"Andre Jute" wrote in message
ups.com...
Some meterhead clown demands to know:
What's wrong with feedback?
and another meterhead clown, on being told by Rudy that:
an OPA2604 ... needs a *thousand* times more feedback
demands to know:
So what?
*****
It ain't rocket science. Explanation taken from:
http://members.lycos.co.uk/fiultra/T...mp%20INDEX.htm
*****
CATCH-22
*****
The customer complained that his new suit didn't fit. 'The sleeves are
too short,' he said to the tailor.
'Yes sir,' said the tailor, 'but if you hold your arm just so, at an
angle as if you're drinking tea with your auntie, it will show just the
right amount of cuff.'
The customer tried it. The tailor was right! 'But what about the other
sleeve? It is definitely too short.'
'Just lower your shoulder, sir. Yes, yes, a little more. Put your foot
out so you can lower your shoulder a little more still. Bend your knee.
Yes, that's it. See how beautifully your suitcoat now fits?'
The customer had to admit the tailor was right. 'Wow. But now the leg
of the pants is all twisted around.'
'That's easily fixed,' said the tailor. 'Just point your other toe
westward, sir, and look over your shoulder to where I am holding up the
hand mirror. See? Doesn't that fit beautifully?'
'Yes,' the customer said doubtfully, 'but-'
'Now would sir like to wear his brilliant new suit or shall we wrap
it?'
The customer was too intimidated to argue. He walked out into the
street in his new suit, his arm crooked as if he were drinking tea, his
other shoulder well down over a bent knee with his foot out to the
side, his other foot pointing westwards, his head twisted back between
hunched shoulders as if complaining to God about a cruel fate.
Behind him he heard a boy say to his father. 'Oh, Daddy, look at the
poor twisted cripple.'
'Hush,' the father said. 'Be grateful the poor man found such a
brilliant tailor.'
****
The Ultrafidelista view of Negative Feedback
by Andre Jute
Negative feedback is the paradigm of modern electronic design. It is
mother's milk to an electronics engineer. He learns to say '100dB of
NFB,' in his sleep before he finishes his first week at the most humble
polytechnic. At the great institutions the professor of feedback is the
most honoured man in the department. In Massachusetts and Minnesota the
feedback guru is the most honoured man on the entire campus, equal in
stature to the football coach. When a guru of transistor high fidelity
(and some in tubes) says, 'I studied under Ron,' one doesn't have to
ask which Ron, one just knows it is the holy name of the prophet of
feedback from the Midwest.
Before I even finished the design of the KISS 300B it was forcefully
suggested by a wannabe guru that with only 50dB more gain (about seven
times as much as is likely to be in the actual design) I can apply 50dB
of negative feedback to linearize my amplifier.
**ALL amplifiers utilise NFB. Every single one. Some do it vai the use of
local feedback loops (like in emitter/cathode degeneration). Some use
internal NFB (such as what occurs within a triode). Some use so-called
'nested feedback' loops, whilst others (the vast majority) use Global
feedback systems. Your KISS 300B uses NFB. Lots of it. Perhaps not as much
as a typical SS amp, but it still uses NFB for linearisation. Without it,
the sound would suck. Big time.
Negative feedback, shorthanded as NFB, is the instant response of the
audio engineering fraternity to all ills, real, perceived,
non-existent. They don't even ask if there is a problem, they swing the
club of NFB regardless. NFB has become a reflex axiom of mainstream
audio design. An audio engineer with his negative feedback is like a
policeman who runs out into the street with his stick and starts
beating a confession out of the first housewife he sees. The difference
is that the policeman is relieved of duty to await punishment and the
audio engineer gets away with it. In the case of the policeman it is
unacceptable behaviour, in the case of the audio engineer so much the
expected norm that no one except the ultrafidelista notice. I guess
that if one in ten million audio amplifiers does not have negative
feedback added, it will be a lot...
**ALL amplifiers utilise NFB for linearisation. Every single one. Not one
amplifier, anywhere, uses Zero NFB. None, nada, zip, zero. So, if you want
to re-state your comment, please do so, but do so ACCURATELY. Define what
types of NFB you are speaking of.
No one asked if my KISS Amp requires linearization. The presumption by
all except those already of the ultrafidelista persuasion was that I
would welcome suggestions about A Good Thing.
