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#1
Posted to rec.audio.tubes
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Distortion products
It seems well known that negative feedback can reduce lower order harmonics, but
increase the higher order ones. Can anyone direct me to references that show how this happens with mathematical analysis? Perhaps a paper in the journal of the AES, or elsewhere; on the web, perhaps? |
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#2
Posted to rec.audio.tubes
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Distortion products
On Apr 1, 4:23*pm, Newbie wrote:
It seems well known that negative feedback can reduce lower order harmonics, but increase the higher order ones. Can anyone direct me to references that show how this happens with mathematical analysis? *Perhaps a paper in the journal of the AES, or elsewhere; on the web, perhaps? Correctly designed NFB reduces all harmonics by linearizing the response. It also reduces all noise generated within the feedback loop. All harmonic generation has a noise equivalent, that is why they are reduced. If the forward gain is A and the feedback gain is B, then the forward gain with NFB is A/(1+AB). A and B are frequency dependent gains (complex variables) but we won't go into that here! If N is the noise out w/o NFB, then with NFB it is 1/(1+AB), i.e. less. NFB also increases bandwidth, but to a limit. NFB also decreases amplifier output impedance - that's a good thing. The only "negative" thing about well designed NFB is the sign of the loop gain! See also http://www.hometheaterhifi.com/volum...october98.html and http://en.wikipedia.org/wiki/Valve_a...back_.28NFB.29 Not an exclusive search.... Hope this helps. Cheers, Roger |
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#3
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Distortion products
"Newbie" It seems well known that negative feedback can reduce lower order harmonics, but increase the higher order ones. ** The operative word here is "can". The missing context is " under what circumstances". Can anyone direct me to references that show how this happens with mathematical analysis? ** How would any "newbie" be any better of with a bunch of incomprehensible math to interpret ( likely wrongly ) instead of gaining an understanding of how amplifiers work ? Perhaps a paper in the journal of the AES, or elsewhere; on the web, perhaps? ** You are some kind of maths freakoid - right ? ...... Phil |
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#4
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Distortion products
On Apr 2, 7:23*am, Newbie wrote:
It seems well known that negative feedback can reduce lower order harmonics, but increase the higher order ones. Can anyone direct me to references that show how this happens with mathematical analysis? *Perhaps a paper in the journal of the AES, or elsewhere; on the web, perhaps? During the last 40 years there have been at least two in depth articles in Wireless World which later became Electronics World. I read all the WW and EW magazines from 1917 to about 1997 and you'll probably find the articles you want to read if you go where there is an archived set of the magazines like I did. In about 1993 I spent days reading in several university library archives to see what had been said about anything to do with audio engineering. Unfortunately I don't have copies of all I read, but the phenomena of increasing distortion spectra after applying NFB is well known, and remains rivetted into my brain cells. The conditions required to "make matters worse" with NFB are about as follows :- The open loop THD distortion of the amp approaches 10% at 2dB below clipping. The frequency of an input test signal is say 1kHz. The open loop bandwidth of the amp is reducing at 6dB/octave above say 1kHz, worst case. The open loop phase shift is high at F above the input signal, but not so high to allow NFB to become positive FB and causing oscillations. A low amount of global NFB is connected, say 10dB. Suppose the THD open loop spectra contains 2H, 3H in declining % amounts in that order. With 10dB NFB applied, and at 2dB below clipping where there is still enough forward gain to drive the signal without clipping, there will be a reduction of 2H, but not as much as one might predict using the formula THD = THD without FB / ( 1 + [A x ß] ) The 2H which appears at the amp output is fed back and intermodulation products of H and 3H are generated because of the nonlinearity of the open loop gain. These add to the mix already there depending on phase and and sure the 2H is reduced, and I don't know what happens to H, but the 3H is only marginally reduced. The 3H when run around the loop generates IMD of 2H and 4H, and these also feed the mix and you get 4H and 6H and so on. These harmonics are called second order harmonic products. Now if the OL phase shift is large and OLG declines after 1kHz then the effective amount of NFB also declines and there are lousy conditions present for reducing THD or any other damn artifacts by NFB. Now if the amount of global NFB is increased a lot, to say 20dB, then the effect of "fixing up the mess" with NFB increases and low numbered H are reduced as theory predicts but second order products become more numerous in number and they might resolve down to very complex composition especially when many signal frequencies are present in music. To get around this problem in SS amps there is usually an enormous total amount of NFB applied. An OP stage might have the output mosfets connected in source follower mode which could be a 20dB application of local NFB and depending on load value. The OP stage might then make only 2% THD at a dB below 100W and the driver stage may make 5%. The total THD will then be approx 5.1%. There may be 52dB of global NFB so we see 0.0127% with GNFB. The OL bandwidth of most SS amps is very high at 100Hz, but above this F we may see the OLG decline at 6dB/ octave or 20dB/decade so that by 10kHz applied GNFB is only +12dB. There is 90 degrees of OL phase lag determined by the capacitor between VAS collector output and VAS base input. Phase shift of 90d is easily corrected while the shift does not exceed 90d, so that the amplifier with GNFB "measures pretty well." If the amp is tested with a 20kHz sine wave 2H is 40kHz, and all the NFB in the world does very little to reduce it because the A reduction is so slight because OLG has become so low. If you examine the characteristics of opamp gain, you'll find out about. Not a huge amount of info is given by makers of audio amps with regard to OLG curves and serious analysis because its such a sad boring story which can only be understood by engineers. In a tubed amp, There may also be 5% THD at 2dB blow clipping, but usually OLG is up to 10kHz before gain tailoring zobel networks reduce gain and reduce the initial F2 pole down a decade and insert a higher F3 pole so that stability with moderate GNFB of say 20dB may be applied. The OL THD of 5% is reduced to 0.5% typically. But where only 10dB of GNFB is applied, there will be second order products which are unresolved to negligible levels and they may be all too audible, bearing in mind that above 100Hz, tones of the same amplitude seem to sound louder as F rises. A 10W SET amp using poor output tube load matching and using 10dB GNFB and which is being forced to use all of its headroom may sound worse than with NFB disconnected. But where the SET is used sensibly with Horn loaded speakers needing only 1/20 of the power, the THD with GNFB is much reduced as *simple* math and theory may predict because initial OLG THD level is less than perhaps 0.8% at 0.5Watts. I find 12dB GNFB is about enough for SET amps so that typically there is 1.3% THD at 10W and 0.2%at 0.5W, and perhaps we may agree that the IMD produced in an amp with predominantly even numbered H products produces a less agonizing sound than the PP amp with odd number H and the same % level at the same PO. In 10W class A and AB PP amps the OL THD might be 1% if triodes are used and at 0.5W OLG THD = 0.2%. The 20dB of GNFB will then reduce the tube amp THD to 0.1% at 10W and 0.02% at 0.5W. The Wiliamson amp is a classic example. There have been other attempts to reduce THD to vanishingly low levels by Professor Ed Cherry with is Nested FB idea, and by the makers of Halcro amps whose schematic does not seem to have entered the public domain where it achieves 