dB of internal or natural NFB. **Then why do you persist in claiming that such an amp can exist with no NFB? You, Trevor Wilson, know that what I actually believe is that most amps work better without *added* negative feedback. **I KNOW that you just published a bunch of lies and half truths. THAT is what I DO know. If you want to publish a correction (which includes the superior linearity of modern BJTs over triodes) then I will support you in that endeavour. And I told you so again: It follows from the argument above that ultrafidelista should choose an intrinsically linear topology and device which does not require added negative feedback to 'linearize' the output. See the "added"? It makes clear to even the rawest newbie what I intend. **No, it does not. I deal daily with newbies (and a goodly number of those who aren't newbies) about the issues surrounding NFB. Most have no idea that ALL amplifiers use NFB of some type for linearisation. Many more have no idea that a triode utilises and internal NFB system. You have merely perpetuated these myths. If you want to explain stuff, then be precise. If you mean local NFB, say so. If you mean Global NFB, say so. If you mean nested NFB, say so. Each feedback system will affect any given amplifier in different ways. As you well (or bloody well should) know. No one who has been in high level tube amps for more than a semester needs it spelled out. **Bull****! I see it every day. Everyone knows the convention is that an amp without added NFB is described as having Zero Negative Feedback. **Piling more bull**** on top, does not make it so. Spell out EXACTLY what you mean when you speak of feedback and there'll be fewer problems. If you mean Zero GLOBAL NFB, then say so. As you well know, not all SS amps utilise Global NFB for linearisation. In fact, they use similar feedback methods to your triode amps. And, surprise surprise, they often provide many of the benefits often ascribed to triode amps, without the obvious shortcomings. But you would not want to actually educate people, would you? You, Trevor Wilson, cannot fail to know it, therefore you are picking desperate nits **Nope. I just get tired of people like you perpetuating lies and half truths. -- and giving away your desperation by hysterically screeching your misleading lie "no such thing as a ZNFB amp" over and over again, probably more than twenty times (those who care can count below). **If you can manage some honesty and accuracy, then I would not nbeed to correct your mistakes (lies?). Your problem is that you know I'm right but that you have thirty years invested in commercially telling people tubes are obsolete. **You have no idea. -- Trevor Wilson www.rageaudio.com.au |
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#7
Posted to rec.audio.tubes,uk.rec.audio,rec.audio.opinion
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The Catch-22 of Negative Feedback aka NFB
"Trevor Wilson" wrote in message ... "Ruud Broens" wrote in message ... "Trevor Wilson" wrote in message ... : : "Ruud Broens" wrote in message : ... : : : we've been through this 'linearity contest' before, here. : whereas you define it in terms of current amplification, : i define it in terms of voltage amplification. : : **In practice, there is no difference. A curent flowing through a resistance : allows a Voltage to be developed = Voltage gain. : : as real-world interfacing in audio is with *voltages*, : doesn't seem unreasonable... : : **The difference is academic. : : : excusez for not having included you on the ss : shortlist. : so, why not build flipper's frontend , completed with that : Toshiba 2SC5200 for power : & report back ? : : **Who's "flipper"? : he, are you playing inspector ClueSo? or something ? get a google clue :-) on rat **I'll ask again: Who's "flipper"? -- Trevor Wilson www.rageaudio.com.au One of the regular posters on RAT. |
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#8
Posted to rec.audio.tubes,uk.rec.audio,rec.audio.opinion
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The Catch-22 of Negative Feedback aka NFB
On 23 Mar 2006 17:37:10 -0800, "Andre Jute" wrote:
Trevor Wilson wrote: (a long commentary on my original post, all of it reproduced in full below) I never said an amp can work without NFB; that's your desperate spin on the matter. In fact, I took part in a long thread which determined that a 300B has about 12-14dB of internal or natural NFB. 'Natural' NFB? You truly are a prize specimen, Jute, you'll tell any lie and make up any fairy tale to support your utterly farcical 'ultrafidelista' mythology. You, Trevor Wilson, know that what I actually believe is that most amps work better without *added* negative feedback. If you believe that, then you're wrong. And I told you so again: It follows from the argument above that ultrafidelista should choose an intrinsically linear topology and device which does not require added negative feedback to 'linearize' the output. See the "added"? It makes clear to even the rawest newbie what I intend. No one who has been in high level tube amps for more than a semester needs it spelled out. Everyone knows the convention is that an amp without added NFB is described as having Zero Negative Feedback. The convention is wrong, this is simply mythology put about by those who are stuck with devices which *cannot* use large amounts of linearising feedback. The bottom line is in the output signal, and the output of a good modern SS amp has *vastly* lower distortion than that of any SET amp. You, Trevor Wilson, cannot fail to know it, therefore you are picking desperate nits -- and giving away your desperation by hysterically screeching your misleading lie "no such thing as a ZNFB amp" over and over again, probably more than twenty times (those who care can count below). Your problem is that you know I'm right but that you have thirty years invested in commercially telling people tubes are obsolete. Jute, you know very little about amplifiers, and you are certainly not right about this. Tubes have been obsolete for the best part of fifty years, except for fashion victims who like big shiny toys that glow in the dark. Andre Jute snip reams of Jute's garbage which he pasted from his hilarious website -- Stewart Pinkerton | Music is Art - Audio is Engineering Posted Via Usenet.com Premium Usenet Newsgroup Services ---------------------------------------------------------- ** SPEED ** RETENTION ** COMPLETION ** ANONYMITY ** ---------------------------------------------------------- http://www.usenet.com |
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#9
Posted to rec.audio.tubes,uk.rec.audio,rec.audio.opinion
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The Catch-22 of Negative Feedback aka NFB
Stewart Pinkerton wrote:. snip Your problem is that you know I'm right but that you have thirty years invested in commercially telling people tubes are obsolete. Jute, you know very little about amplifiers, and you are certainly not right about this. Tubes have been obsolete for the best part of fifty years, except for fashion victims who like big shiny toys that glow in the dark. Andre Jute Jute's a ****penis, but the fact is people buy audio equipment like women buy designer dresses and that isn't going to change. If you want some booty you let her spend her money the way she wants and don't balk. Tubes were the most reliable and best sounding way to build an audio amplifier up until relatively recent times and they still work very well when correctly designed. Solid state amplifiers that are truly well built are not much cheaper than tube ones, and if they made more tube ones the price would come down and quality go up. |
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#10
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The Catch-22 of Negative Feedback aka NFB
I must say, Wilson, I don't understand why you're getting your knickers
in a knot. You're like those bolshies I used to know who claimed that anyone who wasn't willing to kill for "socialism" was only play-acting. Are you really claiming that someone who merely uses the NFB he finds lurking in DHTs can't belong to the club until he repents and uses excessive amounts of loop NFB? Are you really claiming that the use of lots and lots of NFB is a prerequisite for good audio design? Are you really claiming that because a little NFB is a good thing, an unlimited amount must therefore be better, and an infinite amount best of all? Seems a bit immoderate to me, old chap. Your hysterical belief that I am "against" NFB is the product of your unsophisticated literal-mindedness. There is no reason for NFB to be an act of faith, like an on-off switch. For the record, quite contrary to your silly claims about what I said, I believe the little NFB that occurs naturally in triodes and in certain conservatively sanctioned traditional topologies are A Good Thing. To avoid giving you another apoplectic fit, I shan't repeat what I think of the excessive amounts of NFB required to make transistors work at all. If you think you can write a more compelling argument than I can about the evils of excessive NFB, have at it. But every time I see you, like the other techies you merely kibbitz what better men have written. Andre Jute Trevor Wilson wrote: "Andre Jute" wrote in message ups.com... Trevor Wilson wrote: (a long commentary on my original post, all of it reproduced in full below) I never said an amp can work without NFB; **Yes, you did. Several times, in fact. that's your desperate spin on the matter. In fact, I took part in a long thread which determined that a 300B has about 12-14dB of internal or natural NFB. **Then why do you persist in claiming that such an amp can exist with no NFB? You, Trevor Wilson, know that what I actually believe is that most amps work better without *added* negative feedback. **I KNOW that you just published a bunch of lies and half truths. THAT is what I DO know. If you want to publish a correction (which includes the superior linearity of modern BJTs over triodes) then I will support you in that endeavour. And I told you so again: It follows from the argument above that ultrafidelista should choose an intrinsically linear topology and device which does not require added negative feedback to 'linearize' the output. See the "added"? It makes clear to even the rawest newbie what I intend. **No, it does not. I deal daily with newbies (and a goodly number of those who aren't newbies) about the issues surrounding NFB. Most have no idea that ALL amplifiers use NFB of some type for linearisation. Many more have no idea that a triode utilises and internal NFB system. You have merely perpetuated these myths. If you want to explain stuff, then be precise. If you mean local NFB, say so. If you mean Global NFB, say so. If you mean nested NFB, say so. Each feedback system will affect any given amplifier in different ways. As you well (or bloody well should) know. No one who has been in high level tube amps for more than a semester needs it spelled out. **Bull****! I see it every day. Everyone knows the convention is that an amp without added NFB is described as having Zero Negative Feedback. **Piling more bull**** on top, does not make it so. Spell out EXACTLY what you mean when you speak of feedback and there'll be fewer problems. If you mean Zero GLOBAL NFB, then say so. As you well know, not all SS amps utilise Global NFB for linearisation. In fact, they use similar feedback methods to your triode amps. And, surprise surprise, they often provide many of the benefits often ascribed to triode amps, without the obvious shortcomings. But you would not want to actually educate people, would you? You, Trevor Wilson, cannot fail to know it, therefore you are picking desperate nits **Nope. I just get tired of people like you perpetuating lies and half truths. -- and giving away your desperation by hysterically screeching your misleading lie "no such thing as a ZNFB amp" over and over again, probably more than twenty times (those who care can count below). **If you can manage some honesty and accuracy, then I would not nbeed to correct your mistakes (lies?). Your problem is that you know I'm right but that you have thirty years invested in commercially telling people tubes are obsolete. **You have no idea. -- Trevor Wilson www.rageaudio.com.au |
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#11
Posted to rec.audio.tubes,uk.rec.audio,rec.audio.opinion
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The Catch-22 of Negative Feedback aka NFB
"Bret Ludwig" wrote in message oups.com... Stewart Pinkerton wrote:. snip Your problem is that you know I'm right but that you have thirty years invested in commercially telling people tubes are obsolete. Jute, you know very little about amplifiers, and you are certainly not right about this. Tubes have been obsolete for the best part of fifty years, except for fashion victims who like big shiny toys that glow in the dark. Andre Jute Jute's a ****penis, but the fact is people buy audio equipment like women buy designer dresses and that isn't going to change. If you want some booty you let her spend her money the way she wants and don't balk. **Irrelevant. Jute lies to get his point accross. If he told the truth, no one would care either way. When confronted by his obvious lies, he tells more lies to disguise the fact. Tubes were the most reliable and best sounding way to build an audio amplifier up until relatively recent times **Up 'till around 1968, that is quite true. It's now almost FOUR DECADES on. Jute is STILL telling people about transistor amps which have not existed for several decades. and they still work very well when correctly designed. Solid state amplifiers that are truly well built are not much cheaper than tube ones, and if they made more tube ones the price would come down and quality go up. **Utter and complete ********. Due to the presence of the (expensive) output transformers, alone, tube amps are priced well above an approximately equivalent SS one. -- Trevor Wilson www.rageaudio.com.au |
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#12
Posted to rec.audio.tubes,uk.rec.audio,rec.audio.opinion
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The Catch-22 of Negative Feedback aka NFB
"Andre Jute" wrote in message
ups.com How does negative feedback work? Negative feedback is simply a negative voltage fed back from the output to the input amplifying device to offset part of the harmonic distortion which is present as a positive voltage. Wrong. The voltage that is fed back in audio power amps is almost always positive with respect to the input and output of the piece of equipment. Therefore it is wrong to call it a negative voltage. Wrong. The voltage that is fed back in audio power amps is simply a fraction of the output signal. It is not fed back to offset the distortion, but rather is fed back to offset the foreward-going signal voltage. It costs nothing except a loss of gain and a few side effects such as phase shift and possible instability which are well known in the mathematical literature and more or less easily guarded against depending on the level of NFB. Wrong. Negative feedback always reduces phase shift. If misapplied it can cause an amplifier to become less stable, but in fact when properly applied, negative feedback increases stability. Negative feedback increases stability in the sense that it stabilizes the amplifiers most important technical parameters. For example consider the gain of an amplifier. If there is no negative feedback, then the gain of an amplifier is very much exposed to natural variations in the parameters of its active devices whether they be tubes or solid state. For example, the gain of both tubes and transistors can be very sensitive to temperature. With negative feedback, the important parameters of the amplifier are set by a pair of resistors, whose properties can be made to be very stable and independent of temperature. You should get the idea by wrong - Jute has no clue about what negative feedback is, how it works, or what its real benefits are. |
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#13
Posted to rec.audio.tubes,uk.rec.audio,rec.audio.opinion
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The Catch-22 of Negative Feedback aka NFB
"Ruud Broens" wrote in message
we've been through this 'linearity contest' before, here. whereas you define it in terms of current amplification, i define it in terms of voltage amplification. as real-world interfacing in audio is with *voltages*, doesn't seem unreasonable... Negative feedback is generally used to make amplifiers more linear in the voltage domain. It's a highly sucessful strategy. We sent men to the moon, partially based on amplifiers that were linearized with negative feedback. Where's the beef? |