In the face of such overwhelming acceptance by qualified engineers, why
do we as ultrafidelista not take the same easy path of negative
feedback? Especially considering that superficially NFB is easy to
understand and apply.
How does negative feedback work?
Negative feedback is simply a negative voltage fed back from the output
to the input amplifying device to offset part of the harmonic
distortion which is present as a positive voltage. It costs nothing
except a loss of gain and a few side effects such as phase shift and
possible instability which are well known in the mathematical
literature and more or less easily guarded against depending on the
level of NFB.
**A gross oversimplification. And you know it. There are many methods of
applying NFB. Some methods have minimal side effects.
'Wow!' those meeting NFB for the first time will now say, 'Something
for free! I'll grab some of that for my amp.' Hey, I said it, and I am
a professional intellectual, by definition an infinite skeptic. NFB is
a thing of beauty that will draw you in. It is like an electronic
Marxism which admits of no contrary arguments because it has subsumed
them all into The Holy Measurements. To question The Measurements is to
commit heresy. You need to be of strong mind to resist the
blandishments of such a universal panacea and of strong stomach to
withstand the hysterical assaults of the lesser engineers defending
their holy grail. (And when you do get hold of a superior engineer to
explain NFB to you, you need to be high-domed indeed because suddenly
NFB can turn very intricate.)
Unfortunately NFB doesn't come without a price. It levies a cruel
charge on the perceived quality of the sound. Negative feedback is what
gives all those 'blameless' transistor and big PP tube amps their
chillingly unnatural sound.
**Er, there is another possibility: NFB allows those amplifiers to provide
clean, undistorted sound. Lack (or more correctly: INSUFFICIENT) NFB merely
alloows an amplifier to possess a distrotion characteristic which is enjoyed
by many listeners.
ANECDOTE
Many years ago, I service musical instrument amplifiers (almost always tube
types) on a regular basis. One lead guitarist client was very particular
about his favourite amplifier. It had to produce just the right amount of
distortion. It was an ancient 20 Watt amp, with an inbuilt speaker. The amp
was miked to a large (soild state) PA system. He chose the SS PA system
because it did not distort the sound of his little 20 Watt amp.
/ANECDOTE
Then how did NFB come to be such a panacea in amplifier design?
**Er, because it works. Your KISS 300B amp uses NFB. YOU know that NFB
works.
Your guess is as good as mine. Hi-fi design is not prestige work for
engineers, or highly paid. The most talented and best qualified
engineers go into automobiles or military hardware or big construction
projects or computer design.
**And, guess what? All modern automobiles use NFB systems of some complexity
in their fuel injection systems. Same deal with military control systems.
Like it or not, NFB is pretty much essential to the operation of all
electronic systems. Including your KISS 300B amplifier.
The left-overs design amplifiers in the
time they have to spare from writing up specs for requesting a CE mark
for a new electric kettle. Lemmings storming en masse over a cliff come
to mind; such people don't see the necessity of original thought, or
have the mental equipment for it. The exceptions to this rule are
normally audio enthusiasts in charge of their own small audio
manufactories with niche markets; those who grow larger from this base
follow the mainstream mantra of "mo' NFB give lowa' THD" because the
marketing channels demand it from them if they wish to grow. At this
point they usually cease to offer anything different, only the
exclusivity of a very high price. (I know, because a sub-board I
designed for a supplier to the trade turns up in so many very expensive
amps with so many different big names neatly silkscreened on it... it
strikes me as the sort of detail a real designer, as distinct from a
marketer, would take under his own control.) Those very few makers who
will sell you an ultrafi amp without any NFB operate even tinier shops,
usually one man and a cat, just hanging on.
**They lie. ALL amplifiers utilise NFB for linearisation. Every single one.
Including your KISS 300B amp.
The mechanism by which NFB wrecks your sound
Negative feedback at first acquaintance sounds good enough to take to
bed and cuddle. It isn't. It isn't even as simple as a superficial
acquaintance may suggest. Follow the steps with me, from the theory as
she is received to what arrives at your brain as music:
1. In theory NFB reduces all harmonic distortion equally, without
discrimination. Strictly in theory it does not reshape harmonic
distortion by reducing the most objectionable third and higher order
odd harmonic distortion to a greater extent than the relatively
harmless 2nd harmonic.