0.0001% THD at 20kHz, slightly better than you can do I would bet. In a nutshell, SS amps need all the NFB thay might muster, but tube amps get way with much less. Where OL BW is wide and THD is below 1% at clipping, 10dB NFB is OK, but you'll see artifacts at low levels which were not present on the OLG analysis. After reading the papers I have seen I try to make my amps with OLG giving low THD, low phase shift, wide BW at full power and leave very little work for the GNFB to achieve. Sound is fine with NFB. Even if I knew all the math, I'd not make better tube amplifiers because how good they are depends on my understanding of concepts and simple methods to reduce OLG problems. The amount of GNFB applicable around a tube amp is limited by the phase shifts caused by stray C and L in RC couplings and Miller C and in the OPT. All the math in the world won't make tube amps very much more linear than they have been for the last 50 years. In fact, many tube amps are LESS linear than 50 years ago when many makers tried to use 26dB or even 30dB of GNFB so they could quote sales propelling figures. These days output tubes are run with class B amp loading, only 15dB GNFB, and with speakers which are much less sensitive than in 1960. THD and IMD are rarely ever mentioned. In many ways tube amp design has flowed backwards, and the average THD and IMD being listened to has increased +15dB, and difficulty of servicing increased +10dB, while the need for servicing occurs +7dB more often. I raise my hat to all makers charging such high prices and with much more THD/IMD. They are giving me a better chance to sell my amps. And one other thing. Triodes have internal NFB built into them. The NFB is most effective when Ia change is zero, permitting a single triode connected 6550 to produce 100Vrms at only 1% THD. Professor child wrote about this in one of Terman's hard cover old books written in 1937. Much of the understanding of my father's generation was not widely understood 60 years ago, and the written article have been lost, and not uploaded, and now there is the buffoonery of the short attention span ****ter and Farcebook generation where they agree that history may be safely ignored. Patrick Turner. |
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#5
Posted to rec.audio.tubes
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Distortion products
On Sat, 2 Apr 2011 02:55:54 -0700 (PDT), Patrick Turner
wrote: Much of the understanding of my father's generation was not widely understood 60 years ago, and the written article have been lost, and not uploaded, and now there is the buffoonery of the short attention span ****ter and Farcebook generation where they agree that history may be safely ignored. where I live there are/were a few old used book stores that had all those old tube books, with math my (then) calc was blind to. SOMEONE local here bought all the candy, then dumped them. Too often, a 'estate' just dumps what is left because the people running it don't understand/know/care what the stuff is worth. Years ago, a estate dumped about 500 old Boyds teddy bears and my Mom, a long time collector, bought them for chump change. Value now is about 400%+ of pure profit. I bought as many books I could, got a pile of them, yet there ARE sources of said books in .pdf. Yeah, I know, have to have it in ya hands, I agree, but otherwise, I couldn't find them at all. Check Usenet even, so very often for free. Patrick Turner. Great post, as usual. Thanx... JJTj COMING SOON FRITZMANIA 2011 www.fritztronics.com |
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#6
Posted to rec.audio.tubes
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Distortion products
On Apr 1, 9:23*pm, Newbie wrote:
It seems well known that negative feedback can reduce lower order harmonics, but increase the higher order ones. Can anyone direct me to references that show how this happens with mathematical analysis? *Perhaps a paper in the journal of the AES, or elsewhere; on the web, perhaps? Here's an article to help you distinguish between added-on negative feedback and natural negative feedback. http://www.audio-talk.co.uk/fiultra/...dre%20Jute.htm This article explains why added-on negative feedback is a bad thing in tube audio if you have really good speakers; it focuses on how the higher harmonic artifacts are added. http://www.audio-talk.co.uk/fiultra/...dre%20Jute.htm More articles on Harmonic Distortion on the KISS Amps site, available through those pages. Andre Jute Visit Jute on Amps at http://www.audio-talk.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 |
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#7
Posted to rec.audio.tubes
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Distortion products
On Apr 1, 9:23*pm, Newbie wrote:
It seems well known that negative feedback can reduce lower order harmonics, but increase the higher order ones. Can anyone direct me to references that show how this happens with mathematical analysis? *Perhaps a paper in the journal of the AES, or elsewhere; on the web, perhaps? In most things it pays to start at the beginning and proceed as far as you are interested or able - http://www.ece.ucdavis.edu/courses/n...B/feedback.pdf BTW, it is also "well known" that the heavier an object is, the faster it falls. Of course, it's also untrue. |
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#8
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For an authoritative coverage of the subject, see the text 'Electronic & Radio Engineering', Frederick E. Terman, 4th Edition. All covered well in Chapter 6, in particular around p204 then again in Chapter 10, p 326. Lots of power series stuff. For those not familiar with Terman, he was the head of EE at Stanford U. Among his students were both Hewlett & Packard. Also the Varian Brothers, inventors of the Klystron. Later Schockley, co-inventor of the transistor showed up at Stanford. Many spinoffs followed. With heavy weights like these it is no wonder the SFO Bay area became known as Silicon Valley. You can find most of Terman's textbooks at- http://www.alibris.com/booksearch?mt...ederick+emmons And many others. Terman & his work are often found & referenced in RDH4. I have several of Terman's books. They are a great help in understanding the vacuum tube era. A few years ago I built two simple RC amplifiers for making some comparisons of distortion. Both used the same twin triode, it was a 12AU7 I think. One circuit was connected as a mu-follower while the other was a feedback pair. The feedback pair was adjusted so that its gain was the same as the mu-follower circuit. Both showed low distortion on a THD meter, the HP334A. But when a spectrum analyzer was connected to the feedback pair the new higher order distortion components showed up. These are the distortions that are said to cause discomfort. Cheers to all, John |
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#9
Posted to rec.audio.tubes
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Distortion products
On Sat, 02 Apr 2011 14:38:18 -0500, flipper wrote:
On Sat, 2 Apr 2011 02:55:54 -0700 (PDT), Patrick Turner wrote: On Apr 2, 7:23*am, Newbie wrote: It seems well known that negative feedback can reduce lower order harmonics, but increase the higher order ones. Can anyone direct me to references that show how this happens with mathematical analysis? *Perhaps a paper in the journal of the AES, or elsewhere; on the web, perhaps? During the last 40 years there have been at least two in depth articles in Wireless World which later became Electronics World. I read all the WW and EW magazines from 1917 to about 1997 and you'll probably find the articles you want to read if you go where there is an archived set of the magazines like I did. In about 1993 I spent days reading in several university library archives to see what had been said about anything to do with audio engineering. Unfortunately I don't have copies of all I read, but the phenomena of increasing distortion spectra after applying NFB is well known, and remains rivetted into my brain cells. The conditions required to "make matters worse" with NFB are about as follows :- The only condition required to increase higher order harmonics is NFB. Negative feedback can never make harmonics worse. Feedback can make harmonics worse when the phase shifts far enough to make it positive. d |
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#11
