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#1
Posted to rec.audio.tubes
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Explanation still required for triode superiority
In an effort to be agreeable, I tried hard to give you negative
feedback inside the tube as an explanation of the overwhelming superiority of triodes (or trioded pentodes) for audio reproduction, among other reasons because NFB is accessible to many who belong on RAT and is a genetic deformity of the silicon scum whose only purpose on RAT is dissension. NFB is what the silicon slime abuse to make their inadequate components sound passable, and what even tubies inspired by the age of sophisters and cost-accountants use to linearize pentodes. NFB thus has a base level of familiarity which gives it a head start in any black box model intended to explain something to diplomaed quarterwits among the silicon slime as well as the better-educated kibbitzers in my own camp. But fine, you want to reject my explanation, then you must offer a better reason to explain why triodes are such superior amplification devices to anything else, so much more pleasing to the ear, so much more accurate to the cultivated taste. Despite my cracks about the metaphysics of tubes, there *has* to be an electrical reason for the superiority of triodes. But sure, all kinds of input is welcome. Andre Jute |
#2
Posted to rec.audio.tubes
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Explanation still required for triode superiority
Andrew Jute McCoy wrote its usual fallacious arguments: Called "begging the question". Triodes are neither "superior" nor "inferior" when it comes to amplifiers. However, they do have many difficulties. Not the least of which a Flea Power. Expensive to create. Difficult to make operational (as your model so clearly illustrated). So, if one suspends belief sufficiently to actually beg said question, then further discussion is optional. Just like any given Science-Fiction movie/novel/premise. Oh, RIGHT!! McCoy failed in this venue as well. Silly me. Never mind. Peter Wieck Wyncote, PA |
#3
Posted to rec.audio.tubes
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Explanation still required for triode superiority
Le 25 Sep 2006 12:51:12 -0700,
"Bret Ludwig" a écrit : Peter Wieck wrote: Andrew Jute McCoy wrote its usual fallacious arguments: Called "begging the question". Triodes are neither "superior" nor "inferior" when it comes to amplifiers. However, they do have many difficulties. Not the least of which a Flea Power. Expensive to create. Difficult to make operational (as your model so clearly illustrated). Triodes are cheaper to build than beam power tubes of a given size, simpler, and can be used for amplifiers of any size. Most 50 kW AM broadcast transmitters had two triodes in their modulators giving 30 kW audio power in Class B. The common 811 available under $20, though not an ideal tube, can give 200 easy watts for a pair. As can 211s, 845s, etc. The GEC book shows a 1 KW audio amp with a pair of largish glass triodes. Difficult to use? Just give them a filament supply of several amps, preferably from a filament transformer with shunts designed for that tube, a few kV on the plate and enough drive and they work fine. Audio is not only about power and characteristics on the paper, but about sound. Most if not all high power tubes are designed for high frequency transmitters, not for high-fidelity, and they will sound very poorly when used for an audio amp. You will get a much better sound when using 4, 6 or 8 EL34 or 300B as only 2 more powerful tubes. Now, the controversy between pentode and triode is not new. If you look, most of the so called pentodes used in high fidelity audio are in fact beam-tetrodes. The main problem with the pentode is its high output impedance. The main problem with the triode is its low sensibility. The beam tetrode try to get most of the advantages of the triode as its relatively low output impedance and most of the advantages of the pentode as its high sensibility. And the result is a very good tube for power amplification. But if you want to get a good result in audio with a beam tetrode, you must be very careful with the design. A triode design is easier because you don't have the g2. A good practice, if you are using a fixed g2 voltage, is to stabilize it. Just be doing it, the amp will archive at least a 2 time better dynamic at the output. But how many amps on the market are using a stabilized Vg2? Almost no one because it cost more money to build. Another issue, and not only with the beam tetrode, is the quality of the tubes. Take the 6L6. It is hundred of different brands, but only a little part of those brands have a really good quality of manufacturing. It is in theory the same tube as a 807. Make an amp with 2 x 807, 600V DC on the middle of the output transformer when the amp is at full output, 300 V DC on the g2, and replace the 807 by 6L6 tubes. Most of them will just glow up at full output. And already at low level, they will arc inside the tube because of a too high Va, and this will sound like hell in the loudspeaker. Take a preamp tube as the ECC83-12AX7. Buy a few tubes of different brands. You will get as many sound at the output as brands. In fact even more, because with the cheaper brands, different tubes will sound differently. So, when you want to compare tubes, you must not only say the type of the tubes, but the brand too. And when comparing tubes as different as a triode and a pentode, you must talk about the topology of the circuits. A good quality pentode will sound better as a poor quality triode. And a pentode used in a deign that have a good topology will sound better as a triode of the same quality but used in a circuit with a poor design. -- Dominique Michel |
#4
Posted to rec.audio.tubes
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Explanation still required for triode superiority
Hi, Dominique. Welcome to RAT. I'm Andre Jute. This is my thread.
You say: The main problem with the triode is its low sensibility. Actually, the great advantage of the triode (or trioded pentodes like the EL34) is its *high* sensibility with the music, that is the natural, unaffected way it reproduces it. You probably mean "low power". That mainly bothers people who haven't put their minds in gear. Once you have sensitive speakers, 300B are overkill on Lowther horns already. Alternatively, when I wanted a triode single-ended amp to drive Quad ESL63, I built it with transmitting triodes to give 80W from a standard 2Vrms CD output (417A::300B::PSE SV572-3 or -10 does the biz in only three ZNFB stages). If you're willing to pay the price, everything is possible and nothing is a barrier. As examples of why the low power of the commoner triodes is not a problem, here are some bicor horns I built for Lowther drivers: http://members.lycos.co.uk/fiultra/K...20T91HWAF3.jpg to use with either of these 300B http://members.lycos.co.uk/fiultra/T...trafi-crct.jpg http://members.lycos.co.uk/fiultra/T44bis-'Populaire'-crct.jpg but here is 0.3W quality amp that drives the Lowthers very pleasingly indeed: http://members.lycos.co.uk/fiultra/K...0T68MZ417A.jpg http://members.lycos.co.uk/fiultra/t...17acircuit.jpg http://members.lycos.co.uk/fiultra/t68bismzlayout.jpg or if you want good quality and more power for less money, here is a 2W trioded EL34 amp I designed for students: http://members.lycos.co.uk/fiultra/Jute-EL34-SEntry.jpg to use with this inexpensive but very sensitive (and sensible!) speaker: http://members.lycos.co.uk/fiultra/K...Impresario.jpg All becomes clear when you give up being a fashion-victim of people who think more power is better power because they are too lazy or stupid to design within the parameters of the best-sounding components; some are too thick even to work out that it is the sound that matters, not the engineering, and of those we have a surplus already on RAT. You might enjoy "The myth of the Watt:", or then again, not, as the case might be: http://members.lycos.co.uk/fiultra/K...dre%20Jute.htm More on my main site, URL under my sig. 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 Dominique Michel wrote: Le 25 Sep 2006 12:51:12 -0700, "Bret Ludwig" a écrit : Peter Wieck wrote: Andrew Jute McCoy wrote its usual fallacious arguments: Called "begging the question". Triodes are neither "superior" nor "inferior" when it comes to amplifiers. However, they do have many difficulties. Not the least of which a Flea Power. Expensive to create. Difficult to make operational (as your model so clearly illustrated). Triodes are cheaper to build than beam power tubes of a given size, simpler, and can be used for amplifiers of any size. Most 50 kW AM broadcast transmitters had two triodes in their modulators giving 30 kW audio power in Class B. The common 811 available under $20, though not an ideal tube, can give 200 easy watts for a pair. As can 211s, 845s, etc. The GEC book shows a 1 KW audio amp with a pair of largish glass triodes. Difficult to use? Just give them a filament supply of several amps, preferably from a filament transformer with shunts designed for that tube, a few kV on the plate and enough drive and they work fine. Audio is not only about power and characteristics on the paper, but about sound. Most if not all high power tubes are designed for high frequency transmitters, not for high-fidelity, and they will sound very poorly when used for an audio amp. You will get a much better sound when using 4, 6 or 8 EL34 or 300B as only 2 more powerful tubes. Now, the controversy between pentode and triode is not new. If you look, most of the so called pentodes used in high fidelity audio are in fact beam-tetrodes. The main problem with the pentode is its high output impedance. The main problem with the triode is its low sensibility. The beam tetrode try to get most of the advantages of the triode as its relatively low output impedance and most of the advantages of the pentode as its high sensibility. And the result is a very good tube for power amplification. But if you want to get a good result in audio with a beam tetrode, you must be very careful with the design. A triode design is easier because you don't have the g2. A good practice, if you are using a fixed g2 voltage, is to stabilize it. Just be doing it, the amp will archive at least a 2 time better dynamic at the output. But how many amps on the market are using a stabilized Vg2? Almost no one because it cost more money to build. Another issue, and not only with the beam tetrode, is the quality of the tubes. Take the 6L6. It is hundred of different brands, but only a little part of those brands have a really good quality of manufacturing. It is in theory the same tube as a 807. Make an amp with 2 x 807, 600V DC on the middle of the output transformer when the amp is at full output, 300 V DC on the g2, and replace the 807 by 6L6 tubes. Most of them will just glow up at full output. And already at low level, they will arc inside the tube because of a too high Va, and this will sound like hell in the loudspeaker. Take a preamp tube as the ECC83-12AX7. Buy a few tubes of different brands. You will get as many sound at the output as brands. In fact even more, because with the cheaper brands, different tubes will sound differently. So, when you want to compare tubes, you must not only say the type of the tubes, but the brand too. And when comparing tubes as different as a triode and a pentode, you must talk about the topology of the circuits. A good quality pentode will sound better as a poor quality triode. And a pentode used in a deign that have a good topology will sound better as a triode of the same quality but used in a circuit with a poor design. -- Dominique Michel |
#5
Posted to rec.audio.tubes
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Explanation still required for triode superiority
Dominique Michel wrote: Le 25 Sep 2006 12:51:12 -0700, "Bret Ludwig" a écrit : Peter Wieck wrote: Andrew Jute McCoy wrote its usual fallacious arguments: Called "begging the question". Triodes are neither "superior" nor "inferior" when it comes to amplifiers. However, they do have many difficulties. Not the least of which a Flea Power. Expensive to create. Difficult to make operational (as your model so clearly illustrated). Triodes are cheaper to build than beam power tubes of a given size, simpler, and can be used for amplifiers of any size. Most 50 kW AM broadcast transmitters had two triodes in their modulators giving 30 kW audio power in Class B. The common 811 available under $20, though not an ideal tube, can give 200 easy watts for a pair. As can 211s, 845s, etc. The GEC book shows a 1 KW audio amp with a pair of largish glass triodes. Difficult to use? Just give them a filament supply of several amps, preferably from a filament transformer with shunts designed for that tube, a few kV on the plate and enough drive and they work fine. Audio is not only about power and characteristics on the paper, but about sound. Most if not all high power tubes are designed for high frequency transmitters, not for high-fidelity, and they will sound very poorly when used for an audio amp. I doubt this is correct. There are many examples where a tube can be used equally appropriately for RF or AF. The 211 makes a nice transmit tube and can be superlative as an audio tubes. 