**No distortion is harmless. 2nd harmonic *may* be less objectionable to
SOME listeners. It is not harmless. Moreover, NFB reduces odd order
distortion. And even order distortion.
Thus NFB at its theoretically most benign is
already useless in terms of psychoacoustics, as will become clear at
point 4. If you disregard psychoacoustics, as many audio engineers do,
NFB is brilliant in reducing total harmonic distortion to a number as
tiny as you want. You just pile on more NFB.
2. In real life, as distinct from simplified theory, NFB adds artifacts
of its own. Remember, it is a loop. The signal starts at the input and
is amplified by devices until it reaches the output.
**That would depend on the size of the loop. The NFB loop contained with a
triode is small. As is the loop contained with an emitter degeneration
system. As a consequence, the problems tend to be reduced.
From the output a
part of the signal called the negative feedback is fed back to the
input. Here a loop is completed and the combination, less distorted,
reaches the output again, a part of the combination is fed back,
endlessly. The artifacts we want to consider here are created by the
fed-back residue of harmonic distortions adding to both the fundamental
and the distortions already created by the amplifier, then some portion
of the sum of the original and the feedback distortion is fed back
again and added on, until the ooh-ah bird flies up its own fundament.
It looks marginally less disgusting as a recursive mathematical formula
with lots of nested parenthetical parcels of noise being loaded onto
your music. But it is a monkey on the back of your sound, with a
smaller monkey on the back of the first monkey, a still smaller monkey
on the back of the second monkey, and so on ad infinitum. These
additive artifacts are all higher harmonics and the more dominant ones
are all odd.
**Perhaps. What if these artefacts are above (say) 30kHz and below (say)
0.01% of the total? Are they audible? Can you hear less than 0.01% THD,
regardless of the harmonic structure?
Suppose, for the sake of simplicity, a superbly designed
ultrafidelista amp with some second harmonic and zero odd harmonics
before NFB.
**ALL amplifiers utilise NFB. Every single one. Therre is no such thing as
an amplifier which has no NFB.
Add NFB and the second harmonic will be lowered but the
recombinant new loop now contains newly added intermodulation effects
between the fundamental and the residual second harmonic, and that is
third harmonic. In the next cycle a small but nasty dose of fifth
harmonic that wasn't there before is added by interaction between the
still residual second harmonic and reduced newly added third harmonic.
In short, the artifacts NFB adds to the distortion mix are all of the
most harmful kind. But, say the proponents of NFB, so what? Every time
the loop cycles the added artifacts are smaller, even if there are more
of them... The whole affair starts to smell of trying to argue with a
Marxist who simply declares any inconvenient truth 'an anomaly'. (If
this sounds like a mess from which you should run a mile, you have come
to the right conclusion. Start running now. It gets worse.)
**You're making a whole bunch of bad assumptions. Starting with your most
basic, incorrect assumption: That an amplifier exists which does not use
NFB. It ain't so. ALL amplifiers use NFB. It is tthe TYPE and amount of NFB
which varies between amplifiers.
3. We thus arrive at a situation where distortion has been lowered by
NFB but where the most disturbing odd harmonic distortions are still
present to some measure,
**What if those artefacts are below (say) 0.01%?
with the added disadvantage that new and
extremely disturbing artifacts of higher harmonic distortions have been
created by the very process of using negative feedback to lower
distortion. Regardless of the absolute level of THD, or the volume
setting, the mix of harmonics has been adversely affected and now
includes a higher proportion of third and higher harmonics than before
NFB. Let me say that again: after NFB, third and higher harmonics will
make up a greater part of the distortion than before.
**What if those artefacts are below (say) 0.01%?
4. Low volume levels perforce accounts for 99 per cent of audiophile
listening because we all have families or neighbours, and we would like
to keep our ears.
**Cite you evidence to prove this. Additionally, cite the levels that you
are referring to. Is a "low volume level" equivalent to 0.1 Watt, 1 Watt, 10
Watts, or 100 Watts? Please be specific. What is the room size? Speaker
efficiency? The dynamic range of the recordings used?