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Distortion products
"flipper the ****wit " Patrick Turner The conditions required to "make matters worse" with NFB are about as follows :- The only condition required to increase higher order harmonics is NFB. ** That is absolute BULL**** !! ..... Phil |
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#12
Posted to rec.audio.tubes
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Distortion products
"flipper the ****wit " That will certainly screw things up but it has nothing to do with the basic principle of NFB always increasing higher order harmonics. Read Baxandall's paper. ** He says nothing of the sort. As usual, you have taken the comment OUT OF CONTEXT . ..... Phil |
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#13
Posted to rec.audio.tubes
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Distortion products
"flipper is reading challenged " Take it from 'the horses mouth' or, at least, one of the well known thoroughbreds http://www.linearaudio.nl/Documents/...p%20design.pdf I believe it's in issue 5 about 80% of the way down. ** What is ?? One of the oldest CHEATS in the world is to claim that proof of one's point is hidden in some massive pile of data somewhere. When it ain't. ..... Phil |
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#14
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Distortion products
"Don Pearce" Read Baxandall's paper. I've read it. I suggest you read it again. ** No matter how many times someone like " flipper " reads that paper - he will continue to misinterpret it. Cos he ignores the context and always takes the literal meaning of words to be the case. Peter B writes well, but at a level that is intended for engineers and the like to read. Not hobbyists and fools like "flipper". ..... Phil |
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#15
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Distortion products
"flipper" "Conclusions ** The "conclusions" are about what ? A little circuit with a single j-fet (a 2N5456) operated at high level and hence much waveform asymmetry. That is all his " conclusions " apply to. ..... Phil |
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#16
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Distortion products
"flipper is reading challenged "
Take it from 'the horses mouth' or, at least, one of the well known thoroughbreds http://www.linearaudio.nl/Documents/...p%20design.pdf I believe it's in issue 5 about 80% of the way down. ** What is ?? One of the oldest CHEATS in the world is to claim that proof of one's point is hidden in some massive pile of data somewhere. I gave the issue number and rough physical location, ** Not nearly enough. You have to show HOW the material proves YOUR point. ..... Phil |
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#17
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flipper is lying his head off
"flipper" "Conclusions ** The "conclusions" are about what ? What was asked for. ** Oh dear - we are not getting any actual answers. A little circuit with a single j-fet (a 2N5456) operated at high level and hence much waveform asymmetry. That is all his " conclusions " apply to. ** ( snip insane drivel) Game over. ..... Phil |
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#18
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Distortion products
"flipper is reading challenged "
One of the oldest CHEATS in the world is to claim that proof of one's point is hidden in some massive pile of data somewhere. I gave the issue number and rough physical location, ** Not nearly enough. You have to show HOW the material proves YOUR point. I already did. ** Peter B. had a point and made it well - back in 1978. Shame it is nothing like the same one "flipper" posted here. ..... Phil |
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#19
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Distortion products
On Apr 2, 9:17*pm, It's that Guy again...
wrote: On Sat, 2 Apr 2011 02:55:54 -0700 (PDT), Patrick Turner wrote: Much of the understanding of my father's generation was not widely understood 60 years ago, and the written article have been lost, and not uploaded, and now there is the buffoonery of the short attention span ****ter and Farcebook generation where they agree that history may be safely ignored. where I live there are/were a few old used book stores that had all those old tube books, with math my (then) calc was blind to. SOMEONE local here bought all the candy, then dumped them. That's exactly what has happened here. Over the last 15 years since the Internet became mainstream, book sales have plummeted and book shops continue to close. Most second hand book stores have also closed. About all of book traders listed all their books, found out what was worth keeping, and I guess the cream went to the rich cats via Internet sales. Where there is charity book fair you won't find any hard cover stuff on electronics written before 1960. What you may find is the vast mountains of books that if read, may reduce your level of education. Too often, a 'estate' just dumps what is left because the people running it don't understand/know/care what the stuff is worth. The worth of much of a person's collected junk over a lifetime may be less than the cost of dumping it. Most older ppl end up with their little hill of junk, and almost nobody under 40 wants any of it. My mother of 94 has a house fulla crap. Lots of stuff I don't want. When she is Asked Upstairs, I will have to dispose of it all after going through it all and I doubt I'll want anything because I already have a house fulla junk. But she has some big furniture items which might be worth a bit.I have no room for them. I am not emotionally attatched any more. But it is possible she might outlive me and my sister, and I won't have to deal with anything. She used to buy books, half read them, and I asked her to give me a few but she couldn't manage that. OK, she's 94, and it does not matter what she can't do, but all those books will end up going through charity fairs on their way to the tip. There are guys I know with a wall-full of shelves with CDs and LPs, and maybe there are 3,000 recordings, all cost about $20, and that's $60,000 total expense. Their sound gear may have cost them $40,000. When they Drop Off The Perch this $100,000 worth of stuff is unlikely to fetch more than $1,000, so its a -40dB decline in value. Some would say I'm worth $500,000, but after cremation and as a small can of rather dodgy fertilizer I will be worth 50c, which is loss in value of -120dB. My other relatives will buy a car from the loot I leave and forget me easily because they don't ever remember me now. The money will just slosh around the system. Years ago, a estate dumped about 500 old Boyds teddy bears and my Mom, a long time collector, bought them for chump change. Value now is about 400%+ of pure profit. I bought as many books I could, got a pile of them, yet there ARE sources of said books in .pdf. *Yeah, I know, have to have it in ya hands, I agree, but otherwise, I couldn't find them at all. I have not looked around for books online, but there might be something which could anseer your questions. I have a couple of shelves of books I bought between 1993 and 2001, but nothing about THD change when you begin adding NFB. Without those books I'd be really dumb. Some might say I still am dumb, but no matter how much anyone thinks he knows, someone around the corner will know more. But for sure the man who says he knows everything knows nothing. The secret to my survival is to know enough about what I am doing. Once you get to a sufficient knowledge level if you increase knowledge by +20dB, your productivity may not even rise +1dB, and maybe you get distracted by wonderment, and you end up doing -3dB less, losing money while fascinated by some damn thing or other.... Patrick Turner. Check Usenet even, so very often for free. Patrick Turner. Great post, as usual. *Thanx... JJTj * * * * * COMING SOON * * * * FRITZMANIA 2011 * * *www.fritztronics.com |
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#20
Posted to rec.audio.tubes
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Distortion products
On Apr 2, 10:35*pm, mike s wrote:
On Apr 1, 9:23*pm, Newbie wrote: It seems well known that negative feedback can reduce lower order harmonics, but increase the higher order ones. Can anyone direct me to references that show how this happens with mathematical analysis? *Perhaps a paper in the journal of the AES, or elsewhere; on the web, perhaps? In most things it pays to start at the beginning and proceed as far as you are interested or able - http://www.ece.ucdavis.edu/courses/n...B/feedback.pdf BTW, it is also "well known" that the heavier an object is, the faster it falls. *Of course, it's also untrue. Ah, and so it is with common sense. But a 250 Watt class A Krell amplifier will definately reach the ground faster than a pillow when both are chucked out of a 10th story penthouse apartment after having an argument with She Who Must Be Given Attention. They say a man can do a lot. But a woman, wow, she can do a whole lot more DAMAGE. Patrick Turner. |
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#21
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flipper is lying his head off
"flipper is a retard " ** Oh dear - we are not getting any actual answers. Game over. ..... Phil |
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#22
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"flipper is a lying ratbag"
** Peter B. had a point and made it well - back in 1978. Shame it is nothing like the same one "flipper" posted here. .... Phil |
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#23
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"flipper is a lying ratbag"
Says the cripple who knows not 'what'. ** Says a vile, anonymous, psycho cretin who puts tube whatsits in wooden boxes. The sooner someone puts him in one - the better. .... Phil |
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#24
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Distortion products
On Apr 3, 5:34*am, flipper wrote:
On Sat, 2 Apr 2011 05:35:07 -0700 (PDT), mike s wrote: On Apr 1, 9:23*pm, Newbie wrote: It seems well known that negative feedback can reduce lower order harmonics, but increase the higher order ones. Can anyone direct me to references that show how this happens with mathematical analysis? *Perhaps a paper in the journal of the AES, or elsewhere; on the web, perhaps? In most things it pays to start at the beginning and proceed as far as you are interested or able - http://www.ece.ucdavis.edu/courses/n...B/feedback.pdf BTW, it is also "well known" that the heavier an object is, the faster it falls. *Of course, it's also untrue. And, apparently, some people don't believe in gravity at all. NFB does, indeed, increase higher order harmonics, depending on the order of the harmonic and how much NFB is applied. Take it from 'the horses mouth' or, at least, one of the well known thoroughbreds http://www.linearaudio.nl/Documents/...p%20design.pdf This adress took me to a 3.3M .pdf file which has an enormous wealth of useful information. There is a 'page 56' about 80% down the scroll.... And there is the article I once read from Wireless World, December, 1978. It has a graph of harmonic product levels for the amount of NFB applied. I believe it's in issue 5 about 80% of the way down. Note that as the order of harmonic increases so does the amount of NFB required to reduce it back to just the non FB level. However, if it's low enough to begin with (and, fortunately, harmonics are usually created in ever decreasing magnitude, at least in Class A) then even the 'increase' is inaudible, not to mention that above some order N the harmonics lie outside the frequency range of human hearing. Or, a Cyrano Jones opined when negotiating the price of Tribbles: Twice nothing is still nothing.- Indeed. Patrick Turner. |
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#25
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Distortion products
On Sat, 02 Apr 2011 20:28:59 -0500, flipper wrote:
On Sat, 02 Apr 2011 20:33:33 GMT, (Don Pearce) wrote: On Sat, 02 Apr 2011 15:21:55 -0500, flipper wrote: On Sat, 02 Apr 2011 19:46:54 GMT, (Don Pearce) wrote: On Sat, 02 Apr 2011 14:38:18 -0500, flipper wrote: On Sat, 2 Apr 2011 02:55:54 -0700 (PDT), Patrick Turner wrote: On Apr 2, 7:23*am, Newbie wrote: It seems well known that negative feedback can reduce lower order harmonics, but increase the higher order ones. Can anyone direct me to references that show how this happens with mathematical analysis? *Perhaps a paper in the journal of the AES, or elsewhere; on the web, perhaps? During the last 40 years there have been at least two in depth articles in Wireless World which later became Electronics World. I read all the WW and EW magazines from 1917 to about 1997 and you'll probably find the articles you want to read if you go where there is an archived set of the magazines like I did. In about 1993 I spent days reading in several university library archives to see what had been said about anything to do with audio engineering. Unfortunately I don't have copies of all I read, but the phenomena of increasing distortion spectra after applying NFB is well known, and remains rivetted into my brain cells. The conditions required to "make matters worse" with NFB are about as follows :- The only condition required to increase higher order harmonics is NFB. Negative feedback can never make harmonics worse. I didn't say it made "harmonics worse." I said it increases *higher order* harmonics. Exactly - in terms of harmonics, increasing means making worse. No, not 'exactly' because you don't distinguish between *which* "harmonics." THD decreases because the reduction in low order harmonics is greater than the increase in high order harmonics. Feedback can make harmonics worse when the phase shifts far enough to make it positive. That will certainly screw things up but it has nothing to do with the basic principle of NFB always increasing higher order harmonics. Read Baxandall's paper. I've read it. I suggest you read it again. I've read it. I suggest you read it again. I quote "Conclusions . . . A small amount of negative feedback (e.g. 6dB) in a single ended stage, though reducing the second-harmonic distortion, and also the total (unweighted) distortion, by about 6dB, will increase the higher-order distortion and the quality of reproduction may well become worse as judged subjectively. . . . The magnitude of harmonics of extremely high order will be increased by the application of negative feedback, no matter what practical amount of negative feedback is employed, but this is of no consequence if, when thus increased, they are, say, 120dB below the fundamental. . . ." He also explains the mechanism producing the 'new' distortion as the modulator/mixer action of the non-linear amplifier, which we know is non-linear or else we would not be wrapping NFB around the thing trying to 'improve' it's linearity, acting on the fed back harmonic(s). To illustrate he uses the 'simple' case of an amplifier with a 'pure' square law transfer function so that it, sans NFB, produces 'only' the 2'd harmonic. Again I quote "Thus while the amplifier without negative feedback gives nothing but second-harmonic distortion on a single sine-wave input, as soon as a little feedback is applied, a third harmonic output appears. This is not the end of the story, however, for this third harmonic, like the second harmonic, gets fed via the B network into the input circuit, where sum and difference signals are again generated. This time the sum products are at f+3f, which gives a fourth order harmonic, and at 2f+3f, which gives a fifth harmonic. Clearly there is theoretically no end to this process - every new harmonic considered, when fed back, gives rise to harmonics of yet higher order." The mechanism has nothing to do with slew rate limiting or lousy bandwidth, and he explicitly explains his analysis presumes the amplifier is flawless in that regard. It's inherent to NFB because the non-linear amplifier is a mixer. Let me put this as few words as the subject will allow. Global NFB is capable - through intermod - of generating tiny amounts of higher harmonics where none existed. And I do mean tiny. These higher harmonics are, in practice always swamped massively by naturally occurring higher harmonics. These higher harmonics respond normally to the application of NFB and are reduced proportionately to the ratio of open loop to closed loop gain. So no, NFB does not cause higher harmonics to increase. d |
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#26
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Distortion products
On Sun, 03 Apr 2011 03:32:50 -0500, flipper wrote:
Let me put this as few words as the subject will allow. Global NFB is capable - through intermod - of generating tiny amounts of higher harmonics where none existed. And I do mean tiny. These higher harmonics are, in practice always swamped massively by naturally occurring higher harmonics. These higher harmonics respond normally to the application of NFB and are reduced proportionately to the ratio of open loop to closed loop gain. So no, NFB does not cause higher harmonics to increase. d I'll be even more brief. Your 'intuition' is incorrect. Read Baxandall's paper. Nicely argued. I clearly stand corrected. d |
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#27
Posted to rec.audio.tubes
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Distortion products
On Sun, 03 Apr 2011 04:46:27 -0500, flipper wrote:
On Sun, 03 Apr 2011 08:39:55 GMT, (Don Pearce) wrote: On Sun, 03 Apr 2011 03:32:50 -0500, flipper wrote: Let me put this as few words as the subject will allow. Global NFB is capable - through intermod - of generating tiny amounts of higher harmonics where none existed. And I do mean tiny. These higher harmonics are, in practice always swamped massively by naturally occurring higher harmonics. These higher harmonics respond normally to the application of NFB and are reduced proportionately to the ratio of open loop to closed loop gain. So no, NFB does not cause higher harmonics to increase. d I'll be even more brief. Your 'intuition' is incorrect. Read Baxandall's paper. Nicely argued. I clearly stand corrected. I already tried 'argument', including quoting his exact words, but you are immune to a rational discussion, and I see no reason to re type the entire paper when the original is just sitting there for you to read, but if you can't handle the prose he also provided a pretty picture of it. And it won't 'go away' just because you chose to ignore what it says. d I've already told you, I know what it says. Copying and pasting a piece of text does not prove that you have understood it - and you clearly haven't. d |
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#28
Posted to rec.audio.tubes
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Distortion products
On Apr 2, 1:16*pm, Andre Jute wrote:
On Apr 1, 9:23*pm, Newbie wrote: It seems well known that negative feedback can reduce lower order harmonics, but increase the higher order ones. Can anyone direct me to references that show how this happens with mathematical analysis? *Perhaps a paper in the journal of the AES, or elsewhere; on the web, perhaps? Here's an article to help you distinguish between added-on negative feedback and natural negative feedback.http://www.audio-talk.co.uk/fiultra/...dre%20Jute.htm This article explains why added-on negative feedback is a bad thing in tube audio if you have really good speakers; it focuses on how the higher harmonic artifacts are added.http://www.audio-talk.co.uk/fiultra/...dre%20Jute.htm More articles on Harmonic Distortion on the KISS Amps site, available through those pages. This in intriguing in that what Jute has re-invented or re-created is pretty much exactly the type of amplifier that Harold Black was faced with in the 1920s when he invented negative feedback. So it would be interesting to hear from someone with such an amplifier as to what happens if negative feedback is employed. Certainly well designed and built valve amplifiers can perform superbly as domestic audio systems without negative feedback. Though building two with exactly matched characteristics for stereo can be a (not unrewarding) challenge. Of course before the signal reaches the domestic system it's been through many other higher performance amplifiers, all of which will employ NFB. My own system has no global NFB in the output stage, but it uses push pull triodes, driven by another pair of triodes, so it's hardly worth it. And I like the "traditional" sound. http://mike.wepoco.com/Home/retro-ge...lity-Amplifier Andre Jute Visit Jute on Amps at *http://www.audio-talk.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 |
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#29
Posted to rec.audio.tubes
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Distortion products
On Apr 3, 12:04*pm, mike s wrote:
On Apr 2, 1:16*pm, Andre Jute wrote: On Apr 1, 9:23*pm, Newbie wrote: It seems well known that negative feedback can reduce lower order harmonics, but increase the higher order ones. Can anyone direct me to references that show how this happens with mathematical analysis? *Perhaps a paper in the journal of the AES, or elsewhere; on the web, perhaps? Here's an article to help you distinguish between added-on negative feedback and natural negative feedback.http://www.audio-talk.co.uk/fiultra/...dre%20Jute.htm This article explains why added-on negative feedback is a bad thing in tube audio if you have really good speakers; it focuses on how the higher harmonic artifacts are added.http://www.audio-talk.co.uk/fiultra/...dre%20Jute.htm More articles on Harmonic Distortion on the KISS Amps site, available through those pages. This in intriguing in that what Jute has re-invented or re-created is pretty much exactly the type of amplifier that Harold Black was faced with in the 1920s when he invented negative feedback. *So it would be interesting to hear from someone with such an amplifier as to what happens if negative feedback is employed. You should be careful, Mike. My T39, ZNFB SE 300B, would have baffled Black and a whole bunch of his students till well after WW2. They would have thought me mad. Check the WE recommended operating conditions. Now check what I do. I call it my HIGH concept. High voltage, high current, high impedance. I even run huge driver current to absolutely murder Miller impedance (http://www.audio-talk.co.uk/ fiultra/KISS%20125%20by%20Andre%20Jute.htm "Using Slew Rate to make Miller work for you"). The only thing I don't run high is the fils, which I starve about 10% further to tilt the distortion artifacts. This entire scheme is to make a very, very silent amp. At http://www.audio-talk.co.uk/fiultra/...dre%20Jute.htm a standard cookbook amp by a couple of morons dumb enough to challenge me is compared to one of my amps. Of course I leave him for dead in total distortion but note that 0.03% 3rd harmonic on my amp against 0.83% 3rd harmonic on the other fellow's wretched noisemaker. Ouch! That is what all my work is about, tailoring the distortion spectrum to put the 3rd and higher harmonics so far under any likelihood of perception that for practical purposes it is non-existent. I doubt Black would have understood or, if he understood, approved. Certainly well designed and built valve amplifiers can perform superbly as domestic audio systems without negative feedback. *Though building two with exactly matched characteristics for stereo can be a (not unrewarding) challenge. *Of course before the signal reaches the domestic system it's been through many other higher performance amplifiers, all of which will employ NFB. We don't mention that last bit here since the time an audiophile got into the tower with a rifle and started picking us off for being "heretics". My own system has no global NFB in the output stage, but it uses push pull triodes, driven by another pair of triodes, so it's hardly worth it. *And I like the "traditional" sound.http://mike.wepoco.com/Home/retro-ge...ers/Building-a... Yes, one of my favourite amps, my Type 113 Triple Threat EL34 Class A PP EL34 (not on my site as it is commercially licensed), is normally run triode-tied, in which format it is impossible even for professional classical musicians to distinguish from an SE 300B; on fact, they normal prefer the faux triode PP amp. Interesting set of schematics you have there. When I first came into tube amp design over twenty years ago, I touched base there as well. Andre Jute Visit Jute on Amps at http://www.audio-talk.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 |
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#30
Posted to rec.audio.tubes
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Distortion products
On 04/02/11 02:55, Patrick Turner so witilly quipped:
On Apr 2, 7:23 am, wrote: It seems well known that negative feedback can reduce lower order harmonics, but increase the higher order ones. Can anyone direct me to references that show how this happens with mathematical analysis? Perhaps a paper in the journal of the AES, or elsewhere; on the web, perhaps? The conditions required to "make matters worse" with NFB are about as follows :- /me adding summaries to main points and clipping details to condense what is otherwise a very detailed and technical discussion of why this happens. The open loop THD distortion of the amp approaches 10% at 2dB below clipping. putting a large number of high order odd harmonics into the signal The frequency of an input test signal is say 1kHz. so plenty of freq response is left for higher order harmonics The open loop bandwidth of the amp is reducing at 6dB/octave above say 1kHz, worst case. so negative FB has less of an effect at higher frequencies The open loop phase shift is high at F above the input signal, but not so high to allow NFB to become positive FB and causing oscillations. so that you get an effect of having a bit of 'positive FB' for very high frequencies, emphasizing the higher order harmonics A low amount of global NFB is connected, say 10dB. minimizing the positive benefit To get around this problem in SS amps there is usually an enormous total amount of NFB applied. yes - typified by an 'op-amp' design which have high open loop gain and high non-linearity to go with it, with output transistors typically biased very close to class B. Large amounts of NFB needed. FET input helps a lot to avoid mismatching forward transconductance and causing oscillation (don't ask me to do the math, it makes my brain hurt). In a tubed amp, There may also be 5% THD at 2dB below clipping partially correctable by NOT using cheap designs/components. But where only 10dB of GNFB is applied, there will be second order products which are unresolved to negligible levels and they may be all too audible, bearing in mind that above 100Hz, tones of the same amplitude seem to sound louder as F rises. 'louder as F rises' - yeah, sort of. At high volumes all tones 'sound about the same' when you compare 'apparent tone volume' to frequency with equal power. But at lower volumes this is true due to the ear's tendency to 'amplify' midrange tones. At higher frequencies this effect reverses, almost like an old POTS telephone. This was actually part of the design of old-style phones, where the 300-3khz range needed to be as clear as possible, and everything else could be filtered out. Human ears usually aren't that bad, but it's the same basic idea. There have been other attempts to reduce THD to vanishingly low levels by Professor Ed Cherry with is Nested FB idea, and by the makers of Halcro amps whose schematic does not seem to have entered the public domain where it achieves 0.0001% THD at 20kHz, slightly better than you can do I would bet. FYI It's unlikely you can NORMALLY hear anything lower than 0.1% for THD, though I believe the perception threshold is somewhat lower for IM distortion. This would also depend a lot on the nature of the distortion, which is why a lot of people prefer tube audio. 'Nuff. In a nutshell, SS amps need all the NFB thay might muster, but tube amps get way with much less. yes After reading the papers I have seen I try to make my amps with OLG giving low THD, low phase shift, wide BW at full power and leave very little work for the GNFB to achieve. Sound is fine with NFB. works for me. the simplest solutions are usually the best. All the math in the world won't make tube amps very much more linear than they have been for the last 50 years. not surprised. In fact, many tube amps are LESS linear than 50 years ago when many makers tried to use 26dB or even 30dB of GNFB so they could quote sales propelling figures. I'd do that much feedback for better quality and then throw in an extra triode preamp to compensate for lower power stage gain. An extra 12AX7 isn't that expensive. These days output tubes are run with class B amp loading, only 15dB GNFB, and with speakers which are much less sensitive than in 1960. 'less sensitive' speakers? I'll have to disagree with you on THIS one. Modern speakers DO use heavier cones with higher gauss magnets, and you'll need multi-way crossovers to make the best use of them. But I'll take a modern JBL-based 3 or 4-way system over a 1960-design system any day. In many ways tube amp design has flowed backwards hmm... maybe. Then again, maybe not. My expertise is mostly in the area of guitar/keyboard/PA amplifiers, and I think many of these are still using the same basic designs they were using in the 1950's, and that's due to a demand for "that sound". You can find the schematic for a Fender Twin on-line. I'm not a fan of how Fender handles certain aspects of their design (the P-P phase spitter being one of them) but in general it's still about like it was 'back then'. I raise my hat to all makers charging such high prices and with much more THD/IMD. They are giving me a better chance to sell my amps. I can understand your position. And one other thing. Triodes have internal NFB built into them. mmmm, ok that's an interesting way to put it. Much of the understanding of my father's generation was not widely understood 60 years ago, and the written article have been lost, and not uploaded, and now there is the buffoonery of the short attention span ****ter and Farcebook generation where they agree that history may be safely ignored. Nice 'hacker names', '****ter' and 'Farcebook'. I'll have to remember them. FYI the oldest popular electronics I've found online is THIS one: http://www.swtpc.com/mholley/Popular...PE_Apr1955.htm |
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#31
Posted to rec.audio.tubes
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Distortion products
On 04/02/11 15:15, Phil Allison so witilly quipped:
"flipper the ****wit" Patrick Turner The conditions required to "make matters worse" with NFB are about as follows :- The only condition required to increase higher order harmonics is NFB. ** That is absolute BULL**** !! uh, did you REALLY read it? |
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#32
Posted to rec.audio.tubes
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Distortion products
On 04/02/11 13:21, flipper so witilly quipped:
On Sat, 02 Apr 2011 19:46:54 GMT, (Don Pearce) wrote: On Sat, 02 Apr 2011 14:38:18 -0500, wrote: On Sat, 2 Apr 2011 02:55:54 -0700 (PDT), Patrick Turner wrote: On Apr 2, 7:23 am, wrote: It seems well known that negative feedback can reduce lower order harmonics, but increase the higher order ones. Can anyone direct me to references that show how this happens with mathematical analysis? Perhaps a paper in the journal of the AES, or elsewhere; on the web, perhaps? During the last 40 years there have been at least two in depth articles in Wireless World which later became Electronics World. I read all the WW and EW magazines from 1917 to about 1997 and you'll probably find the articles you want to read if you go where there is an archived set of the magazines like I did. In about 1993 I spent days reading in several university library archives to see what had been said about anything to do with audio engineering. Unfortunately I don't have copies of all I read, but the phenomena of increasing distortion spectra after applying NFB is well known, and remains rivetted into my brain cells. The conditions required to "make matters worse" with NFB are about as follows :- The only condition required to increase higher order harmonics is NFB. Negative feedback can never make harmonics worse. I didn't say it made "harmonics worse." I said it increases *higher order* harmonics. THD decreases because the reduction in low order harmonics is greater than the increase in high order harmonics. I can believe that high order harmonics would increase RELATIVE TO THD but not in actual power/amplitude as a component of the output. What you describe sounds like negative feedback CAUSING high harmonic distortion, under ALL conditions. If you consider an ideal unity gain amplifier with infinite open loop gain, this could not be the case. Alternately, in the case of a wide bandwidth high open loop gain amplifier with low distortion, and large negative feedback, higher distortion for high harmonics (over open loop) is also highly unlikely. Other cases, as described by Patrick, make sense (and these are driven by specific design inadequacies, phase shift at high freq's and low open loop gain at high freq's being the biggest contributing factors). If you can uniquely identify the cause in the 'ideal' and 'high gain high bandwidth low distortion plus large NFB' case, please let me know. Thanks. Otherwise I'll have to proceed on the basis that overall distortion (including higher harmonics) goes DOWN with sufficient negative feedback. |
