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Re KISS 123 by Andre Jute
Google in the new Beta newsreader shows many messages in this thread
but the reply button doesn't work. In the old version not one message shows but at least you can make a new message. Let's hope it threads this reply. - AJ Patrick Turner wrote: I feel compelled to make a reply about NFB, but rather than tediously work down through the main body of the post, I shall leave it together, and place a reply at the bottom where it repectfully belongs..... Yo, Patrick, You make good case for your view that NFB is a useful bodge. I don't want to argue point by point with you, splitting hairs, since I agree with much of what you say. The key is that 'everything is relative'. Below my sig is my original post and your entire post. I shall have something very little to say about phase when in a few articles I come to various checks an amateur should perform on his design before he builds it. Of course, in two stage amps such as ultrafidelista build, phase is hardly a consideration. But you should be careful when interpreting KISS 123 to keep sight of the context and not misinterpret statements made inside a particular context and from a particular viewpoint as beliefs carved on tablets, valid for all situations. I am not personally against all NFB. For instance, my T113 EL34 PP amp was conceived from the beginning with a switch which permits pentode/UL/triode operation and variable NFB, a setup that allows one to study noise and its reduction by both natural and add-on loop NFB in various configurations with minimum inconvenience and maximum repeatability. I noted in KISS 123 a preference for one of its configurations by listeners whose ears I trust; I didn't say it was the most silent. The context in KISS 123 is that I am explaining to the faithful, whom I don't actually want to make their choice on faith but instead on fact and conscious expression of taste, why in an ultra-fidelity amp we, perversely in the view of some, do not worry about what others may consider *the potential* for substantial distortion, because in the context the cure is very bad indeed. (As you point out, the potential doesn't actually eventuate, but that has never stopped hairsplitters, and it is possible for a pair of congenital idiots trying very hard indeed to exceed their earthly quota of stupidity to design an absolutely wretched 300B amp as in the Bubbaland 300B example, an SE amp with so much third and higher HD, NFB would never have saved it from the mercy-killing it so richly deserved.) At the risk of sounding arrogant, I am explaining to people already committed to spending the money to do the job right from the word go, first for high-rent space and speakers and then for rare devices and associated components (film power caps, high-quality iron, more iron for choke input filters, etc) why, after we decide to sacrifice more than half the possible power of an expensive device for extra silence, the bodge of NFB becomes totally unnecessary. I hope you won't mind if I point out that to the ultrafidelista a cathode follower is 'good' NFB and therefore not counted against the designer's place in heaven. . . In fact, in a large part of that niche you can be excommunicated for mentioning in passing that a cathode follower is a feedback topology. They won't believe you. When I was younger I used to make myself very unpopular with the maniac fringes by pointing out that cathode followers are negative feedback devices, that bypassing cathode resistors have negative feedback implications, that SRPP far from being constant current devices are cathode negative feedback bootstrap stacks, and such other technical truths they didn't want to hear. (1) KISS 123 is all about *added-on* loop or global feedback, a subtext the ultrafidelista understand only too well. I am really surprised that you have not heard for yourself that NFB works its evil most diligently at lower listening levels. The theoretical framework stands without question (but, of course, that is no final argument since the theoretical framework of NFB stood for years without question). However, it is a clear effect in listening tests. It seems to me that at thundering levels the enemies of fidelity can get away with any old ****. Personally, I instantly suspect any audiophile or manufacturer showing off his gear who wants to play at high volumes. It is at the fractional powers that you can most easily hear the true quality of the sound. Even at low volumes the bass content of music will effectively demonstrate the power delivery of the amp. In any event, you can always turn the wick up after you are satisfied with low-level performance. It is my experience that an amp which does not sound right at low level and cannot be gimmicked right without oodles of NFB (we can talk about an acceptable level of NFB relative to various common circumstances) will never be satisfactory. That is another argument for sensitive speakers (because they permit the choice of intrinsically linear devices to be operated in the narrow band of their utmost linearity without added artifice) and against silicon. Andre Jute Andre Jute wrote: KISS 123 by Andre Jute This text is copyright Andre Jute, 2001, 2004 and may not be reproduced except in the thread KISS xxx on rec.audio.tubes. If you have arrived late to this project, you can get an overview at http://members.lycos.co.uk/fiultra/The KISS Amp INDEX.htm or with uncooperative browsers at http://members.lycos.co.uk/fiultra/T...