813 is another..... You will get a much better sound when using 4, 6 or 8 EL34 or 300B as only 2 more powerful tubes. Ain't necessarily so... Now, the controversy between pentode and triode is not new. If you look, most of the so called pentodes used in high fidelity audio are in fact beam-tetrodes. The main problem with the pentode is its high output impedance. The main problem with the triode is its low sensibility. Perhaps you mean sensitivity, or voltage gain. Its a very sensible choice for an output device.... The beam tetrode try to get most of the advantages of the triode as its relatively low output impedance and most of the advantages of the pentode as its high sensibility. And the result is a very good tube for power amplification. ??? But if you want to get a good result in audio with a beam tetrode, you must be very careful with the design. A triode design is easier because you don't have the g2. A good practice, if you are using a fixed g2 voltage, is to stabilize it. Just be doing it, the amp will archive at least a 2 time better dynamic at the output. But how many amps on the market are using a stabilized Vg2? The use of a slow time constant with RC filter used for g2 supply is all that is required for AB audio amps with multi grids because in fact the Eg2 variation at average power = 1/10 of clipping power is very small. Almost no one because it cost more money to build. I shunt regulate my screen supplies. A few 75V x 5 watt zener diodes and a power resistor of 10W is usually all that is needed. Another issue, and not only with the beam tetrode, is the quality of the tubes. Take the 6L6. It is hundred of different brands, but only a little part of those brands have a really good quality of manufacturing. It is in theory the same tube as a 807. Make an amp with 2 x 807, 600V DC on the middle of the output transformer when the amp is at full output, 300 V DC on the g2, and replace the 807 by 6L6 tubes. Most of them will just glow up at full output. And already at low level, they will arc inside the tube because of a too high Va, and this will sound like hell in the loudspeaker. Maybe depends which type of 6L6 one uses.... Take a preamp tube as the ECC83-12AX7. Buy a few tubes of different brands. You will get as many sound at the output as brands. In fact even more, because with the cheaper brands, different tubes will sound differently. So, when you want to compare tubes, you must not only say the type of the tubes, but the brand too. And when comparing tubes as different as a triode and a pentode, you must talk about the topology of the circuits. A good quality pentode will sound better as a poor quality triode. And a pentode used in a deign that have a good topology will sound better as a triode of the same quality but used in a circuit with a poor design. -- Dominique Michel Tubes brands do make different sound amoung the same type number and using pentodes in lieu of triodes ditto. Tubecraft involves many variables around simple ideas. Its all so much easier than class D DIY PWM amps.... Patrick Turner. |
#6
Posted to rec.audio.tubes
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Explanation still required for triode superiority
On 24 Sep 2006 16:18:06 -0700, "Andre Jute" wrote:
In an effort to be agreeable, I tried hard to give you negative feedback inside the tube as an explanation of the overwhelming superiority of triodes (or trioded pentodes) for audio reproduction, among other reasons because NFB is accessible to many who belong on RAT and is a genetic deformity of the silicon scum whose only purpose on RAT is dissension. NFB is what the silicon slime abuse to make their inadequate components sound passable, and what even tubies inspired by the age of sophisters and cost-accountants use to linearize pentodes. NFB thus has a base level of familiarity which gives it a head start in any black box model intended to explain something to diplomaed quarterwits among the silicon slime as well as the better-educated kibbitzers in my own camp. But fine, you want to reject my explanation, then you must offer a better reason to explain why triodes are such superior amplification devices to anything else, so much more pleasing to the ear, so much more accurate to the cultivated taste. Despite my cracks about the metaphysics of tubes, there *has* to be an electrical reason for the superiority of triodes. But sure, all kinds of input is welcome. Andre Jute I know the answer! It's distance!! In a triode, there is a great distance between elements, forcing the electrons to be linear! In a transistor, things are so close and crowded that any electron going through has to grab a few buddies to take along... and the more going through, even more extras tag along - clearly exponential! And in ICs, which sound even worse then discrete transistors, things are even closer, so sound lots worse because they are even more exponential!! Simple. |
#7
Posted to rec.audio.tubes
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Explanation still required for triode superiority
On 24 Sep 2006 16:18:06 -0700, "Andre Jute" wrote:
But fine, you want to reject my explanation, then you must offer a better reason to explain why triodes are such superior amplification devices to anything else, so much more pleasing to the ear, so much more accurate to the cultivated taste. If, by "you", you mean the newsgroup in general, who "must" offer a reason, then prepare yourself for quite a... what's the word? Anyway, Caveat Emptor (Latin for "take a flashlight into the supposedly empty cave") and Habeas Corpus (Latin for "habanero sauce is good on the corpse"). Lotsa misinformation abounds. But if by "you" you mean "me", who's been the bitchiest about the topic, then, no, I don't have any pat answers. I can offer a couple topics for possible discussion, but the topic doesn't lend itself to pat answers. And, I would generally agree about your conclusion, at least in appropriate circumstances. Not all circumstances are appropriate, YMMV, yadayada. But, yeah. Much thanks, as always, Chris Hornbeck |
#8
Posted to rec.audio.tubes
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Explanation still required for triode superiority
Chris Hornbeck wrote: On 24 Sep 2006 16:18:06 -0700, "Andre Jute" wrote: But fine, you want to reject my explanation, then you must offer a better reason to explain why triodes are such superior amplification devices to anything else, so much more pleasing to the ear, so much more accurate to the cultivated taste. If, by "you", you mean the newsgroup in general, who "must" offer a reason, then prepare yourself for quite a... what's the word? Anyway, Caveat Emptor (Latin for "take a flashlight into the supposedly empty cave") and Habeas Corpus (Latin for "habanero sauce is good on the corpse"). Lotsa misinformation abounds. I'd say "****'em" but their minds are clearly syphiclically degenerated (the NFB of the immoral) so touching them physically may expose one to contamination. As for "must", sure thing. The most amazing thing about the amateur flamers on RAT is not their incompetence in both electronics and polemics but there blind hypocrisy, best seen in their pretension of being "scientific". But if by "you" you mean "me", who's been the bitchiest about the topic, then, no, I don't have any pat answers. I can offer a couple topics for possible discussion, but the topic doesn't lend itself to pat answers. Bring on your topics, Chris. We could do with some fresh air in RAT. And, I would generally agree about your conclusion, at least in appropriate circumstances. Not all circumstances are appropriate, YMMV, yadayada. But, yeah. Which conclusion? That "triodes sound better" or that "there must be an electrical explanation"? Much thanks, as always, Chris Hornbeck Andre Jute "Take the money and run" -- Len Deighton |
#9
Posted to rec.audio.tubes
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Explanation still required for triode superiority
Andre Jute wrote: Chris Hornbeck wrote: On 24 Sep 2006 16:18:06 -0700, "Andre Jute" wrote: But fine, you want to reject my explanation, then you must offer a better reason to explain why triodes are such superior amplification devices to anything else, so much more pleasing to the ear, so much more accurate to the cultivated taste. If, by "you", you mean the newsgroup in general, who "must" offer a reason, then prepare yourself for quite a... what's the word? Anyway, Caveat Emptor (Latin for "take a flashlight into the supposedly empty cave") and Habeas Corpus (Latin for "habanero sauce is good on the corpse"). Lotsa misinformation abounds. I'd say "****'em" but their minds are clearly syphiclically degenerated (the NFB of the immoral) so touching them physically may expose one to contamination. As for "must", sure thing. The most amazing thing about the amateur flamers on RAT is not their incompetence in both electronics and polemics but there blind hypocrisy, best seen in their pretension of being "scientific". And down with IBM's ViaVoice too for that "there" for "their". In the same sentence as "pretension" too! -- AJ But if by "you" you mean "me", who's been the bitchiest about the topic, then, no, I don't have any pat answers. I can offer a couple topics for possible discussion, but the topic doesn't lend itself to pat answers. Bring on your topics, Chris. We could do with some fresh air in RAT. And, I would generally agree about your conclusion, at least in appropriate circumstances. Not all circumstances are appropriate, YMMV, yadayada. But, yeah. Which conclusion? That "triodes sound better" or that "there must be an electrical explanation"? Much thanks, as always, Chris Hornbeck Andre Jute "Take the money and run" -- Len Deighton |
#10
Posted to rec.audio.tubes
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Explanation still required for triode superiority
Andre Jute wrote: Which conclusion? That "triodes sound better" That's your opinion. or that "there must be an electrical explanation"? Of course there must be. Added low order distortion of course. It's basically an 'effect' like SRS or BBE 'sound enhancement'. With toobs you can't turn if off though Graham |
#11
Posted to rec.audio.tubes
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Explanation still required for triode superiority
Eeyore wrote: Andre Jute wrote: Which conclusion? That "triodes sound better" That's your opinion. or that "there must be an electrical explanation"? Of course there must be. Added low order distortion of course. It's basically an 'effect' like SRS or BBE 'sound enhancement'. With toobs you can't turn if off though Graham I am not sure triodes are superior. They are just 3 terminal devices with inbuilt NFB. In the 1950s and 60s in many recording studios, gear was used that was mostly designed by cloth eared engineers with 100% regard only for numbers and such gear had SHIRTLOADS of pentodes and beam tetrodes and transformers and NFB. We hear the results today on well recorded and preserved vinyl recordings. Not a digit or PN junction used anywhere. Great stuff when it turned out right. Was it dependant on triodes? maybe, but also maybe not. Why would any engineer use 3 triodes where two pentodes would do? The final link for us lesser latterday mortals is from recording to speaker, and triodes are a good choice. Not necessarily superior IMHO, and i say that after trying such tubes as the 13Ei SEUL with mild NFB that I believe will give ANY triode amp some real competion, providing the power ceiling is the same. As a later SE amp development I tried a quad of humble cheap EH 6CA7, actually Sovtek prettied up, to get 35 watts SE with CFB, and again the measurements were NOT typically inferior to PP designs and the sound was detailed, sparkling and natural sounding and all that anyone may wish for. I have built SET amps with 2A3 or 300B, and found they gave the best 8 and 4 watts i have ever heard with 7 db of NFB to reduce Rout compared to tubes like a single EL84 in pentode or a single EL34 in pentode, respectively, but with 20dB of NFB needed to get the same Rout. The CFB connection transcends the UL connection which transcends pentode. Triode is somewhere in their between CFB and UL IMHO. Experimenters may experiment to verify/discount my opinions. See my web pages for more on such matters. http://www.turneraudio.com.au Patrick Turner. |
#12
Posted to rec.audio.tubes
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Explanation still required for triode superiority
On 25 Sep 2006 03:09:39 -0700, "Andre Jute" wrote:
Bring on your topics, Chris. We could do with some fresh air in RAT. Some folks who I'd hoped to be refreshing have turned out to be terminally boring. Sorry for having encouraged 'em. We live and don't learn... or something. Again, sorry, all. No time this week to give any adequate or deserving response to yours or Henry's provocative posts, but, if I may, a few possibilities: 1. Unweighted THD is useless as an indicator of quality. 2. Unweighted IMD " " " 3. We make totally unwarranted assumptions of monotonicity. This is a fatal flaw in our thinking, and we all do it, all the time. 4. We make often unwarranted assumptions about input signal bandwidth. 5. We *always* *without exception* forget to properly weight the importance of simply being upstream in the signal path. Always. 6. Loud voices will say that everything (that doesn't include the dreaded vacuum valve, scourge of nations) sounds the same. This has recently been expanded to include transistor amplifiers from the early 1970's, by certain especially vocal ideologes, in another thread, this very week. Against such a religious fervor, nothing can stand. Been reading about weaponry this week; no real purpose; haven't needed a weapon since discharged from the Army in 1972; just looking, ya know? It seems that the US military converted from the Colt .45 caliber pistol in standard issue since 1911(!) to a Berreta of some NATO gauge in the mid 1980's. But it seems it doesn't work, despite all the high powered analysis. Ya shoot sombody with it, but they keep comin'. What's so sadly wrong with the story is that the US military went through the whole exact same hand-wringing back 100+ years ago in the Phillippines, leading to the adoption of, wait for it, the .45 caliber. Modeling and analysis are essential; but the map is not the world. Which conclusion? That "triodes sound better" or that "there must be an electrical explanation"? Strong agreement with the former; strong disagreement ('cause that ain't science!) with the latter. "Take the money and run" -- Len Deighton And also... who? Bob Segar? Somebody like that, anyway. Much thanks, as always, Chris Hornbeck |
#13
Posted to rec.audio.tubes