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#14
Posted to rec.audio.tubes,uk.rec.audio,rec.audio.opinion
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The Catch-22 of Negative Feedback aka NFB
"Arny Krueger" wrote in message ... : "Ruud Broens" wrote in message : : : we've been through this 'linearity contest' before, here. : whereas you define it in terms of current amplification, : i define it in terms of voltage amplification. : as real-world interfacing in audio is with *voltages*, : doesn't seem unreasonable... : : : Negative feedback is generally used to make amplifiers more linear in the : voltage domain. It's a highly sucessful strategy. We sent men to the moon, : partially based on amplifiers that were linearized with negative feedback. : : Where's the beef? : they 'beef' is, your banana needs 1K more straightening' than my banana - why keep claiming it is "superior" ? :-) Rudy |
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#15
Posted to rec.audio.tubes,uk.rec.audio,rec.audio.opinion
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The Catch-22 of Negative Feedback aka NFB
: we've been through this 'linearity contest' before, here. : whereas you define it in terms of current amplification, : i define it in terms of voltage amplification. : as real-world interfacing in audio is with *voltages*, : doesn't seem unreasonable... : : : Negative feedback is generally used to make amplifiers more linear in the : voltage domain. It's a highly sucessful strategy. We sent men to the moon, : partially based on amplifiers that were linearized with negative feedback. Uhhh, power amplifiers work on complex loads in the power domain? |
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#16
Posted to rec.audio.tubes,uk.rec.audio,rec.audio.opinion
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The Catch-22 of Negative Feedback aka NFB
On Fri, 24 Mar 2006 12:59:37 +0100, "Ruud Broens"
wrote: "Arny Krueger" wrote in message ... : "Ruud Broens" wrote in message : : : we've been through this 'linearity contest' before, here. : whereas you define it in terms of current amplification, : i define it in terms of voltage amplification. : as real-world interfacing in audio is with *voltages*, : doesn't seem unreasonable... : : : Negative feedback is generally used to make amplifiers more linear in the : voltage domain. It's a highly sucessful strategy. We sent men to the moon, : partially based on amplifiers that were linearized with negative feedback. : : Where's the beef? : they 'beef' is, your banana needs 1K more straightening' than my banana - why keep claiming it is "superior" ? Because what matters is the output of the *final* circuit - the banana is perfectly straight in the opamp, but your tube circuit is still bendy. The opamp is *designed* to use large amounts of linearising feedback, something that simply isn't an option with the inherently much lower open-loop gain of tubes. -- Stewart Pinkerton | Music is Art - Audio is Engineering Posted Via Usenet.com Premium Usenet Newsgroup Services ---------------------------------------------------------- ** SPEED ** RETENTION ** COMPLETION ** ANONYMITY ** ---------------------------------------------------------- http://www.usenet.com |
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#17
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The Catch-22 of Negative Feedback aka NFB
On 24 Mar 2006 00:46:36 -0800, "Andre Jute" wrote:
I must say, Wilson, I don't understand why you're getting your knickers in a knot. You're like those bolshies I used to know who claimed that anyone who wasn't willing to kill for "socialism" was only play-acting. Are you really claiming that someone who merely uses the NFB he finds lurking in DHTs can't belong to the club until he repents and uses excessive amounts of loop NFB? Are you really claiming that the use of lots and lots of NFB is a prerequisite for good audio design? Are you really claiming that because a little NFB is a good thing, an unlimited amount must therefore be better, and an infinite amount best of all? Seems a bit immoderate to me, old chap. Your hysterical belief that I am "against" NFB is the product of your unsophisticated literal-mindedness. There is no reason for NFB to be an act of faith, like an on-off switch. For the record, quite contrary to your silly claims about what I said, I believe the little NFB that occurs naturally in triodes and in certain conservatively sanctioned traditional topologies are A Good Thing. To avoid giving you another apoplectic fit, I shan't repeat what I think of the excessive amounts of NFB required to make transistors work at all. As ever, you have no idea what you're talking about. A modern power transistor such as the 2SC2922 is *more* linear than a 300B, *without* any feedback. With feedback, due to its gain of 100, it can achieve linearity far beyond the wildest dreams of the 'ultrafidelista'. That, of course, is why KISASS will kick the ass of KISS any day of the week. -- Stewart Pinkerton | Music is Art - Audio is Engineering Posted Via Usenet.com Premium Usenet Newsgroup Services ---------------------------------------------------------- ** SPEED ** RETENTION ** COMPLETION ** ANONYMITY ** ---------------------------------------------------------- http://www.usenet.com |
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#18
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The Catch-22 of Negative Feedback aka NFB
"Stewart Pinkerton" wrote in message ... : On Fri, 24 Mar 2006 12:59:37 +0100, "Ruud Broens" : wrote: : : "Arny Krueger" wrote in message : ... : : "Ruud Broens" wrote in message : : : : : : we've been through this 'linearity contest' before, here. : : whereas you define it in terms of current amplification, : : i define it in terms of voltage amplification. : : as real-world interfacing in audio is with *voltages*, : : doesn't seem unreasonable... : : : : : : Negative feedback is generally used to make amplifiers more linear in the : : voltage domain. It's a highly sucessful strategy. We sent men to the moon, : : partially based on amplifiers that were linearized with negative feedback. : : : : Where's the beef? : : : they 'beef' is, your banana needs 1K more straightening' than my banana - : why keep claiming it is "superior" ? : : Because what matters is the output of the *final* circuit - the banana : is perfectly straight in the opamp, but your tube circuit is still : bendy. yeah, right, as in 0.007 % distortion kinda bendy. must be what they call 'the scottish bend', then R. The opamp is *designed* to use large amounts of linearising : feedback, something that simply isn't an option with the inherently : much lower open-loop gain of tubes. : -- : : Stewart Pinkerton | Music is Art - Audio is Engineering : : Posted Via Usenet.com Premium Usenet Newsgroup Services : ---------------------------------------------------------- : ** SPEED ** RETENTION ** COMPLETION ** ANONYMITY ** : ---------------------------------------------------------- : http://www.usenet.com |
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#19
Posted to rec.audio.tubes,uk.rec.audio,rec.audio.opinion
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The Catch-22 of Negative Feedback aka NFB
The details of transistor amplifiers have changed immensely and the
devices have improved, but the physics are still the same as when John F. Kennedy was holding up operations at Idlewild while pounding on the Monroe Doctrine in SAM 26000. Bipolar transistors are still low impedance, current controlled, current controlling devices. |
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#20
Posted to rec.audio.tubes,uk.rec.audio,rec.audio.opinion
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The Catch-22 of Negative Feedback aka NFB
"Stewart Pinkerton" wrote in message ... : On Fri, 24 Mar 2006 17:57:12 +0100, "Ruud Broens" : wrote: : : : : Where's the beef? : : : : : they 'beef' is, your banana needs 1K more straightening' than my banana - : : why keep claiming it is "superior" ? : : : : Because what matters is the output of the *final* circuit - the banana : : is perfectly straight in the opamp, but your tube circuit is still : : bendy. : : yeah, right, as in 0.007 % distortion kinda bendy. : must be what they call 'the scottish bend', : then : : Be more specific about the tubed circuit you claim has this level of : distortion. : eehhrrmm, don't think that earlier suggestion of a scottish detective will work out, after all, thanks for detecting, not even well above noise level, lot'sZZ :-) Rudy |
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#21
Posted to rec.audio.tubes,uk.rec.audio,rec.audio.opinion
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The Catch-22 of Negative Feedback aka NFB
Trevor Wilson wrote:
Incorrect and innappropriate application of NFB can damage an amplifier's performance. Now you're getting it, sonny. Next, try not to describe as a liar anyone who doesn't instantly subscribe to your fanatical faith in Blow Jobs from Transvestites (BJTs) and soon the rest of us might take you seriously. Andre Jute Visit Jute on Amps at http://members.lycos.co.uk/fiultra/ "wonderfully well written and reasoned information for the tube audio constructor" John Broskie TubeCAD & GlassWare "an unbelievably comprehensive web site containing vital gems of wisdom" Stuart Perry Hi-Fi News & Record Review Trevor Wilson wrote: "Andre Jute" wrote in message oups.com... I must say, Wilson, I don't understand why you're getting your knickers in a knot. **They're not and I'm not. I am, however, merely pointing out your more obvious lies. You're like those bolshies I used to know who claimed that anyone who wasn't willing to kill for "socialism" was only play-acting. Are you really claiming that someone who merely uses the NFB he finds lurking in DHTs can't belong to the club until he repents and uses excessive amounts of loop NFB? **Nope. I merely stated that ALL amplifiers utilise NFB for linearisation. You, however, claimed that your amplifiers do not. Are you really claiming that the use of lots and lots of NFB is a prerequisite for good audio design? **Nope. I merely stated that ALL amplifiers utilise lots and lots of NFB for linearisation. Every single one of them. What differs is HOW that NFB is applied (and, of course, how much). Of course, you would not want to actually educate your readers, would you? You would prefer to keep them confused by your more obvious lies. Are you really claiming that because a little NFB is a good thing, an unlimited amount must therefore be better, and an infinite amount best of all? **Where did I claim anything of the sort? Be precise in your cite. What I have stated many times, is that CORRECT and APPROPRIATE application of NFB will make an amplifier a good one. Incorrect and innappropriate application of NFB can damage an amplifier's performance. Seems a bit immoderate to me, old chap. **Wait for the answer, dickhead. Your hysterical belief that I am "against" NFB is the product of your unsophisticated literal-mindedness. **YOU stated it, not me. YOU claimed that certain amplifiers utilised no NFB. That is a lie. ALL amplifiers utilise NFB for linearisation. Every single one. What differs is the type and amount. You, of course, would not want to tell your readers this little fact, would you? There is no reason for NFB to be an act of faith, like an on-off switch. For the record, quite contrary to your silly claims about what I said, I believe the little NFB that occurs naturally in triodes and in certain conservatively sanctioned traditional topologies are A Good Thing. **LIke I said: ALL amplifiers utilise NFB for linearisation. Every single one. To avoid giving you another apoplectic fit, I shan't repeat what I think of the excessive amounts of NFB required to make transistors work at all. **And again, you lie. It is not necessary to use "excessive amounts" of NFB to make a transistor work at all. In fact, modern transistors are more linear than tubes and can, therefore, operate with less feedback, if required. Because transistors are so cheap and reliable, most manufacturers tend to use lots of them and, consequently, lots of NFB to linearise the whole shebang. If you think you can write a more compelling argument than I can about the evils of excessive NFB, have at it. But every time I see you, like the other techies you merely kibbitz what better men have written. **I just point out your lies. Nothing more. -- Trevor Wilson www.rageaudio.com.au |
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#22
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The Catch-22 of Negative Feedback aka NFB
Ruud Broens wrote: "Trevor Wilson" wrote in message ... : : "Ruud Broens" wrote in message : ... : : : we've been through this 'linearity contest' before, here. : whereas you define it in terms of current amplification, : i define it in terms of voltage amplification. : : **In practice, there is no difference. A curent flowing through a resistance : allows a Voltage to be developed = Voltage gain. : : as real-world interfacing in audio is with *voltages*, : doesn't seem unreasonable... : : **The difference is academic. : : : excusez for not having included you on the ss : shortlist. : so, why not build flipper's frontend , completed with that : Toshiba 2SC5200 for power : & report back ? : : **Who's "flipper"? : he, are you playing inspector ClueSo? or something ? get a google clue :-) on rat R. : -- : Trevor Wilson : www.rageaudio.com.au Hooo is de Fleeper?? Maybeez he is dee BBIIGGG Fishy in de water, and he SPLASH all over you with heez tail and make you so wet all over. Patrick Turner. : : |
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#23
Posted to rec.audio.tubes,uk.rec.audio,rec.audio.opinion
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The Catch-22 of Negative Feedback aka NFB
Arny "I spoke in error" Krueger next tries hairsplitting: "Andre Jute" wrote in message ups.com How does negative feedback work? Negative feedback is simply a negative voltage fed back from the output to the input amplifying device to offset part of the harmonic distortion which is present as a positive voltage. Wrong. The voltage that is fed back in audio power amps is almost always positive with respect to the input and output of the piece of equipment. Therefore it is wrong to call it a negative voltage. If you can't win the argument, redefine the standard terms. Wrong. The voltage that is fed back in audio power amps is simply a fraction of the output signal. It is not fed back to offset the distortion, but rather is fed back to offset the foreward-going signal voltage. If you can't win the argument, redefine it so narrowly that you can split hairs about a single tiny piece. It costs nothing except a loss of gain and a few side effects such as phase shift and possible instability which are well known in the mathematical literature and more or less easily guarded against depending on the level of NFB. Wrong. Negative feedback always reduces phase shift. If misapplied it can cause an amplifier to become less stable, but in fact when properly applied, negative feedback increases stability. If you can't win the argument on the specifics, shift ground to the generality. State that misapplication will always be misapplication, as if you have just discovered the dull truism. Negative feedback increases stability in the sense that it stabilizes the amplifiers most important technical parameters. For example consider the gain of an amplifier. If there is no negative feedback, then the gain of an amplifier is very much exposed to natural variations in the parameters of its active devices whether they be tubes or solid state. For example, the gain of both tubes and transistors can be very sensitive to temperature. With negative feedback, the important parameters of the amplifier are set by a pair of resistors, whose properties can be made to be very stable and independent of temperature. If you can't win the argument on its merits, drag in a lot of extraneous considerations and pretend that each of them is the main problem. You should get the idea by wrong - Jute has no clue about what negative feedback is, how it works, or what its real benefits are. "You should get the idea by wrong" -- duh, Arny? Another Freudian slip? It tells us poor little Arny, an old man, is still so childish that, regardless of the facts, he insists on being right and everyone else being wrong. What a moron. If you want to be a hairdresser, Krueger, first you should realize that the purpose of hairdressing is not to promote and increase split ends but to prevent them. The same in audio with distortion. I thought you claimed to be an engineer... Andre Jute In audio less is hedonism |
