"Ruud Broens" said:


see my upcoming movie: Tweak City
, , starring Kyra


Does it perhaps involve aspirins, for when she 'pretends' having a
headache?


I recommend Mundorf aspirins for critical applications ;-)

--

- Never argue with idiots, they drag you down their level and beat you with experience. -

"Sander deWaal" wrote in message
...
: "Ruud Broens" said:
:
:
: see my upcoming movie: Tweak City
: , , starring Kyra
:
:
: Does it perhaps involve aspirins, for when she 'pretends' having a
: headache?
:
:
: I recommend Mundorf aspirins for critical applications ;-)
:
: --
:
: - Never argue with idiots, they drag you down their level and beat you with
experience. -

I agree,
that could make of the difference between
knightly & dayly

Ry

Patrick Turner wrote:
Andre Jute wrote:

I must say, Wilson, I don't understand why you're getting your knickers
in a knot. You're like those bolshies I used to know who claimed that
anyone who wasn't willing to kill for "socialism" was only play-acting.
Are you really claiming that someone who merely uses the NFB he finds
lurking in DHTs can't belong to the club until he repents and uses
excessive amounts of loop NFB? Are you really claiming that the use of
lots and lots of NFB is a prerequisite for good audio design? Are you
really claiming that because a little NFB is a good thing, an unlimited
amount must therefore be better, and an infinite amount best of all?
Seems a bit immoderate to me, old chap.

You will find yourself arguing at a man who has a head full of solid concrete.
Trev will never see the whole picture, and has never designed or built any amp in
the last 30 years.

He's mindless, for sure. All those mindlessly insistent claims that I
said something I didn't say -- right next to the place where I said
exactly the opposite.

I try not to bother arguing the same old tired BS time after time.
He just likes hanging out in news groups and Bull****ting.
He has NEVER once contributed a single article of constructive advice on tubecraft
at r.a.t.

Doesn't surprise me.

Your hysterical belief that I am "against" NFB is the product of your
unsophisticated literal-mindedness. There is no reason for NFB to be an
act of faith, like an on-off switch. For the record, quite contrary to
your silly claims about what I said, I believe the little NFB that
occurs naturally in triodes and in certain conservatively sanctioned
traditional topologies are A Good Thing. To avoid giving you another
apoplectic fit, I shan't repeat what I think of the excessive amounts
of NFB required to make transistors work at all.

If you think you can write a more compelling argument than I can about
the evils of excessive NFB, have at it. But every time I see you, like
the other techies you merely kibbitz what better men have written.

I am now doing 845 amps, two per channel for 50 watts each.
I expect music to be real fine. Damping factor good, noise low,
bandwidth wide, distortions negligible and maybe I can add 6db of global NFB but
I doubt any difference due to some **added** NFB will be heard.

You probably won't need the NFB. The biggest problem with 845 is not
deciding how much NFB to use but steeling yourself to let them out of
the door. 845 are God's own tube. I really hesitated when I needed the
space, wondering if I shouldn't keep the simple SET 845 rather than my
Millennium's End SV572-xx amps.

Trev has never done anything like this and simply doesn't understand.

It's not so difficult to open your mind to new experiences and new
ideas. But first one has to stop being a smartarse, and that I don't
think poor old Trevor can manage.

With 845, just no need for the 106dB of added NFB used in SS amps.
I don't care a bit about what Trev says.

I'm fast reaching the same conclusion.

Patrick Turner.

Andre Jute

Stewart Pinkerton wrote:
On Fri, 24 Mar 2006 17:57:12 +0100, "Ruud Broens"
wrote:


"Stewart Pinkerton" wrote in message
.. .
: On Fri, 24 Mar 2006 12:59:37 +0100, "Ruud Broens"
: wrote:
:
: "Arny Krueger" wrote in message
: ...
: : "Ruud Broens" wrote in message
: :
: :
: : we've been through this 'linearity contest' before, here.
: : whereas you define it in terms of current amplification,
: : i define it in terms of voltage amplification.
: : as real-world interfacing in audio is with *voltages*,
: : doesn't seem unreasonable...
: :
: :
: : Negative feedback is generally used to make amplifiers more linear in the
: : voltage domain. It's a highly sucessful strategy. We sent men to the moon,
: : partially based on amplifiers that were linearized with negative feedback.
: :
: : Where's the beef?
: :
: they 'beef' is, your banana needs 1K more straightening' than my banana -
: why keep claiming it is "superior" ?
:
: Because what matters is the output of the *final* circuit - the banana
: is perfectly straight in the opamp, but your tube circuit is still
: bendy.

yeah, right, as in 0.007 % distortion kinda bendy.
must be what they call 'the scottish bend',
then

Be more specific about the tubed circuit you claim has this level of
distortion.

