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Patrick Turner
 
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Default KISS 123 by Andre Jute: Why the KISS 300B is ZNFB

I feel compelled to make a reply about NFB, but rather than tediously
work down through the main body of the post,
I shall leave it together, and place a reply at the bottom where it repectfully belongs.....

Andre Jute wrote:

KISS 123 by Andre Jute



This text is copyright Andre Jute, 2001, 2004 and may not be reproduced except
in the thread KISS xxx on rec.audio.tubes. If you have arrived late to this
project, you can get an overview at

http://members.lycos.co.uk/fiultra/The KISS Amp INDEX.htm

or with uncooperative browsers at

http://members.lycos.co.uk/fiultra/T...%20INDEX.htmor by finding
a file called KISS 100.



*****



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.'



****



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 suggested 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 it 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 cost 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
those 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 equipment. The exceptions to this rule are normally 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. They don't 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 relative 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
appear to 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, 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 chosen 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 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.



I think a couple of facts about the music we all listen to should be
added to the discussion.

Let us take ourselves back to 1960, when the bulk of recording gear
was tubed.
There were some truly breathtaking performances at that time,
and some of the recordings of a young Joan Sutherland on vinyl that I have would make a
man's heart melt.

On monday afternoons, the ABC Classic FM station broadcasts 'music from the archives'
and I guess the sources are / were from tapes, vinyl, and probably placed onto a digital
medium.
Despite the radio stations processing, many ancient recordings sound as fresh as if they
were made yesterday.

Just about all recordings of any merit were made using all tubed equipment, and a
all that gear had a fair amount of NFB used in the microphone amps, recording tape decks,
mixing desks, cutting head amps et all.

There was considerable possiblity for noise and distortion to enter the signal path
along the way, and one tool used to minimise it was NFB, along with
some adherence to the idea of getting it right before the NFB was added.
Pentode tubes were used throughout much of the 1960 gear, and these always produced
more thd and had poorer spectral content, and as a pentode has a high Ra, NFB was needed
to reduce the Ro of all the stages in the music production chain.
NFB was also used to keep the gains of multiple channels about exactly equal,
rather difficult to do without NFB, since gain varies with tube samples.
Then transformer coupling was also used, so another avenue for the iron caused thd to creep
in.
So its a miracle that the gems of 1960 made it to still be worth listening to.

Some of the material recorded then simply was trash, or mush, and not worth the listen, and
a testament
to getting things wrong.

If one was to add up all the factors influencing the analog sound quality in 1960,
it would be a long, long list, and NFB is but one of the items on that list.

Since we recognize some of the gems from those days as being gems, and we know
considerable NFB was used, then why should we be fanatically against NFB?????
Would that not lead us to conclude that some NFB may be quite OK?

After 1960, the recording industry began in earnest to throw out the ageing tube gear which
was
bulky, expensive to maintain, and costly to power, and which didn't have the bells and
whistles of the new
cool solid state analog gear.

I believe much sound quality took a nose dive at that change.
A typical discrete transistor working with a volt with 15 volt rails is
working as hard as a tube producing 10v with 150v rails, except
that the bjt still manages to produce a whole lot more thd.
But transistors had bags of gain, so a lot more NFB was able to be applied, and so it was,
and then opamps came into use, and these little black grubs such as the 741
had umpteen devices within, and 60 dB of NFB was routinely used in a loop
around them.

This freed engineers to cascade as many opamps as they wished.
100 opamps in series each 0.001% thd will still measure 0.01%
at the end of the daisy chain.
I cannot say how the music will sound after sending it through so many devices
but the engineers would have us believe it'd sound fine, and indistinguishable to
sound passed through a low thd triode, or just one opamp with NFB.

Before I say I believe those engineers are pulling my leg, I'd want the experiment
done with the 100 opamps, so I could hear the results.

In the pop music industry, it has always been routine to add serious distortions of numerous
kinds to all the sound,
since the sound has been compressed, de-essed, equ'd, and many instrument amps
work at over 10% thd all the time, up to 40% thd....
Now there are al these analog and digital effects added....

So talking about zero FB at the end of the chain when considering pop music is a
bit accademic to me, since what does 0.3% of 2H do when 5% of mixed grunge has already
been added?
Its a bit like offering a carriage for a pig to ride in.

Because we are stuck with large amounts of fine recordings made with gear using NFB
I just cannot accept that NFB is universally evil wherever it is applied.

In the case of a 300B, and all triodes, there is already an internal electrostatic NFB loop.

We shouls consider the basic action in a triode.

The grid tries to turn on the plate current by going positive, and more electrons flow to
the anode from the cathode.
The anode responds to the load current and its voltage goes more negative, and this
reduction
of the anode potential tends to cause less current to flow to the anode.
So the grid and the anode itself both affect the electron flow.

If say a +ve distortion voltage appears at the anode, this is also applied to the electron
stream,
and it tends to cause an increase in electron flow, and the increased distortion current
flow
causes a -ve going load voltage, so the +ve going distortion voltage is opposed
by a simultaneously acting current in the triode.

This scheme of feedback is goverened by the laws of nature, and not by
external loop paths in the amp. It is an imperfect loop of applied NFB since even with such
FB
there is still about 4% of mainly 2H at 5 watts from the 300B, and there are other products
there but at a lower level, and in fact if we have good measuring gear,
you'd be surprised how grubby a 300B really is.
But that's near full power, at the lower levels the spectra is more benign,
and at all levels its the 2H which is the highest.
Its grubby until you think about a 6L6 in beam tetrode without NFB or a transistor....

But properly set up, there is no need to add any more FB to a triode amp
than already exists in the tube to start with.
There is a lot about Child's Law concerning electrostatic behaviour in thermionic
devices, but the keen amoungst you take yourself to a library and read the books of the
1930s
which spells out in more detail all I have said about NFB in triode amps.

At the end of the day, the 300B and other triodes make nice enough amplifiers
with the FB within them, and there is no need to gild the lily by adding more NFB.



Feedback networks never entirely remove distortion.
All amplifying devices produce harmonic distortion.
To have distortion reduction, there must be distortion in the first place
so that it can be fed back to the input and amplified to oppose its own production.

Consider the basic amplifier, with an open loop gain ( gain without a NFB loop connected )
of say 10.

Let us suppose we feed back 1/5 of the output signal to one of the two inputs that every
amplifier has. This is usually done with a reistance divider between the output and 0V,
and the junction is where a replica of the output signal exists, but its 1/5 of the
output voltage.

Let's say we have +20v of output voltage.

Then to make that voltage, +2v is needed between the two inputs of the amp,
which in a tube amp is often the grid and cathode of V1, which is working in differential
mode.

+4 volts is the fed back voltage, and in our tube amp's case it is applied to V1 cathode.

