Home |
Search |
Today's Posts |
#1
Posted to rec.audio.tech
|
|||
|
|||
The Old "Feedback Is Bad" Lie
In the past we were told that negative feedback in an amplifier (power
amplifier) was bad. I believe the old charge was that it produced excessive "transient intermodulation distortion." Can someone please explain, using as much engineering-speak as necessary (i.e., don't sugar-coat it - assume an audience of electrical engineers) what this was all about? -- % Randy Yates % "Midnight, on the water... %% Fuquay-Varina, NC % I saw... the ocean's daughter." %%% 919-577-9882 % 'Can't Get It Out Of My Head' %%%% % *El Dorado*, Electric Light Orchestra http://www.digitalsignallabs.com |
#2
Posted to rec.audio.tech
|
|||
|
|||
The Old "Feedback Is Bad" Lie
"Randy Yates" wrote in message
In the past we were told that negative feedback in an amplifier (power amplifier) was bad. It is pretty hard to build an amplifier without at least some local negative feedback. Our local Usenet audio group eggspurt on the evils of negative feedback is an Aussie named Trevor Wilson. He understands that it is hard to build an amplifier with at least some local negative feedback, and so in the past he has criticized amplifiers with loop feedback. He represented an Aussie firm named ME that built power amps with allegedly zero loop feedback. Needless to say, they had to jump through some oretty expensive hoops to build amps that way. So, let's clarify this question to stipulate that it was loop feedback that is the purported evil. I believe the old charge was that it produced excessive "transient intermodulation distortion." AKA TIM. Or, as some have called it, slewing-induced distortion. AKA SID. Can someone please explain, using as much engineering-speak as necessary (i.e., don't sugar-coat it - assume an audience of electrical engineers) what this was all about? Transient intermodulation distortion (TIMD) in a feedback amplifier is associated with nonlinearity in the stages that precede the forward-path dominant pole. The forward-path dominant pole is usually introduced into the amplifier's circuitry to render it stable under a wide range of configurations and operations. Transient intermodulation distortion is stimulated by input signals whose slew rate is in excess of that which the amplifier can amplify linearly. Transient intermodulation distortion is not necessarily caused by loop feedback, and engineering design criteria have been established in the literature to avoid it in audio amplifiers with loop feedback. |
#3
Posted to rec.audio.tech
|
|||
|
|||
The Old "Feedback Is Bad" Lie
On Fri, 11 Jan 2008 21:49:20 -0500, Randy Yates
wrote: In the past we were told that negative feedback in an amplifier (power amplifier) was bad. I believe the old charge was that it produced excessive "transient intermodulation distortion." Can someone please explain, using as much engineering-speak as necessary (i.e., don't sugar-coat it - assume an audience of electrical engineers) what this was all about? The classic 1970's description went: an amplifier is modelled as an input transconductance stage driving a capacitor, the value determined by what's necessary for single dominant pole stability, a big-ass gain stage, and followers. The limiting condition is when all the input transconductance stage's current is used in charging and discharging the capacitor, slewing. But the reactive loadline causes distortion effects at lower levels, called, tada, TIM, or various other names. "SID" (slewing induced distortion) was also used. This hasn't magically disappeared as an amplifier design issue, but awareness of the importance of degenerating the transconductance of the input stage, and much better devices, have helped move it off the hot seat. Although it was often framed within the context of an issue of feedback, it never really was. It was really an issue of input stage transconductance in a single dominant pole feedback amplifier. Trimming 2/3 of my response, because I can go on forever, sorry, Much thanks, as always, Chris Hornbeck "There's little that's impossible, but it becomes more complicated if you move between different systems." - Mike Rivers, in another context |
#4
Posted to rec.audio.tech
|
|||
|
|||
The Old "Feedback Is Bad" Lie
Randy Yates wrote: In the past we were told that negative feedback in an amplifier (power amplifier) was bad. I believe the old charge was that it produced excessive "transient intermodulation distortion." Can someone please explain, using as much engineering-speak as necessary (i.e., don't sugar-coat it - assume an audience of electrical engineers) what this was all about? It was about a bunch of idiots talking nonsense. Their success in talking nonsense was so fantastic that they finally progressed to suggesting that sound quality can be improved by having a digital clock on the same circuit as your amplifier and lying rocks on your cables ( Tice Clock and Shakti Stones). The vast majority of commentators on audio matters are completely technically incompetent. Graham |
#5
Posted to rec.audio.tech
|
|||
|
|||
The Old "Feedback Is Bad" Lie
Chris Hornbeck wrote: Randy Yates wrote: In the past we were told that negative feedback in an amplifier (power amplifier) was bad. I believe the old charge was that it produced excessive "transient intermodulation distortion." Can someone please explain, using as much engineering-speak as necessary (i.e., don't sugar-coat it - assume an audience of electrical engineers) what this was all about? The classic 1970's description went: an amplifier is modelled as an input transconductance stage driving a capacitor, the value determined by what's necessary for single dominant pole stability, a big-ass gain stage, and followers. YUK ! Things have improved somewhat since then. Graham |
#6
Posted to rec.audio.tech
|
|||
|
|||
The Old "Feedback Is Bad" Lie
In article , Randy Yates