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#24
Posted to rec.audio.tubes,uk.rec.audio,rec.audio.opinion
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The Catch-22 of Negative Feedback aka NFB
Patrick Turner wrote: Ruud Broens wrote: "Trevor Wilson" wrote in message ... : : "Ruud Broens" wrote in message : ... : : : we've been through this 'linearity contest' before, here. : whereas you define it in terms of current amplification, : i define it in terms of voltage amplification. : : **In practice, there is no difference. A curent flowing through a resistance : allows a Voltage to be developed = Voltage gain. : : as real-world interfacing in audio is with *voltages*, : doesn't seem unreasonable... : : **The difference is academic. : : : excusez for not having included you on the ss : shortlist. : so, why not build flipper's frontend , completed with that : Toshiba 2SC5200 for power : & report back ? : : **Who's "flipper"? : he, are you playing inspector ClueSo? or something ? get a google clue :-) on rat R. : -- : Trevor Wilson : www.rageaudio.com.au Hooo is de Fleeper?? Maybeez he is dee BBIIGGG Fishy in de water, and he SPLASH all over you with heez tail and make you so wet all over. Patrick Turner. Poor Trevor is wet all over already. The name of his firm, Rage Audio, already tells you much about his character. The inchoate mouthfoaming when he meets the slightest resistance is unnecessary confirmation. Here's another truth to enrage poor Trevor: It would be easy to show from the published texts that he and I are in agreement on NFB if only he will open his mind. I don't suppose he will, though. Andre Jute |
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#25
Posted to rec.audio.tubes,uk.rec.audio,rec.audio.opinion
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The Catch-22 of Negative Feedback aka NFB
Andre Jute said to DebatingTradeBorg: I thought you claimed to be an engineer... Krooger has amended that claim. He now claims to be "Master of the Debating Trade". Much more prestigious in his eyes. -- A day without Krooger is like a day without arsenic. |
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#26
Posted to rec.audio.tubes,uk.rec.audio,rec.audio.opinion
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The Catch-22 of Negative Feedback aka NFB
"Andre Jute" wrote in message ups.com... Some meterhead clown demands to know: What's wrong with feedback? and another meterhead clown, on being told by Rudy that: an OPA2604 ... needs a *thousand* times more feedback demands to know: So what? ***** It ain't rocket science. Explanation taken from: http://members.lycos.co.uk/fiultra/T...mp%20INDEX.htm ***** CATCH-22 ***** The customer complained that his new suit didn't fit. 'The sleeves are too short,' he said to the tailor. 'Yes sir,' said the tailor, 'but if you hold your arm just so, at an angle as if you're drinking tea with your auntie, it will show just the right amount of cuff.' The customer tried it. The tailor was right! 'But what about the other sleeve? It is definitely too short.' 'Just lower your shoulder, sir. Yes, yes, a little more. Put your foot out so you can lower your shoulder a little more still. Bend your knee. Yes, that's it. See how beautifully your suitcoat now fits?' The customer had to admit the tailor was right. 'Wow. But now the leg of the pants is all twisted around.' 'That's easily fixed,' said the tailor. 'Just point your other toe westward, sir, and look over your shoulder to where I am holding up the hand mirror. See? Doesn't that fit beautifully?' 'Yes,' the customer said doubtfully, 'but-' 'Now would sir like to wear his brilliant new suit or shall we wrap it?' The customer was too intimidated to argue. He walked out into the street in his new suit, his arm crooked as if he were drinking tea, his other shoulder well down over a bent knee with his foot out to the side, his other foot pointing westwards, his head twisted back between hunched shoulders as if complaining to God about a cruel fate. Behind him he heard a boy say to his father. 'Oh, Daddy, look at the poor twisted cripple.' 'Hush,' the father said. 'Be grateful the poor man found such a brilliant tailor.' **** The Ultrafidelista view of Negative Feedback by Andre Jute Negative feedback is the paradigm of modern electronic design. It is mother's milk to an electronics engineer. He learns to say '100dB of NFB,' in his sleep before he finishes his first week at the most humble polytechnic. At the great institutions the professor of feedback is the most honoured man in the department. In Massachusetts and Minnesota the feedback guru is the most honoured man on the entire campus, equal in stature to the football coach. When a guru of transistor high fidelity (and some in tubes) says, 'I studied under Ron,' one doesn't have to ask which Ron, one just knows it is the holy name of the prophet of feedback from the Midwest. Before I even finished the design of the KISS 300B it was forcefully suggested by a wannabe guru that with only 50dB more gain (about seven times as much as is likely to be in the actual design) I can apply 50dB of negative feedback to linearize my amplifier. Negative feedback, shorthanded as NFB, is the instant response of the audio engineering fraternity to all ills, real, perceived, non-existent. They don't even ask if there is a problem, they swing the club of NFB regardless. NFB has become a reflex axiom of mainstream audio design. An audio engineer with his negative feedback is like a policeman who runs out into the street with his stick and starts beating a confession out of the first housewife he sees. The difference is that the policeman is relieved of duty to await punishment and the audio engineer gets away with it. In the case of the policeman it is unacceptable behaviour, in the case of the audio engineer so much the expected norm that no one except the ultrafidelista notice. I guess that if one in ten million audio amplifiers does not have negative feedback added, it will be a lot... No one asked if my KISS Amp requires linearization. The presumption by all except those already of the ultrafidelista persuasion was that I would welcome suggestions about A Good Thing. In the face of such overwhelming acceptance by qualified engineers, why do we as ultrafidelista not take the same easy path of negative feedback? Especially considering that superficially NFB is easy to understand and apply. How does negative feedback work? Negative feedback is simply a negative voltage fed back from the output to the input amplifying device to offset part of the harmonic distortion which is present as a positive voltage. It costs nothing except a loss of gain and a few side effects such as phase shift and possible instability which are well known in the mathematical literature and more or less easily guarded against depending on the level of NFB. 'Wow!' those meeting NFB for the first time will now say, 'Something for free! I'll grab some of that for my amp.' Hey, I said it, and I am a professional intellectual, by definition an infinite skeptic. NFB is a thing of beauty that will draw you in. It is like an electronic Marxism which admits of no contrary arguments because it has subsumed them all into The Holy Measurements. To question The Measurements is to commit heresy. You need to be of strong mind to resist the blandishments of such a universal panacea and of strong stomach to withstand the hysterical assaults of the lesser engineers defending their holy grail. (And when you do get hold of a superior engineer to explain NFB to you, you need to be high-domed indeed because suddenly NFB can turn very intricate.) Unfortunately NFB doesn't come without a price. It levies a cruel charge on the perceived quality of the sound. Negative feedback is what gives all those 'blameless' transistor and big PP tube amps their chillingly unnatural sound. Then how did NFB come to be such a panacea in amplifier design? Your guess is as good as mine. Hi-fi design is not prestige work for engineers, or highly paid. The most talented and best qualified engineers go into automobiles or military hardware or big construction projects or computer design. The left-overs design amplifiers in the time they have to spare from writing up specs for requesting a CE mark for a new electric kettle. Lemmings storming en masse over a cliff come to mind; such people don't see the necessity of original thought, or have the mental equipment for it. The exceptions to this rule are normally audio enthusiasts in charge of their own small audio manufactories with niche markets; those who grow larger from this base follow the mainstream mantra of "mo' NFB give lowa' THD" because the marketing channels demand it from them if they wish to grow. At this point they usually cease to offer anything different, only the exclusivity of a very high price. (I know, because a sub-board I designed for a supplier to the trade turns up in so many very expensive amps with so many different big names neatly silkscreened on it... it strikes me as the sort of detail a real designer, as distinct from a marketer, would take under his own control.) Those very few makers who will sell you an ultrafi amp without any NFB operate even tinier shops, usually one man and a cat, just hanging on. The mechanism by which NFB wrecks your sound Negative feedback at first acquaintance sounds good enough to take to bed and cuddle. It isn't. It isn't even as simple as a superficial acquaintance may suggest. Follow the steps with me, from the theory as she is received to what arrives at your brain as music: 1. In theory NFB reduces all harmonic distortion equally, without discrimination. Strictly in theory it does not reshape harmonic distortion by reducing the most objectionable third and higher order odd harmonic distortion to a greater extent than the relatively harmless 2nd harmonic. Thus NFB at its theoretically most benign is already useless in terms of psychoacoustics, as will become clear at point 4. If you disregard psychoacoustics, as many audio engineers do, NFB is brilliant in reducing total harmonic distortion to a number as tiny as you want. You just pile on more NFB. 2. In real life, as distinct from simplified theory, NFB adds artifacts of its own. Remember, it is a loop. The signal starts at the input and is amplified by devices until it reaches the output. From the output a part of the signal called the negative feedback is fed back to the input. Here a loop is completed and the combination, less distorted, reaches the output again, a part of the combination is fed back, endlessly. The artifacts we want to consider here are created by the fed-back residue of harmonic distortions adding to both the fundamental and the distortions already created by the amplifier, then some portion of the sum of the original and the feedback distortion is fed back again and added on, until the ooh-ah bird flies up its own fundament. It looks marginally less disgusting as a recursive mathematical formula with lots of nested parenthetical parcels of noise being loaded onto your music. But it is a monkey on the back of your sound, with a smaller monkey on the back of the first monkey, a still smaller monkey on the back of the second monkey, and so on ad infinitum. These additive artifacts are all higher harmonics and the more dominant ones are all odd. Suppose, for the sake of simplicity, a superbly designed ultrafidelista amp with some second harmonic and zero odd harmonics before NFB. Add NFB and the second harmonic will be lowered but the recombinant new loop now contains newly added intermodulation effects between the fundamental and the residual second harmonic, and that is third harmonic. In the next cycle a small but nasty dose of fifth harmonic that wasn't there before is added by interaction between the still residual second harmonic and reduced newly added third harmonic. In short, the artifacts NFB adds to the distortion mix are all of the most harmful kind. But, say the proponents of NFB, so what? Every time the loop cycles the added artifacts are smaller, even if there are more of them... The whole affair starts to smell of trying to argue with a Marxist who simply declares any inconvenient truth 'an anomaly'. (If this sounds like a mess from which you should run a mile, you have come to the right conclusion. Start running now. It gets worse.) 3. We thus arrive at a situation where distortion has been lowered by NFB but where the most disturbing odd harmonic distortions are still present to some measure, with the added disadvantage that new and extremely disturbing artifacts of higher harmonic distortions have been created by the very process of using negative feedback to lower distortion. Regardless of the absolute level of THD, or the volume setting, the mix of harmonics has been adversely affected and now includes a higher proportion of third and higher harmonics than before NFB. Let me say that again: after NFB, third and higher harmonics will make up a greater part of the distortion than before. 4. Low volume levels perforce accounts for 99 per cent of audiophile listening because we all have families or neighbours, and we would like to keep our ears. Unfortunately for the lowest common denominator of hi-fi designer, the one who specifies NFB as a conditioned response much like Pavlov's dogs slavered when the bell rang, human physiology and psycho-acoustic response is such that odd harmonics are disproportionately more disturbing at lower than at higher listening levels. This inescapable effect is independent of definition of 'listening level.' At the 110dB in-room SPL (only 14dB louder than an automatic riveter!) advocated by the already deaf Transient Overload Elite known on newsgroups as the Borg, this poisonous concoction of original distortions and NFB recombinant artifacts will be least disturbing (and soon not heard at all!). At any lower level perceived interference of this harmonics cocktail with the music will increase in inverse proportion to the volume level. At low volume levels the artifacts generated by NFB will by their nature as higher harmonic distortions be disproportionately far more disturbing. At these normal listening levels 0.75 per cent of second harmonic distortion may be below the threshhold of perception for sophisticated listeners, whereas tiny amounts of third and higher odd harmonic distortions grate. And they still use Negative Feedback? Are they stupid? No, they are not stupid. Most of them march to the drum of a cost accountant on whom we wouldn't spit if he were alight. NFB is as cheap in money terms as it is expensive in terms of perceived quality of music. We shall come to those who claim to be sympathetic to high-fidelity but insist on devices which do not work without NFB, who have another devious answer. Here, meanwhile, for you to keep in mind, is a single-sentence summary of a complicated interdisciplinary argument: The case against NFB is that for 99 per cent of listening the NFB cure is worse than the disease. But surely we don't have to do anything so stupid? It follows from the argument above that ultrafidelista should choose an intrinsically linear topology and device which does not require added negative feedback to 'linearize' the output. The intrinsically linear device is the thermionic tube in either its triode form or as a pentode hogtied to work as a triode, which can be a most pleasing alternative both economically and sonically. The topology is often single-ended operation, chosen also for several other reasons described elsewhere in these articles, including KISS; if the chosen topology is push-pull operation, which is more difficult but far from impossible to arrange without NFB, operation should be specified as Class A1. Inside the argued case above lies too the overwhelming reason to accept the potential small disadvantage that may accompany the preferred topology in comparison to the discarded alternatives. The disadvantage is of course the potential for a residual second harmonic that measures high by transistor or NFB tube standards. (Note the word potential. With a conservatively designed DHT amp the potential problem should not arise.) The ultrafidelista, who are as keen on silent amps as anyone else, accept this small potential difficulty because it is the lesser evil compared to