So therefore the input voltage to the amp has to be +6v at the input grid, since
the +4v and +2v make a total of 6v.

Since we know all amplifiers have distortion, let us suppose our hypothetical triode example
amp
has +1% thd with NFB and when we measure it at 20v output.

Notice how I have placed a + or - sign in front of the voltages concerned.

The sign indicates the relative phase of the sine wave signal voltage in our amp.

A +ve signal will have a 180 degree phase difference to a -ve signal, ie, the
-ve signal is an exact same version of the +ve signal, its just turned upside down.

Now we know +1% Dn ( distortion ) occurs at our amp's output, so there must be
+0.2% at the feedback input of our amp, due to the voltage divider resistors.

There is none of this Dn in the input signal.

So there is effectively -0.2% Dn appearing between the two input terminals, and its
amplified
10 times by the amp to produce -2% Dn at the output.

This doesn't make any sense, because we measured +1% Dn.

So how come we observe this?

The answer is simple, there had to be +3% of Dn to begin with without the NFB application.

Then when the -2% Dn is applied, it subtracts from the +3% Dn, leaving +1%.

So by applying the NFB, we reduce the Dn from +3% to +1%.

If we had an open loop gain of 100 instead of only 10, the amount of meaured Dn would be a
lot lower,
and I leave you all to work out what the open loop Dn would have been if we measured
0.1% with the above amp, but with a gain of 100.

Now much has been written about the evils of NFB, including quite a lot above this reply.

To add to that, people have said that NFB is "trying to catch the horse after it has
bolted".

But in the case I just hypotheticised the feedback is instantaneous.
The transit time for signals through a well designed triode amp
will result in less than a few degrees of phase lag at 20 kHz.

Some claim the delays in an amp cannot be corrected, but I assure you that
phase delays can be largely corrected by NFB in all but the worst designed amps.

Would someone prefer the phase lagged signal rather than one which had its timing
more accurately reproduced as a result of NFB?

As F rises, the phase lag increases more rapidly, and an F is reached where a fed back
voltage
has a phase which causes oscillations.
Similarly, a phase lead can occur at LF, and oscillations can occur.
This isn't a failure of NFB, its a failure of the human designer to take enough factors into
consideration
so that NFB allows perfect stability regardless of any F or load reactance.

Having mentioned phase delay, or lagging phase, its easy to imagine.

Phase lead is harder, because the phase is advanced ahead of the reference phase of the
input signal.
How can this be?
Does the electrical signal arrive at its destination before it set out on its journey?
We know that's impossible, and its beyond my time allottment to explain that, so off the the
library
with you all if you don't know about phase relationships, and you wanna fiddle with
amplifiers!


I have explained in other posts recently that NFB causes harmonics to appear where they they
didn't
exist before FB was applied.
Andre has explained again above, and its by way of the intermodulation process that
additional spectra are added to the signal when NFB is used.

In the case above of the amp making 20v, the 0.2% Dn is applied to
the amp input and is amplified to subtract from the open loop Dn.
In this process, this 0.2% is also distorted, and that second lot of Dn signal is fed back,
amplified to oppose itself,
and I leave the math experts amoungst you to work out what the exact figures are produced
of "secondary effect" Dn.
The second order products are usually low enough to have not the slightest worry about their
presence
if the Dn without FB was low to begin with.

You see the above 20v amp may well have 1% of 2H in the output,
but there will also be 3,4,5,6,7,8,9H etc if we care to measure well enough
down to the -100 dB level.

The second order added products will often be at similar levels of the existing
non 2H content of our amp, and at such low levels that they are inaudible.



All this talk about NFB leaves me a little puzzled, because
last year 3 others and myself tested several different 6CG7 tubes in an SET preamp,
and recently I had the chance to measure this amp and found the 2H was less than 0.01%
at all levels used on the day.

Yet almost vastly different sonic timbres were heard with the same test recordings.
The sound changes were not due to measurements imho.
The preamp had no loop NFB in its gain stage, but it did have a cathode follower
buffered output, which is a case of consideable NFB, but totally benign, since the CF
would have only had about 0.001% thd, and all mainly 2H.
The best tubes were Telefunken NOS, Mullard next best, and Sovtek the worst.

0.01% thd and the resulting imd is such a tiny signal at a speaker compared to a
1 watt signal level that if the thd/imd could appear at the speaker
without the original wanted signal present, the Dn could not possibly be heard if one works
out all the power and SPL involved.
Everything is relative, let's not forget that.

I cannot conclude that NFB is an evil thing, and that it does the worst damage to music
at the low levels most folks listen at.
I have demoed too many amps to too many people to have a firm view, since
any crowd of audiophiles will say different things to me about a system, and
much of it is in conflict, like some may say the bass is a bit shy, others will say
the bass is really heavy, so is it their hearing, or where they sat to listen?

But thankfully, they wanted to stay to listen all night, and didn't wanna throw me out
of their venues.
I have seen a crowd of them melt away early when PP triode amps using only 6 dB NFB
were used. They were much happier with my amps with half the tube count in UL
and with 16 dB of NFB.

I know that the NFB haters have reasons why they think its a bad thing.
There is a lot there above in print.....
But is it because of subconscious rebellion about the need for corrective efforts?
Does anyone believe in Original Sin, and that when Adam and Eve sinned,
they were damned, and cast out of the Garden, and had to await a Redeemer?
What BS! Must we suffer a godly imposed inferiority complex?
So those who love freedom say there isn't a need for NFB, because the triode hasn't
sinned...
Why must we be obsessed with corrective measures when innocence should be let free from such

measures? Can we not be free of assumed guilt?
Gee, I have heard a few triode amps that did do a bit of sinning, and a bigger dose of NFB
might have improved them.
Anyway, the God Of Triodes put the NFB into triodes, and tonight I will not
argue with a god about what he did when he made the laws of physics.

And if all those amps used in 1960 had not been looped with NFB, maybe you would have
ended up with recordings that were the finest sample of mud.

NFB may well be a "bodge", and I assume that means plastering over the cracks,
bandaiding a cut finger, etc, but in some cases, its the best bodge we've got
where something is needed.

Halcro amps must use so much NFB that they get 0.0001% thd at 200 watts.
The HK audio club said after a Halcro demo, "Ah, Halcro, it like 300B, but go louder.."
I admire their inscrutability.

Patrick Turner.






  #2   Report Post  
Ian Iveson
 
Posts: n/a
Default

"Patrick Turner" wrote

In the case of a 300B, and all triodes, there is already an
internal electrostatic NFB loop.


What you call feedback is a matter of definition, not physics or god
or nature.

The usual definition would not normally include your idea of how a
triode works. If the output is taken from the anode, then the output
is a result of a self-regulating system with the point of dynamic
equilibrium modified by the Vgk input. There is no feedback to the
*input* (Vgk), which is why it would not fall under the usual
definition of a feedback control system.