--

It's there in the RAT thread Hybrid circuit-CQ something or other.

If you think you can write a more compelling argument than I can about
the evils of excessive NFB, have at it. But every time I see you, like
the other techies you merely kibbitz what better men have written.

I am now doing 845 amps, two per channel for 50 watts each.
I expect music to be real fine. Damping factor good, noise low,
bandwidth wide, distortions negligible and maybe I can add 6db of global NFB but
I doubt any difference due to some **added** NFB will be heard.

You probably won't need the NFB. The biggest problem with 845 is not
deciding how much NFB to use but steeling yourself to let them out of
the door. 845 are God's own tube. I really hesitated when I needed the
space, wondering if I shouldn't keep the simple SET 845 rather than my
Millennium's End SV572-xx amps.

And I am using KR845.....

I think I'll use sepaarate cathode biasing for each 845, tubes and a trioded EL34
driving a 1:1
IST but with a cap shunting the anode to secondary winding to make sure the
HF transfer is near perfect without the leakage inductance having any effect.
Input will be 6SN7, but maybe could be 6SL7 µ follower



Trev has never done anything like this and simply doesn't understand.

It's not so difficult to open your mind to new experiences and new
ideas. But first one has to stop being a smartarse, and that I don't
think poor old Trevor can manage.

He regards SET amplification as a form of hi-fi illegitimacy.
Calling people *******s for owning and preferring SE amps to his fav brands of solid
state
makes him look a fool.



With 845, just no need for the 106dB of added NFB used in SS amps.
I don't care a bit about what Trev says.

I'm fast reaching the same conclusion.

Patrick Turner.

Andre Jute

A colleague in Sydney is doing an SE amp with 6 x GM70 for 120 watts.
But it will be switchable to PP......its doable, 5 very good switches are needed, and I
did the OPT design.

Whether the GM70 is better than 845 is unknown.

I like challenges. The OPT for 845 has 3 times the turns that my 300 watt PP amps have,
and the wire is thinner for 845, and my eyes are not getting younger....

Patrick Turner,

On 24 Mar 2006 14:01:45 -0800, "Andre Jute" wrote:

Patrick Turner wrote:

I am now doing 845 amps, two per channel for 50 watts each.
I expect music to be real fine. Damping factor good, noise low,
bandwidth wide, distortions negligible and maybe I can add 6db of global NFB but
I doubt any difference due to some **added** NFB will be heard.

Depends on the speakers, the reduced output impedance may be useful.

You probably won't need the NFB. The biggest problem with 845 is not
deciding how much NFB to use but steeling yourself to let them out of
the door. 845 are God's own tube. I really hesitated when I needed the
space, wondering if I shouldn't keep the simple SET 845 rather than my
Millennium's End SV572-xx amps.

It's an excellent tube, as tubes go, but of course we've had seventy
years of progress since then.......

Trev has never done anything like this and simply doesn't understand.

It's not so difficult to open your mind to new experiences and new
ideas. But first one has to stop being a smartarse, and that I don't
think poor old Trevor can manage.

You of course would be the master of this art - except that your arse
is the smartest part of you.

With 845, just no need for the 106dB of added NFB used in SS amps.
I don't care a bit about what Trev says.

With SS, no need for the massive cost of the output transformer for 50
watts of single-ended tube amplification, plus of course you get much
better linearity than even the mighty 845 can manage. A really good
60-watt SS amp is very easy to implement these days - and you can buy
one for the cost of a pair of those OPTs...............

--

Stewart Pinkerton | Music is Art - Audio is Engineering

Posted Via Usenet.com Premium Usenet Newsgroup Services
----------------------------------------------------------
** SPEED ** RETENTION ** COMPLETION ** ANONYMITY **
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"Bret Ludwig" wrote in message
oups.com
Ruud Broens wrote:

snip


The opamp is *designed* to use large amounts of
linearising
feedback, something that simply isn't an option with
the inherently much lower open-loop gain of tubes.

Philbrick and Julie were building perfectly proper op
amps with tubes as early as 1943, but thanks for playing.

Actually, tubed operational amps, especially the early ones (the specific
ones you're mentioning), were horrors. If the noise and drift didn't get
you, the short parts life would.

Operational amplifiers were originally designed as parts for analog
computers. When SS op amps for analog computers became commercial, analog
computer experts sighed a huge sigh of relief. They were stable, reliable,
and quiet.

John Deans wrote:

"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.

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...

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.

'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.

Then how did NFB come to be such a panacea in amplifier design?
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. 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.

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. 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. 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. Suppose, for the sake of simplicity, a superbly designed
ultrafidelista amp with some second harmonic and zero odd harmonics
before 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.)