NFB. Unbelievers (largely unwashed, according to reports) sneer that ultrafidelista like this approach because of the 'added euphonics', which is bow-wow techie talk for the warmth a big chunk of second harmonic lays on a zero negative feedback single-ended amplifier. But competent design can easily reduce the level of second harmonic to below the level of perception without the need for NFB and its deleterious after-effects. In any event, it is your amplifier. You paid for it. You have a right to tune it as you please. The key thing is to get rid of NFB and to understand why you did it. Can we prove any of this scientifically? We have already. All of this is the technical subtext to my longtime contention that what the ultrafidelista hear and love is not a directly heated triode sound as is claimed by many enthusiasts but a Class A1, ZNFB sound. (Admittedly, as we have seen above, the right sound is virtually guaranteed with a ZNFB DHT SE amp of conservative provenance but may have to be developed the hard way with more economical or higher-power contenders.) In comparative ABX tests conducted over a number of years, I found that professional musicians, certified golden ears, choose the triode-linked Class A1 PP ZNFB EL34 whenever it is present in the test over all other contenders including SE 300B and 'blameless' high-NFB silicon. Science also proceeds by pure reason. Ultrafidelista have long doubted whether what engineers insist we measure (the absolute level of distortion, THD) predicts success in audio gear. This is the full circle, because I have just proven by logical, individually uncontested steps that what matters, once a certain modest level of silence is assured to an amplifier, is not the absolute level of disharmonics but their composition. The same proof demonstrates that a more beneficial distribution follows instantly from doing without NFB. But transistor amps won't work at all without NFB! That is not our problem. Those who choose inefficient speakers and consequently are forced to accept monstrous amps made possible only by gigadeciBels of NFB, will receive our sympathy - and the music they deserve. Engineering hangers-on of transistor attempts at high fidelity, where the measure of success is vanishing THD rather than sonic hedonism, pretend to be enthusiasts for NFB. To make it work for them, they have attempted to change the rules so that we won't hear what their treasured NFB does to our sound. They sneer that low level listening, which 99 per cent of us prefer and where NFB does most to wreck the sound, is 'easy listening' and therefore not permissible. According to them we should all be forced to listen at the high volume level which suits NFB amps, which they call 'realistic'. This is a contemptible circular argument, only too characteristic of a fascist mentality in a part of the audiophile spectrum which wants to prescribe their arid vision without regard for our enjoyment. We can recommend a good tailor to them. It hurts every time you wear his suit. No pain, no gain, fellers! In summary Almost everyone listens at low level most of the time. NFB wrecks everybody's sound at all levels but most wretchedly at normal listening levels. We started out with a contemptible circular argument and we have met another along the way. We can now put both in context: An 'engineer' who designs an amplifier which does not work perfectly without negative feedback is like a tailor cutting the suit incompetently and then demanding that you walk like a cripple to make it fit, so that everyone can admire the brilliance of your tailor. Negative feedback is a bodge. That is why it is despicable to the ultrafidelista. ******* More Zero Negative Feedback amplifiers at Jute on Amps http://members.lycos.co.uk/fiultra/JUTE%20ON%20AMPS.htm The KISS Amp project, which explains much more, is here http://members.lycos.co.uk/fiultra/T...mp%20INDEX.htm and its schematics and transfer curves are here http://members.lycos.co.uk/fiultra/KISS%20190.htm ******** THE VOLTAGES IN THIS AMP WILL KILL YOU. GET EXPERIENCED SUPERVISION IF IT IS YOUR FIRST TUBE AMP All text and illustration is Copyright © Andre Jute 2001, 2004, 2006 and may not be reproduced except in the thread KISS xxx on rec.audio.tubes |
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#27
Posted to rec.audio.tubes,uk.rec.audio,rec.audio.opinion
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The Catch-22 of Negative Feedback aka NFB
Andre Jute wrote: I must say, Wilson, I don't understand why you're getting your knickers in a knot. You're like those bolshies I used to know who claimed that anyone who wasn't willing to kill for "socialism" was only play-acting. Are you really claiming that someone who merely uses the NFB he finds lurking in DHTs can't belong to the club until he repents and uses excessive amounts of loop NFB? Are you really claiming that the use of lots and lots of NFB is a prerequisite for good audio design? Are you really claiming that because a little NFB is a good thing, an unlimited amount must therefore be better, and an infinite amount best of all? Seems a bit immoderate to me, old chap. You will find yourself arguing at a man who has a head full of solid concrete. Trev will never see the whole picture, and has never designed or built any amp in the last 30 years. I try not to bother arguing the same old tired BS time after time. He just likes hanging out in news groups and Bull****ting. He has NEVER once contributed a single article of constructive advice on tubecraft at r.a.t. Your hysterical belief that I am "against" NFB is the product of your unsophisticated literal-mindedness. There is no reason for NFB to be an act of faith, like an on-off switch. For the record, quite contrary to your silly claims about what I said, I believe the little NFB that occurs naturally in triodes and in certain conservatively sanctioned traditional topologies are A Good Thing. To avoid giving you another apoplectic fit, I shan't repeat what I think of the excessive amounts of NFB required to make transistors work at all. If you think you can write a more compelling argument than I can about the evils of excessive NFB, have at it. But every time I see you, like the other techies you merely kibbitz what better men have written. I am now doing 845 amps, two per channel for 50 watts each. I expect music to be real fine. Damping factor good, noise low, bandwidth wide, distortions negligible and maybe I can add 6db of global NFB but I doubt any difference due to some **added** NFB will be heard. Trev has never done anything like this and simply doesn't understand. With 845, just no need for the 106dB of added NFB used in SS amps. I don't care a bit about what Trev says. Patrick Turner. Andre Jute Trevor Wilson wrote: "Andre Jute" wrote in message ups.com... Trevor Wilson wrote: (a long commentary on my original post, all of it reproduced in full below) I never said an amp can work without NFB; **Yes, you did. Several times, in fact. that's your desperate spin on the matter. In fact, I took part in a long thread which determined that a 300B has about 12-14dB of internal or natural NFB. **Then why do you persist in claiming that such an amp can exist with no NFB? You, Trevor Wilson, know that what I actually believe is that most amps work better without *added* negative feedback. **I KNOW that you just published a bunch of lies and half truths. THAT is what I DO know. If you want to publish a correction (which includes the superior linearity of modern BJTs over triodes) then I will support you in that endeavour. And I told you so again: It follows from the argument above that ultrafidelista should choose an intrinsically linear topology and device which does not require added negative feedback to 'linearize' the output. See the "added"? It makes clear to even the rawest newbie what I intend. **No, it does not. I deal daily with newbies (and a goodly number of those who aren't newbies) about the issues surrounding NFB. Most have no idea that ALL amplifiers use NFB of some type for linearisation. Many more have no idea that a triode utilises and internal NFB system. You have merely perpetuated these myths. If you want to explain stuff, then be precise. If you mean local NFB, say so. If you mean Global NFB, say so. If you mean nested NFB, say so. Each feedback system will affect any given amplifier in different ways. As you well (or bloody well should) know. No one who has been in high level tube amps for more than a semester needs it spelled out. **Bull****! I see it every day. Everyone knows the convention is that an amp without added NFB is described as having Zero Negative Feedback. **Piling more bull**** on top, does not make it so. Spell out EXACTLY what you mean when you speak of feedback and there'll be fewer problems. If you mean Zero GLOBAL NFB, then say so. As you well know, not all SS amps utilise Global NFB for linearisation. In fact, they use similar feedback methods to your triode amps. And, surprise surprise, they often provide many of the benefits often ascribed to triode amps, without the obvious shortcomings. But you would not want to actually educate people, would you? You, Trevor Wilson, cannot fail to know it, therefore you are picking desperate nits **Nope. I just get tired of people like you perpetuating lies and half truths. -- and giving away your desperation by hysterically screeching your misleading lie "no such thing as a ZNFB amp" over and over again, probably more than twenty times (those who care can count below). **If you can manage some honesty and accuracy, then I would not nbeed to correct your mistakes (lies?). Your problem is that you know I'm right but that you have thirty years invested in commercially telling people tubes are obsolete. **You have no idea. -- Trevor Wilson www.rageaudio.com.au |
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#28
Posted to rec.audio.tubes,uk.rec.audio,rec.audio.opinion
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The Catch-22 of Negative Feedback aka NFB
Just another Hi Fi nut who has built an amp thinks it sounds good but is it
accurate or a nice sound negative feedback is almost necessary in transistor amps used well it works fine and much has been written on good transistor amp design I will put my pro transistor amps against most valve amps anyway Tinted glasses make the world look better but are they accurate .. "Andre Jute" wrote in message ups.com... Some meterhead clown demands to know: What's wrong with feedback? and another meterhead clown, on being told by Rudy that: an OPA2604 ... needs a *thousand* times more feedback demands to know: So what? ***** It ain't rocket science. Explanation taken from: http://members.lycos.co.uk/fiultra/T...mp%20INDEX.htm ***** CATCH-22 ***** The customer complained that his new suit didn't fit. 'The sleeves are too short,' he said to the tailor. 'Yes sir,' said the tailor, 'but if you hold your arm just so, at an angle as if you're drinking tea with your auntie, it will show just the right amount of cuff.' The customer tried it. The tailor was right! 'But what about the other sleeve? It is definitely too short.' 'Just lower your shoulder, sir. Yes, yes, a little more. Put your foot out so you can lower your shoulder a little more still. Bend your knee. Yes, that's it. See how beautifully your suitcoat now fits?' The customer had to admit the tailor was right. 'Wow. But now the leg of the pants is all twisted around.' 'That's easily fixed,' said the tailor. 'Just point your other toe westward, sir, and look over your shoulder to where I am holding up the hand mirror. See? Doesn't that fit beautifully?' 'Yes,' the customer said doubtfully, 'but-' 'Now would sir like to wear his brilliant new suit or shall we wrap it?' The customer was too intimidated to argue. He walked out into the street in his new suit, his arm crooked as if he were drinking tea, his other shoulder well down over a bent knee with his foot out to the side, his other foot pointing westwards, his head twisted back between hunched shoulders as if complaining to God about a cruel fate. Behind him he heard a boy say to his father. 'Oh, Daddy, look at the poor twisted cripple.' 'Hush,' the father said. 'Be grateful the poor man found such a brilliant tailor.' **** The Ultrafidelista view of Negative Feedback by Andre Jute Negative feedback is the paradigm of modern electronic design. It is mother's milk to an electronics engineer. He learns to say '100dB of NFB,' in his sleep before he finishes his first week at the most humble polytechnic. At the great institutions the professor of feedback is the most honoured man in the department. In Massachusetts and Minnesota the feedback guru is the most honoured man on the entire campus, equal in stature to the football coach. When a guru of transistor high fidelity (and some in tubes) says, 'I studied under Ron,' one doesn't have to ask which Ron, one just knows it is the holy name of the prophet of feedback from the Midwest. Before I even finished the design of the KISS 300B it was forcefully suggested by a wannabe guru that with only 50dB more gain (about seven times as much as is likely to be in the actual design) I can apply 50dB of negative feedback to linearize my amplifier. Negative feedback, shorthanded as NFB, is the instant response of the audio engineering fraternity to all ills, real, perceived, non-existent. They don't even ask if there is a problem, they swing the club of NFB regardless. NFB has become a reflex axiom of mainstream audio design. An audio engineer with his negative feedback is like a policeman who runs out into the street with his stick and starts beating a confession out of the first housewife he sees. The difference is that the policeman is relieved of duty to await punishment and the audio engineer gets away with it. In the case of the policeman it is unacceptable behaviour, in the case of the audio engineer so much the expected norm that no one except the ultrafidelista notice. I guess that if one in ten million audio amplifiers does not have negative feedback added, it will be a lot... No one asked if my KISS Amp requires linearization. The presumption by all except those already of the ultrafidelista persuasion was that I would welcome suggestions about A Good Thing. In the face of such overwhelming acceptance by qualified engineers, why do we as ultrafidelista not take the same easy path of negative feedback? Especially considering that superficially NFB is easy to understand and apply. How does negative feedback work? Negative feedback is simply a negative voltage fed back from the output to the input amplifying device to offset part of the harmonic distortion which is present as a positive voltage. It costs nothing except a loss of gain and a few side effects such as phase shift and possible instability which are well known in the mathematical literature and more or less easily guarded against depending on the level of NFB. 