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Explanation still required for triode superiority
Chris Hornbeck wrote: 1. Unweighted THD is useless as an indicator of quality. So why do you think I regularly mention the order of the distortion ? Graham |
#14
Posted to rec.audio.tubes
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Explanation still required for triode superiority
On Tue, 26 Sep 2006 05:07:55 +0100, Eeyore
wrote: 1. Unweighted THD is useless as an indicator of quality. So why do you think I regularly mention the order of the distortion ? Wanna *not* be a major disappointment? Post something positive and educational on the topic. Right here; right now. Lord knows you're capable, and all the latterday endless cheap shots are beneath you. Step up to the plate and take a swing! This *could* be a very interesting newsgroup again someday. Gonna take everybody; no slackers. As always, Chris Hornbeck |
#15
Posted to rec.audio.tubes
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Explanation still required for triode superiority
Chris Hornbeck wrote: On 25 Sep 2006 03:09:39 -0700, "Andre Jute" wrote: Bring on your topics, Chris. We could do with some fresh air in RAT. [various snips] if I may, a few possibilities: 1. Unweighted THD is useless as an indicator of quality. 2. Unweighted IMD " " " No argument about points 1 and 2; I just normally put it more emotively. 3. We make totally unwarranted assumptions of monotonicity. This is a fatal flaw in our thinking, and we all do it, all the time. 4. We make often unwarranted assumptions about input signal bandwidth. 5. We *always* *without exception* forget to properly weight the importance of simply being upstream in the signal path. Always. When you find the time, please elaborate on points 3, 4 and 5. 6. Loud voices will say that everything (that doesn't include the dreaded vacuum valve, scourge of nations) sounds the same. This has recently been expanded to include transistor amplifiers from the early 1970's, by certain especially vocal ideologes, in another thread, this very week. Against such a religious fervor, nothing can stand. This relates back to points 1 and 2, where for judgement has been substituted the belief that when the master tape is reproduced with under-x per cent of THD, the engineer has faithfully performed his service. In human terms, elevating a single measure of goodness so high is inspired by fear, a desire to control events (can't let a bunch of arty-farties substitute taste for what engineers "know"), which is also a form of fear, and of course the largest fear-reflex of them all is religion, the defense against fear of the unknown darkness. Modeling and analysis are essential; but the map is not the world. Actually, the only °essential* is having some method of deciding where you want to arrive. High fidelity went wrong long before Mr Leak's inspired marketing terminology (Point One) became an engineering article of faith, but that set the seal on the decline. Which conclusion? That "triodes sound better" or that "there must be an electrical explanation"? Strong agreement with the former; That's why we are here. strong disagreement ('cause that ain't science!) with the latter. Eh? Surely a thermionic valve is nothing but a bunch of electrical impulses created by vacuum, wire and electricity? Whatever happens in there, regardless of whether we can see it or not, regardless of what we call the result, *must* perforce have an *electrical* explanation. °That* is science. Anything else would make me uncomfortable -- and me a certified witchdoctor! "Take the money and run" -- Len Deighton And also... who? Bob Segar? Somebody like that, anyway. Woody Allen. From of an early movie about a hapless bank robber. Len Deighton, one of the best novelists ever to work in the thriller genre, was referring to a novelist's relationships with "moom pitcher pipple". Much thanks, as always, Chris Hornbeck 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 |
#16
Posted to rec.audio.tubes
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Explanation still required for triode superiority
Andre Jute wrote: This relates back to points 1 and 2, where for judgement has been substituted the belief that when the master tape is reproduced with under-x per cent of THD, the engineer has faithfully performed his service. What master tape ? There's sod all tape used these days ! Graham |
#17
Posted to rec.audio.tubes
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Explanation still required for triode superiority
On 26 Sep 2006 15:30:25 -0700, "Andre Jute" wrote:
3. We make totally unwarranted assumptions of monotonicity. This is a fatal flaw in our thinking, and we all do it, all the time. 4. We make often unwarranted assumptions about input signal bandwidth. 5. We *always* *without exception* forget to properly weight the importance of simply being upstream in the signal path. Always. elaborate on points 3, 4 and 5. We're always fighting the last war. In my on-war-footing and yet war-unravaged country, this seems particularly unwise, but still unavoidable. Progress in any sufficiently mature technology is less a matter of perfecting the existing technology than in identifying the flaws in the model by which the technology was created. Any second rate modern military power could have ruled the world of even only a hundred years ago. And, conversely, the Number One Military Might Big Kahuna in the contemporary world can't defeat a determined insurgency in a militarily defeated and occupied country. (Or two). Defining the problem is everything. Defining the problem in the context of the last war is foolish, but seemingly unavoidable. Failure to recognize this trap is potentially fatal militarily, and a blind alley technically. Which conclusion? That "triodes sound better" or that "there must be an electrical explanation"? strong disagreement ('cause that ain't science!) with the latter. Eh? Surely a thermionic valve is nothing but a bunch of electrical impulses created by vacuum, wire and electricity? Whatever happens in there, regardless of whether we can see it or not, regardless of what we call the result, *must* perforce have an *electrical* explanation. °That* is science. Anything else would make me uncomfortable -- and me a certified witchdoctor! Gotta be more careful with my phraseology. It really depends on what "be" be. Arf! Have I heard a technically compelling explanation for ..some.. of the things that I nonetheless believe to be both true and ultimately explainable to me in terms that I can accept as real? No. Do I believe that explanations acceptable to me exist and are awaiting discovery? Yes. Science is the quest(ion) itself, so we really aren't in disagreement. And, I'll try to work up some more keystrokes about my favorite bete noirs, monotonicity and operating level later, 'gator. Much thanks, as always, Chris Hornbeck |
#18
Posted to rec.audio.tubes
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Explanation still required for triode superiority
Somebody said, and Andre replied..... Modeling and analysis are essential; but the map is not the world. Actually, the only °essential* is having some method of deciding where you want to arrive. High fidelity went wrong long before Mr Leak's inspired marketing terminology (Point One) became an engineering article of faith, but that set the seal on the decline. So what happened before Leak that was the beginning of the fidelity decline? Invention of pentodes and bean tetrodes? Beginning of FB use? Indirect heating of cathodes? Patrick Turner. |
#19
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Explanation still required for triode superiority
"Impedance is futile, you will be simulated into the triode of the
Borg"... :-) |
#20
Posted to rec.audio.tubes
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Explanation still required for triode superiority
Andre Jute wrote:
In an effort to be agreeable, I tried hard to give you negative feedback inside the tube as an explanation of the overwhelming superiority of triodes (or trioded pentodes) for audio reproduction, among other reasons because NFB is accessible to many who belong on RAT and is a genetic deformity of the silicon scum whose only purpose on RAT is dissension. NFB is what the silicon slime abuse to make their inadequate components sound passable, and what even tubies inspired by the age of sophisters and cost-accountants use to linearize pentodes. NFB thus has a base level of familiarity which gives it a head start in any black box model intended to explain something to diplomaed quarterwits among the silicon slime as well as the better-educated kibbitzers in my own camp. But fine, you want to reject my explanation, then you must offer a better reason to explain why triodes are such superior amplification devices to anything else, so much more pleasing to the ear, so much more accurate to the cultivated taste. Despite my cracks about the metaphysics of tubes, there *has* to be an electrical reason for the superiority of triodes. But sure, all kinds of input is welcome. Andre Jute Andre, I think you are too quick to dismiss the idea that feedback transforms amplitude distortions into phase-smearing, like Otala claimed in his talk/paper (assuming a paper ever followed the talk!). When Patrick was defending feedback, he mentioned that poorly designed amps sound like crap when they have lots of feedback, but that if you fix them up a bit, meaning get rid of much of their excessive nonlinearities -- read, *amplitude* nonlinearities -- then adding feedback sounds okay. Now, if we eliminate the idea that feedback produces random noise -- and we *know* that it reduces amplitude non-linearities -- then the only distortion mechanism left, I believe, is phase-shifting. However, I want to describe the usual "constant 20 degrees phase lag at 40 KHz" as "phase-shifting," and a dynamic, microsecond to microsecond shifting back and forth of one frequency relative to another as "phase-smearing," or "time-smearing." What Otala was saying is that applying feedback to circuits with lots of open-loop distortions, which are (I believe) almost always amplitude distortions, converts these distortions into a back and forth smearing of the high frequencies relative to the low frequencies (and it may smear both in terms of the delay through the amp). Nor can we assume that this time-smearing is a simple function of the low frequency amplitude, because it is probably proportional to the magnitude of the distortions, as well as the LF amplitude: VERY non-musical. When enough feedback is applied to badly designed amps, the amplitude distortions become quite small, so why did Patrick find that they sounded much worse than the same amp sounds when touched up enough to reduce the larger open-loop amplitude distortions? As you say, *something* is wrong, and if it isn't high amplitude distortions (it can't be), and if feedback doesn't produce spurious noises, then the only thing left is exactly what Otala said, time-smearing. In essence, feedback *connects* two things that are normally separate in an amp, namely amplitude distortions, and phase-smearing. It achieves a balance between these two, a balance which is determined by the speed of the amp, the amount of feedback, and the amount of amplitude distortion. Contrary to what you say, Otala was *not* referring to TIM, and transistors did not become so much faster after 1980 than the ones used in his '73 article to make the problem he described go away. Yes, the amps had to be designed well enough to avoid TIM, but that was not a real problem even in '73 *if* you knew what you were doing. The problem he described in '80 was quite different, and even high MHz tubes are subject to it. The interesting things, assuming that feedback problems are indeed time-smearing (regardless of whether this comes from the conversion of amplitude distortions), are one, a single tone will reveal nothing of this, giving very low THD numbers, and two, multiple tones should show something, although looking at it in the amplitude realm will only show higher than expected IMD. There should be a fairly easy way to test to see if this really produces time-smearing. In general, we put a 4 volt 60 Hz signal and a 10 mV 20 KHz signal into an amp with lots of feedback, preferably using non-linear sections of the amplifying devices (say, use two 12AX7 type triodes where the plate curves vary from widely spaced to closely spaced, and use about 60 to 80 dB of feedback to get the overall gain down to 1). Use a high pass filter to see only the 20 KHz signal, and use the 20 KHz signal from the generator to trigger the 'scope. If time-smearing exists, then the 20 KHz signal will appear "fuzzy" on the scope, since it is being shifted back and forth. Repeat without the 60 Hz signal to make sure the time-smearing isn't coming from somewhere else, and then also divide the output from each tube down so that you get the same amplitude output *without* using feedback, and see if the time-smearing goes away. Finally, if possible test over a region where the curves are fairly linear, to see if that also reduces the time-smearing. If so, you have proof that feedback transformed the amplitude distortion into time-smearing. I will try to do this, but I have little time, less energy, and not the best test bench in the world, so it may be a while! If you or someone else both can and wants to try this, I suspect we will get an answer much faster than if I do it. Note that we can use pentodes or transistors, too, but using solid state has the disadvantage of possibly introducing other complications due to the poor quality silicon parasitic capacitances. Also, I am suggesting the use of very different magnitudes as well as frequencies for the two signals, in case equal magnitudes somehow avoids this problem, or at least masks it from this test. And again, contrary to what Patrick said, the idea that *if* this were true, then by now someone would have already tries it, and the results would have