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, 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.

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. 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.

And they still use Negative Feedback? Are they 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.

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. 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. The topology is often single-ended
operation, chosen also for several other reasons described elsewhere in
these articles, including KISS; 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. 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. 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. 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. 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.

Can we prove any of this scientifically?
We have already. 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.

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. 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.

But transistor amps won't work at all without NFB!
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.

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'. 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. NFB wrecks
everybody's sound at all levels but most wretchedly at normal listening
levels. 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.

*******
More Zero Negative Feedback amplifiers at Jute on Amps
http://members.lycos.co.uk/fiultra/JUTE%20ON%20AMPS.htm
The KISS Amp project, which explains much more, is here
http://members.lycos.co.uk/fiultra/T...mp%20INDEX.htm
and its schematics and transfer curves are here
http://members.lycos.co.uk/fiultra/KISS%20190.htm

********
THE VOLTAGES IN THIS AMP WILL KILL YOU.
GET EXPERIENCED SUPERVISION IF IT IS YOUR FIRST TUBE AMP

All text and illustration is Copyright © Andre Jute 2001, 2004, 2006
and may not be reproduced except in the thread KISS xxx on
rec.audio.tubes

Bunk. JLS

John Deans wrote:

"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.

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...

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.

'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.

Then how did NFB come to be such a panacea in amplifier design?
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. 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.

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. 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. 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. Suppose, for the sake of simplicity, a superbly designed
ultrafidelista amp with some second harmonic and zero odd harmonics
before 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.)

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, 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.

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. 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.

And they still use Negative Feedback? Are they 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.

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. 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. The topology is often single-ended
operation, chosen also for several other reasons described elsewhere in
these articles, including KISS; 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. 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. 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. 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. 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.

Can we prove any of this scientifically?
We have already. 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.

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. 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.

But transistor amps won't work at all without NFB!
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.

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'. 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. NFB wrecks
everybody's sound at all levels but most wretchedly at normal listening
levels. 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.

*******
More Zero Negative Feedback amplifiers at Jute on Amps
http://members.lycos.co.uk/fiultra/JUTE%20ON%20AMPS.htm
The KISS Amp project, which explains much more, is here
http://members.lycos.co.uk/fiultra/T...mp%20INDEX.htm
and its schematics and transfer curves are here
http://members.lycos.co.uk/fiultra/KISS%20190.htm

********
THE VOLTAGES IN THIS AMP WILL KILL YOU.
GET EXPERIENCED SUPERVISION IF IT IS YOUR FIRST TUBE AMP

All text and illustration is Copyright © Andre Jute 2001, 2004, 2006
and may not be reproduced except in the thread KISS xxx on
rec.audio.tubes

Bunk. JLS

Chris Hornbeck wrote

Negative feedback always reduces phase shift.

Except, of course, when it doesn't. Feedback of *any* kind
always reduces the stability of the system. Small signal
open loop amplitude and timing errors are reduced in
proportion to the gain reduction; beyond the limits
where gain is reduced...

Could have done with perhaps another word or two here. It's not the most common
way of putting it...

In general I was impressed with Arnie's succinct and, dare I say, sane
contribution. Of course you are right to point out the constraints. Arnie
doesn't always cross his tees.

In fact I would be happy to adopt a combination of your contributions as a
working definition of "feedback". Anything I can't connect a wire to is
metaphysics.


I bring this up because we all have a mindset that
feedback is an unavoidable necessity in electronics,
especially in larger stages driving loudspeakers.
But, strangely, we have no such mindset concerning
loudspeakers themselves, which have one or two
orders of magnitude larger distortions of all kinds,
(and introduce a few new ones unique to themselves),
yet we never insist on feedback around our loudspeakers.

Some do, surely?

If misapplied it can
cause an amplifier to become less stable, but in fact when properly applied,
negative feedback increases stability.

Sophistry...

What, from Arnie? It's just his definition of stability is a little unstable.

Feedback of any kind always decreases stability.

Thanks. And thanks again.

In the sense that everyone always means the term.

True of "stability", more or less, but sadly not of "feedback".

And, there is no such thing as unconditional stability.
There is only conditional stability, and that only by
constraining circumstances. Life's a bitch; like that.

And again. My life's kind of OK though, I have to admit.

For example, the
gain of both tubes and transistors can be very sensitive to temperature.
With negative feedback, the important parameters of the amplifier are set
by a pair of resistors, whose properties can be made to be very stable and
independent of temperature.

Perhaps not the best example, because vacuum tubes operate
in their own very high temperature environment and were made
to tolerances impossible for semiconductors, buy yeah, a
feedback loop of a pair of resistors is a pretty durned
good reference.

Wot, no compensation? Is that possible?

cheers, Ian