'Wow!' those meeting NFB for the first time will now say, 'Something for free! I'll grab some of that for my amp.' Hey, I said it, and I am a professional intellectual, by definition an infinite skeptic. NFB is a thing of beauty that will draw you in. It is like an electronic Marxism which admits of no contrary arguments because it has subsumed them all into The Holy Measurements. To question The Measurements is to commit heresy. You need to be of strong mind to resist the blandishments of such a universal panacea and of strong stomach to withstand the hysterical assaults of the lesser engineers defending their holy grail. (And when you do get hold of a superior engineer to explain NFB to you, you need to be high-domed indeed because suddenly NFB can turn very intricate.) Unfortunately NFB doesn't come without a price. It levies a cruel charge on the perceived quality of the sound. Negative feedback is what gives all those 'blameless' transistor and big PP tube amps their chillingly unnatural sound. Then how did NFB come to be such a panacea in amplifier design? Your guess is as good as mine. Hi-fi design is not prestige work for engineers, or highly paid. The most talented and best qualified engineers go into automobiles or military hardware or big construction projects or computer design. The left-overs design amplifiers in the time they have to spare from writing up specs for requesting a CE mark for a new electric kettle. Lemmings storming en masse over a cliff come to mind; such people don't see the necessity of original thought, or have the mental equipment for it. The exceptions to this rule are normally audio enthusiasts in charge of their own small audio manufactories with niche markets; those who grow larger from this base follow the mainstream mantra of "mo' NFB give lowa' THD" because the marketing channels demand it from them if they wish to grow. At this point they usually cease to offer anything different, only the exclusivity of a very high price. (I know, because a sub-board I designed for a supplier to the trade turns up in so many very expensive amps with so many different big names neatly silkscreened on it... it strikes me as the sort of detail a real designer, as distinct from a marketer, would take under his own control.) Those very few makers who will sell you an ultrafi amp without any NFB operate even tinier shops, usually one man and a cat, just hanging on. The mechanism by which NFB wrecks your sound Negative feedback at first acquaintance sounds good enough to take to bed and cuddle. It isn't. It isn't even as simple as a superficial acquaintance may suggest. Follow the steps with me, from the theory as she is received to what arrives at your brain as music: 1. In theory NFB reduces all harmonic distortion equally, without discrimination. Strictly in theory it does not reshape harmonic distortion by reducing the most objectionable third and higher order odd harmonic distortion to a greater extent than the relatively harmless 2nd harmonic. Thus NFB at its theoretically most benign is already useless in terms of psychoacoustics, as will become clear at point 4. If you disregard psychoacoustics, as many audio engineers do, NFB is brilliant in reducing total harmonic distortion to a number as tiny as you want. You just pile on more NFB. 2. In real life, as distinct from simplified theory, NFB adds artifacts of its own. Remember, it is a loop. The signal starts at the input and is amplified by devices until it reaches the output. From the output a part of the signal called the negative feedback is fed back to the input. Here a loop is completed and the combination, less distorted, reaches the output again, a part of the combination is fed back, endlessly. The artifacts we want to consider here are created by the fed-back residue of harmonic distortions adding to both the fundamental and the distortions already created by the amplifier, then some portion of the sum of the original and the feedback distortion is fed back again and added on, until the ooh-ah bird flies up its own fundament. It looks marginally less disgusting as a recursive mathematical formula with lots of nested parenthetical parcels of noise being loaded onto your music. But it is a monkey on the back of your sound, with a smaller monkey on the back of the first monkey, a still smaller monkey on the back of the second monkey, and so on ad infinitum. These additive artifacts are all higher harmonics and the more dominant ones are all odd. Suppose, for the sake of simplicity, a superbly designed ultrafidelista amp with some second harmonic and zero odd harmonics before NFB. Add NFB and the second harmonic will be lowered but the recombinant new loop now contains newly added intermodulation effects between the fundamental and the residual second harmonic, and that is third harmonic. In the next cycle a small but nasty dose of fifth harmonic that wasn't there before is added by interaction between the still residual second harmonic and reduced newly added third harmonic. In short, the artifacts NFB adds to the distortion mix are all of the most harmful kind. But, say the proponents of NFB, so what? Every time the loop cycles the added artifacts are smaller, even if there are more of them... The whole affair starts to smell of trying to argue with a Marxist who simply declares any inconvenient truth 'an anomaly'. (If this sounds like a mess from which you should run a mile, you have come to the right conclusion. Start running now. It gets worse.) 3. We thus arrive at a situation where distortion has been lowered by NFB but where the most disturbing odd harmonic distortions are still present to some measure, with the added disadvantage that new and extremely disturbing artifacts of higher harmonic distortions have been created by the very process of using negative feedback to lower distortion. Regardless of the absolute level of THD, or the volume setting, the mix of harmonics has been adversely affected and now includes a higher proportion of third and higher harmonics than before NFB. Let me say that again: after NFB, third and higher harmonics will make up a greater part of the distortion than before. 4. Low volume levels perforce accounts for 99 per cent of audiophile listening because we all have families or neighbours, and we would like to keep our ears. Unfortunately for the lowest common denominator of hi-fi designer, the one who specifies NFB as a conditioned response much like Pavlov's dogs slavered when the bell rang, human physiology and psycho-acoustic response is such that odd harmonics are disproportionately more disturbing at lower than at higher listening levels. This inescapable effect is independent of definition of 'listening level.' At the 110dB in-room SPL (only 14dB louder than an automatic riveter!) advocated by the already deaf Transient Overload Elite known on newsgroups as the Borg, this poisonous concoction of original distortions and NFB recombinant artifacts will be least disturbing (and soon not heard at all!). At any lower level perceived interference of this harmonics cocktail with the music will increase in inverse proportion to the volume level. At low volume levels the artifacts generated by NFB will by their nature as higher harmonic distortions be disproportionately far more disturbing. At these normal listening levels 0.75 per cent of second harmonic distortion may be below the threshhold of perception for sophisticated listeners, whereas tiny amounts of third and higher odd harmonic distortions grate. And they still use Negative Feedback? Are they stupid? No, they are not stupid. Most of them march to the drum of a cost accountant on whom we wouldn't spit if he were alight. NFB is as cheap in money terms as it is expensive in terms of perceived quality of music. We shall come to those who claim to be sympathetic to high-fidelity but insist on devices which do not work without NFB, who have another devious answer. Here, meanwhile, for you to keep in mind, is a single-sentence summary of a complicated interdisciplinary argument: The case against NFB is that for 99 per cent of listening the NFB cure is worse than the disease. But surely we don't have to do anything so stupid? It follows from the argument above that ultrafidelista should choose an intrinsically linear topology and device which does not require added negative feedback to 'linearize' the output. The intrinsically linear device is the thermionic tube in either its triode form or as a pentode hogtied to work as a triode, which can be a most pleasing alternative both economically and sonically. The topology is often single-ended operation, chosen also for several other reasons described elsewhere in these articles, including KISS; if the chosen topology is push-pull operation, which is more difficult but far from impossible to arrange without NFB, operation should be specified as Class A1. Inside the argued case above lies too the overwhelming reason to accept the potential small disadvantage that may accompany the preferred topology in comparison to the discarded alternatives. The disadvantage is of course the potential for a residual second harmonic that measures high by transistor or NFB tube standards. (Note the word potential. With a conservatively designed DHT amp the potential problem should not arise.) The ultrafidelista, who are as keen on silent amps as anyone else, accept this small potential difficulty because it is the lesser evil compared to NFB. Unbelievers (largely unwashed, according to reports) sneer that ultrafidelista like this approach because of the 'added euphonics', which is bow-wow techie talk for the warmth a big chunk of second harmonic lays on a zero negative feedback single-ended amplifier. But competent design can easily reduce the level of second harmonic to below the level of perception without the need for NFB and its deleterious after-effects. In any event, it is your amplifier. You paid for it. You have a right to tune it as you please. The key thing is to get rid of NFB and to understand why you did it. Can we prove any of this scientifically? We have already. All of this is the technical subtext to my longtime contention that what the ultrafidelista hear and love is not a directly heated triode sound as is claimed by many enthusiasts but a Class A1, ZNFB sound. (Admittedly, as we have seen above, the right sound is virtually guaranteed with a ZNFB DHT SE amp of conservative provenance but may have to be developed the hard way with more economical or higher-power contenders.) In comparative ABX tests conducted over a number of years, I found that professional musicians, certified golden ears, choose the triode-linked Class A1 PP ZNFB EL34 whenever it is present in the test over all other contenders including SE 300B and 'blameless' high-NFB silicon. Science also proceeds by pure reason. Ultrafidelista have long doubted whether what engineers insist we measure (the absolute level of distortion, THD) predicts success in audio gear. This is the full circle, because I have just proven by logical, individually uncontested steps that what matters, once a certain modest level of silence is assured to an amplifier, is not the absolute level of disharmonics but their composition. The same proof demonstrates that a more beneficial distribution follows instantly from doing without NFB. But transistor amps won't work at all without NFB! That is not our problem. Those who choose inefficient speakers and consequently are forced to accept monstrous amps made possible only by gigadeciBels of NFB, will receive our sympathy - and the music they deserve. Engineering hangers-on of transistor attempts at high fidelity, where the measure of success is vanishing THD rather than sonic hedonism, pretend to be enthusiasts for NFB. To make it work for them, they have attempted to change the rules so that we won't hear what their treasured NFB does to our sound. They sneer that low level listening, which 99 per cent of us prefer and where NFB does most to wreck the sound, is 'easy listening' and therefore not permissible. According to them we should all be forced to listen at the high volume level which suits NFB amps, which they call 'realistic'. This is a contemptible circular argument, only too characteristic of a fascist mentality in a part of the audiophile spectrum which wants to prescribe their arid vision without regard for our enjoyment. We can recommend a good tailor to them. It hurts every time you wear his suit. No pain, no gain, fellers! In summary Almost everyone listens at low level most of the time. NFB wrecks everybody's sound at all levels but most wretchedly at normal listening levels. We started out with a contemptible circular argument and we have met another along the way. We can now put both in context: An 'engineer' who designs an amplifier which does not work perfectly without negative feedback is like a tailor cutting the suit incompetently and then demanding that you walk like a cripple to make it fit, so that everyone can admire the brilliance of your tailor. Negative feedback is a bodge. That is why it is despicable to the ultrafidelista. ******* More Zero Negative Feedback amplifiers at Jute on Amps http://members.lycos.co.uk/fiultra/JUTE%20ON%20AMPS.htm The KISS Amp project, which explains much more, is here http://members.lycos.co.uk/fiultra/T...mp%20INDEX.htm and its schematics and transfer curves are here http://members.lycos.co.uk/fiultra/KISS%20190.htm ******** THE VOLTAGES IN THIS AMP WILL KILL YOU. GET EXPERIENCED SUPERVISION IF IT IS YOUR FIRST TUBE AMP All text and illustration is Copyright © Andre Jute 2001, 2004, 2006 and may not be reproduced except in the thread KISS xxx on rec.audio.tubes |
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#29
Posted to rec.audio.tubes,uk.rec.audio,rec.audio.opinion
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The Catch-22 of Negative Feedback aka NFB
On Fri, 24 Mar 2006 19:49:18 +0100, "Ruud Broens"
wrote: "Stewart Pinkerton" wrote in message .. . : On Fri, 24 Mar 2006 17:57:12 +0100, "Ruud Broens" : wrote: : : : : Where's the beef? : : : : : they 'beef' is, your banana needs 1K more straightening' than my banana - : : why keep claiming it is "superior" ? : : : : Because what matters is the output of the *final* circuit - the banana : : is perfectly straight in the opamp, but your tube circuit is still : : bendy. : : yeah, right, as in 0.007 % distortion kinda bendy. : must be what they call 'the scottish bend', : then : : Be more specific about the tubed circuit you claim has this level of : distortion. : eehhrrmm, don't think that earlier suggestion of a scottish detective will work out, after all, thanks for detecting, not even well above noise level, lot'sZZ In other words, you made it up. -- Stewart Pinkerton | Music is Art - Audio is Engineering |
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#30
Posted to rec.audio.tubes,uk.rec.audio,rec.audio.opinion
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The Catch-22 of Negative Feedback aka NFB
"Stewart Pinkerton" wrote in message ... : On Fri, 24 Mar 2006 19:49:18 +0100, "Ruud Broens" : wrote: : : : "Stewart Pinkerton" wrote in message : .. . : : On Fri, 24 Mar 2006 17:57:12 +0100, "Ruud Broens" : : wrote: : : : : : : Where's the beef? : : : : : : : they 'beef' is, your banana needs 1K more straightening' than my banana - : : : why keep claiming it is "superior" ? : : : : : : Because what matters is the output of the *final* circuit - the banana : : : is perfectly straight in the opamp, but your tube circuit is still : : : bendy. : : : : yeah, right, as in 0.007 % distortion kinda bendy. : : must be what they call 'the scottish bend', : : then : : : : Be more specific about the tubed circuit you claim has this level of : : distortion. : : : : eehhrrmm, : don't think that earlier suggestion of a scottish detective will : work out, : after all, : : thanks for detecting, : not even well above noise level, : lot'sZZ : : In other words, you made it up. : -- : : Stewart Pinkerton | Music is Art - Audio is Engineering as my track record is pretty impressive in that respect, i can afford this one: see my upcoming movie: Tweak City , , starring Kyra evidently, Rudy |
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#31
Posted to uk.rec.audio,rec.audio.opinion
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The Catch-22 of Negative Feedback aka NFB
"Ruud Broens" said:
see my upcoming movie: Tweak City , , starring Kyra Does it perhaps involve aspirins, for when she 'pretends' having a headache? I recommend Mundorf aspirins for critical applications ;-) -- - Never argue with idiots, they drag you down their level and beat you with experience. - |
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#32
Posted to uk.rec.audio,rec.audio.opinion
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The Catch-22 of Negative Feedback aka NFB
"Sander deWaal" wrote in message ... : "Ruud Broens" said: : : : see my upcoming movie: Tweak City : , , starring Kyra : : : Does it perhaps involve aspirins, for when she 'pretends' having a : headache? : : : I recommend Mundorf aspirins for critical applications ;-) : : -- : : - Never argue with idiots, they drag you down their level and beat you with experience. - I agree, that could make of the difference between knightly & dayly Ry |
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#33
Posted to rec.audio.tubes,uk.rec.audio,rec.audio.opinion
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The Catch-22 of Negative Feedback aka NFB