wrote: In the past we were told that negative feedback in an amplifier (power amplifier) was bad. I believe the old charge was that it produced excessive "transient intermodulation distortion." Can someone please explain, using as much engineering-speak as necessary (i.e., don't sugar-coat it - assume an audience of electrical engineers) what this was all about? If you stick a signal that's beyond its bandwidth capability into an amplifier, that amp will "outrun" its own feedback loop in an attempt to handle the signal (i.e. some stage -- usually the input one -- will either saturate or cut off before the correcting feedback can arrive to prevent that). The result is distortion, but only until the feedback *does* arrive; hence *Transient* Intermodulation Distortion. The solution is to band-limit the signal *before* it gets to the amp, or alternately, to use an amp with a large enough bandwidth. In this kind of situation, an amplifier's bandwidth can vary with signal level, so that an amp may perform fine for low-level signals, but not for high-level ones. Isaac |
#7
Posted to rec.audio.tech
|
|||
|
|||
The Old "Feedback Is Bad" Lie
isw wrote: Randy Yates wrote: In the past we were told that negative feedback in an amplifier (power amplifier) was bad. I believe the old charge was that it produced excessive "transient intermodulation distortion." Can someone please explain, using as much engineering-speak as necessary (i.e., don't sugar-coat it - assume an audience of electrical engineers) what this was all about? If you stick a signal that's beyond its bandwidth capability into an amplifier, Where would you get one of those (signals) ? that amp will "outrun" its own feedback loop in an attempt to handle the signal (i.e. some stage -- usually the input one -- will either saturate or cut off before the correcting feedback can arrive to prevent that). The result is distortion, but only until the feedback *does* arrive; hence *Transient* Intermodulation Distortion. And of course with audio band-limited signals and decent modern circuitry such a thing NEVER happens. National's recent LM4562 and LME49710 family op-amps have no less than 55 MHz gain-bandwidth. Talk about overkill ! Another popular myth dies a death. Graham |
#8
Posted to rec.audio.tech
|
|||
|
|||
The Old "Feedback Is Bad" Lie
On Sat, 12 Jan 2008 04:11:45 +0000, Eeyore
wrote: The classic 1970's description went: an amplifier is modelled as an input transconductance stage driving a capacitor, the value determined by what's necessary for single dominant pole stability, a big-ass gain stage, and followers. YUK ! Things have improved somewhat since then. Well, yeah, I'd hope so. But the previous Governor of my home Snake (in the United Snakes of America) just won the Iowa presidential caucus for his party (the current ruling party) and he doesn't even believe in Darwin's mid-19th century version of evolution. Your glass may be half full; I'm thirsty. "Improved" is local. But on a lighter note, the mindset of considering stage gain and compensating capacitance together in an amplifier design is so ingrained these days (post-Matti Otala) that we take it all for given. Ain't so; somebody did it!, back in the day. I'd be very interested in your current thoughts about intrinsic linearity in general. I've certainly enjoyed reading your various comments over the last several years, which seem to have evolved philosophically into a more and more agnostic viewpoint. Comments? Much thanks, as always, Chris Hornbeck "There's little that's impossible, but it becomes more complicated if you move between different systems." - Mike Rivers, in another context |
#9
Posted to rec.audio.tech
|
|||
|
|||
The Old "Feedback Is Bad" Lie
On Fri, 11 Jan 2008 21:22:06 -0800, isw wrote:
If you stick a signal that's beyond its bandwidth capability into an amplifier, that amp will "outrun" its own feedback loop in an attempt to handle the signal (i.e. some stage -- usually the input one -- will either saturate or cut off before the correcting feedback can arrive to prevent that). The result is distortion, but only until the feedback *does* arrive; hence *Transient* Intermodulation Distortion. The solution is to band-limit the signal *before* it gets to the amp, or alternately, to use an amp with a large enough bandwidth. In this kind of situation, an amplifier's bandwidth can vary with signal level, so that an amp may perform fine for low-level signals, but not for high-level ones. There are three situations, in the modern world, where amplifiers have to deal with high slew rate and/or possibly out of "band" signals, but get no respect: Phono equalizers are fed a rich diet of massively eq'd, and then velocity-sensitively-reproduced impulses. Oh, yeah, they have to play some music, also. Very tough gig. Microphone preamps are another. Insanely large input dynamic range and an expectation for similar performance at all input signal levels. Easy to do if somebody else is signing the checks, but getting easier over the years. And, maybe the most relevant of all to Randy's OP: the D/A convertor summing junction. Insanely large and fast switching impulses must be (literally) integrated by a feedback amplifier of very little or negative closed-loop gain (a condition that can only aggravate the slewing issues of that integrating stage. It's a Beotch. These before-any-possible-bandlimiting amplifiers are the fun 'uns, and here the 1970's heads-up!s are still both relevant and often overlooked. Engineering doesn't change, but it grows - like that. Much thanks, as always, Chris Hornbeck "There's little that's impossible, but it becomes more complicated if you move between different systems." - Mike Rivers, in another context |
#10
Posted to rec.audio.tech
|
|||
|
|||
The Old "Feedback Is Bad" Lie
Chris Hornbeck wrote in
: Trimming 2/3 of my response, because I can go on forever, sorry, That was interesting as far as it went; I wouldn't mind hearing more on the subject (both input stage design and feedback issues). --Damon |
Reply |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
"AKAI", "KURZWEIL", "ROLAND", DVDs and CDs | Audio Opinions |