become widely know, is just naive. The human race simply isn't that intelligent, at least not yet. An interesting conclusion, assuming this feedback time-smear mechanism exists, is that the output of a feedback amp does *not* match the input when multiple signals exist! However, our normal tests cannot see this, since they tend to focus on one frequency at a time. I believe that Patrick, as well as Otala, said that using local feedback to achieve good open loop linearity, combined with some global feedback, tends to sound pretty good. This makes sense for two reasons, first because although the local feedback will produce phase-smearing, it does so at *very* high speed, which I believe directly reduces the amount of smearing (common sense says that as the speed of devices approaches infinity, feedback becomes "perfect"). Second, the result does contain some mis-match between input and output, even given the high speed of local feedback, but now the global feedback will not only reduce the remaining "normal" amplitude distortions, it should also reduce the time-smearing produced by the local feedback, since this time-smearing will still produce an error signal for the global feedback. Of course, this reduction in time-smearing will itself produce more time-smearing, but it should be a case of 0.1 x 0.1 = 0.01, so the final degree of time-smearing is reduced by "dividing up" the total feedback into local plus global. Finally, you saw the review of Otala that Phil Allison gave, and I think you were as impressed by it as I was (although it looked bad to PA). When a man who was as talented, knowledgeable, and honest as Otala produces a PROOF that negative feedback *always* transforms the amplitude distortions of the open loop into phase smears of the closed loop, we should not dismiss it as simply a problem of "old devices," a problem that a slight increase in speed can make go away, especially when the discussion that led to his analysis (the Audio Critic BS session) included a lot of talk about vacuum tubes, and how their speed advantage made it easier to use them with feedback. We really should do at least one or two tests before dismissing his conclusions. Again, I will try to do so, but if you want to know anytime soon, you should probably rely on someone else. Phil |
#21
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Explanation still required for triode superiority
I've read your arguments, Phil. Their subtext is methods to make NFB
more usable, less damaging to reproduced sound. That is a legitimate outlook for Otala, who has to fit into an existing industrial environment where NFB is an ineradicable part of that dominant electronics cost-accounting system in which the explicit purpose of NFB is to make cheap parts tolerable. But that has buggerall to do with me as a DIYer. I can afford to take the far simpler, more fundamental course of making my amps so superior in design and components right from the start -- that I can do without NFB and without the damage that NFB does. So I design amps without NFB, period. I shortcut the problem of NFB and eliminate it before it arises. My amps are ultra-silent without NFB; they do not need NFB for any purpose whatsoever. Since one of my ZNFB amps is capable of 80W when in its PSE mode, more than enough to drive electrostatic panels to power-rivetter volume, I take the view that any designer who requires NFB to make his hi-fi amps work has either permitted cost-accountants to bully him, is an impressionable fashion victim, or is too thick to put his mind in gear, tick one or more boxes. The rest is interesting speculation but not of such consuming interest to me that I will spend a morning setting up bench experiments to prove the details of something I already know: that NFB smears the sound. A complete summary of my view on NFB can be found here http://members.lycos.co.uk/fiultra/K...dre%20Jute.htm and here http://members.lycos.co.uk/fiultra/K...dre%20Jute.htm I haven't changed my mind one jot or tittle over the recent discussion. I have heard absolutely nothing that proves a contrary case. That is not to say you may not be right, that the psycho-acoustic effect which gives Class A1 ZNFB (or very low NFB) amps their distinct superiority is a subliminal reaction to the HF phase- smearing in NFB amps that you say Otala posits. I just haven't seen any proof yet, and know that, when I do see such proof, I will consider it of intellectual interest -- and continue to build the sort of ZNFB amps I have always built, in which NFB is excluded for its amplitude smearing at frequencies starting below 100c/s, so that HF phase smearing, if it exists in NFB tube amps, is no danger to my sound. 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 Phil wrote: Andre Jute wrote: In an effort to be agreeable, I tried hard to give you negative feedback inside the tube as an explanation of the overwhelming superiority of triodes (or trioded pentodes) for audio reproduction, among other reasons because NFB is accessible to many who belong on RAT and is a genetic deformity of the silicon scum whose only purpose on RAT is dissension. NFB is what the silicon slime abuse to make their inadequate components sound passable, and what even tubies inspired by the age of sophisters and cost-accountants use to linearize pentodes. NFB thus has a base level of familiarity which gives it a head start in any black box model intended to explain something to diplomaed quarterwits among the silicon slime as well as the better-educated kibbitzers in my own camp. But fine, you want to reject my explanation, then you must offer a better reason to explain why triodes are such superior amplification devices to anything else, so much more pleasing to the ear, so much more accurate to the cultivated taste. Despite my cracks about the metaphysics of tubes, there *has* to be an electrical reason for the superiority of triodes. But sure, all kinds of input is welcome. Andre Jute Andre, I think you are too quick to dismiss the idea that feedback transforms amplitude distortions into phase-smearing, like Otala claimed in his talk/paper (assuming a paper ever followed the talk!). When Patrick was defending feedback, he mentioned that poorly designed amps sound like crap when they have lots of feedback, but that if you fix them up a bit, meaning get rid of much of their excessive nonlinearities -- read, *amplitude* nonlinearities -- then adding feedback sounds okay. Now, if we eliminate the idea that feedback produces random noise -- and we *know* that it reduces amplitude non-linearities -- then the only distortion mechanism left, I believe, is phase-shifting. However, I want to describe the usual "constant 20 degrees phase lag at 40 KHz" as "phase-shifting," and a dynamic, microsecond to microsecond shifting back and forth of one frequency relative to another as "phase-smearing," or "time-smearing." What Otala was saying is that applying feedback to circuits with lots of open-loop distortions, which are (I believe) almost always amplitude distortions, converts these distortions into a back and forth smearing of the high frequencies relative to the low frequencies (and it may smear both in terms of the delay through the amp). Nor can we assume that this time-smearing is a simple function of the low frequency amplitude, because it is probably proportional to the magnitude of the distortions, as well as the LF amplitude: VERY non-musical. When enough feedback is applied to badly designed amps, the amplitude distortions become quite small, so why did Patrick find that they sounded much worse than the same amp sounds when touched up enough to reduce the larger open-loop amplitude distortions? As you say, *something* is wrong, and if it isn't high amplitude distortions (it can't be), and if feedback doesn't produce spurious noises, then the only thing left is exactly what Otala said, time-smearing. In essence, feedback *connects* two things that are normally separate in an amp, namely amplitude distortions, and phase-smearing. It achieves a balance between these two, a balance which is determined by the speed of the amp, the amount of feedback, and the amount of amplitude distortion. Contrary to what you say, Otala was *not* referring to TIM, and transistors did not become so much faster after 1980 than the ones used in his '73 article to make the problem he described go away. Yes, the amps had to be designed well enough to avoid TIM, but that was not a real problem even in '73 *if* you knew what you were doing. The problem he described in '80 was quite different, and even high MHz tubes are subject to it. The interesting things, assuming that feedback problems are indeed time-smearing (regardless of whether this comes from the conversion of amplitude distortions), are one, a single tone will reveal nothing of this, giving very low THD numbers, and two, multiple tones should show something, although looking at it in the amplitude realm will only show higher than expected IMD. There should be a fairly easy way to test to see if this really produces time-smearing. In general, we put a 4 volt 60 Hz signal and a 10 mV 20 KHz signal into an amp with lots of feedback, preferably using non-linear sections of the amplifying devices (say, use two 12AX7 type triodes where the plate curves vary from widely spaced to closely spaced, and use about 60 to 80 dB of feedback to get the overall gain down to 1). Use a high pass filter to see only the 20 KHz signal, and use the 20 KHz signal from the generator to trigger the 'scope. If time-smearing exists, then the 20 KHz signal will appear "fuzzy" on the scope, since it is being shifted back and forth. Repeat without the 60 Hz signal to make sure the time-smearing isn't coming from somewhere else, and then also divide the output from each tube down so that you get the same amplitude output *without* using feedback, and see if the time-smearing goes away. Finally, if possible test over a region where the curves are fairly linear, to see if that also reduces the time-smearing. If so, you have proof that feedback transformed the amplitude distortion into time-smearing. I will try to do this, but I have little time, less energy, and not the best test bench in the world, so it may be a while! If you or someone else both can and wants to try this, I suspect we will get an answer much faster than if I do it. Note that we can use pentodes or transistors, too, but using solid state has the disadvantage of possibly introducing other complications due to the poor quality silicon parasitic capacitances. Also, I am suggesting the use of very different magnitudes as well as frequencies for the two signals, in case equal magnitudes somehow avoids this problem, or at least masks it from this test. And again, contrary to what Patrick said, the idea that *if* this were true, then by now someone would have already tries it, and the results would have become widely know, is just naive. The human race simply isn't that intelligent, at least not yet. An interesting conclusion, assuming this feedback time-smear mechanism exists, is that the output of a feedback amp does *not* match the input when multiple signals exist! However, our normal tests cannot see this, since they tend to focus on one frequency at a time. I believe that Patrick, as well as Otala, said that using local feedback to achieve good open loop linearity, combined with some global feedback, tends to sound pretty good. This makes sense for two reasons, first because although the local feedback will produce phase-smearing, it does so at *very* high speed, which I believe directly reduces the amount of smearing (common sense says that as the speed of devices approaches infinity, feedback becomes "perfect"). Second, the result does contain some mis-match between input and output, even given the high speed of local feedback, but now the global feedback will not only reduce the remaining "normal" amplitude distortions, it should also reduce the time-smearing produced by the local feedback, since this time-smearing will still produce an error signal for the global feedback. Of course, this reduction in time-smearing will itself produce more time-smearing, but it should be a case of 0.1 x 0.1 = 0.01, so the final degree of time-smearing is reduced by "dividing up" the total feedback into local plus global. Finally, you saw the review of Otala that Phil Allison gave, and I think you were as impressed by it as I was (although it looked bad to PA). When a man who was as talented, knowledgeable, and honest as Otala produces a PROOF that negative feedback *always* transforms the amplitude distortions of the open loop into phase smears of the closed loop, we should not dismiss it as simply a problem of "old devices," a problem that a slight increase in speed can make go away, especially when the discussion that led to his analysis (the Audio Critic BS session) included a lot of talk about vacuum tubes, and how their speed advantage made it easier to use them with feedback. We really should do at least one or two tests before dismissing his conclusions. Again, I will try to do so, but if you want to know anytime soon, you should probably rely on someone else. Phil |
#22
Posted to rec.audio.tubes
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Explanation still required for triode superiority
Andre Jute wrote:
I've read your arguments, Phil. Their subtext is methods to make NFB more usable, less damaging to reproduced sound. That is a legitimate outlook for Otala, who has to fit into an existing industrial environment where NFB is an ineradicable part of that dominant electronics cost-accounting system in which the explicit purpose of NFB is to make cheap parts tolerable. But that has buggerall to do with me as a DIYer. I can afford to take the far simpler, more fundamental course of making my amps so superior in design and components right from the start -- that I can do without NFB and without the damage that NFB does. So I design amps without NFB, period. I shortcut the problem of NFB and eliminate it before it arises. My amps are ultra-silent without NFB; they do not need NFB for any purpose whatsoever. Since one of my ZNFB amps is capable of 80W when in its PSE mode, more than enough to drive electrostatic panels to power-rivetter volume, I take the view that any designer who requires NFB to make his hi-fi amps work has either permitted cost-accountants to bully him, is an impressionable fashion victim, or is too thick to put his mind in gear, tick one or more boxes. The rest is interesting speculation but not of such consuming interest to me that I will spend a morning setting up bench experiments to prove the details of something I already know: that NFB smears the sound. A complete summary of my view on NFB can be found here http://members.lycos.co.uk/fiultra/K...dre%20Jute.htm and here http://members.lycos.co.uk/fiultra/K...dre%20Jute.htm I haven't changed my mind one jot or tittle over the recent discussion. I have heard absolutely nothing that proves a contrary case. That is not to say you may not be right, that the psycho-acoustic effect which gives Class A1 ZNFB (or very low NFB) amps their distinct superiority is a subliminal reaction to the HF phase- smearing in NFB amps that you say Otala posits. I just haven't seen any proof yet, and know that, when I do see such proof, I will consider it of intellectual interest -- and continue to build the sort of ZNFB amps I have always built, in which NFB is excluded for its amplitude smearing at frequencies starting below 100c/s, so that HF phase smearing, if it exists in NFB tube amps, is no danger to my sound. Be fair now, you didn't ask, "Why should I build high feedback pentode amps," you asked, "Why do do [no feedback] triode amps sound better?" I merely attempted to give you an answer. You believe that triodes have an internal feedback mechanism, and you wondered why they (still) sound better. The basic core of my response/answer is that (1) negative feedback transforms relatively benign amplitude distortion into much less musical phase distortion, and (2) either triodes do *not* have this distortion mechanism, or it occurs at such staggeringly high frequencies that triodes can "get away with it," since the amount of phase distortion produced decreases as the high frequency limit increases. And I proposed a test, which may not work anyway, and which apparently only I have any interest in performing! However, I believe you when you say you already know that feedback smears the sound somehow, and don't need a test to "justify" your decisions, which is a stand I do respect. For a while there, all of us had to face serious criticisms for saying that in our own experience, cables do sound different, and it really is a mark of character to stand up and say something that is true, but "officially" stupid. If only the "official" beliefs didn't so often turn out to be the ones that are actually stupid, we could all be mindless sheep, and be better off for it! ;-) However, there are sometimes good reasons for more fully understanding something. If we assume for a moment that my analysis -- which is basically my attempt to guess at the rest of what Otala was saying, since I have yet to see the full text -- is correct, then several interesting things follow. First, since a threshold below which we cannot hear phase distortion realistically *must* exist -- and again, for Patrick's benefit, this is phase-smearing, and *not* the simple phase shifting which feedback does correct -- we should be able to add some feedback and get "all gain, no pain." Patrick said that he added 6 dB to lower output Z, and it sounded fantastic, as opposed to a 20 dB version of the same amp. Well, maybe this is completely true, but if we *know* that it is true, then, for example, amps that use high-mu transmitter tubes with positive grid drive and a bit of feedback to get the Zout down begin to make sense, especially when the feedback is used in a two-stage configuration that does not include the output transformer, meaning that it can have a *very* high upper frequency limit (you don't need to "dumb down" a stage like you often need to do with a three-stage to prevent oscillation), which limits the damage feedback can do to sub-threshold levels. Many people report that they LOVE the sound of these things, but an unjustified, in this case, bias against *any* use of feedback could prevent us from even trying one. Or, let's look at the home builder who wants to make a solid state amp -- what the hell -- or at least one with a SS output stage. There are basically two forms of feedback, the normal one, and the "active-error" version described by J. R. MacDonald and others. The active-error version only "corrects" the output when an actual error exists, whereas the standard version has to correct the open-loop gain even when the load is a steady resistance and the devices are behaving with perfect linearity. If tests show more phase-smearing with the standard version than with the active-error version, well, I know which version I would want to use, or have in a new television. As a bit of a side note, with better sounding SS output stages, maybe we can more easily hear the advantage, assuming one exists, of using a tube to produce the error signal (a tube doesn't have poor quality parasitic capacitances to potentially mess up the low level information). All of this may sound like something only of interest to home builders, but at least some manufacturers actually would be happy to produce noticeably better sounding products, if they could do so for about the same money! If EE's in general become aware of the full characteristics of feedback -- and if home builders start to do this, many EE's and high end manufacturers will indeed follow, eventually -- then we might actually see better products in cars and TV's. No, I'm not saying do this so that we will get better products, but a good understanding of what is needed to make better audio products does tend to help everyone, sooner or later. Low output Z triodes are in fact the theoretically best audio devices at this time, sound-wise, but they require an expensive, heavy, big, high quality output transformer, and that will always limit their use. Or, maybe someone just wants to write an article for AudioXpress about the true nature of feedback, and how to best use it, if its use cannot be avoided! Phil 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 Phil wrote: Andre Jute wrote: In an effort to be agreeable, I tried hard to give you negative feedback inside the tube as an explanation of the overwhelming superiority of triodes (or trioded pentodes) for audio reproduction, among other reasons because NFB is accessible to many who belong on RAT and is a genetic deformity of the silicon scum whose only purpose on RAT is dissension. NFB is what the silicon slime abuse to make their inadequate components sound passable, and what even tubies inspired by the age of sophisters and cost-accountants use to linearize pentodes. NFB thus has a base level of familiarity which gives it a head start in any black box model intended to explain something to diplomaed quarterwits among the silicon slime as well as the better-educated kibbitzers in my own camp. But fine, you want to reject my explanation, then you must offer a better reason to explain why triodes are such superior amplification devices to anything else, so much more pleasing to the ear, so much more accurate to the cultivated taste. Despite my cracks about the metaphysics of tubes, there *has* to be an electrical reason for the superiority of triodes. But sure, all kinds of input is welcome. Andre Jute Andre, I think you are too quick to dismiss the idea that feedback transforms amplitude distortions into phase-smearing, like Otala claimed in his talk/paper (assuming a paper ever followed the talk!). When Patrick was defending feedback, he mentioned that poorly designed amps sound like crap when they have lots of feedback, but that if you fix them up a bit, meaning get rid of much of their excessive nonlinearities -- read, *amplitude* nonlinearities -- then adding feedback sounds okay. Now, if we eliminate the idea that feedback produces random noise -- and we *know* that it reduces amplitude non-linearities -- then the only distortion mechanism left, I believe, is phase-shifting. However, I want to describe the usual "constant 20 degrees phase lag at 40 KHz" as "phase-shifting," and a dynamic, microsecond to microsecond shifting back and forth of one frequency relative to another as "phase-smearing," or "time-smearing." What Otala was saying is that applying feedback to circuits with lots of open-loop distortions, which are (I believe) almost always amplitude distortions, converts these distortions into a back and forth smearing of the high frequencies relative to the low frequencies (and it may smear both in terms of the delay through the amp). Nor can we assume that this time-smearing is a simple function of the low frequency amplitude, because it is probably proportional to the magnitude of the distortions, as well as the LF amplitude: VERY non-musical. When enough feedback is applied to badly designed amps, the amplitude distortions become quite small, so why did Patrick find that they sounded much worse than the same amp sounds when touched up enough to reduce the larger open-loop amplitude distortions? As you say, *something* is wrong, and if it isn't high amplitude distortions (it can't be), and if feedback doesn't produce spurious noises, then the only thing left is exactly what Otala said, time-smearing. In essence, feedback *connects* two things that are normally separate in an amp, namely amplitude distortions, and phase-smearing. It achieves a balance between these two, a balance which is determined by the speed of the amp, the amount of feedback, and the amount of amplitude distortion. Contrary to what you say, Otala was *not* referring to TIM, and transistors did not become so much faster after 1980 than the ones used in his '73 article to make the problem he described go away. Yes, the amps had to be designed well enough to avoid TIM, but that was not a real problem even in '73 *if* you knew what you were doing. The problem he described in '80 was quite different, and even high MHz tubes are subject to it. The interesting things, assuming that feedback problems are indeed time-smearing (regardless of whether this comes from the conversion of amplitude distortions), are one, a single tone will reveal nothing of this, giving very low THD numbers, and two, multiple tones should show something, although looking at it in the amplitude realm will only show higher than expected IMD. There should be a fairly easy way to test to see if this really produces time-smearing. In general, we put a 4 volt 60 Hz signal and a 10 mV 20 KHz signal into an amp with lots of feedback, preferably using non-linear sections of the amplifying devices (say, use two 12AX7 type triodes where the plate curves vary from widely spaced to closely spaced, and use about 60 to 80 dB of feedback to get the overall gain down to 1). Use a high pass filter to see only the 20 KHz signal, and use the 20 KHz signal from the generator to trigger the 'scope. If time-smearing exists, then the 20 KHz signal will appear "fuzzy" on the scope, since it is being shifted back and forth. Repeat without the 60 Hz signal to make sure the time-smearing isn't coming from somewhere else, and then also divide the output from each tube down so that you get the same amplitude output *without* using feedback, and see if the time-smearing goes away. Finally, if possible test over a region where the curves are fairly linear, to see if that also reduces the time-smearing. If so, you have proof that feedback transformed the amplitude distortion into time-smearing. I will try to do this, but I have little time, less energy, and not the best test bench in the world, so it may be a while! If you or someone else both can and wants to try this, I suspect we will get an answer much faster than if I do it. Note that we can use pentodes or transistors, too, but using solid state has the disadvantage of possibly introducing other complications due to the poor quality silicon parasitic capacitances. Also, I am suggesting the use of very different magnitudes as well as frequencies for the two signals, in case equal magnitudes somehow avoids this problem, or at least masks it from this test. And again, contrary to what Patrick said, the idea that *if* this were true, then by now someone would have already tries it, and the results would have become widely know, is just naive. The human race simply isn't that intelligent, at least not yet. An interesting conclusion, assuming this feedback time-smear mechanism exists, is that the output of a feedback amp does *not* match the input when multiple signals exist! However, our normal tests cannot see this, since they tend to focus on one frequency at a time. I believe that Patrick, as well as Otala, said that using local feedback to achieve good open loop linearity, combined with some global feedback, tends to sound pretty good. This makes sense for two reasons, first because although the local feedback will produce phase-smearing, it does so at *very* high speed, which I believe directly reduces the amount of smearing (common sense says that as the speed of devices approaches infinity, feedback becomes "perfect"). Second, the result does contain some mis-match between input and output, even given the high speed of local feedback, but now the global feedback will not only reduce the remaining "normal" amplitude distortions, it should also reduce the time-smearing produced by the local feedback, since this time-smearing will still produce an error signal for the global feedback. Of course, this reduction in time-smearing will itself produce more time-smearing, but it should be a case of 0.1 x 0.1 = 0.01, so the final degree of time-smearing is reduced by "dividing up" the total feedback into local plus global. Finally, you saw the review of Otala that Phil Allison gave, and I think you were as impressed by it as I was (although it looked bad to PA). When a man who was as talented, knowledgeable, and honest as Otala produces a PROOF that negative feedback *always* transforms the amplitude distortions of the open loop into phase smears of the closed loop, we should not dismiss it as simply a problem of "old devices," a problem that a slight increase in speed can make go away, especially when the discussion that led to his analysis (the Audio Critic BS session) included a lot of talk about vacuum tubes, and how their speed advantage made it easier to use them with feedback. We really should do at least one or two tests before dismissing his conclusions. Again, I will try to do so, but if you want to know anytime soon, you should probably rely on someone else. Phil |