According to your thinking, a resistor, or a pot, would equally be a
feedback control system.

Until you get your head around this point, the rest is not worth
comment.

cheers, Ian


  #3   Report Post  
Chris Morriss
 
Posts: n/a
Default

In message , Ian Iveson
writes
"Patrick Turner" wrote

In the case of a 300B, and all triodes, there is already an
internal electrostatic NFB loop.


What you call feedback is a matter of definition, not physics or god
or nature.

The usual definition would not normally include your idea of how a
triode works. If the output is taken from the anode, then the output
is a result of a self-regulating system with the point of dynamic
equilibrium modified by the Vgk input. There is no feedback to the
*input* (Vgk), which is why it would not fall under the usual
definition of a feedback control system.

According to your thinking, a resistor, or a pot, would equally be a
feedback control system.

Until you get your head around this point, the rest is not worth
comment.

cheers, Ian



Yes there is NFB. Consider this: The grid of the triode is fed from a
non-zero source impedance. The signal at the anode is inverted. There
is capacitance between the anode and the grid.

OK?

So we have internal capacitive NFB, which rolls of the gain at high
frequencies. Typical, and expected triode behaviour.

So what would you call it?
--
Chris Morriss
  #4   Report Post  
Stewart Pinkerton
 
Posts: n/a
Default

On Mon, 13 Dec 2004 20:03:58 GMT, "Ian Iveson"
wrote:

"Patrick Turner" wrote

In the case of a 300B, and all triodes, there is already an
internal electrostatic NFB loop.


What you call feedback is a matter of definition, not physics or god
or nature.

The usual definition would not normally include your idea of how a
triode works. If the output is taken from the anode, then the output
is a result of a self-regulating system with the point of dynamic
equilibrium modified by the Vgk input. There is no feedback to the
*input* (Vgk), which is why it would not fall under the usual
definition of a feedback control system.

According to your thinking, a resistor, or a pot, would equally be a
feedback control system.

Until you get your head around this point, the rest is not worth
comment.


He has the same problem with cathode/emitter/source followers.
--

Stewart Pinkerton | Music is Art - Audio is Engineering
  #5   Report Post  
Sander deWaal
 
Posts: n/a
Default

Chris Morriss said:

Yes there is NFB. Consider this: The grid of the triode is fed from a
non-zero source impedance. The signal at the anode is inverted. There
is capacitance between the anode and the grid.


OK?


So we have internal capacitive NFB, which rolls of the gain at high
frequencies. Typical, and expected triode behaviour.


So what would you call it?


Miller effect.

--
Sander de Waal
" SOA of a KT88? Sufficient. "


  #6   Report Post  
Chris Morriss
 
Posts: n/a
Default

In message , Sander deWaal
writes
Chris Morriss said:

Yes there is NFB. Consider this: The grid of the triode is fed from a
non-zero source impedance. The signal at the anode is inverted. There
is capacitance between the anode and the grid.


OK?


So we have internal capacitive NFB, which rolls of the gain at high
frequencies. Typical, and expected triode behaviour.


So what would you call it?


Miller effect.


Of course, and the Miller effect has an effect because of............?
(Here's a clue, the words negative, feed and back may just come into it)
--
Chris Morriss
  #7   Report Post  
Sander deWaal
 
Posts: n/a
Default

Chris Morriss said:

Yes there is NFB. Consider this: The grid of the triode is fed from a
non-zero source impedance. The signal at the anode is inverted. There
is capacitance between the anode and the grid.


OK?


So we have internal capacitive NFB, which rolls of the gain at high
frequencies. Typical, and expected triode behaviour.


So what would you call it?


Miller effect.



Of course, and the Miller effect has an effect because of............?
(Here's a clue, the words negative, feed and back may just come into it)



;-))))))))))))))

--
Sander de Waal
" SOA of a KT88? Sufficient. "
  #9   Report Post  
Ian Iveson
 
Posts: n/a
Default

"Chris Morriss" wrote

Of course, and the Miller effect has an effect because
of............? (Here's a clue, the words negative, feed and back
may just come into it)


Now you are being cheeky as well as daft. Source impedance is the
answer to your question. Source impedance is not part of the valve.

The Miller effect is not what Patrick was talking about. For him,
the characteristics of the triode are shaped by what he calls
feedback. He is confusing the effect of the load current on Va, and
hence on the output, even at DC.

You say that

Yes there is NFB. Consider this: The grid of the triode is fed
from a
non-zero source impedance.


The feedback due to the miller effect is independent of the source
impedance, or it would not be a characteristic of the valve. The
*effect* of the feedback does depend on the source impedance.

A triode fed from an effectively zero source impedance would still
display the effect that Patrick is talking about, presumably,
because he doesn't mention source impedance. But there would be no
Miller effect under such external conditions.

I remember when Patrick was mugged and dragged kicking and screaming
into this foolish and misleading notion. He was right and they made
him wrong. I am trying to help him escape. You are muddying the
water.

Read his post and come back to me if you can make sense of it.

cheers, Ian


  #10   Report Post  
Ian Iveson
 
Posts: n/a
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"John Byrns" wrote

... I would not have built the
KISS amplifier with the 300B as you suggested because I don't have
the
parts in my junk box, but this news may inspire me to build a KISS
amplifier using triode strapped EL34s the parts for which I do
have. I
will await your description of the driver and power supply to see
how well
those sections match the parts I have in my junk box


John, you could design any number of equally "good" amplifiers with
your eyes shut.

What's the fawning for?

cheers, Ian




  #11   Report Post  
Stewart Pinkerton
 
Posts: n/a
Default

On Mon, 13 Dec 2004 21:57:20 +0000, Chris Morriss
wrote:

In message , Sander deWaal
writes
Chris Morriss said:

Yes there is NFB. Consider this: The grid of the triode is fed from a
non-zero source impedance. The signal at the anode is inverted. There
is capacitance between the anode and the grid.


OK?


So we have internal capacitive NFB, which rolls of the gain at high
frequencies. Typical, and expected triode behaviour.


So what would you call it?


Miller effect.


Of course, and the Miller effect has an effect because of............?
(Here's a clue, the words negative, feed and back may just come into it)


Quite so, but it's inherent to the device, so should not IMO be
counted as NFB in a circuit sense, since you don't get the loop delay
problems which occur with global feedback. The same argument of course
applies to the cathode/source/emitter follower, since while there is
100% voltage feedback, there is no current feedback whatever. I don't
count this as NFB, simply changing the mode of operation.