%20INDEX.htmor by finding a file called KISS 100. ***** 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.' **** 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 suggested 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 it 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 cost 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 those 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 equipment. The exceptions to this rule are normally 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. They don't 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 relative 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 appear to 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, 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 chosen 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 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. I think a couple of facts about the music we all listen to should be added to the discussion. Let us take ourselves back to 1960, when the bulk of recording gear was tubed. There were some truly breathtaking performances at that time, and some of the recordings of a young Joan Sutherland on vinyl that I have would make a man's heart melt. On monday afternoons, the ABC Classic FM station broadcasts 'music from the archives' and I guess the sources are / were from tapes, vinyl, and probably placed onto a digital medium. Despite the radio stations processing, many ancient recordings sound as fresh as if they were made yesterday. Just about all recordings of any merit were made using all tubed equipment, and a all that gear had a fair amount of NFB used in the microphone amps, recording tape decks, mixing desks, cutting head amps et all. There was considerable possiblity for noise and distortion to enter the signal path along the way, and one tool used to minimise it was NFB, along with some adherence to the idea of getting it right before the NFB was added. Pentode tubes were used throughout much of the 1960 gear, and these always produced more thd and had poorer spectral content, and as a pentode has a high Ra, NFB was needed to reduce the Ro of all the stages in the music production chain. NFB was also used to keep the gains of multiple channels about exactly equal, rather difficult to do without NFB, since gain varies with tube samples. Then transformer coupling was also used, so another avenue for the iron caused thd to creep in. So its a miracle that the gems of 1960 made it to still be worth listening to. Some of the material recorded then simply was trash, or mush, and not worth the listen, and a testament to getting things wrong. If one was to add up all the factors influencing the analog sound quality in 1960, it would be a long, long list, and NFB is but one of the items on that list. Since we recognize some of the gems from those days as being gems, and we know considerable NFB was used, then why should we be fanatically against NFB????? Would that not lead us to conclude that some NFB may be quite OK? After 1960, the recording industry began in earnest to throw out the ageing tube gear which was bulky, expensive to maintain, and costly to power, and which didn't have the bells and whistles of the new cool solid state analog gear. I believe much sound quality took a nose dive at that change. A typical discrete transistor working with a volt with 15 volt rails is working as hard as a tube producing 10v with 150v rails, except that the bjt still manages to produce a whole lot more thd. But transistors had bags of gain, so a lot more NFB was able to be applied, and so it was, and then opamps came into use, and these little black grubs such as the 741 had umpteen devices within, and 60 dB of NFB was routinely used in a loop around them. This freed engineers to cascade as many opamps as they wished. 100 opamps in series each 0.001% thd will still measure 0.01% at the end of the daisy chain. I cannot say how the music will sound after sending it through so many devices but the engineers would have us believe it'd sound fine, and indistinguishable to sound passed through a low thd triode, or just one opamp with NFB. Before I say I believe those engineers are pulling my leg, I'd want the experiment done with the 100 opamps, so I could hear the results. In the pop music industry, it has always been routine to add serious distortions of numerous kinds to all the sound, since the sound has been compressed, de-essed, equ'd, and many instrument amps work at over 10% thd all the time, up to 40% thd.... Now there are al these analog and digital effects added.... So talking about zero FB at the end of the chain when considering pop music is a bit accademic to me, since what does 0.3% of 2H do when 5% of mixed grunge has already been added? Its a bit like offering a carriage for a pig to ride in. Because