#23
Posted to rec.audio.tubes
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Explanation still required for triode superiority
Phil wrote: Andre Jute wrote: I've read your arguments, Phil. Their subtext is methods to make NFB more usable, less damaging to reproduced sound. That is a legitimate outlook for Otala, who has to fit into an existing industrial environment where NFB is an ineradicable part of that dominant electronics cost-accounting system in which the explicit purpose of NFB is to make cheap parts tolerable. But that has buggerall to do with me as a DIYer. I can afford to take the far simpler, more fundamental course of making my amps so superior in design and components right from the start -- that I can do without NFB and without the damage that NFB does. So I design amps without NFB, period. I shortcut the problem of NFB and eliminate it before it arises. My amps are ultra-silent without NFB; they do not need NFB for any purpose whatsoever. Since one of my ZNFB amps is capable of 80W when in its PSE mode, more than enough to drive electrostatic panels to power-rivetter volume, I take the view that any designer who requires NFB to make his hi-fi amps work has either permitted cost-accountants to bully him, is an impressionable fashion victim, or is too thick to put his mind in gear, tick one or more boxes. The rest is interesting speculation but not of such consuming interest to me that I will spend a morning setting up bench experiments to prove the details of something I already know: that NFB smears the sound. A complete summary of my view on NFB can be found here http://members.lycos.co.uk/fiultra/K...dre%20Jute.htm and here http://members.lycos.co.uk/fiultra/K...dre%20Jute.htm I haven't changed my mind one jot or tittle over the recent discussion. I have heard absolutely nothing that proves a contrary case. That is not to say you may not be right, that the psycho-acoustic effect which gives Class A1 ZNFB (or very low NFB) amps their distinct superiority is a subliminal reaction to the HF phase- smearing in NFB amps that you say Otala posits. I just haven't seen any proof yet, and know that, when I do see such proof, I will consider it of intellectual interest -- and continue to build the sort of ZNFB amps I have always built, in which NFB is excluded for its amplitude smearing at frequencies starting below 100c/s, so that HF phase smearing, if it exists in NFB tube amps, is no danger to my sound. Be fair now, you didn't ask, "Why should I build high feedback pentode amps," But I do build pentode amps, if without the "high feedback". The best amp I ever designed, my Type 114 "Triple Threat" is a PP EL34 with a pentode/ultralinear/triode switch and NFB tunable from zero to about 6dB in the latest iteration but up to 20dB in some early versions. Of course, it is my "best" amp only when operated in the trioded ZNFB mode, but the choice is in the hands of the builder. you asked, "Why do do [no feedback] triode amps sound better?" I merely attempted to give you an answer. You believe that triodes have an internal feedback mechanism, and you wondered why they (still) sound better. My point is a little more subtle. I see an effect which looks like NFB happening between the terminals of a triode; I'm happy to call it internal or natural NFB until someone else offers an explanation that justifies giving it another name; no one offers such an explanation, merely negative objections to naming it NFB; until they pull their finger from their arse and say something positive (and more cogent) than Patrick, who *does* have a case, I shall not change my mind just because they're "agin". You should also understand that some of these clowns are agin not for any good reason but only because it is *me* saying so; Pasternack, for instance, and the usual sockpuppets of the Magnequest Scum, whose disruptive presence on RAT I dispensed with wholesale a few years ago ("Jute is wrong even if he is right, isn't he?" one of them, Ron Bales, complained pitifully!). Pasternack, for instance, has been repeatedly caught out lying on professional matters "in my zeal to flame Andre" -- which is his own sickening excuse in his own words. The basic core of my response/answer is that (1) negative feedback transforms relatively benign amplitude distortion into much less musical phase distortion, and (2) either triodes do *not* have this distortion mechanism, or it occurs at such staggeringly high frequencies that triodes can "get away with it," since the amount of phase distortion produced decreases as the high frequency limit increases. I don't need to go that sophisticated. By far the most popular of my loudspeaker designs (of the published ones; an expensive licensed speak sells amazingly well in the Far East considering the wretchedly high price dictated by the cost of the drivers) is an economy fullranger using a guitar driver, in most installations that I know of with the tweeter disconnected or, on my advice, never fitted. http://members.lycos.co.uk/fiultra/K...Impresario.jpg It has been years since I have been impressed with bandwidth as measure of audio goodness, and I don't just mean excessive bandwidth, I mean the upper end of what is commonly called the "audio spectrum". Most people can't hear it even when they're young. Most people into hi-fi are over middle age; they're lucky if they can hear past 12kHz. In any event, Top C on a piano is a frequency of 4186c/s; over 70% of the energy of that note (over half the decibel value referred to the fundamental's energy) will be in the 2nd and 3rd harmonics, which takes you just past 12kHz. (Try an experiment: filter out everything below the 3rd harmonic of top C, 12558c/s, and listen to what remains. It is horrid, just white noise.) By the way, the same argument of harmonic weight in the production of sound applies to the bass end and well into the midbass: an open G string on a violin played with medium intensity has only 0.1 per cent of its energy in the fundamental -- which is 196c/s; this is the basis for my thesis that the human ear reconstitutes the fundamental from the dominant harmonics, where the energy is. The upshot is that the lower extension of the so-called "audio range" is another engineering chimera that has nothing whatsoever to do with music and everything to do with a bunch of arrogant engineers sitting in a room feeding of each other's testosterone while setting standards that should instead have been set by psychologists or musicologists or, in fact, anyone but engineers. In practice, it means that you can roll speakers off quite high, where the bass will "measure" modestly but sound very, very clean, and they will sound better over the long term than those one-note boof-boff big-bass abominations so beloved of "audiophiles" and "engineers" alike. Peter Walker didn't build his ESL63 any bigger than 45c/s bass (the last pair I measured was 8dB down at 32c/s) because to the ear even a 16c/s organ note, if on the recording at all, will sound startlingly precise because all the energy are in the first two harmonics above the fundamental. More, drivers with a natural high frequency mechanism, like the classic Lowther driver, can sound screechy in the treble precisely because it is not a fundamentally natural noise http://members.lycos.co.uk/fiultra/K...20T91HWAF3.jpg Lowthers with the whizzer tweaked by the insertion of cotton wool behind it or by stiffening with C37, always sound more "natural" even as the measurements start looking less impressive. (I prefer the waterfall anyway, but only after I have run it through a smoothing program because the small glitches are a distraction to the eye but not the ear.) And I proposed a test, which may not work anyway, and which apparently only I have any interest in performing! However, I believe you when you say you already know that feedback smears the sound somehow, and don't need a test to "justify" your decisions, which is a stand I do respect. Hang on a minute. I have demonstrated smearing in the frequency band I'm interested in. I've done it again and again and again, theoretically by mathematics, by measurement with instruments, graphically, and by placebo listening tests (what the pretentious call ABX). http://members.lycos.co.uk/fiultra/K...dre%20Jute.htm http://members.lycos.co.uk/fiultra/K...dre%20Jute.htm I have no problem standing by my taste when taste is the question (for about ten years I was the most widely read music critic in the world with a column syndicated to 9.2m readers every week) but when science has an answer I am as keen as the next man (and clearly keener than the diplomaed quarterwits on RAT) to find the correct answer by the proper scientific method. For a while there, all of us had to face serious criticisms for saying that in our own experience, cables do sound different, Sure, I believe you can hear cables, under two very specific conditions. Again, my conclusion is based on a scientific test. I flew to a different country and in a warehouse spliced huge reels of cable until I had the several of different construction of a length that an engineer (a proper one, not one of the local clowns) calculated for me should be audible by the laws of physics. They were, barely. The other condition under which cables are audible also answers to the laws of physics: when components in the audio chain are mismatched, the resistance and capacitance on a particular cable can make the combination sound better or worse, hence "cable is audible". I personally use Cardas golden section multi-diameter cable because the concept appeals to me: my Impresario speak is designed on Phi, the formulaic base of the golden section. http://members.lycos.co.uk/fiultra/K...Impresario.jpg and it really is a mark of character to stand up and say something that is true, but "officially" stupid. Man, you got enough problems already. You really don't want my character as well: I was sent into exile from my motherland for saying the statistical basis of apartheid was flawed (everyone could see it only twenty years after I said it), and later twice hunted by assassins sent by the apartheid government for a couple of my books. http://members.lycos.co.uk/fiultra/THE%20WRITER'S%20HOUSE.html As an economist I was academically haunted by being the only monetarist in keynesian countries in a keynesian age, as a psychologist in a Freudian age thought very odd, most unreliable, a loose cannon on deck for saying that Freud was a literary giant but didn't know **** about human character. Today, of course, everyone knows I am right and was right then. That was nothing to the filth heaped on me for saying the Common Agricultural Policy of the European Community was a wasteful abomination, or pointing out that the case against DDT was and is unproven, or that "global warming" is an official lie (I've been saying it since the 1960s when I had a running joke in a satarical newspaper column about the missing hole in the ozone layer). If only the "official" beliefs didn't so often turn out to be the ones that are actually stupid, we could all be mindless sheep, and be better off for it! ;-) Nah, I like being "the most dangerous class", as Lenin called reformers. Sure, the price has been high, but the alternative would have been dull. However, there are sometimes good reasons for more fully understanding something. If we assume for a moment that my analysis -- which is basically my attempt to guess at the rest of what Otala was saying, since I have yet to see the full text -- is correct, then several interesting things follow. First, since a threshold below which we cannot hear phase distortion realistically *must* exist There is no *must* about any threshhold. We're discussing the known preference for Class A1 triode sound by an important niche of knowledgeable audiophiles -- despite the known fact that it measures worse than the alternatives on quite a few parameters. Even if those parameters are at present driven to ludicrous lengths by the engineers, it is clear that the highest level of triodes -- DHT, ZNFB, SE or Class A1 trioded ZNFB PP pentodes -- will never come within a magnitude of current technology, at least not on the meter. The preference must be explained by some subliminal effect. The problem with subliminal effects is that they recede with experience, that is, that they're educable; for instance, the limit at which distortion now becomes audible is lower than it was when Olsen first studied the subject 70 years ago. -- and again, for Patrick's benefit, this is phase-smearing, and *not* the simple phase shifting which feedback does correct -- we should be able to add some feedback and get "all gain, no pain." You're still on someone else's agenda, trying to make NFB usable. My amps started sounding brilliant the day I rejected NFB on principle. Patrick said that he added 6 dB to lower output Z, and it sounded fantastic, as opposed to a 20 dB version of the same amp. Hang on a minute. Patrick said that he found that 6dB of NFB didn't degrade the sound audibly -- a matter of taste for him and his client which we must permit them to enjoy in peace; I shall just say I tend to believe that 6dB is very likely a common audibility