--

Stewart Pinkerton | Music is Art - Audio is Engineering
  #12   Report Post  
Patrick Turner
 
Posts: n/a
Default



Chris Morriss wrote:

In message , Ian Iveson
writes
"Patrick Turner" wrote

In the case of a 300B, and all triodes, there is already an
internal electrostatic NFB loop.


What you call feedback is a matter of definition, not physics or god
or nature.

The usual definition would not normally include your idea of how a
triode works. If the output is taken from the anode, then the output
is a result of a self-regulating system with the point of dynamic
equilibrium modified by the Vgk input. There is no feedback to the
*input* (Vgk), which is why it would not fall under the usual
definition of a feedback control system.

According to your thinking, a resistor, or a pot, would equally be a
feedback control system.

Until you get your head around this point, the rest is not worth
comment.

cheers, Ian



Yes there is NFB. Consider this: The grid of the triode is fed from a
non-zero source impedance. The signal at the anode is inverted. There
is capacitance between the anode and the grid.

OK?

So we have internal capacitive NFB, which rolls of the gain at high
frequencies. Typical, and expected triode behaviour.

So what would you call it?
--
Chris Morriss


The feedback that occurs in triodes is not just because of the capacitances
between electrodes
in the triode.

The FB I said occurs at LF, and as the 1930s scholars defined it is due to
electrostatic effects
on the electron stream by anode and grid.

I have to go out right now, it'll take some time to read the latest thread
replies fully....

Patrick Turner.


  #13   Report Post  
John Stewart
 
Posts: n/a
Default

Chris Morriss wrote:

In message , Ian Iveson
writes
"Patrick Turner" wrote

In the case of a 300B, and all triodes, there is already an
internal electrostatic NFB loop.


What you call feedback is a matter of definition, not physics or god
or nature.

The usual definition would not normally include your idea of how a
triode works. If the output is taken from the anode, then the output
is a result of a self-regulating system with the point of dynamic
equilibrium modified by the Vgk input. There is no feedback to the
*input* (Vgk), which is why it would not fall under the usual
definition of a feedback control system.

According to your thinking, a resistor, or a pot, would equally be a
feedback control system.

Until you get your head around this point, the rest is not worth
comment.

cheers, Ian



Yes there is NFB. Consider this: The grid of the triode is fed from a
non-zero source impedance. The signal at the anode is inverted. There
is capacitance between the anode and the grid.

OK?

So we have internal capacitive NFB, which rolls of the gain at high
frequencies. Typical, and expected triode behaviour.

So what would you call it?
--
Chris Morriss


Go To ABSE for the real explanation of NFB in triodes.
There are three pages. The file is about 1000K.

Cheers to all, John Stewart


  #14   Report Post  
Patrick Turner
 
Posts: n/a
Default



Ian Iveson wrote:

"Patrick Turner" wrote

In the case of a 300B, and all triodes, there is already an
internal electrostatic NFB loop.


What you call feedback is a matter of definition, not physics or god
or nature.

The usual definition would not normally include your idea of how a
triode works. If the output is taken from the anode, then the output
is a result of a self-regulating system with the point of dynamic
equilibrium modified by the Vgk input. There is no feedback to the
*input* (Vgk), which is why it would not fall under the usual
definition of a feedback control system.


Child, in Terman's 1937 book describes what he calls "self regulation"
in the triode. Maybe you should read it.

But it appears to me there is an electrostatic divider network between
the output, the anode
and grid, the input, and when there is no load at the anode, there is no
gain, so the gm
is at its maximum value.
But when a very high load is connected, or indeed a CCS, gm = 0.0 A/V
because there is no
current change in the triode with a CCS. That is the effect of the
voltage feedback.




According to your thinking, a resistor, or a pot, would equally be a
feedback control system.


Nope.



Until you get your head around this point, the rest is not worth
comment.


Fair enough.

About 2 years ago, Dennis Assenayev tried to convince everyone thet FB
existed in triodes.
He was such a clumsy teacher, but finally he convinced me.
The full discussion is there in the archives.

Dennis also maintained the FB in triodes was via a non linear transfer
mechanism,
since plate current varies with the voltage taken to the power of 2/3.
I'm not too clear about that, you'd have to check the old books for the
math.
Dennis believed that tubes working in pentode or beam tetrode mode with
CFB windings, like Quad II was
a better way to apply FB locally than rely on the triodic mechanism.
Trouble is then you have the parasitic reactances to worry over.
So one has to wind OPTs with low capaciatnce and leakage.
The screen is then taken to a fixed voltage, so some of the linear FB
is applied to the screen, and this tends to change the spectra of the
outcome towards triodes.
But I digress, we are focused on the FB in triodes, or the lack of it,
if perhaps you are to believed.

Patrick Turner.



cheers, Ian


  #15   Report Post  
Patrick Turner
 
Posts: n/a
Default



Stewart Pinkerton wrote:

On Mon, 13 Dec 2004 20:03:58 GMT, "Ian Iveson"
wrote:

"Patrick Turner" wrote

In the case of a 300B, and all triodes, there is already an
internal electrostatic NFB loop.


What you call feedback is a matter of definition, not physics or god
or nature.

The usual definition would not normally include your idea of how a
triode works. If the output is taken from the anode, then the output
is a result of a self-regulating system with the point of dynamic
equilibrium modified by the Vgk input. There is no feedback to the
*input* (Vgk), which is why it would not fall under the usual
definition of a feedback control system.

According to your thinking, a resistor, or a pot, would equally be a
feedback control system.

Until you get your head around this point, the rest is not worth
comment.


He has the same problem with cathode/emitter/source followers.


I do not have the same problem at all.

EF of SF or CF is a sample of an external loop of series voltage negative
feedback.

The internal FB in a triode only happens in a triode.

They have yet to build an SS device which has the same action as a
triode,
but operating at amps, not milliamps, and which has gain, and which
has an output impedance lower than a speaker load.

Patrick Turner.


--

Stewart Pinkerton | Music is Art - Audio is Engineering




  #16   Report Post  
Patrick Turner
 
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Default



Sander deWaal wrote:

Chris Morriss said:

Yes there is NFB. Consider this: The grid of the triode is fed from a
non-zero source impedance. The signal at the anode is inverted. There
is capacitance between the anode and the grid.


OK?


So we have internal capacitive NFB, which rolls of the gain at high
frequencies. Typical, and expected triode behaviour.


So what would you call it?


Miller effect.


The electrostatic action in triodes acts at DC to HF.
The field effect of a voltage isn't frequency dependant.
capacitance and the field effcts in a triode are two separate identities.

Patrick Turner.



--
Sander de Waal
" SOA of a KT88? Sufficient. "


  #17   Report Post  
Patrick Turner
 
Posts: n/a
Default



Chris Morriss wrote:

In message , Sander deWaal
writes
Chris Morriss said:

Yes there is NFB. Consider this: The grid of the triode is fed from a
non-zero source impedance. The signal at the anode is inverted. There
is capacitance between the anode and the grid.