we are stuck with large amounts of fine recordings made with gear using NFB I just cannot accept that NFB is universally evil wherever it is applied. In the case of a 300B, and all triodes, there is already an internal electrostatic NFB loop. We shouls consider the basic action in a triode. The grid tries to turn on the plate current by going positive, and more electrons flow to the anode from the cathode. The anode responds to the load current and its voltage goes more negative, and this reduction of the anode potential tends to cause less current to flow to the anode. So the grid and the anode itself both affect the electron flow. If say a +ve distortion voltage appears at the anode, this is also applied to the electron stream, and it tends to cause an increase in electron flow, and the increased distortion current flow causes a -ve going load voltage, so the +ve going distortion voltage is opposed by a simultaneously acting current in the triode. This scheme of feedback is goverened by the laws of nature, and not by external loop paths in the amp. It is an imperfect loop of applied NFB since even with such FB there is still about 4% of mainly 2H at 5 watts from the 300B, and there are other products there but at a lower level, and in fact if we have good measuring gear, you'd be surprised how grubby a 300B really is. But that's near full power, at the lower levels the spectra is more benign, and at all levels its the 2H which is the highest. Its grubby until you think about a 6L6 in beam tetrode without NFB or a transistor.... But properly set up, there is no need to add any more FB to a triode amp than already exists in the tube to start with. There is a lot about Child's Law concerning electrostatic behaviour in thermionic devices, but the keen amoungst you take yourself to a library and read the books of the 1930s which spells out in more detail all I have said about NFB in triode amps. At the end of the day, the 300B and other triodes make nice enough amplifiers with the FB within them, and there is no need to gild the lily by adding more NFB. Feedback networks never entirely remove distortion. All amplifying devices produce harmonic distortion. To have distortion reduction, there must be distortion in the first place so that it can be fed back to the input and amplified to oppose its own production. Consider the basic amplifier, with an open loop gain ( gain without a NFB loop connected ) of say 10. Let us suppose we feed back 1/5 of the output signal to one of the two inputs that every amplifier has. This is usually done with a reistance divider between the output and 0V, and the junction is where a replica of the output signal exists, but its 1/5 of the output voltage. Let's say we have +20v of output voltage. Then to make that voltage, +2v is needed between the two inputs of the amp, which in a tube amp is often the grid and cathode of V1, which is working in differential mode. +4 volts is the fed back voltage, and in our tube amp's case it is applied to V1 cathode. So therefore the input voltage to the amp has to be +6v at the input grid, since the +4v and +2v make a total of 6v. Since we know all amplifiers have distortion, let us suppose our hypothetical triode example amp has +1% thd with NFB and when we measure it at 20v output. Notice how I have placed a + or - sign in front of the voltages concerned. The sign indicates the relative phase of the sine wave signal voltage in our amp. A +ve signal will have a 180 degree phase difference to a -ve signal, ie, the -ve signal is an exact same version of the +ve signal, its just turned upside down. Now we know +1% Dn ( distortion ) occurs at our amp's output, so there must be +0.2% at the feedback input of our amp, due to the voltage divider resistors. There is none of this Dn in the input signal. So there is effectively -0.2% Dn appearing between the two input terminals, and its amplified 10 times by the amp to produce -2% Dn at the output. This doesn't make any sense, because we measured +1% Dn. So how come we observe this? The answer is simple, there had to be +3% of Dn to begin with without the NFB application. Then when the -2% Dn is applied, it subtracts from the +3% Dn, leaving +1%. So by applying the NFB, we reduce the Dn from +3% to +1%. If we had an open loop gain of 100 instead of only 10, the amount of meaured Dn would be a lot lower, and I leave you all to work out what the open loop Dn would have been if we measured 0.1% with the above amp, but with a gain of 100. Now much has been written about the evils of NFB, including quite a lot above this reply. To add to that, people have said that NFB is "trying to catch the horse after it has bolted". But in the case I just hypotheticised the feedback is instantaneous. The transit time for signals through a well designed triode amp will result in less than a few degrees of phase lag at 20 kHz. Some claim the delays in an amp cannot be corrected, but I assure you that phase delays can be largely corrected by NFB in all but the worst designed amps. Would someone prefer the phase lagged signal rather than one which had its timing more accurately reproduced as a result of NFB? As F rises, the