limit. Patrick also made the point that the NFB lowers the output impedance, on which he puts a high priority *because he builds amps for sale, often to be used with unknown speakers". Anyone who can design his speakers first can also design them so that output impedance and the consequent need for NFB is less; amps can be designed to be speaker-friendly without NFB as long as you know from the beginning that NFB will be excluded. Well, maybe this is completely true, but if we *know* that it is true, then, for example, amps that use high-mu transmitter tubes with positive grid drive and a bit of feedback to get the Zout down begin to make sense, Been there, done that. You're still on someone else's agenda, this time "mo' powa' is betta powa'". It's bullcrap. Even at the lower end of the same argument, a 211 doesn't sound nearly as good as an 845 when both are built to give the same power, simply because the 211 must be driven across the 0V bias line into A2 while the 845 stays strictly in A1. It's another example of a subliminal effect on the ear when very little difference shows on the measuring instrument. especially when the feedback is used in a two-stage configuration that does not include the output transformer, meaning that it can have a *very* high upper frequency limit (you don't need to "dumb down" a stage like you often need to do with a three-stage to prevent oscillation), which limits the damage feedback can do to sub-threshold levels. Many people report that they LOVE the sound of these things, but an unjustified, in this case, bias against *any* use of feedback could prevent us from even trying one. No. This isn't a bias against NFB. This is a preference for staying in Class A1. Or, let's look at the home builder who wants to make a solid state amp -- what the hell -- or at least one with a SS output stage. Sander swears by his hybrid amps. I play SS amps often; the one I like best is the Quad 405 Mk II, because it is so livable. You might check Google for the threads when Stewart Pinkerton, a Poopie Stevenson type clown but with a smidgin more class, challenged me to a design contest. In theory he was supposed to design a silicon amp to beat my 300B SE amp, or at least produce something that sounded close to it. http://members.lycos.co.uk/fiultra/T...trafi-crct.jpg http://members.lycos.co.uk/fiultra/T44bis-'Populaire'-crct.jpg Even with months of help and coaching from John Byrns and Patrick Turner, what Pinko produced was such a botch that not even he wanted to build it. Bored with waiting for the interminable process to run its fractious course, I designed my own simple SS amp and showed how it could be developed to sound something like a good tube amp by driving it into class A. I'm playing it right now. Thanks for reminding me to plug it in and try it again; for the cost it can't be beat! http://members.lycos.co.uk/fiultra/K...dre%20Jute.htm http://members.lycos.co.uk/fiultra/K...dre%20Jute.htm http://members.lycos.co.uk/fiultra/K...20mGBschem.jpg http://members.lycos.co.uk/fiultra/K...%20mGBmatr.jpg http://members.lycos.co.uk/fiultra/K...%20NoBleed.jpg Basically, I'm saying that, if there is phase-smearing as you claim, I also think you're right, that it will be inaudible. I am more interested in what is audible, even if only subliminally. The rest is of theoretical interest and my time is limited. Thanks for the entertainment. Andre Jute More at http://members.lycos.co.uk/fiultra/JUTE%20ON%20AMPS.htm and http://members.lycos.co.uk/fiultra/T...mp%20INDEX.htm There are basically two forms of feedback, the normal one, and the "active-error" version described by J. R. MacDonald and others. The active-error version only "corrects" the output when an actual error exists, whereas the standard version has to correct the open-loop gain even when the load is a steady resistance and the devices are behaving with perfect linearity. If tests show more phase-smearing with the standard version than with the active-error version, well, I know which version I would want to use, or have in a new television. As a bit of a side note, with better sounding SS output stages, maybe we can more easily hear the advantage, assuming one exists, of using a tube to produce the error signal (a tube doesn't have poor quality parasitic capacitances to potentially mess up the low level information). All of this may sound like something only of interest to home builders, but at least some manufacturers actually would be happy to produce noticeably better sounding products, if they could do so for about the same money! If EE's in general become aware of the full characteristics of feedback -- and if home builders start to do this, many EE's and high end manufacturers will indeed follow, eventually -- then we might actually see better products in cars and TV's. No, I'm not saying do this so that we will get better products, but a good understanding of what is needed to make better audio products does tend to help everyone, sooner or later. Low output Z triodes are in fact the theoretically best audio devices at this time, sound-wise, but they require an expensive, heavy, big, high quality output transformer, and that will always limit their use. Or, maybe someone just wants to write an article for AudioXpress about the true nature of feedback, and how to best use it, if its use cannot be avoided! Phil 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 Phil wrote: Andre Jute wrote: In an effort to be agreeable, I tried hard to give you negative feedback inside the tube as an explanation of the overwhelming superiority of triodes (or trioded pentodes) for audio reproduction, among other reasons because NFB is accessible to many who belong on RAT and is a genetic deformity of the silicon scum whose only purpose on RAT is dissension. NFB is what the silicon slime abuse to make their inadequate components sound passable, and what even tubies inspired by the age of sophisters and cost-accountants use to linearize pentodes. NFB thus has a base level of familiarity which gives it a head start in any black box model intended to explain something to diplomaed quarterwits among the silicon slime as well as the better-educated kibbitzers in my own camp. But fine, you want to reject my explanation, then you must offer a better reason to explain why triodes are such superior amplification devices to anything else, so much more pleasing to the ear, so much more accurate to the cultivated taste. Despite my cracks about the metaphysics of tubes, there *has* to be an electrical reason for the superiority of triodes. But sure, all kinds of input is welcome. Andre Jute Andre, I think you are too quick to dismiss the idea that feedback transforms amplitude distortions into phase-smearing, like Otala claimed in his talk/paper (assuming a paper ever followed the talk!). When Patrick was defending feedback, he mentioned that poorly designed amps sound like crap when they have lots of feedback, but that if you fix them up a bit, meaning get rid of much of their excessive nonlinearities -- read, *amplitude* nonlinearities -- then adding feedback sounds okay. Now, if we eliminate the idea that feedback produces random noise -- and we *know* that it reduces amplitude non-linearities -- then the only distortion mechanism left, I believe, is phase-shifting. However, I want to describe the usual "constant 20 degrees phase lag at 40 KHz" as "phase-shifting," and a dynamic, microsecond to microsecond shifting back and forth of one frequency relative to another as "phase-smearing," or "time-smearing." What Otala was saying is that applying feedback to circuits with lots of open-loop distortions, which are (I believe) almost always amplitude distortions, converts these distortions into a back and forth smearing of the high frequencies relative to the low frequencies (and it may smear both in terms of the delay through the amp). Nor can we assume that this time-smearing is a simple function of the low frequency amplitude, because it is probably proportional to the magnitude of the distortions, as well as the LF amplitude: VERY non-musical. When enough feedback is applied to badly designed amps, the amplitude distortions become quite small, so why did Patrick find that they sounded much worse than the same amp sounds when touched up enough to reduce the larger open-loop amplitude distortions? As you say, *something* is wrong, and if it isn't high amplitude distortions (it can't be), and if feedback doesn't produce spurious noises, then the only thing left is exactly what Otala said, time-smearing. In essence, feedback *connects* two things that are normally separate in an amp, namely amplitude distortions, and phase-smearing. It achieves a balance between these two, a balance which is determined by the speed of the amp, the amount of feedback, and the amount of amplitude distortion. Contrary to what you say, Otala was *not* referring to TIM, and transistors did not become so much faster after 1980 than the ones used in his '73 article to make the problem he described go away. Yes, the amps had to be designed well enough to avoid TIM, but that was not a real problem even in '73 *if* you knew what you were doing. The problem he described in '80 was quite different, and even high MHz tubes are subject to it. The interesting things, assuming that feedback problems are indeed time-smearing (regardless of whether this comes from the conversion of amplitude distortions), are one, a single tone will reveal nothing of this, giving very low THD numbers, and two, multiple tones should show something, although looking at it in the amplitude realm will only show higher than expected IMD. There should be a fairly easy way to test to see if this really produces time-smearing. In general, we put a 4 volt 60 Hz signal and a 10 mV 20 KHz signal into an amp with lots of feedback, preferably using non-linear sections of the amplifying devices (say, use two 12AX7 type triodes where the plate curves vary from widely spaced to closely spaced, and use about 60 to 80 dB of feedback to get the overall gain down to 1). Use a high pass filter to see only the 20 KHz signal, and use the 20 KHz signal from the generator to trigger the 'scope. If time-smearing exists, then the 20 KHz signal will appear "fuzzy" on the scope, since it is being shifted back and forth. Repeat without the 60 Hz signal to make sure the time-smearing isn't coming from somewhere else, and then also divide the output from each tube down so that you get the same amplitude output *without* using feedback, and see if the time-smearing goes away. Finally, if possible test over a region where the curves are fairly linear, to see if that also reduces the time-smearing. If so, you have proof that feedback transformed the amplitude distortion into time-smearing. I will try to do this, but I have little time, less energy, and not the best test bench in the world, so it may be a while! If you or someone else both can and wants to try this, I suspect we will get an answer much faster than if I do it. Note that we can use pentodes or transistors, too, but using solid state has the disadvantage of possibly introducing other complications due to the poor quality silicon parasitic capacitances. Also, I am suggesting the use of very different magnitudes as well as frequencies for the two signals, in case equal magnitudes somehow avoids this problem, or at least masks it from this test. And again, contrary to what Patrick said, the idea that *if* this were true, then by now someone would have already tries it, and the results would have become widely know, is just naive. The human race simply isn't that intelligent, at least not yet. An interesting conclusion, assuming this feedback time-smear mechanism exists, is that the output of a feedback amp does *not* match the input when multiple signals exist! However, our normal tests cannot see this, since they tend to focus on one frequency at a time. I believe that Patrick, as well as Otala, said that using local feedback to achieve good open loop linearity, combined with some global feedback, tends to sound pretty good. This makes sense for two reasons, first because although the local feedback will produce phase-smearing, it does so at *very* high speed, which I believe directly reduces the amount of smearing (common sense says that as the speed of devices approaches infinity, feedback becomes "perfect"). Second, the result does contain some mis-match between input and output, even given the high speed of local feedback, but now the global feedback will not only reduce the remaining "normal" amplitude distortions, it should also reduce the time-smearing produced by the local feedback, since this time-smearing will still produce an error signal for the global feedback. Of course, this reduction in time-smearing will itself produce more time-smearing, but it should be a case of 0.1 x 0.1 = 0.01, so the final degree of time-smearing is reduced by "dividing up" the total feedback into local plus global. Finally, you saw the review of Otala that Phil Allison gave, and I think you were as impressed by it as I was (although it looked bad to PA). When a man who was as talented, knowledgeable, and honest as Otala produces a PROOF that negative feedback *always* transforms the amplitude distortions of the open loop into phase smears of the closed loop, we should not dismiss it as simply a problem of "old devices," a problem that a slight increase in speed can make go away, especially when the discussion that led to his analysis (the Audio Critic BS session) included a lot of talk about vacuum tubes, and how their speed advantage made it easier to use them with feedback. We really should do at least one or two tests before dismissing his conclusions. Again, I will try to do so, but if you want to know anytime soon, you should probably rely on someone else. Phil |
#24
Posted to rec.audio.tubes
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Explanation still required for triode superiority
"Andre Jute" wrote in message ups.com...