OK?


So we have internal capacitive NFB, which rolls of the gain at high
frequencies. Typical, and expected triode behaviour.


So what would you call it?


Miller effect.


Of course, and the Miller effect has an effect because of............?
(Here's a clue, the words negative, feed and back may just come into it)


Because there are finite amounts of capacitances in a triode between all the
electrodes,
AC operation at any F does cause charge and discharge currents to flow.

If the triode has increasing current changes due to capacitances and the
presence of AC,
propelled by the signal change in voltages, and you have a CCS supply to the
anode,
what is the structure of the current paths?
There will be current flowing in and out of the grid circuit, the same change
will
be occuring at the cathode, but none at the anode, which has a CCS load.
Add an R load, and it gets much more complicated.
I don't have the time to do your research.

But tubes are primarily voltage devices, and they don't need any current
change to
act as gain devices, like bjts do.

One can further prod your minds by saying that a normal R loaded tube has a
positive RL.

But we can easily arrange for a negative RL, where the -ve going anode has a
reducing
current flow, and the +ve going anode has an increasing current.
Best triode linearity is usually with what is between a positive and negative
RL, ie, a constant current source.
Beam pentode tubes have enormous gain and near constant gm because there is
next to no
electrostatic FB, because the screen shields the electron stream from feeling
the changing force
of the changing anode voltage. So pentodes have more constant gm.
And because next to no FB is present, they have far higher gain, and a lot
more thd.
Same goes for bjts and mosfets, lots of gain, lots of thd, because next to
no internal FB is operating.

Nature didn't present us with semi conducting substances which have a linear
voltage to current
transfer function.

Patrick Turner.






--
Chris Morriss


  #18   Report Post  
Patrick Turner
 
Posts: n/a
Default



Ian Iveson wrote:

"Chris Morriss" wrote

Of course, and the Miller effect has an effect because
of............? (Here's a clue, the words negative, feed and back
may just come into it)


Now you are being cheeky as well as daft. Source impedance is the
answer to your question. Source impedance is not part of the valve.

The Miller effect is not what Patrick was talking about. For him,
the characteristics of the triode are shaped by what he calls
feedback. He is confusing the effect of the load current on Va, and
hence on the output, even at DC.


I am not confused at all, its you.

The current flow in any tube, including a triode does not have any
feedback effect at all.

The anode ***voltage*** changes in a triode have a profound effect on
the current flow.

Think of the triode set up at idle with no input signal.
Apply a +ve signal to the anode from some low impedance source.
The increase in voltage increases the current flow.
The voltage change divided by the current change = the plate resistance
in ohms.
Its usually a lower figure than the load with which is to be used with
the triode.

But with a pentode, the current change is perhaps 20 times less for the
same voltage change at the anode,
and that's because the anode voltage change is prevented from affecting
its own current flow.
So Ra of the pentode is much higher, 12kohms with an EL34, as opposed to
1,50 ohms
when the tube is strapped in triode, and the anode charge is allowed to
affect the
electron stream just as if the screen was a metal cylinder anode of the
same dia as the screen.

The differences between a signal pentode Ra and trioded Ra in a 6AU6 is
far greater than in most power tubes.

Patrick Turner.





You say that

Yes there is NFB. Consider this: The grid of the triode is fed
from a
non-zero source impedance.


The feedback due to the miller effect is independent of the source
impedance, or it would not be a characteristic of the valve. The
*effect* of the feedback does depend on the source impedance.

A triode fed from an effectively zero source impedance would still
display the effect that Patrick is talking about, presumably,
because he doesn't mention source impedance. But there would be no
Miller effect under such external conditions.

I remember when Patrick was mugged and dragged kicking and screaming
into this foolish and misleading notion. He was right and they made
him wrong. I am trying to help him escape. You are muddying the
water.

Read his post and come back to me if you can make sense of it.

cheers, Ian


  #19   Report Post  
Patrick Turner
 
Posts: n/a
Default



Stewart Pinkerton wrote:

On Mon, 13 Dec 2004 21:57:20 +0000, Chris Morriss
wrote:

In message , Sander deWaal
writes
Chris Morriss said:

Yes there is NFB. Consider this: The grid of the triode is fed from a
non-zero source impedance. The signal at the anode is inverted. There
is capacitance between the anode and the grid.

OK?

So we have internal capacitive NFB, which rolls of the gain at high
frequencies. Typical, and expected triode behaviour.

So what would you call it?

Miller effect.


Of course, and the Miller effect has an effect because of............?
(Here's a clue, the words negative, feed and back may just come into it)


Quite so, but it's inherent to the device, so should not IMO be
counted as NFB in a circuit sense, since you don't get the loop delay
problems which occur with global feedback. The same argument of course
applies to the cathode/source/emitter follower, since while there is
100% voltage feedback, there is no current feedback whatever. I don't
count this as NFB, simply changing the mode of operation.


You completely contradict yourself.

You say there is 100% voltage NFB, then you say this isn't NFB.

Global NFB in all amps is series voltage NFB, distinct from the other 3 main
types of FB,
being shunt voltage NFB, series current NFB, and we won't list varieties of
positive FB.

But its a voltage source that we want at the output of our amps, so the only
type of NFB ever used is series voltage NFB, and follower connections
are a prime example of such NFB and its in all the text books
if you don't believe me.

There is no way you can bull**** about "mode of operation", and think that
you'll get away with it.

I believe you are trying desperately to somehow make out that bjts and mosfets

etc are more inherently linear than tubes.

Without NFB, they are very non linear voltage to current devices.




Patrick Turner.



--

Stewart Pinkerton | Music is Art - Audio is Engineering


  #20   Report Post  
Ian Iveson
 
Posts: n/a
Default

"John Stewart" wrote

Go To ABSE for the real explanation of NFB in triodes.
There are three pages. The file is about 1000K.


Real? It is a contrivance, surely. Lots of pages...what about kiss?
Who's knife was it?

As I have said, any system, including a perfect resistor, can be
expressed in the canonical form of a feedback control system. It is
a matter of definition, just maths, not science.

Whether it is useful to do so depends on delay. If there is no delay
between input and feedback at the summing point, then there is no
need for all the rigmarole of feedback analysis because a simple
formula can be used to express the relationship between input and
output.

The classic formula relating input to output of a triode includes no
term for time. There is no need to think of it as a feedback control
system any more than you would a perfect resistor.

That is why in practice, those who avoid nfb are happy with triodes.
There is no sensible meaning for "instantaneous feedback".

The Miller capacitance is however seen by the classic formula as
external, and being a connection from output to input, it qualifies
as feedback with respect to that formula, and it is convenient to
treat it as such, in spice modelling for example. The effect depends
on the input impedance, however, which then must also be considered
part of the system as a necessary component for the production and
summing of input and feedback voltages.