Patrick Turner wrote: Andre Jute wrote: I must say, Wilson, I don't understand why you're getting your knickers in a knot. You're like those bolshies I used to know who claimed that anyone who wasn't willing to kill for "socialism" was only play-acting. Are you really claiming that someone who merely uses the NFB he finds lurking in DHTs can't belong to the club until he repents and uses excessive amounts of loop NFB? Are you really claiming that the use of lots and lots of NFB is a prerequisite for good audio design? Are you really claiming that because a little NFB is a good thing, an unlimited amount must therefore be better, and an infinite amount best of all? Seems a bit immoderate to me, old chap. You will find yourself arguing at a man who has a head full of solid concrete. Trev will never see the whole picture, and has never designed or built any amp in the last 30 years. He's mindless, for sure. All those mindlessly insistent claims that I said something I didn't say -- right next to the place where I said exactly the opposite. I try not to bother arguing the same old tired BS time after time. He just likes hanging out in news groups and Bull****ting. He has NEVER once contributed a single article of constructive advice on tubecraft at r.a.t. Doesn't surprise me. Your hysterical belief that I am "against" NFB is the product of your unsophisticated literal-mindedness. There is no reason for NFB to be an act of faith, like an on-off switch. For the record, quite contrary to your silly claims about what I said, I believe the little NFB that occurs naturally in triodes and in certain conservatively sanctioned traditional topologies are A Good Thing. To avoid giving you another apoplectic fit, I shan't repeat what I think of the excessive amounts of NFB required to make transistors work at all. If you think you can write a more compelling argument than I can about the evils of excessive NFB, have at it. But every time I see you, like the other techies you merely kibbitz what better men have written. I am now doing 845 amps, two per channel for 50 watts each. I expect music to be real fine. Damping factor good, noise low, bandwidth wide, distortions negligible and maybe I can add 6db of global NFB but I doubt any difference due to some **added** NFB will be heard. You probably won't need the NFB. The biggest problem with 845 is not deciding how much NFB to use but steeling yourself to let them out of the door. 845 are God's own tube. I really hesitated when I needed the space, wondering if I shouldn't keep the simple SET 845 rather than my Millennium's End SV572-xx amps. Trev has never done anything like this and simply doesn't understand. It's not so difficult to open your mind to new experiences and new ideas. But first one has to stop being a smartarse, and that I don't think poor old Trevor can manage. With 845, just no need for the 106dB of added NFB used in SS amps. I don't care a bit about what Trev says. I'm fast reaching the same conclusion. Patrick Turner. Andre Jute |
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#34
Posted to rec.audio.tubes,uk.rec.audio,rec.audio.opinion
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The Catch-22 of Negative Feedback aka NFB
Stewart Pinkerton wrote: On Fri, 24 Mar 2006 17:57:12 +0100, "Ruud Broens" wrote: "Stewart Pinkerton" wrote in message .. . : On Fri, 24 Mar 2006 12:59:37 +0100, "Ruud Broens" : wrote: : : "Arny Krueger" wrote in message : ... : : "Ruud Broens" wrote in message : : : : : : we've been through this 'linearity contest' before, here. : : whereas you define it in terms of current amplification, : : i define it in terms of voltage amplification. : : as real-world interfacing in audio is with *voltages*, : : doesn't seem unreasonable... : : : : : : Negative feedback is generally used to make amplifiers more linear in the : : voltage domain. It's a highly sucessful strategy. We sent men to the moon, : : partially based on amplifiers that were linearized with negative feedback. : : : : Where's the beef? : : : they 'beef' is, your banana needs 1K more straightening' than my banana - : why keep claiming it is "superior" ? : : Because what matters is the output of the *final* circuit - the banana : is perfectly straight in the opamp, but your tube circuit is still : bendy. yeah, right, as in 0.007 % distortion kinda bendy. must be what they call 'the scottish bend', then Be more specific about the tubed circuit you claim has this level of distortion. -- It's there in the RAT thread Hybrid circuit-CQ something or other. |
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#35
Posted to rec.audio.tubes,uk.rec.audio,rec.audio.opinion
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The Catch-22 of Negative Feedback aka NFB
If you think you can write a more compelling argument than I can about the evils of excessive NFB, have at it. But every time I see you, like the other techies you merely kibbitz what better men have written. I am now doing 845 amps, two per channel for 50 watts each. I expect music to be real fine. Damping factor good, noise low, bandwidth wide, distortions negligible and maybe I can add 6db of global NFB but I doubt any difference due to some **added** NFB will be heard. You probably won't need the NFB. The biggest problem with 845 is not deciding how much NFB to use but steeling yourself to let them out of the door. 845 are God's own tube. I really hesitated when I needed the space, wondering if I shouldn't keep the simple SET 845 rather than my Millennium's End SV572-xx amps. And I am using KR845..... I think I'll use sepaarate cathode biasing for each 845, tubes and a trioded EL34 driving a 1:1 IST but with a cap shunting the anode to secondary winding to make sure the HF transfer is near perfect without the leakage inductance having any effect. Input will be 6SN7, but maybe could be 6SL7 µ follower Trev has never done anything like this and simply doesn't understand. It's not so difficult to open your mind to new experiences and new ideas. But first one has to stop being a smartarse, and that I don't think poor old Trevor can manage. He regards SET amplification as a form of hi-fi illegitimacy. Calling people *******s for owning and preferring SE amps to his fav brands of solid state makes him look a fool. With 845, just no need for the 106dB of added NFB used in SS amps. I don't care a bit about what Trev says. I'm fast reaching the same conclusion. Patrick Turner. Andre Jute A colleague in Sydney is doing an SE amp with 6 x GM70 for 120 watts. But it will be switchable to PP......its doable, 5 very good switches are needed, and I did the OPT design. Whether the GM70 is better than 845 is unknown. I like challenges. The OPT for 845 has 3 times the turns that my 300 watt PP amps have, and the wire is thinner for 845, and my eyes are not getting younger.... Patrick Turner, |
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#36
Posted to rec.audio.tubes,uk.rec.audio,rec.audio.opinion
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The Catch-22 of Negative Feedback aka NFB
On 24 Mar 2006 14:01:45 -0800, "Andre Jute" wrote:
Patrick Turner wrote: I am now doing 845 amps, two per channel for 50 watts each. I expect music to be real fine. Damping factor good, noise low, bandwidth wide, distortions negligible and maybe I can add 6db of global NFB but I doubt any difference due to some **added** NFB will be heard. Depends on the speakers, the reduced output impedance may be useful. You probably won't need the NFB. The biggest problem with 845 is not deciding how much NFB to use but steeling yourself to let them out of the door. 845 are God's own tube. I really hesitated when I needed the space, wondering if I shouldn't keep the simple SET 845 rather than my Millennium's End SV572-xx amps. It's an excellent tube, as tubes go, but of course we've had seventy years of progress since then....... Trev has never done anything like this and simply doesn't understand. It's not so difficult to open your mind to new experiences and new ideas. But first one has to stop being a smartarse, and that I don't think poor old Trevor can manage. You of course would be the master of this art - except that your arse is the smartest part of you. With 845, just no need for the 106dB of added NFB used in SS amps. I don't care a bit about what Trev says. With SS, no need for the massive cost of the output transformer for 50 watts of single-ended tube amplification, plus of course you get much better linearity than even the mighty 845 can manage. A really good 60-watt SS amp is very easy to implement these days - and you can buy one for the cost of a pair of those OPTs............... -- Stewart Pinkerton | Music is Art - Audio is Engineering Posted Via Usenet.com Premium Usenet Newsgroup Services ---------------------------------------------------------- ** SPEED ** RETENTION ** COMPLETION ** ANONYMITY ** ---------------------------------------------------------- http://www.usenet.com |
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#37
Posted to rec.audio.tubes,uk.rec.audio,rec.audio.opinion
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The Catch-22 of Negative Feedback aka NFB
"Bret Ludwig" wrote in message
oups.com Ruud Broens wrote: snip The opamp is *designed* to use large amounts of linearising feedback, something that simply isn't an option with the inherently much lower open-loop gain of tubes. Philbrick and Julie were building perfectly proper op amps with tubes as early as 1943, but thanks for playing. Actually, tubed operational amps, especially the early ones (the specific ones you're mentioning), were horrors. If the noise and drift didn't get you, the short parts life would. Operational amplifiers were originally designed as parts for analog computers. When SS op amps for analog computers became commercial, analog computer experts sighed a huge sigh of relief. They were stable, reliable, and quiet. |
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#38
Posted to rec.audio.tubes,uk.rec.audio,rec.audio.opinion
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The Catch-22 of Negative Feedback aka NFB
John Deans wrote:
"Andre Jute" wrote in message ups.com... Some meterhead clown demands to know: What's wrong with feedback? and another meterhead clown, on being told by Rudy that: an OPA2604 ... needs a *thousand* times more feedback demands to know: So what? ***** It ain't rocket science. Explanation taken from: http://members.lycos.co.uk/fiultra/T...mp%20INDEX.htm ***** CATCH-22 ***** The customer complained that his new suit didn't fit. 'The sleeves are too short,' he said to the tailor. 'Yes sir,' said the tailor, 'but if you hold your arm just so, at an angle as if you're drinking tea with your auntie, it will show just the right amount of cuff.' The customer tried it. The tailor was right! 'But what about the other sleeve? It is definitely too short.' 'Just lower your shoulder, sir. Yes, yes, a little more. Put your foot out so you can lower your shoulder a little more still. Bend your knee. Yes, that's it. See how beautifully your suitcoat now fits?' The customer had to admit the tailor was right. 'Wow. But now the leg of the pants is all twisted around.' 'That's easily fixed,' said the tailor. 'Just point your other toe westward, sir, and look over your shoulder to where I am holding up the hand mirror. See? Doesn't that fit beautifully?' 'Yes,' the customer said doubtfully, 'but-' 'Now would sir like to wear his brilliant new suit or shall we wrap it?' The customer was too intimidated to argue. He walked out into the street in his new suit, his arm crooked as if he were drinking tea, his other shoulder well down over a bent knee with his foot out to the side, his other foot pointing westwards, his head twisted back between hunched shoulders as if complaining to God about a cruel fate. Behind him he heard a boy say to his father. 'Oh, Daddy, look at the poor twisted cripple.' 'Hush,' the father said. 'Be grateful the poor man found such a brilliant tailor.' **** The Ultrafidelista view of Negative Feedback by Andre Jute Negative feedback is the paradigm of modern electronic design. It is mother's milk to an electronics engineer. He learns to say '100dB of NFB,' in his sleep before he finishes his first week at the most humble polytechnic. At the great institutions the professor of feedback is the most honoured man in the department. In Massachusetts and Minnesota the feedback guru is the most honoured man on the entire campus, equal in stature to the football coach. When a guru of transistor high fidelity (and some in tubes) says, 'I studied under Ron,' one doesn't have to ask which Ron, one just knows it is the holy name of the prophet of feedback from the Midwest. Before I even finished the design of the KISS 300B it was forcefully suggested by a wannabe guru that with only 50dB more gain (about seven times as much as is likely to be in the actual design) I can apply 50dB of negative feedback to linearize my amplifier. Negative feedback, shorthanded as NFB, is the instant response of the audio engineering fraternity to all ills, real, perceived, non-existent. They don't even ask if there is a problem, they swing the club of NFB regardless. NFB has become a reflex axiom of mainstream audio design. An audio engineer with his negative feedback is like a policeman who runs out into the street with his stick and starts beating a confession out of the first housewife he sees. The difference is that the policeman is relieved of duty to await punishment and the audio engineer gets away with it. In the case of the policeman it is unacceptable behaviour, in the case of the audio engineer so much the expected norm that no one except the ultrafidelista notice. I guess that if one in ten million audio amplifiers does not have negative feedback added, it will be a lot... No one asked if my KISS Amp requires linearization. The presumption by all except those already of the ultrafidelista persuasion was that I would welcome suggestions about A Good Thing. In the face of such overwhelming acceptance by qualified engineers, why do we as ultrafidelista not take the same easy path of negative feedback? Especially considering that superficially NFB is easy to understand and apply. How does negative feedback work? Negative feedback is simply a negative voltage fed back from the output to the input amplifying device to offset part of the harmonic distortion which is present as a positive voltage. It costs nothing except a loss of gain and a few side effects such as phase shift and possible instability which are well known in the mathematical literature and more or less easily guarded against depending on the level of NFB. 'Wow!' those meeting NFB for the first time will now say, 'Something for free! I'll grab some of that for my amp.' Hey, I said it, and I am a professional intellectual, by definition an infinite skeptic. NFB is a thing of beauty that will draw you in. It is like an electronic Marxism which admits of no contrary arguments because it has subsumed them all into The Holy Measurements. To question The Measurements is to commit heresy. You need to be of strong mind to resist the blandishments of such a universal panacea and of strong stomach to withstand the hysterical assaults of the lesser engineers defending their holy grail. (And when you do get hold of a superior engineer to explain NFB to you, you need to be high-domed indeed because suddenly NFB can turn very intricate.) Unfortunately NFB doesn't come without a price. It levies a cruel charge on the perceived quality of the sound. Negative feedback is what gives all those 'blameless' transistor and big PP tube amps their chillingly unnatural sound. Then how did NFB come to be such a panacea in amplifier design? Your guess is as good as mine. Hi-fi design is not prestige work for engineers, or highly paid. The most talented and best qualified engineers go into automobiles or military hardware or big construction projects or computer design. The left-overs design amplifiers in the time they have to spare from writing up specs for requesting a CE mark for a new electric kettle. Lemmings storming en masse over a cliff come to mind; such people don't see the necessity of original thought, or have the mental equipment for it. The exceptions to this rule are normally audio enthusiasts in charge of their own small audio manufactories with niche markets; those who grow larger from this base follow the mainstream mantra of "mo' NFB give lowa' THD" because the marketing channels demand it from them if they wish to grow. At this point they usually cease to offer anything different, only the exclusivity of a very high price. (I know, because a sub-board I designed for a supplier to the trade turns up in so many very expensive amps with so many different big names neatly silkscreened on it... it strikes me as the sort of detail a real designer, as distinct from a marketer, would take under his own control.) Those very few makers who will sell you an ultrafi amp without any NFB operate even tinier shops, usually one man and a cat, just hanging on. The mechanism by which NFB wrecks your sound Negative feedback at first acquaintance sounds good enough to take to bed and cuddle. It isn't. It isn't even as simple as a superficial acquaintance may suggest. Follow the steps with me, from the theory as she is received to what arrives at your brain as music: 1. In