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#33
Posted to rec.audio.tubes
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Distortion products
On Mon, 04 Apr 2011 11:24:07 -0700, Big Bad Bob
wrote: On 04/02/11 02:55, Patrick Turner so witilly quipped: You might at least spell wittily correctly - please? d |
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#34
Posted to rec.audio.tubes
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Distortion products
On 04/04/11 12:56, Don Pearce so witilly quipped:
On Mon, 04 Apr 2011 11:24:07 -0700, Big Bad Bob wrote: On 04/02/11 02:55, Patrick Turner so witilly quipped: You might at least spell wittily correctly - please? it's that stupid computer of mine. it can't spell, add, type, or answer my questions without lying to me. stupid computer. |
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#35
Posted to rec.audio.tubes
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Distortion products
On 04/02/11 05:16, Andre Jute so witilly quipped:
This article explains why added-on negative feedback is a bad thing in tube audio if you have really good speakers; it focuses on how the higher harmonic artifacts are added. http://www.audio-talk.co.uk/fiultra/...dre%20Jute.htm don't believe EVERYTHING you read on the intarweb About half of what this guy says is pure opinion. I see no math to back it up. basic 'servo theory' should easily disprove it. |
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#36
Posted to rec.audio.tubes
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Distortion products
On 04/04/11 09:56, Andre Jute so witilly quipped:
At http://www.audio-talk.co.uk/fiultra/...dre%20Jute.htm a standard cookbook amp by a couple of morons dumb enough to challenge me is compared to one of my amps. Of course I leave him for dead in total distortion but note that 0.03% 3rd harmonic on my amp against 0.83% 3rd harmonic on the other fellow's wretched noisemaker. Ouch! That is what all my work is about, tailoring the distortion spectrum to put the 3rd and higher harmonics so far under any likelihood of perception that for practical purposes it is non-existent. I doubt Black would have understood or, if he understood, approved. how's your frequency response? No resonance or phase shift issues I take it? Keep in mind that NFB is used for a LOT of things, reduction of distortion being ONE of those. Also keep in mind that tube characteristics will change over time, and if you want consistent performance for many years of frequent or continuous operation, you'll consider NFB as being one of those things you'll put into your existing amplifiers to deal with it. And I think you'll also find that your overall THD will be even LOWER. By the way - don't forget that a triode tube has a form of 'negative feedback' built into it (grid position with respect to plate and cathode being one of the factors) that limits ideal gain to the 'mu' factor of the triode. Also a self-bias circuit without a bypass cap in the cathode will add NFB, limiting gain to Rp/Rk [I prefer stage gain that's significantly below the mu factor, like a gain of 20 with a mu of 100] and also improving linearity at the same time. So you add a few more pre-amp stages, make sure your input stages use low noise components, add plenty of "global" NFB to give you a nice flat frequency response and low distortion, and the end result should outperform yours without too much trouble, especially at the edge conditions. That's what the really high end units are doing, last I checked. FYI - pump a square wave through your amp (under load), at 50Hz and 10khz, and if it doesn't look 'square' on an o-scope, it's because you don't have any NFB. Phase distortion can be bad, too. Just remember two words: servo theory - With NFB you're amplifying the _DIFFERENCE_ signal between NFB and input by the OL amplification factor, and _NOT_ just 'adding distortion' to the signal. Like an op-amp. Yeah. You can limit the effects of additional 'added distortion' by putting various kinds of negative feedback into the individual stages, and that would be NECESSARY to prevent the conditions you describe ("adding" distortion by distorting the distortion that's part of the feedback). And if you use AB2 bias on P-P output WITH screen taps (which are a form of NFB), your low signal level distortion from the output stage should be VERY low. Anyway, these are all 'the basics' of amp design. |
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#37
Posted to rec.audio.tubes
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Distortion products
On Apr 4, 5:56*pm, Andre Jute wrote:
On Apr 3, 12:04*pm, mike s wrote: ... This in intriguing in that what Jute has re-invented or re-created is pretty much exactly the type of amplifier that Harold Black was faced with in the 1920s when he invented negative feedback. *So it would be interesting to hear from someone with such an amplifier as to what happens if negative feedback is employed. You should be careful, Mike. My T39, ZNFB SE 300B, would have baffled Black and a whole bunch of his students till well after WW2. They would have thought me mad. Check the WE recommended operating conditions. Now check what I do. I call it my HIGH concept. High voltage, high current, high impedance. I even run huge driver current to absolutely murder Miller impedance (http://www.audio-talk.co.uk/ fiultra/KISS%20125%20by%20Andre%20Jute.htm "Using Slew Rate to make Miller work for you"). The only thing I don't run high is the fils, which I starve about 10% further to tilt the distortion artifacts. This entire scheme is to make a very, very silent amp. I did notice that some of the component values were rather "off", but was too polite to comment on it. I expect Black, as an engineer, would have thought us all mad by the old engineering standard of - "An engineer does for a shilling what a fool does for a pound". Starving filaments is interesting. It's another of those things I think about from time to time, but there are always other more pressing things to do. What made me wonder about it is that my preamp uses EF37A (Mullard version of the 6J7) which I still have quite a lot of despite selling many on Ebay. EF37A pentode was also sold as ME1400 electrometer pentode. As ME1400 it was rated with a heater voltage of 4.5V rather than the usual 6.3V but even as a triode the gain is formidable so NFB would seem necessary :-) Athttp://www.audio-talk.co.uk/fiultra/KISS%20114%20by%20Andre%20Jute.htm a standard cookbook amp by a couple of morons dumb enough to challenge me is compared to one of my amps. Of course I leave him for dead in total distortion but note that 0.03% 3rd harmonic on my amp against 0.83% 3rd harmonic on the other fellow's wretched noisemaker. Ouch! That is what all my work is about, tailoring the distortion spectrum to put the 3rd and higher harmonics so far under any likelihood of perception that for practical purposes it is non-existent. I doubt Black would have understood or, if he understood, approved. Certainly well designed and built valve amplifiers can perform superbly as domestic audio systems without negative feedback. *Though building two with exactly matched characteristics for stereo can be a (not unrewarding) challenge. *Of course before the signal reaches the domestic system it's been through many other higher performance amplifiers, all of which will employ NFB. We don't mention that last bit here since the time an audiophile got into the tower with a rifle and started picking us off for being "heretics". My own system has no global NFB in the output stage, but it uses push pull triodes, driven by another pair of triodes, so it's hardly worth it. *And I like the "traditional" sound.http://mike.wepoco.com/Home/retro-ge...ers/Building-a... Yes, one of my favourite amps, my Type 113 Triple Threat EL34 Class A PP EL34 (not on my site as it is commercially licensed), is normally run triode-tied, in which format it is impossible even for professional classical musicians to distinguish from an SE 300B; on fact, they normal prefer the faux triode PP amp. Williamson states that triode strapped KT66 was almost exactly the same as PX25 and since he worked for the manufacturer I expect he was right. Whether my KT44s will perform as well in the long term I don't know, but they're cheap enough to replace at present and I can always switch to 807s if I have a problem. Interesting set of schematics you have there. When I first came into tube amp design over twenty years ago, I touched base there as well. I've acquired most copies of Wireless World from the birth of this amplifier in 1934 to the Williamson and on into the 1950s. They're an enjoyable read on winter evenings and from time to time I'll scan interesting bits and add them to my website. Andre Jute Visit Jute on Amps at *http://www.audio-talk.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 |