Once you include all the necessary circuit to define the Miller
effect, is it correct to say there is delay in the loop? And then is
it correct to say that this feedback control system is part of the
valve? I think yes and no.

I feel it is important to stress the importance of loop delay.
Without it, most objections to nfb disappear. Just wondering offhand
if all would...does the scattering of distortion products that
usually results from nfb depend on delay?

cheers, Ian




  #21   Report Post  
Ian Iveson
 
Posts: n/a
Default

"Patrick Turner" wrote

I am not confused at all, its you.

The current flow in any tube, including a triode does not have any
feedback effect at all.

The anode ***voltage*** changes in a triode have a profound effect
on
the current flow.

Think of the triode set up at idle with no input signal.
[etc. etc....see below]



Well that's all very well but it is not feedback in the usual
meaning of the term. Where is the input summed with the feedback
loop? Quantity of argument doesn't substitute for simple fact.

cheers, Ian


Now you are being cheeky as well as daft. Source impedance is the
answer to your question. Source impedance is not part of the
valve.

The Miller effect is not what Patrick was talking about. For him,
the characteristics of the triode are shaped by what he calls
feedback. He is confusing the effect of the load current on Va,
and
hence on the output, even at DC.


Apply a +ve signal to the anode from some low impedance source.
The increase in voltage increases the current flow.
The voltage change divided by the current change = the plate
resistance
in ohms.
Its usually a lower figure than the load with which is to be used
with
the triode.

But with a pentode, the current change is perhaps 20 times less
for the
same voltage change at the anode,
and that's because the anode voltage change is prevented from
affecting
its own current flow.
So Ra of the pentode is much higher, 12kohms with an EL34, as
opposed to
1,50 ohms
when the tube is strapped in triode, and the anode charge is
allowed to
affect the
electron stream just as if the screen was a metal cylinder anode
of the
same dia as the screen.

The differences between a signal pentode Ra and trioded Ra in a
6AU6 is
far greater than in most power tubes.

Patrick Turner.





You say that

Yes there is NFB. Consider this: The grid of the triode is
fed
from a
non-zero source impedance.


The feedback due to the miller effect is independent of the
source
impedance, or it would not be a characteristic of the valve. The
*effect* of the feedback does depend on the source impedance.

A triode fed from an effectively zero source impedance would
still
display the effect that Patrick is talking about, presumably,
because he doesn't mention source impedance. But there would be
no
Miller effect under such external conditions.

I remember when Patrick was mugged and dragged kicking and
screaming
into this foolish and misleading notion. He was right and they
made
him wrong. I am trying to help him escape. You are muddying the
water.

Read his post and come back to me if you can make sense of it.

cheers, Ian




  #22   Report Post  
Chris Hornbeck
 
Posts: n/a
Default

On Tue, 14 Dec 2004 17:08:34 GMT, "Ian Iveson"
wrote:

...does the scattering of distortion products that
usually results from nfb depend on delay?


No, only on open-loop gain variations.

Many good points; thanks,

Chris Hornbeck
"If we gave your unit armor, we'd have to give
it to everybody" -Big Don
  #23   Report Post  
John Byrns
 
Posts: n/a
Default

In article , "Ian Iveson"
wrote:

Well that's all very well but it is not feedback in the usual
meaning of the term. Where is the input summed with the feedback
loop? Quantity of argument doesn't substitute for simple fact.


The input voltage at the grid, and the output voltage (feedback) at the
anode each create and electric field within the tube, the summation of the
two opposing electric fields within the tube determines the anode current,
it is a simple negative feedback system.

I estimate that Andre's 300B KISS amplifier has about 14 dB of negative
feedback resulting from this effect.


Regards,

John Byrns


Surf my web pages at, http://users.rcn.com/jbyrns/
  #24   Report Post  
Patrick Turner
 
Posts: n/a
Default



Ian Iveson wrote:

"Patrick Turner" wrote

I am not confused at all, its you.

The current flow in any tube, including a triode does not have any
feedback effect at all.

The anode ***voltage*** changes in a triode have a profound effect
on
the current flow.

Think of the triode set up at idle with no input signal.
[etc. etc....see below]


Well that's all very well but it is not feedback in the usual
meaning of the term. Where is the input summed with the feedback
loop? Quantity of argument doesn't substitute for simple fact.

cheers, Ian


The input and output signal is summed within the triode.

If the applied +ve voltage at the grid of the triode acts to increase
the current,
the anode voltage change, a -ve voltage, tries to reduce the current.
The sum of the two applied voltages (or electrostatic fields ) is what
actually
changes the current flow.
There is a summing of voltage fields. The difference between the
fields is what is amplified, and its no different to
the two signals going into a differential amp at the input of most amps.

Patrick Turner.






Now you are being cheeky as well as daft. Source impedance is the
answer to your question. Source impedance is not part of the
valve.

The Miller effect is not what Patrick was talking about. For him,
the characteristics of the triode are shaped by what he calls
feedback. He is confusing the effect of the load current on Va,
and
hence on the output, even at DC.


Apply a +ve signal to the anode from some low impedance source.
The increase in voltage increases the current flow.
The voltage change divided by the current change = the plate
resistance
in ohms.
Its usually a lower figure than the load with which is to be used
with
the triode.

But with a pentode, the current change is perhaps 20 times less
for the
same voltage change at the anode,
and that's because the anode voltage change is prevented from
affecting
its own current flow.
So Ra of the pentode is much higher, 12kohms with an EL34, as
opposed to
1,50 ohms
when the tube is strapped in triode, and the anode charge is
allowed to
affect the
electron stream just as if the screen was a metal cylinder anode
of the
same dia as the screen.

The differences between a signal pentode Ra and trioded Ra in a
6AU6 is
far greater than in most power tubes.

Patrick Turner.





You say that

Yes there is NFB. Consider this: The grid of the triode is
fed
from a
non-zero source impedance.

The feedback due to the miller effect is independent of the
source
impedance, or it would not be a characteristic of the valve. The
*effect* of the feedback does depend on the source impedance.

A triode fed from an effectively zero source impedance would
still
display the effect that Patrick is talking about, presumably,
because he doesn't mention source impedance. But there would be
no
Miller effect under such external conditions.

I remember when Patrick was mugged and dragged kicking and
screaming
into this foolish and misleading notion. He was right and they
made
him wrong. I am trying to help him escape. You are muddying the
water.

Read his post and come back to me if you can make sense of it.

cheers, Ian



  #25   Report Post  
Ian Iveson
 
Posts: n/a
Default

"John Byrns" wrote

The input voltage at the grid, and the output voltage (feedback)
at the
anode each create and electric field within the tube, the
summation of the
two opposing electric fields within the tube determines the anode
current,
it is a simple negative feedback system.