theory NFB reduces all harmonic distortion equally, without discrimination. Strictly in theory it does not reshape harmonic distortion by reducing the most objectionable third and higher order odd harmonic distortion to a greater extent than the relatively harmless 2nd harmonic. Thus NFB at its theoretically most benign is already useless in terms of psychoacoustics, as will become clear at point 4. If you disregard psychoacoustics, as many audio engineers do, NFB is brilliant in reducing total harmonic distortion to a number as tiny as you want. You just pile on more NFB. 2. In real life, as distinct from simplified theory, NFB adds artifacts of its own. Remember, it is a loop. The signal starts at the input and is amplified by devices until it reaches the output. From the output a part of the signal called the negative feedback is fed back to the input. Here a loop is completed and the combination, less distorted, reaches the output again, a part of the combination is fed back, endlessly. The artifacts we want to consider here are created by the fed-back residue of harmonic distortions adding to both the fundamental and the distortions already created by the amplifier, then some portion of the sum of the original and the feedback distortion is fed back again and added on, until the ooh-ah bird flies up its own fundament. It looks marginally less disgusting as a recursive mathematical formula with lots of nested parenthetical parcels of noise being loaded onto your music. But it is a monkey on the back of your sound, with a smaller monkey on the back of the first monkey, a still smaller monkey on the back of the second monkey, and so on ad infinitum. These additive artifacts are all higher harmonics and the more dominant ones are all odd. Suppose, for the sake of simplicity, a superbly designed ultrafidelista amp with some second harmonic and zero odd harmonics before NFB. Add NFB and the second harmonic will be lowered but the recombinant new loop now contains newly added intermodulation effects between the fundamental and the residual second harmonic, and that is third harmonic. In the next cycle a small but nasty dose of fifth harmonic that wasn't there before is added by interaction between the still residual second harmonic and reduced newly added third harmonic. In short, the artifacts NFB adds to the distortion mix are all of the most harmful kind. But, say the proponents of NFB, so what? Every time the loop cycles the added artifacts are smaller, even if there are more of them... The whole affair starts to smell of trying to argue with a Marxist who simply declares any inconvenient truth 'an anomaly'. (If this sounds like a mess from which you should run a mile, you have come to the right conclusion. Start running now. It gets worse.) 3. We thus arrive at a situation where distortion has been lowered by NFB but where the most disturbing odd harmonic distortions are still present to some measure, with the added disadvantage that new and extremely disturbing artifacts of higher harmonic distortions have been created by the very process of using negative feedback to lower distortion. Regardless of the absolute level of THD, or the volume setting, the mix of harmonics has been adversely affected and now includes a higher proportion of third and higher harmonics than before NFB. Let me say that again: after NFB, third and higher harmonics will make up a greater part of the distortion than before. 4. Low volume levels perforce accounts for 99 per cent of audiophile listening because we all have families or neighbours, and we would like to keep our ears. Unfortunately for the lowest common denominator of hi-fi designer, the one who specifies NFB as a conditioned response much like Pavlov's dogs slavered when the bell rang, human physiology and psycho-acoustic response is such that odd harmonics are disproportionately more disturbing at lower than at higher listening levels. This inescapable effect is independent of definition of 'listening level.' At the 110dB in-room SPL (only 14dB louder than an automatic riveter!) advocated by the already deaf Transient Overload Elite known on newsgroups as the Borg, this poisonous concoction of original distortions and NFB recombinant artifacts will be least disturbing (and soon not heard at all!). At any lower level perceived interference of this harmonics cocktail with the music will increase in inverse proportion to the volume level. At low volume levels the artifacts generated by NFB will by their nature as higher harmonic distortions be disproportionately far more disturbing. At these normal listening levels 0.75 per cent of second harmonic distortion may be below the threshhold of perception for sophisticated listeners, whereas tiny amounts of third and higher odd harmonic distortions grate. And they still use Negative Feedback? Are they stupid? No, they are not stupid. Most of them march to the drum of a cost accountant on whom we wouldn't spit if he were alight. NFB is as cheap in money terms as it is expensive in terms of perceived quality of music. We shall come to those who claim to be sympathetic to high-fidelity but insist on devices which do not work without NFB, who have another devious answer. Here, meanwhile, for you to keep in mind, is a single-sentence summary of a complicated interdisciplinary argument: The case against NFB is that for 99 per cent of listening the NFB cure is worse than the disease. But surely we don't have to do anything so stupid? It follows from the argument above that ultrafidelista should choose an intrinsically linear topology and device which does not require added negative feedback to 'linearize' the output. The intrinsically linear device is the thermionic tube in either its triode form or as a pentode hogtied to work as a triode, which can be a most pleasing alternative both economically and sonically. The topology is often single-ended operation, chosen also for several other reasons described elsewhere in these articles, including KISS; if the chosen topology is push-pull operation, which is more difficult but far from impossible to arrange without NFB, operation should be specified as Class A1. Inside the argued case above lies too the overwhelming reason to accept the potential small disadvantage that may accompany the preferred topology in comparison to the discarded alternatives. The disadvantage is of course the potential for a residual second harmonic that measures high by transistor or NFB tube standards. (Note the word potential. With a conservatively designed DHT amp the potential problem should not arise.) The ultrafidelista, who are as keen on silent amps as anyone else, accept this small potential difficulty because it is the lesser evil compared to NFB. Unbelievers (largely unwashed, according to reports) sneer that ultrafidelista like this approach because of the 'added euphonics', which is bow-wow techie talk for the warmth a big chunk of second harmonic lays on a zero negative feedback single-ended amplifier. But competent design can easily reduce the level of second harmonic to below the level of perception without the need for NFB and its deleterious after-effects. In any event, it is your amplifier. You paid for it. You have a right to tune it as you please. The key thing is to get rid of NFB and to understand why you did it. Can we prove any of this scientifically? We have already. All of this is the technical subtext to my longtime contention that what the ultrafidelista hear and love is not a directly heated triode sound as is claimed by many enthusiasts but a Class A1, ZNFB sound. (Admittedly, as we have seen above, the right sound is virtually guaranteed with a ZNFB DHT SE amp of conservative provenance but may have to be developed the hard way with more economical or higher-power contenders.) In comparative ABX tests conducted over a number of years, I found that professional musicians, certified golden ears, choose the triode-linked Class A1 PP ZNFB EL34 whenever it is present in the test over all other contenders including SE 300B and 'blameless' high-NFB silicon. Science also proceeds by pure reason. Ultrafidelista have long doubted whether what engineers insist we measure (the absolute level of distortion, THD) predicts success in audio gear. This is the full circle, because I have just proven by logical, individually uncontested steps that what matters, once a certain modest level of silence is assured to an amplifier, is not the absolute level of disharmonics but their composition. The same proof demonstrates that a more beneficial distribution follows instantly from doing without NFB. But transistor amps won't work at all without NFB! That is not our problem. Those who choose inefficient speakers and consequently are forced to accept monstrous amps made possible only by gigadeciBels of NFB, will receive our sympathy - and the music they deserve. Engineering hangers-on of transistor attempts at high fidelity, where the measure of success is vanishing THD rather than sonic hedonism, pretend to be enthusiasts for NFB. To make it work for them, they have attempted to change the rules so that we won't hear what their treasured NFB does to our sound. They sneer that low level listening, which 99 per cent of us prefer and where NFB does most to wreck the sound, is 'easy listening' and therefore not permissible. According to them we should all be forced to listen at the high volume level which suits NFB amps, which they call 'realistic'. This is a contemptible circular argument, only too characteristic of a fascist mentality in a part of the audiophile spectrum which wants to prescribe their arid vision without regard for our enjoyment. We can recommend a good tailor to them. It hurts every time you wear his suit. No pain, no gain, fellers! In summary Almost everyone listens at low level most of the time. NFB wrecks everybody's sound at all levels but most wretchedly at normal listening levels. We started out with a contemptible circular argument and we have met another along the way. We can now put both in context: An 'engineer' who designs an amplifier which does not work perfectly without negative feedback is like a tailor cutting the suit incompetently and then demanding that you walk like a cripple to make it fit, so that everyone can admire the brilliance of your tailor. Negative feedback is a bodge. That is why it is despicable to the ultrafidelista. ******* More Zero Negative Feedback amplifiers at Jute on Amps http://members.lycos.co.uk/fiultra/JUTE%20ON%20AMPS.htm The KISS Amp project, which explains much more, is here http://members.lycos.co.uk/fiultra/T...mp%20INDEX.htm and its schematics and transfer curves are here http://members.lycos.co.uk/fiultra/KISS%20190.htm ******** THE VOLTAGES IN THIS AMP WILL KILL YOU. GET EXPERIENCED SUPERVISION IF IT IS YOUR FIRST TUBE AMP All text and illustration is Copyright © Andre Jute 2001, 2004, 2006 and may not be reproduced except in the thread KISS xxx on rec.audio.tubes Bunk. JLS |
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#39
Posted to rec.audio.tubes,uk.rec.audio,rec.audio.opinion,rec.audio.tubes
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The Catch-22 of Negative Feedback aka NFB
John Deans wrote: "Andre Jute" wrote in message ups.com... Some meterhead clown demands to know: What's wrong with feedback? and another meterhead clown, on being told by Rudy that: an OPA2604 ... needs a *thousand* times more feedback demands to know: So what? ***** It ain't rocket science. Explanation taken from: http://members.lycos.co.uk/fiultra/T...mp%20INDEX.htm ***** CATCH-22 ***** The customer complained that his new suit didn't fit. 'The sleeves are too short,' he said to the tailor. 'Yes sir,' said the tailor, 'but if you hold your arm just so, at an angle as if you're drinking tea with your auntie, it will show just the right amount of cuff.' The customer tried it. The tailor was right! 'But what about the other sleeve? It is definitely too short.' 'Just lower your shoulder, sir. Yes, yes, a little more. Put your foot out so you can lower your shoulder a little more still. Bend your knee. Yes, that's it. See how beautifully your suitcoat now fits?' The customer had to admit the tailor was right. 'Wow. But now the leg of the pants is all twisted around.' 'That's easily fixed,' said the tailor. 'Just point your other toe westward, sir, and look over your shoulder to where I am holding up the hand mirror. See? Doesn't that fit beautifully?' 'Yes,' the customer said doubtfully, 'but-' 'Now would sir like to wear his brilliant new suit or shall we wrap it?' The customer was too intimidated to argue. He walked out into the street in his new suit, his arm crooked as if he were drinking tea, his other shoulder well down over a bent knee with his foot out to the side, his other foot pointing westwards, his head twisted back between hunched shoulders as if complaining to God about a cruel fate. Behind him he heard a boy say to his father. 'Oh, Daddy, look at the poor twisted cripple.' 'Hush,' the father said. 'Be grateful the poor man found such a brilliant tailor.' **** The Ultrafidelista view of Negative Feedback by Andre Jute Negative feedback is the paradigm of modern electronic design. It is mother's milk to an electronics engineer. He learns to say '100dB of NFB,' in his sleep before he finishes his first week at the most humble polytechnic. At the great institutions the professor of feedback is the most honoured man in the department. In Massachusetts and Minnesota the feedback guru is the most honoured man on the entire campus, equal in stature to the football coach. When a guru of transistor high fidelity (and some in tubes) says, 'I studied under Ron,' one doesn't have to ask which Ron, one just knows it is the holy name of the prophet of feedback from the Midwest. Before I even finished the design of the KISS 300B it was forcefully suggested by a wannabe guru that with only 50dB more gain (about seven times as much as is likely to be in the actual design) I can apply 50dB of negative feedback to linearize my amplifier. Negative feedback, shorthanded as NFB, is the instant response of the audio engineering fraternity to all ills, real, perceived, non-existent. They don't even ask if there is a problem, they swing the club of NFB regardless. NFB has become a reflex axiom of mainstream audio design. An audio engineer with his negative feedback is like a policeman who runs out into the street with his stick and starts beating a confession out of the first housewife he sees. The difference is that the policeman is relieved of duty to await punishment and the audio engineer gets away with it. In the case of the policeman it is unacceptable behaviour, in the case of the audio engineer so much the expected norm that no one except the ultrafidelista notice. I guess that if one in ten million audio amplifiers does not have negative feedback added, it will be a lot... No one asked if my KISS Amp requires linearization. The presumption by all except those already of the ultrafidelista persuasion was that I would welcome suggestions about A Good Thing. In the face of such overwhelming acceptance by qualified engineers, why do we as ultrafidelista not take the same easy path of negative feedback? Especially considering that superficially NFB is easy to understand and apply. How does negative feedback work? Negative feedback is simply a negative voltage fed back from the output to the input amplifying device to offset part of the harmonic distortion which is present as a positive voltage. It costs nothing except a loss of gain and a few side effects such as phase shift and possible instability which are well known in the mathematical literature and more or less easily guarded against depending on the level of NFB. 