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Randy Yates Randy Yates is offline
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Default 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?
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Arny Krueger Arny Krueger is offline
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Default 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.


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Chris Hornbeck Chris Hornbeck is offline
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Default 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
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Eeyore Eeyore is offline
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Default 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

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Eeyore Eeyore is offline
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Default 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



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isw isw is offline
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Default 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
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Eeyore Eeyore is offline
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Default 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

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Chris Hornbeck Chris Hornbeck is offline
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Default 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
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Chris Hornbeck Chris Hornbeck is offline
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Default 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
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Damon Hill[_2_] Damon Hill[_2_] is offline
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Default 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

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