phase lag increases more rapidly, and an F is reached where a fed back voltage has a phase which causes oscillations. Similarly, a phase lead can occur at LF, and oscillations can occur. This isn't a failure of NFB, its a failure of the human designer to take enough factors into consideration so that NFB allows perfect stability regardless of any F or load reactance. Having mentioned phase delay, or lagging phase, its easy to imagine. Phase lead is harder, because the phase is advanced ahead of the reference phase of the input signal. How can this be? Does the electrical signal arrive at its destination before it set out on its journey? We know that's impossible, and its beyond my time allottment to explain that, so off the the library with you all if you don't know about phase relationships, and you wanna fiddle with amplifiers! I have explained in other posts recently that NFB causes harmonics to appear where they they didn't exist before FB was applied. Andre has explained again above, and its by way of the intermodulation process that additional spectra are added to the signal when NFB is used. In the case above of the amp making 20v, the 0.2% Dn is applied to the amp input and is amplified to subtract from the open loop Dn. In this process, this 0.2% is also distorted, and that second lot of Dn signal is fed back, amplified to oppose itself, and I leave the math experts amoungst you to work out what the exact figures are produced of "secondary effect" Dn. The second order products are usually low enough to have not the slightest worry about their presence if the Dn without FB was low to begin with. You see the above 20v amp may well have 1% of 2H in the output, but there will also be 3,4,5,6,7,8,9H etc if we care to measure well enough down to the -100 dB level. The second order added products will often be at similar levels of the existing non 2H content of our amp, and at such low levels that they are inaudible. All this talk about NFB leaves me a little puzzled, because last year 3 others and myself tested several different 6CG7 tubes in an SET preamp, and recently I had the chance to measure this amp and found the 2H was less than 0.01% at all levels used on the day. Yet almost vastly different sonic timbres were heard with the same test recordings. The sound changes were not due to measurements imho. The preamp had no loop NFB in its gain stage, but it did have a cathode follower buffered output, which is a case of consideable NFB, but totally benign, since the CF would have only had about 0.001% thd, and all mainly 2H. The best tubes were Telefunken NOS, Mullard next best, and Sovtek the worst. 0.01% thd and the resulting imd is such a tiny signal at a speaker compared to a 1 watt signal level that if the thd/imd could appear at the speaker without the original wanted signal present, the Dn could not possibly be heard if one works out all the power and SPL involved. Everything is relative, let's not forget that. I cannot conclude that NFB is an evil thing, and that it does the worst damage to music at the low levels most folks listen at. I have demoed too many amps to too many people to have a firm view, since any crowd of audiophiles will say different things to me about a system, and much of it is in conflict, like some may say the bass is a bit shy, others will say the bass is really heavy, so is it their hearing, or where they sat to listen? But thankfully, they wanted to stay to listen all night, and didn't wanna throw me out of their venues. I have seen a crowd of them melt away early when PP triode amps using only 6 dB NFB were used. They were much happier with my amps with half the tube count in UL and with 16 dB of NFB. I know that the NFB haters have reasons why they think its a bad thing. There is a lot there above in print..... But is it because of subconscious rebellion about the need for corrective efforts? Does anyone believe in Original Sin, and that when Adam and Eve sinned, they were damned, and cast out of the Garden, and had to await a Redeemer? What BS! Must we suffer a godly imposed inferiority complex? So those who love freedom say there isn't a need for NFB, because the triode hasn't sinned... Why must we be obsessed with corrective measures when innocence should be let free from such measures? Can we not be free of assumed guilt? Gee, I have heard a few triode amps that did do a bit of sinning, and a bigger dose of NFB might have improved them. Anyway, the God Of Triodes put the NFB into triodes, and tonight I will not argue with a god about what he did when he made the laws of physics. And if all those amps used in 1960 had not been looped with NFB, maybe you would have ended up with recordings that were the finest sample of mud. NFB may well be a "bodge", and I assume that means plastering over the cracks, bandaiding a cut finger, etc, but in some cases, its the best bodge we've got where something is needed. Halcro amps must use so much NFB that they get 0.0001% thd at 200 watts. The HK audio club said after a Halcro demo, "Ah, Halcro, it like 300B, but go louder.." I admire their inscrutability. Patrick Turner. |
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