John,

How can I make it more clear?

You can call it feedback if you want. For it to be feedback in the
usual sense of the word applied to a control system, which is what
an amplifier is, there must be

**summation of input and feedback signals**

Other summations don't qualify.

Interaction of anode and grid gives rise to the miller effect, we
have dealt with that.

I estimate that Andre's 300B KISS amplifier has about 14 dB of
negative
feedback resulting from this effect.


Haven't seen it. Seems a wild claim to me. The thing is, as I have
said a few times now, the meaning of feedback is a matter of
definition. It seems a wild claim because it would not normally be
claimed. Hence it falls outside the normal definition. QED.

Neither is your definition useful. The response of a triode can be
expressed as a time-invariant function. The effects of
interelectrode capacitance and electrode inductance depend on the
circuit, but in normal triode connection with a grounded cathode
only the Miller capacitance is a path from output to input, and only
becomes feedback in the context of the particular circuit attached
to the grid.

Put a constant voltage across a perfect resistor and the current
will rise to produce a voltage equal to the input voltage. You could
contrive to say that it is a feedback control system because the
resulting current arises from a comparison of input and output
voltage. It would not be a new discovery, and you will find a
perfect resistor as an example in the introductory chapters of most
student texts on feedback control systems. You would not add
anything useful to anyone's understanding of resistors. There would
simply be no point. Ohms law is all we need. The books use the
resistor as an example of how *everything* can be considered a
feedback control system. The rest of the book will restrict itself
to examples where it is useful to do so. After all, if everything
truly is a feedback control system, the name becomes meaningless for
the purpose of identification, and so there can be none. At night,
all cows are black.

The available descriptions of valve behaviour...all the published
curves and stuff, don't require transformation. You don't need to
add any stuff about fields inside...you just need the curves.

Is a capacitor a feedback control system? Or a choke? Why not? And
if all these components are feedback control systems, perhaps you
better add a few more dB to your estimate.

cheers, Ian




  #26   Report Post  
Ian Iveson
 
Posts: n/a
Default

"Patrick Turner" wrote

The input and output signal is summed within the triode.

If the applied +ve voltage at the grid of the triode acts to
increase
the current,
the anode voltage change, a -ve voltage, tries to reduce the
current.
The sum of the two applied voltages (or electrostatic fields ) is
what
actually
changes the current flow.
There is a summing of voltage fields. The difference between the
fields is what is amplified, and its no different to
the two signals going into a differential amp at the input of most
amps.

Nonsense!

The input signal is to the grid, referenced to ground, not to the
anode. Of course a change in Va effects the current, but it does not
impinge on Vg normally except via the miller capacitance.

A diode works the same way. The effect is between cathode and anode,
essentially. It is modified by the grid in the case of the triode,
but the anode voltage only effects the *grid* voltage, which is the
input, via the miller capacitance.

Just wondering BTW if there are such things as diodes with screens?

Anyway, how can you say your nfb is internal when it requires an
external load? Without a load it disappears.

And conversely, if I use an effectively zero input impedance at the
grid, then there can be no feedback to it, and the triode will still
behave like a triode. Your so-called nfb will not disappear then.
Only the miller effect will disappear.

cheers, Ian


  #27   Report Post  
Patrick Turner
 
Posts: n/a
Default



Ian Iveson wrote:

"Patrick Turner" wrote

The input and output signal is summed within the triode.

If the applied +ve voltage at the grid of the triode acts to
increase
the current,
the anode voltage change, a -ve voltage, tries to reduce the
current.
The sum of the two applied voltages (or electrostatic fields ) is
what
actually
changes the current flow.
There is a summing of voltage fields. The difference between the
fields is what is amplified, and its no different to
the two signals going into a differential amp at the input of most
amps.

Nonsense!

The input signal is to the grid, referenced to ground, not to the
anode. Of course a change in Va effects the current, but it does not
impinge on Vg normally except via the miller capacitance.


The grid remains a high impedance input port of what is a differential
amp
if you consider the triode as a perfect pentode with an internal
loop of electrostatic NFB.
The electrostatic FB network works rather like a resistance shunt FB
network between the
anode and grid, but its unseen and inside the tube, and works like an
imaginary resistance divider,
but it has no real resistance, since if it did, you'd have input
currents, but we don't,
all we have is an input voltage field, and then the field from the anode
sums with the applied grid field
to make a net field which controls the flow from the cathode.



A diode works the same way. The effect is between cathode and anode,
essentially. It is modified by the grid in the case of the triode,
but the anode voltage only effects the *grid* voltage, which is the
input, via the miller capacitance.


Its no use talking about diodes. We have a triode to deal with.


Just wondering BTW if there are such things as diodes with screens?


You could build one.
Just build a tetrode without a control grid.
But its easier to just use a pentode and connect the grid to the
cathode,
and the "screen" then can be used as a control grid for what will
be a high plate resistance triode with low U, low Gm, and
pretty useless, except some folks have done just this to great effect,
Tim DeParavicini designed an SE pentode amp with screen drive, and lotsa
NFB,
and got good reviews.


Anyway, how can you say your nfb is internal when it requires an
external load? Without a load it disappears.


The FB is internal, and it depends on the signal changes at the anode
to get any FB.

Its the same as any amp, short the output, and there is no FB.
No sound either.


And conversely, if I use an effectively zero input impedance at the
grid, then there can be no feedback to it, and the triode will still
behave like a triode. Your so-called nfb will not disappear then.
Only the miller effect will disappear.


Well, with a grounded grid in a normall biased tube, what you have is a
diode.
The only thing that changes Ia is the change of Ea.
By keeping G1 at various fixed selected values of voltage and by forcing
the anode to change its voltage,
and measuring the current,
we get the anode curves for the data sheets.

Diodes have the same curves as triodes, but with a diode, there is only
one curve,
since there is no grid.

Pentodes on the other hand act like there is very little electrostatic
feedback,
but let's not complicate the discussion any more than it is.



cheers, Ian


  #28   Report Post  
Ian Iveson
 
Posts: n/a
Default

"Patrick Turner" wrote

[below]


I don't believe you are trying to understand, Patrick. As Dylan put
it, you try to hide what you don't know to begin with. Glad to see
Positively 4th Street is on his latest "best of" cd.

Oh, and before I forget...not a knife, but a razor. Occam's, I
think. Can't remember where it fits in though...

One more time, two points.

1***Mathematically, any system can be contrived to be a feedback
system. That means everything, including every part of an amplifier,
including the chassis and the shelf it sits on, including the sun
and the moon and the stars and all the fishes in the sea.