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#38
Posted to rec.audio.tubes
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On Apr 4, 9:19*pm, Big Bad Bob BigBadBob-at-mrp3-
wrote: On 04/02/11 05:16, Andre Jute so witilly quipped: This article explains why added-on negative feedback is a bad thing in tube audio if you have really good speakers; it focuses on how the higher harmonic artifacts are added. http://www.audio-talk.co.uk/fiultra/...dre%20Jute.htm don't believe EVERYTHING you read on the intarweb About half of what this guy says is pure opinion. *I see no math to back it up. *basic 'servo theory' should easily disprove it. That's a humorous article, Bubba. The math is elsewhere on the site. But by all means bring on your math. We had a clown here called Henry Pasternack who was last seen running away with his halls in one hand and his MSEE from Stanford in the other. I sincerely hope you're better prepared than he was. |
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#39
Posted to rec.audio.tubes
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Distortion products
"Big Bad Bob" On 04/02/11 15:15, Phil Allison so witilly quipped: "flipper the ****wit" Patrick Turner The conditions required to "make matters worse" with NFB are about as follows :- The only condition required to increase higher order harmonics is NFB. ** That is absolute BULL**** !! uh, did you REALLY read it? ** The comment as presented is absolutely false. ..... Phil |
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#40
Posted to rec.audio.tubes
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Distortion products
On Apr 4, 9:41*pm, Big Bad Bob BigBadBob-at-mrp3-
wrote: On 04/04/11 09:56, Andre Jute so witilly quipped: Athttp://www.audio-talk.co.uk/fiultra/KISS%20114%20by%20Andre%20Jute.htm a standard cookbook amp by a couple of morons dumb enough to challenge me is compared to one of my amps. Of course I leave him for dead in total distortion but note that 0.03% 3rd harmonic on my amp against 0.83% 3rd harmonic on the other fellow's wretched noisemaker. Ouch! That is what all my work is about, tailoring the distortion spectrum to put the 3rd and higher harmonics so far under any likelihood of perception that for practical purposes it is non-existent. I doubt Black would have understood or, if he understood, approved. how's your frequency response? Limited by the iron both as a selfstanding amp and as a booster for my 75W SE/PSE kilovolt transmitter tube amp. *No resonance or phase shift issues I take it? * Why should there be. You're making very large assumptions here, Bubba, when you should be studying the circuit instead. I don't build amps with your assumptions. Keep in mind that NFB is used for a LOT of things, reduction of distortion being ONE of those. * Uh-huh. So what? I'm not interested in those other things. I'm not even interested in reduction of distortion by NFB. I don't need it. Also keep in mind that tube characteristics will change over time, and if you want consistent performance for many years of frequent or continuous operation, you'll consider NFB as being one of those things you'll put into your existing amplifiers to deal with it. * Why? And I think you'll also find that your overall THD will be even LOWER. I'm sure that's important to you. But, d'you see, Bubba, my amp starts out on the blank sheet with the statement, "I don't care **** about THD because I know mine will be lower than yours. What I care about is lowering the proportion of 3rd and higher harmonics. And I know how to do that too without NFB." Now, explain to me again why I need you and your NFB, Bubba. We get a guy like you running in here every few years, knowing better than I do, throwing out wild statements like yours, rote-learned at some third-rate polytechnic. Without fail, they leave with their tails between their legs. By the way - don't forget that a triode tube has a form of 'negative feedback' built into it (grid position with respect to plate and cathode being one of the factors) that limits ideal gain to the 'mu' factor of the triode. * My netsite has only said so for about 20 years. The article you condemn actually says so and refers the reader to more detail on my netsite. You should pay attention, Bubba. Also a self-bias circuit without a bypass cap in the cathode will add NFB, limiting gain to Rp/Rk [I prefer stage gain that's significantly below the mu factor, like a gain of 20 with a mu of 100] and also improving linearity at the same time. * We all have our preferences. I wonder why it is only mine that the clowns come to sneer at. So you add a few more pre-amp stages, make sure your input stages You should learn to read, Bubba. The concept for my amp is KISS. It stands for Keep it Simple, Stupid, and it is a mnemonic for people like you. I don't need more stages because I want only 3.8W out of an 8W tube. You really should calculate up the circuit before you start pronouncing on it, Bubba. use low noise components, add plenty of "global" NFB to give you a nice flat frequency response and low distortion, and the end result should outperform yours without too much trouble, Well then, build it and show us, and then we'll get some musicians in and play your amp and mine behind a curtain and discover which they think sounds more natural. especially at the edge conditions. *That's what the really high end units are doing, last I checked. Who's interested in emulating what everyone else is doing? You're projecting your dreams unto me. You clearly don't know what my equipment consists of. I can afford any amp I want. I build my own because what I want isn't out there. FYI - pump a square wave through your amp (under load), at 50Hz and 10khz, and if it doesn't look 'square' on an o-scope, it's because you don't have any NFB. *Phase distortion can be bad, too. I'm not in the kindergarten class at wherever you teach, Bubba. Just remember two words: *servo theory - With NFB you're amplifying the _DIFFERENCE_ signal between NFB and input by the OL amplification factor, and _NOT_ just 'adding distortion' to the signal. * I'm still not in the kindie class, Bubba. Like an op-amp. *Yeah. * Perhaps you should go away and go live with that op-amp before you suffer an accident to your bluster here on RAT. You can limit the effects of additional 'added distortion' by putting various kinds of negative feedback into the individual stages, Gee. first lots of Global Negative FeedBack, now lots of little monkeys to cling to the back of the big monkey on the back of my sound. Thanks, but no thanks, I already have a chimpanzee for a pet, I don't want these ugly NFB monkeys. and that would be NECESSARY to prevent the conditions you describe ("adding" distortion by distorting the distortion that's part of the feedback). * Good golly, the monkeys on the backs of other monkeys are *compulsory*. Now I've heard everything. Bring back Dumb and Dumber, Arny and Poopie, all is forgiven, there's an even bigger idiot on RAT. And if you use AB2 bias on P-P output WITH screen taps (which are a form of NFB), your low signal level distortion from the output stage should be VERY low. Oh dear. KISS means Keep It Simple, Stupid. The concept is for an SE amp. The concept for every amp in my collection is Class A1 sound. Why don't you take your Class AB2 and stick it where it hurts, and stick your NFB by screen taps the same place. Anyway, these are all 'the basics' of amp design. What an offensive jerk you are, Bubba. WTF makes you think I need you to tell me "the basics"? If you're smart, you'll apologize and shut up. If you're not, I'll destroy your little self-confidence for life. I'm bored with fools coming here and reading me lectures from the depth of their ignorance. Andre Jute Visit Jute on Amps at http://www.audio-talk.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 |
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