'Wow!' those meeting NFB for the first time will now say, 'Something for free! I'll grab some of that for my amp.' Hey, I said it, and I am a professional intellectual, by definition an infinite skeptic. NFB is a thing of beauty that will draw you in. It is like an electronic Marxism which admits of no contrary arguments because it has subsumed them all into The Holy Measurements. To question The Measurements is to commit heresy. You need to be of strong mind to resist the blandishments of such a universal panacea and of strong stomach to withstand the hysterical assaults of the lesser engineers defending their holy grail. (And when you do get hold of a superior engineer to explain NFB to you, you need to be high-domed indeed because suddenly NFB can turn very intricate.) Unfortunately NFB doesn't come without a price. It levies a cruel charge on the perceived quality of the sound. Negative feedback is what gives all those 'blameless' transistor and big PP tube amps their chillingly unnatural sound. Then how did NFB come to be such a panacea in amplifier design? Your guess is as good as mine. Hi-fi design is not prestige work for engineers, or highly paid. The most talented and best qualified engineers go into automobiles or military hardware or big construction projects or computer design. The left-overs design amplifiers in the time they have to spare from writing up specs for requesting a CE mark for a new electric kettle. Lemmings storming en masse over a cliff come to mind; such people don't see the necessity of original thought, or have the mental equipment for it. The exceptions to this rule are normally audio enthusiasts in charge of their own small audio manufactories with niche markets; those who grow larger from this base follow the mainstream mantra of "mo' NFB give lowa' THD" because the marketing channels demand it from them if they wish to grow. At this point they usually cease to offer anything different, only the exclusivity of a very high price. (I know, because a sub-board I designed for a supplier to the trade turns up in so many very expensive amps with so many different big names neatly silkscreened on it... it strikes me as the sort of detail a real designer, as distinct from a marketer, would take under his own control.) Those very few makers who will sell you an ultrafi amp without any NFB operate even tinier shops, usually one man and a cat, just hanging on. The mechanism by which NFB wrecks your sound Negative feedback at first acquaintance sounds good enough to take to bed and cuddle. It isn't. It isn't even as simple as a superficial acquaintance may suggest. Follow the steps with me, from the theory as she is received to what arrives at your brain as music: 1. In theory NFB reduces all harmonic distortion equally, without discrimination. Strictly in theory it does not reshape harmonic distortion by reducing the most objectionable third and higher order odd harmonic distortion to a greater extent than the relatively harmless 2nd harmonic. Thus NFB at its theoretically most benign is already useless in terms of psychoacoustics, as will become clear at point 4. If you disregard psychoacoustics, as many audio engineers do, NFB is brilliant in reducing total harmonic distortion to a number as tiny as you want. You just pile on more NFB. 2. In real life, as distinct from simplified theory, NFB adds artifacts of its own. Remember, it is a loop. The signal starts at the input and is amplified by devices until it reaches the output. From the output a part of the signal called the negative feedback is fed back to the input. Here a loop is completed and the combination, less distorted, reaches the output again, a part of the combination is fed back, endlessly. The artifacts we want to consider here are created by the fed-back residue of harmonic distortions adding to both the fundamental and the distortions already created by the amplifier, then some portion of the sum of the original and the feedback distortion is fed back again and added on, until the ooh-ah bird flies up its own fundament. It looks marginally less disgusting as a recursive mathematical formula with lots of nested parenthetical parcels of noise being loaded onto your music. But it is a monkey on the back of your sound, with a smaller monkey on the back of the first monkey, a still smaller monkey on the back of the second monkey, and so on ad infinitum. These additive artifacts are all higher harmonics and the more dominant ones are all odd. Suppose, for the sake of simplicity, a superbly designed ultrafidelista amp with some second harmonic and zero odd harmonics before NFB. Add NFB and the second harmonic will be lowered but the recombinant new loop now contains newly added intermodulation effects between the fundamental and the residual second harmonic, and that is third harmonic. In the next cycle a small but nasty dose of fifth harmonic that wasn't there before is added by interaction between the still residual second harmonic and reduced newly added third harmonic. In short, the artifacts NFB adds to the distortion mix are all of the most harmful kind. But, say the proponents of NFB, so what? Every time the loop cycles the added artifacts are smaller, even if there are more of them... The whole affair starts to smell of trying to argue with a Marxist who simply declares any inconvenient truth 'an anomaly'. (If this sounds like a mess from which you should run a mile, you have come to the right conclusion. Start running now. It gets worse.) 3. We thus arrive at a situation where distortion has been lowered by NFB but where the most disturbing odd harmonic distortions are still present to some measure, with the added disadvantage that new and extremely disturbing artifacts of higher harmonic distortions have been created by the very process of using negative feedback to lower distortion. Regardless of the absolute level of THD, or the volume setting, the mix of harmonics has been adversely affected and now includes a higher proportion of third and higher harmonics than before NFB. Let me say that again: after NFB, third and higher harmonics will make up a greater part of the distortion than before. 4. Low volume levels perforce accounts for 99 per cent of audiophile listening because we all have families or neighbours, and we would like to keep our ears. Unfortunately for the lowest common denominator of hi-fi designer, the one who specifies NFB as a conditioned response much like Pavlov's dogs slavered when the bell rang, human physiology and psycho-acoustic response is such that odd harmonics are disproportionately more disturbing at lower than at higher listening levels. This inescapable effect is independent of definition of 'listening level.' At the 110dB in-room SPL (only 14dB louder than an automatic riveter!) advocated by the already deaf Transient Overload Elite known on newsgroups as the Borg, this poisonous concoction of original distortions and NFB recombinant artifacts will be least disturbing (and soon not heard at all!). At any lower level perceived interference of this harmonics cocktail with the music will increase in inverse proportion to the volume level. At low volume levels the artifacts generated by NFB will by their nature as higher harmonic distortions be disproportionately far more disturbing. At these normal listening levels 0.75 per cent of second harmonic distortion may be below the threshhold of perception for sophisticated listeners, whereas tiny amounts of third and higher odd harmonic distortions grate. And they still use Negative Feedback? Are they stupid? No, they are not stupid. Most of them march to the drum of a cost accountant on whom we wouldn't spit if he were alight. NFB is as cheap in money terms as it is expensive in terms of perceived quality of music. We shall come to those who claim to be sympathetic to high-fidelity but insist on devices which do not work without NFB, who have another devious answer. Here, meanwhile, for you to keep in mind, is a single-sentence summary of a complicated interdisciplinary argument: The case against NFB is that for 99 per cent of listening the NFB cure is worse than the disease. But surely we don't have to do anything so stupid? It follows from the argument above that ultrafidelista should choose an intrinsically linear topology and device which does not require added negative feedback to 'linearize' the output. The intrinsically linear device is the thermionic tube in either its triode form or as a pentode hogtied to work as a triode, which can be a most pleasing alternative both economically and sonically. The topology is often single-ended operation, chosen also for several other reasons described elsewhere in these articles, including KISS; if the chosen topology is push-pull operation, which is more difficult but far from impossible to arrange without NFB, operation should be specified as Class A1. Inside the argued case above lies too the overwhelming reason to accept the potential small disadvantage that may accompany the preferred topology in comparison to the discarded alternatives. The disadvantage is of course the potential for a residual second harmonic that measures high by transistor or NFB tube standards. (Note the word potential. With a conservatively designed DHT amp the potential problem should not arise.) The ultrafidelista, who are as keen on silent amps as anyone else, accept this small potential difficulty because it is the lesser evil compared to NFB. Unbelievers (largely unwashed, according to reports) sneer that ultrafidelista like this approach because of the 'added euphonics', which is bow-wow techie talk for the warmth a big chunk of second harmonic lays on a zero negative feedback single-ended amplifier. But competent design can easily reduce the level of second harmonic to below the level of perception without the need for NFB and its deleterious after-effects. In any event, it is your amplifier. You paid for it. You have a right to tune it as you please. The key thing is to get rid of NFB and to understand why you did it. Can we prove any of this scientifically? We have already. All of this is the technical subtext to my longtime contention that what the ultrafidelista hear and love is not a directly heated triode sound as is claimed by many enthusiasts but a Class A1, ZNFB sound. (Admittedly, as we have seen above, the right sound is virtually guaranteed with a ZNFB DHT SE amp of conservative provenance but may have to be developed the hard way with more economical or higher-power contenders.) In comparative ABX tests conducted over a number of years, I found that professional musicians, certified golden ears, choose the triode-linked Class A1 PP ZNFB EL34 whenever it is present in the test over all other contenders including SE 300B and 'blameless' high-NFB silicon. Science also proceeds by pure reason. Ultrafidelista have long doubted whether what engineers insist we measure (the absolute level of distortion, THD) predicts success in audio gear. This is the full circle, because I have just proven by logical, individually uncontested steps that what matters, once a certain modest level of silence is assured to an amplifier, is not the absolute level of disharmonics but their composition. The same proof demonstrates that a more beneficial distribution follows instantly from doing without NFB. But transistor amps won't work at all without NFB! That is not our problem. Those who choose inefficient speakers and consequently are forced to accept monstrous amps made possible only by gigadeciBels of NFB, will receive our sympathy - and the music they deserve. Engineering hangers-on of transistor attempts at high fidelity, where the measure of success is vanishing THD rather than sonic hedonism, pretend to be enthusiasts for NFB. To make it work for them, they have attempted to change the rules so that we won't hear what their treasured NFB does to our sound. They sneer that low level listening, which 99 per cent of us prefer and where NFB does most to wreck the sound, is 'easy listening' and therefore not permissible. According to them we should all be forced to listen at the high volume level which suits NFB amps, which they call 'realistic'. This is a contemptible circular argument, only too characteristic of a fascist mentality in a part of the audiophile spectrum which wants to prescribe their arid vision without regard for our enjoyment. We can recommend a good tailor to them. It hurts every time you wear his suit. No pain, no gain, fellers! In summary Almost everyone listens at low level most of the time. NFB wrecks everybody's sound at all levels but most wretchedly at normal listening levels. We started out with a contemptible circular argument and we have met another along the way. We can now put both in context: An 'engineer' who designs an amplifier which does not work perfectly without negative feedback is like a tailor cutting the suit incompetently and then demanding that you walk like a cripple to make it fit, so that everyone can admire the brilliance of your tailor. Negative feedback is a bodge. That is why it is despicable to the ultrafidelista. ******* More Zero Negative Feedback amplifiers at Jute on Amps http://members.lycos.co.uk/fiultra/JUTE%20ON%20AMPS.htm The KISS Amp project, which explains much more, is here http://members.lycos.co.uk/fiultra/T...mp%20INDEX.htm and its schematics and transfer curves are here http://members.lycos.co.uk/fiultra/KISS%20190.htm ******** THE VOLTAGES IN THIS AMP WILL KILL YOU. GET EXPERIENCED SUPERVISION IF IT IS YOUR FIRST TUBE AMP All text and illustration is Copyright © Andre Jute 2001, 2004, 2006 and may not be reproduced except in the thread KISS xxx on rec.audio.tubes Bunk. JLS |
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#40
Posted to rec.audio.tubes,uk.rec.audio,rec.audio.opinion
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The Catch-22 of Negative Feedback aka NFB
"Andre Jute" wrote in message oups.com... Trevor Wilson wrote: Incorrect and innappropriate application of NFB can damage an amplifier's performance. Now you're getting it, sonny. **Then say so. Stop telling half-truths. There is NOTHING wrong with NFB, as long as it is appropriately applied. Next, try not to describe as a liar anyone who doesn't instantly subscribe to your fanatical faith in Blow Jobs from Transvestites (BJTs) and soon the rest of us might take you seriously. **I only tell it like it is. Here are a few of your lies (some of which you have backtracked on): "Negative feedback, shorthanded as NFB, is the instant response of the audio engineering fraternity to all ills, real, perceived, non-existent." "How does negative feedback work? Negative feedback is simply a negative voltage fed back from the output to the input amplifying device to offset part of the harmonic distortion which is present as a positive voltage. It costs nothing except a loss of gain and a few side effects such as phase shift and possible instability which are well known in the mathematical literature and more or less easily guarded against depending on the level of NFB." "Negative feedback is what gives all those 'blameless' transistor and big PP tube amps their chillingly unnatural sound." "(I know, because a sub-board I designed for a supplier to the trade turns up in so many very expensive amps with so many different big names neatly silkscreened on it..." " Suppose, for the sake of simplicity, a superbly designed ultrafidelista amp with some second harmonic and zero odd harmonics before NFB." "Let me say that again: after NFB, third and higher harmonics will make up a greater part of the distortion than before." "Low volume levels perforce accounts for 99 per cent of audiophile listening because we all have families or neighbours, and we would like to keep our ears." "And they still use Negative Feedback?" "The case against NFB is that for 99 per cent of listening the NFB cure is worse than the disease." "It follows from the argument above that ultrafidelista should choose an intrinsically linear topology and device which does not require added negative feedback to 'linearize' the output." "The intrinsically linear device is the thermionic tube in either its triode form or as a pentode hogtied to work as a triode, which can be a most pleasing alternative both economically and sonically." "In comparative ABX tests conducted over a number of years, I found that professional musicians, certified golden ears, choose the triode-linked Class A1 PP ZNFB EL34 whenever it is present in the test over all other contenders including SE 300B and 'blameless' high-NFB silicon." "But transistor amps won't work at all without NFB!" "Engineering hangers-on of transistor attempts at high fidelity, where the measure of success is vanishing THD rather than sonic hedonism, pretend to be enthusiasts for NFB." "They sneer that low level listening, which 99 per cent of us prefer" "According to them we should all be forced to listen at the high volume level which suits NFB amps, which they call 'realistic'." "This is a contemptible circular argument, only too characteristic of a fascist mentality in a part of the audiophile spectrum which wants to prescribe their arid vision without regard for our enjoyment." "Almost everyone listens at low level most of the time." "Negative feedback is a bodge." Would you care to amend these lies and state PRECISELY what you mean? -- Trevor Wilson www.rageaudio.com.au |
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Linear Mode