My point is a little more subtle. I see an effect which looks like NFB happening between the terminals of a triode; I'm happy to call it internal or natural NFB until someone else offers an explanation that justifies giving it another name; no one offers such an explanation, merely negative objections to naming it NFB; until they pull their finger from their arse and say something positive (and more cogent) than Patrick, who *does* have a case, I shall not change my mind just because they're "agin". The effect you see is the change in plate current with plate voltage. This is exactly the definition of plate resistance, which is both the name and the explanation that have been in standard use for the past seven decades or so. In actual fact there is no external observation you can make of the triode that proves, or even suggests, there is internal negative feedback at work inside the tube. Everything you see can be more simply and economically explained by the Thevenin or Norton models. Thevenin, Norton, and the NFB conjecture are all just fictional models that help to describe or predict the tube's external behavior. None of them represents the actual physical processes at work inside the tube, and it is a mistake to claim otherwise. The Child-Langmuir equation, which is based on the distribution of electric fields and charge density inside the tube is another model, and arguably as good an explanation of tube behavior as we need. There's nothing in it that depends on negative feedback, either. This is fundamentally an engineering debate and the arguments are necessarily framed in engineering terms. The fact that technical language is used to express a point doesn't make the point true by default. The assumptions and inferences have to be grounded in fact and logic to be valid. The challenge is to define who's qualified to decide what is factual and logical in an engineering argument. An engineer perhaps? I would be delighted to debate this subject, point by point, in front of a qualified and neutral judge (or panel of judges) from the tube audio engineering community. How about Steve Bench, John Atwood, Lynn Olson, or Morgan Jones? -Henry |
#25
Posted to rec.audio.tubes
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Explanation still required for triode superiority
Phil wrote: Andre Jute wrote: In an effort to be agreeable, I tried hard to give you negative feedback inside the tube as an explanation of the overwhelming superiority of triodes (or trioded pentodes) for audio reproduction, among other reasons because NFB is accessible to many who belong on RAT and is a genetic deformity of the silicon scum whose only purpose on RAT is dissension. NFB is what the silicon slime abuse to make their inadequate components sound passable, and what even tubies inspired by the age of sophisters and cost-accountants use to linearize pentodes. NFB thus has a base level of familiarity which gives it a head start in any black box model intended to explain something to diplomaed quarterwits among the silicon slime as well as the better-educated kibbitzers in my own camp. But fine, you want to reject my explanation, then you must offer a better reason to explain why triodes are such superior amplification devices to anything else, so much more pleasing to the ear, so much more accurate to the cultivated taste. Despite my cracks about the metaphysics of tubes, there *has* to be an electrical reason for the superiority of triodes. But sure, all kinds of input is welcome. Andre Jute Andre, I think you are too quick to dismiss the idea that feedback transforms amplitude distortions into phase-smearing, like Otala claimed in his talk/paper (assuming a paper ever followed the talk!). When Patrick was defending feedback, he mentioned that poorly designed amps sound like crap when they have lots of feedback, but that if you fix them up a bit, meaning get rid of much of their excessive nonlinearities -- read, *amplitude* nonlinearities -- then adding feedback sounds okay. Now, if we eliminate the idea that feedback produces random noise -- and we *know* that it reduces amplitude non-linearities -- then the only distortion mechanism left, I believe, is phase-shifting. However, I want to describe the usual "constant 20 degrees phase lag at 40 KHz" as "phase-shifting," and a dynamic, microsecond to microsecond shifting back and forth of one frequency relative to another as "phase-smearing," or "time-smearing." What Otala was saying is that applying feedback to circuits with lots of open-loop distortions, which are (I believe) almost always amplitude distortions, converts these distortions into a back and forth smearing of the high frequencies relative to the low frequencies (and it may smear both in terms of the delay through the amp). Nor can we assume that this time-smearing is a simple function of the low frequency amplitude, because it is probably proportional to the magnitude of the distortions, as well as the LF amplitude: VERY non-musical. When enough feedback is applied to badly designed amps, the amplitude distortions become quite small, so why did Patrick find that they sounded much worse than the same amp sounds when touched up enough to reduce the larger open-loop amplitude distortions? As you say, *something* is wrong, and if it isn't high amplitude distortions (it can't be), and if feedback doesn't produce spurious noises, then the only thing left is exactly what Otala said, time-smearing. In essence, feedback *connects* two things that are normally separate in an amp, namely amplitude distortions, and phase-smearing. It achieves a balance between these two, a balance which is determined by the speed of the amp, the amount of feedback, and the amount of amplitude distortion. Contrary to what you say, Otala was *not* referring to TIM, and transistors did not become so much faster after 1980 than the ones used in his '73 article to make the problem he described go away. Yes, the amps had to be designed well enough to avoid TIM, but that was not a real problem even in '73 *if* you knew what you were doing. The problem he described in '80 was quite different, and even high MHz tubes are subject to it. The interesting things, assuming that feedback problems are indeed time-smearing (regardless of whether this comes from the conversion of amplitude distortions), are one, a single tone will reveal nothing of this, giving very low THD numbers, and two, multiple tones should show something, although looking at it in the amplitude realm will only show higher than expected IMD. There should be a fairly easy way to test to see if this really produces time-smearing. In general, we put a 4 volt 60 Hz signal and a 10 mV 20 KHz signal into an amp with lots of feedback, preferably using non-linear sections of the amplifying devices (say, use two 12AX7 type triodes where the plate curves vary from widely spaced to closely spaced, and use about 60 to 80 dB of feedback to get the overall gain down to 1). Use a high pass filter to see only the 20 KHz signal, and use the 20 KHz signal from the generator to trigger the 'scope. If time-smearing exists, then the 20 KHz signal will appear "fuzzy" on the scope, since it is being shifted back and forth. Repeat without the 60 Hz signal to make sure the time-smearing isn't coming from somewhere else, and then also divide the output from each tube down so that you get the same amplitude output *without* using feedback, and see if the time-smearing goes away. Finally, if possible test over a region where the curves are fairly linear, to see if that also reduces the time-smearing. If so, you have proof that feedback transformed the amplitude distortion into time-smearing. I will try to do this, but I have little time, less energy, and not the best test bench in the world, so it may be a while! If you or someone else both can and wants to try this, I suspect we will get an answer much faster than if I do it. Note that we can use pentodes or transistors, too, but using solid state has the disadvantage of possibly introducing other complications due to the poor quality silicon parasitic capacitances. Also, I am suggesting the use of very different magnitudes as well as frequencies for the two signals, in case equal magnitudes somehow avoids this problem, or at least masks it from this test. And again, contrary to what Patrick said, the idea that *if* this were true, then by now someone would have already tries it, and the results would have become widely know, is just naive. The human race simply isn't that intelligent, at least not yet. An interesting conclusion, assuming this feedback time-smear mechanism exists, is that the output of a feedback amp does *not* match the input when multiple signals exist! However, our normal tests cannot see this, since they tend to focus on one frequency at a time. I believe that Patrick, as well as Otala, said that using local feedback to achieve good open loop linearity, combined with some global feedback, tends to sound pretty good. This makes sense for two reasons, first because although the local feedback will produce phase-smearing, it does so at *very* high speed, which I believe directly reduces the amount of smearing (common sense says that as the speed of devices approaches infinity, feedback becomes "perfect"). Second, the result does contain some mis-match between input and output, even given the high speed of local feedback, but now the global feedback will not only reduce the remaining "normal" amplitude distortions, it should also reduce the time-smearing produced by the local feedback, since this time-smearing will still produce an error signal for the global feedback. Of course, this reduction in time-smearing will itself produce more time-smearing, but it should be a case of 0.1 x 0.1 = 0.01, so the final degree of time-smearing is reduced by "dividing up" the total feedback into local plus global. Finally, you saw the review of Otala that Phil Allison gave, and I think you were as impressed by it as I was (although it looked bad to PA). When a man who was as talented, knowledgeable, and honest as Otala produces a PROOF that negative feedback *always* transforms the amplitude distortions of the open loop into phase smears of the closed loop, we should not dismiss it as simply a problem of "old devices," a problem that a slight increase in speed can make go away, especially when the discussion that led to his analysis (the Audio Critic BS session) included a lot of talk about vacuum tubes, and how their speed advantage made it easier to use them with feedback. We really should do at least one or two tests before dismissing his conclusions. Again, I will try to do so, but if you want to know anytime soon, you should probably rely on someone else. Phil I have seen no evidence of the time smearing you are talking about. The essence of time smearing you speak of when testing say 5kHz with 60Hz present as a larger signal is that the 60Hz affects the devices as a changing reactance load on the devices so that the phase of the 5kHz waves are phase advanced and phase lagged alternatively 60 times persecond. Phase modulation and FM modulation was gained deliberately in reactance tube modulators which exploited the change in gm with Ia in a tube thus shifting the F of an oscillator or the phase of an RF carrier. There is much about thei is old books. But dynamic phase shift of a fraction of a 5kHz wave I have not seen due to dynamic action by a lower F. The NFB reduces ALL artifacts and such phase shifting is reduced in the open loop character of an amp where it allegedly should exist. I repeatedly gave the conditions needed where the application of NFB didn't make any improvement to the sound, and made little difference to the measured THD artifacts especially when weighted for audibibilty, and increased the number of artifacts significantly. Far greater minds than I have spelled it all out in Wireless World years ago. I suggest you read all your local university library archives containing the magazine with its brilliant audio articles between when it first appeared in 1917 to now. ( I assume your local uni isn't full of football magazines in the archives ). I have only read and copied out the audio stuff up to about 1996. In all of this literature on the effects NFB there wasn't much about FB causing dynamic phase shift that I can recall. Perhaps if you read what I read you'll find something I missed. Seriously, methinks you NEED to do some real study. Perhaps you'd like to re-iterate what I said to all about open loop bw, phase shift, and the amount of applied NFB and the amount of open loop THD. The fuzziness you say is observed when viewing the HF wave in a cascaded 12AX7 amp where the gain has been reduced to 1 is hard to believe. A cascaded pair of 12AX7 would have an open loop gain = at least 3,000, and applying FB to reduce gain to 1.0 = 70dB of applied NFB. Any dynamic phase shifting in any signal before FB is applied should be easily visible / measurable / quantifiable before NFB is connected, but after a reduction of 70dB it would most definately be invisible on a CRO. Do offer my sincerest respects to Mr Otala and ask him to tell you all about what you appear to maybe not understand as well as he may. I suspect you instead remain delighted by the jargon and terminology around the subject rather than staying with the cold hard facts about applied NFB and its effects and the conditions under which it is applied, all of which cannot easily be dealt with without being utterly precise at all times, which seems right considering the books and magazine articles which have been written about NFB so far. Patrick Turner. |
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Explanation still required for triode superiority
"Phil" wrote in message ... Andre, I think you are too quick to dismiss the idea that feedback transforms amplitude distortions into phase-smearing, There is no reliable theoretical or measurable support for this idea. There's no support based on reliable listening tests. In contrast, there are zillions of examples of equipment with tons of negative feedback that has marvelous phase accuracy, both measured and heard. Proven theory says this is exactly as it should be. How thoroughly does something need to be disproved to be abandoned? |
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Explanation still required for triode superiority
Arny Krueger wrote: "Phil" wrote in message ... Andre, I think you are too quick to dismiss the idea that feedback transforms amplitude distortions into phase-smearing, There is no reliable theoretical or measurable support for this idea. There's no support based on reliable listening tests. In contrast, there are zillions of examples of equipment with tons of negative feedback that has marvelous phase accuracy, both measured and heard. Proven theory says this is exactly as it should be. How thoroughly does something need to be disproved to be abandoned? Furthermore it can easily be shown to be yet another myth by the application of mathematics ! Graham |
#28
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Explanation still required for triode superiority
Andre Jute wrote: In an effort to be agreeable, I tried hard to give you negative feedback inside the tube as an explanation of the overwhelming superiority of triodes (or trioded pentodes) for audio reproduction, among other reasons because NFB is accessible to many who belong on RAT and is a genetic deformity of the silicon scum whose only purpose on RAT is dissension. NFB is what the silicon slime abuse to make their inadequate components sound passable, and what even tubies inspired by the age of sophisters and cost-accountants use to linearize pentodes. NFB thus has a base level of familiarity which gives it a head start in any black box model intended to explain something to diplomaed quarterwits among the silicon slime as well as the better-educated kibbitzers in my own camp. But fine, you want to reject my explanation, then you must offer a better reason to explain why triodes are such superior amplification devices to anything else, so much more pleasing to the ear, so much more accurate to the cultivated taste. Despite my cracks about the metaphysics of tubes, there *has* to be an electrical reason for the superiority of triodes. But sure, all kinds of input is welcome. Andre Jute There are now 136 messages in the thread I started with the post above. In all that bandwidth, all that shouting, all that bad temper, no one has taken up the challenge to offer a better reason [than internal negative feedback] to explain why triodes are such superior amplification devices to anything else, so much more pleasing to the ear, so much more accurate to the cultivated taste All that we have seen is that Patrick Turner and I have solidified our faith in our transconductance model substantially and that John Byrns, arriving late to the thread (a wise man to absent himself from this boring round about the mulberry bush), has further emphasized the native appeal of the transconductance model graphically. None of us deny there may be another explanation. We just say that NFB inside the triode seems a logical explanation, and the easiest to assume since we are already familiar with the mechanism and math, and because it so readily relates the triode to the less attractive pentode. That's a whole bunch of persuasive practical reasons already. We don't need faith for our explanation, just the knowledge that it works for us. Against that, you mantra-chanting gaggle of naysayers offer us what precisely? Some basic principles that Plodnick, having regaled us with unlikely tales of being Terman's annointed successor, can't even copy out right and refuses to transform into any useful form, probably because he doesn't know how. That's hardly a productive use of so much bandwidth and time. The depth of the supporting arguments for our position, and the experience backing it, is also very striking when compared to the thin, unnourishing broth of half-digested and, in some cases halfbaked or even plain perverse, theories you're throwing against our solid bulwark. Andre Jute Habit is the nursery of errors. -- Victor Hugo |
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