Unfortunately for the lowest common denominator of
hi-fi designer, the one who specifies NFB as a conditioned response
much like Pavlov's dogs slavered when the bell rang, human physiology
and psycho-acoustic response is such that odd harmonics are
disproportionately more disturbing at lower than at higher listening
levels. This inescapable effect is independent of definition of
'listening level.' At the 110dB in-room SPL (only 14dB louder than an
automatic riveter!) advocated by the already deaf Transient Overload
Elite known on newsgroups as the Borg, this poisonous concoction of
original distortions and NFB recombinant artifacts will be least
disturbing (and soon not heard at all!). At any lower level perceived
interference of this harmonics cocktail with the music will increase in
inverse proportion to the volume level. At low volume levels the
artifacts generated by NFB will by their nature as higher harmonic
distortions be disproportionately far more disturbing. At these normal
listening levels 0.75 per cent of second harmonic distortion may be
below the threshhold of perception for sophisticated listeners, whereas
tiny amounts of third and higher odd harmonic distortions grate.
**How much odd harmonic distortion? 0.01%? More? Less?
And they still use Negative Feedback?
**All amplifiers use NFB. Every single one.
Are they stupid?
**Your own KISS 300B amplifier uses NFB. Does that make you stupid?
No, they are not stupid. Most of them march to the drum of a cost
accountant on whom we wouldn't spit if he were alight. NFB is as cheap
in money terms as it is expensive in terms of perceived quality of
music. We shall come to those who claim to be sympathetic to
high-fidelity but insist on devices which do not work without NFB, who
have another devious answer. Here, meanwhile, for you to keep in mind,
is a single-sentence summary of a complicated interdisciplinary
argument:
The case against NFB is that for 99 per cent of listening the NFB cure
is worse than the disease.
**ALL amplifiers use NFB. Every single one. If you imagine that one exists
which does not use NFB, then present your evidence.
But surely we don't have to do anything so stupid?
It follows from the argument above that ultrafidelista should choose an
intrinsically linear topology and device which does not require added
negative feedback to 'linearize' the output.
**No such device, nor system exists.
The intrinsically linear
device is the thermionic tube in either its triode form or as a pentode
hogtied to work as a triode, which can be a most pleasing alternative
both economically and sonically.
**Incorrect. On several counts. Triodes utilise an internal NFB system to
linearise their operation. Modern BJTs are more linear than triodes. Yes,
MORE linear. Even quite high power BJTs are very, very linear indeed. More
linear than triodes and MUCH more linear than the accused Pentodes. Get with
the programme and look at the spec sheets (run the curves yourself, if you
wish) of (say) the Toshiba 2SC5200. From less than 0.01A all the way past 3
Amps, it demonstates bugger all variation in gain. Let's see a triode (any
triode) come close to that feat. Modern BJTs are phenomenally linear.
Something which most tube nuts seem blissfully unaware of of, since they
lost interestin transistors back in 1970, when single diffused, low
frequency and highly non-linear devices were the only things available. Move
into the 21st Century and you will se a whole new world of very impressive
devices available to the designer.
The topology is often single-ended
operation, chosen also for several other reasons described elsewhere in
these articles, including KISS;
**Of course. Some listeners enjoy distortion. PP reduces distortion. No
downsides. Just less distortion.
if the chosen topology is push-pull
operation, which is more difficult but far from impossible to arrange
without NFB, operation should be specified as Class A1.
**Why? With suitably matched devices (impossible with tubes, I admit), one
need only organise enough bias current to keep operation in the linear part
of the amplification curve. Any more is wasteful.
Inside the
argued case above lies too the overwhelming reason to accept the
potential small disadvantage that may accompany the preferred topology
in comparison to the discarded alternatives. The disadvantage is of
course the potential for a residual second harmonic that measures high
by transistor or NFB tube standards. (Note the word potential. With a
conservatively designed DHT amp the potential problem should not
arise.)
The ultrafidelista, who are as keen on silent amps as anyone else,
accept this small potential difficulty because it is the lesser evil
compared to NFB.
**ALL amplifiers use NFB. Every single one.
Unbelievers (largely unwashed, according to reports)
sneer that ultrafidelista like this approach because of the 'added
euphonics', which is bow-wow techie talk for the warmth a big chunk of
second harmonic lays on a zero negative feedback single-ended
amplifier.
**There is nothing that a SE design can do that a PP design cannot do
cheaper, better and more efficiently.
But competent design can easily reduce the level of second
harmonic to below the level of perception without the need for NFB and
its deleterious after-effects.
**All amplifier use NFB. Every single one.
In any event, it is your amplifier. You
paid for it. You have a right to tune it as you please. The key thing
is to get rid of NFB and to understand why you did it.
**You can't "get rid" of NFB. Particularly in triode amplifiers. It is built
right into the triode. If you "get rid" of the feedback in a triode, you end
up with a pentode. I prefer to call that an abomination.
Can we prove any of this scientifically?
We have already.
**You first need to prove that an amplifier eixists which can operate
without NFB.
All of this is the technical subtext to my longtime
contention that what the ultrafidelista hear and love is not a directly
heated triode sound as is claimed by many enthusiasts but a Class A1,
ZNFB sound. (Admittedly, as we have seen above, the right sound is
virtually guaranteed with a ZNFB DHT SE amp of conservative provenance
but may have to be developed the hard way with more economical or
higher-power contenders.) In comparative ABX tests conducted over a
number of years, I found that professional musicians, certified golden
ears, choose the triode-linked Class A1 PP ZNFB EL34 whenever it is
present in the test over all other contenders including SE 300B and
'blameless' high-NFB silicon.
**Cite please. Cite SPECIFIC amplifiers used. Cite operating conditions.
Science also proceeds by pure reason. Ultrafidelista have long doubted
whether what engineers insist we measure (the absolute level of
distortion, THD) predicts success in audio gear.
**Duh. It is reasonable assumed by most that levels of THD below (say) 0.1%
are pretty much inaudible. Reductions below this level are pretty much
inconsequential. Conversely, levels ABOVE (say) 0.5% are not
inconsequential. They are audible to many listeners.
This is the full
circle, because I have just proven by logical, individually uncontested
steps that what matters, once a certain modest level of silence is
assured to an amplifier, is not the absolute level of disharmonics but
their composition. The same proof demonstrates that a more beneficial
distribution follows instantly from doing without NFB.
**Nope. Your train of logic is utterly and completely flawed. In several
ways:
* ALL amplifiers use NFB. Every single one.
* Amplifiers of different types, can use widely different types of NFB
application. You have been spectacularly vague about the different types of
NFB available to designers.
* Modern transistors (specifically output transistors) are MUCH more linear
than any tube. Including triodes.
But transistor amps won't work at all without NFB!
**NO AMPLIFIER WORKS WITHOUT NFB. Not one.
That is not our problem. Those who choose inefficient speakers and
consequently are forced to accept monstrous amps made possible only by
gigadeciBels of NFB, will receive our sympathy - and the music they
deserve.
**It is extremely costly and difficult to build high efficiency speakers,
which also possess a wide bandwidth and good dispersion characteristics. If
you know of any, then cite them.
Engineering hangers-on of transistor attempts at high fidelity, where
the measure of success is vanishing THD rather than sonic hedonism,
pretend to be enthusiasts for NFB. To make it work for them, they have
attempted to change the rules so that we won't hear what their
treasured NFB does to our sound. They sneer that low level listening,
which 99 per cent of us prefer and where NFB does most to wreck the
sound, is 'easy listening' and therefore not permissible. According to
them we should all be forced to listen at the high volume level which
suits NFB amps, which they call 'realistic'.
**I know of no listeners who are "forced" to do anything.
This is a contemptible
circular argument, only too characteristic of a fascist mentality in a
part of the audiophile spectrum which wants to prescribe their arid
vision without regard for our enjoyment.
We can recommend a good tailor to them. It hurts every time you wear
his suit. No pain, no gain, fellers!
In summary
Almost everyone listens at low level most of the time.
**Prove it.
NFB wrecks
everybody's sound at all levels but most wretchedly at normal listening
levels.
**Even your own KISS 300B amp, which uses NFB? How curious.
We started out with a contemptible circular argument and we
have met another along the way. We can now put both in context:
An 'engineer' who designs an amplifier which does not work perfectly
without negative feedback is like a tailor cutting the suit
incompetently and then demanding that you walk like a cripple to make
it fit, so that everyone can admire the brilliance of your tailor.
Negative feedback is a bodge. That is why it is despicable to the
ultrafidelista.
**ALL amplifiers use NFB for linearisation. Every single one.
--
Trevor Wilson
www.rageaudio.com.au
The Catch-22 of Negative Feedback aka NFB
Linear Mode