2***For a system to be a feedback control system in the usual sense,
as in "12dB of nfb", there must be summation of input and feedback
control signals at the input. If a triode is driven with an
effectively zero input impedance to the grid, there can be no
feedback of control signal to the grid. (The control signal is a
voltage, so any feedback signal to be summed with it must also be a
voltage, but you can't sum voltages at the input if it is driven
with a zero impedance source. Such a source is easy to
arrange...amps designed for A2 operation use them.) If triode
behaviour really results from feedback to the grid, then this would
defeat it. Yet it still behaves like a triode. Hence its behaviour
cannot be explained in terms of feedback to the input. It is
therefore not a feedback control system.

But you know in your heart that John's estimate of nfb in a SET amp
is ludicrous. You are being silly, all three of you, and you know
it. Not for the first time either.

A diode with a screen grid would make a CCS. If you held the screen
at constant voltage and attached the load to the anode. And if the
screen of a tetrode were called the anode, and the anode were called
a collector, or sink, or somesuch, then ppl would not be so confused
about the tetrode or pentode, which is constructed by putting lots
of holes in the anode and adding another electrode behind it to
prevent the electricity gathering and molesting its characteristics.

cheers, Ian

The input signal is to the grid, referenced to ground, not to the
anode. Of course a change in Va effects the current, but it does
not
impinge on Vg normally except via the miller capacitance.


The grid remains a high impedance input port of what is a
differential
amp
if you consider the triode as a perfect pentode with an internal
loop of electrostatic NFB.
The electrostatic FB network works rather like a resistance shunt
FB
network between the
anode and grid, but its unseen and inside the tube, and works like
an
imaginary resistance divider,
but it has no real resistance, since if it did, you'd have input
currents, but we don't,
all we have is an input voltage field, and then the field from the
anode
sums with the applied grid field
to make a net field which controls the flow from the cathode.



A diode works the same way. The effect is between cathode and
anode,
essentially. It is modified by the grid in the case of the
triode,
but the anode voltage only effects the *grid* voltage, which is
the
input, via the miller capacitance.


Its no use talking about diodes. We have a triode to deal with.


Just wondering BTW if there are such things as diodes with
screens?


You could build one.
Just build a tetrode without a control grid.
But its easier to just use a pentode and connect the grid to the
cathode,
and the "screen" then can be used as a control grid for what will
be a high plate resistance triode with low U, low Gm, and
pretty useless, except some folks have done just this to great
effect,
Tim DeParavicini designed an SE pentode amp with screen drive, and
lotsa
NFB,
and got good reviews.


Anyway, how can you say your nfb is internal when it requires an
external load? Without a load it disappears.


The FB is internal, and it depends on the signal changes at the
anode
to get any FB.

Its the same as any amp, short the output, and there is no FB.
No sound either.


And conversely, if I use an effectively zero input impedance at
the
grid, then there can be no feedback to it, and the triode will
still
behave like a triode. Your so-called nfb will not disappear then.
Only the miller effect will disappear.


Well, with a grounded grid in a normall biased tube, what you have
is a
diode.
The only thing that changes Ia is the change of Ea.
By keeping G1 at various fixed selected values of voltage and by
forcing
the anode to change its voltage,
and measuring the current,
we get the anode curves for the data sheets.

Diodes have the same curves as triodes, but with a diode, there is
only
one curve,
since there is no grid.

Pentodes on the other hand act like there is very little
electrostatic
feedback,
but let's not complicate the discussion any more than it is.



cheers, Ian




  #29   Report Post  
John Byrns
 
Posts: n/a
Default


Hi Ian,

I have two problems with your view of "NFB".

First were is it written that in order to have "NFB" you must sum
voltages? I have seen analog "NFB" systems where the signals being summed
were represented by air pressure, so why not create "NFB" by summing
electric fields? In the triode the input and output voltages are
transduced to electric fields, and those electric fields are then combined
within the triode to create "NFB".

Second, what is wrong with doing the summing within the tube, why must the
summing be external? Taking your reasoning to the extreme, virtually no
audio amplifiers that exist today use "NFB", because the summing is done
internally to the amplifier. By your reasoning, for an amplifier to have
"NFB" the summing must be done in a circuit external to the amplifier
housing, for example by a rat's nest of resistors hanging off the input
and output terminals.


Regards,

John Byrns


In article , "Ian Iveson"
wrote:

"Patrick Turner" wrote

[below]


I don't believe you are trying to understand, Patrick. As Dylan put
it, you try to hide what you don't know to begin with. Glad to see
Positively 4th Street is on his latest "best of" cd.

Oh, and before I forget...not a knife, but a razor. Occam's, I
think. Can't remember where it fits in though...

One more time, two points.

1***Mathematically, any system can be contrived to be a feedback
system. That means everything, including every part of an amplifier,
including the chassis and the shelf it sits on, including the sun
and the moon and the stars and all the fishes in the sea.

2***For a system to be a feedback control system in the usual sense,
as in "12dB of nfb", there must be summation of input and feedback
control signals at the input. If a triode is driven with an
effectively zero input impedance to the grid, there can be no
feedback of control signal to the grid. (The control signal is a
voltage, so any feedback signal to be summed with it must also be a
voltage, but you can't sum voltages at the input if it is driven
with a zero impedance source. Such a source is easy to
arrange...amps designed for A2 operation use them.) If triode
behaviour really results from feedback to the grid, then this would
defeat it. Yet it still behaves like a triode. Hence its behaviour
cannot be explained in terms of feedback to the input. It is
therefore not a feedback control system.

But you know in your heart that John's estimate of nfb in a SET amp
is ludicrous. You are being silly, all three of you, and you know
it. Not for the first time either.

A diode with a screen grid would make a CCS. If you held the screen
at constant voltage and attached the load to the anode. And if the
screen of a tetrode were called the anode, and the anode were called
a collector, or sink, or somesuch, then ppl would not be so confused
about the tetrode or pentode, which is constructed by putting lots
of holes in the anode and adding another electrode behind it to
prevent the electricity gathering and molesting its characteristics.

cheers, Ian

The input signal is to the grid, referenced to ground, not to the
anode. Of course a change in Va effects the current, but it does
not
impinge on Vg normally except via the miller capacitance.


The grid remains a high impedance input port of what is a
differential
amp
if you consider the triode as a perfect pentode with an internal
loop of electrostatic NFB.
The electrostatic FB network works rather like a resistance shunt
FB
network between the
anode and grid, but its unseen and inside the tube, and works like
an
imaginary resistance divider,
but it has no real resistance, since if it did, you'd have input
currents, but we don't,
all we have is an input voltage field, and then the field from the
anode
sums with the applied grid field
to make a net field which controls the flow from the cathode.



A diode works the same way. The effect is between cathode and
anode,
essentially. It is modified by the grid in the case of the
triode,
but the anode voltage only effects the *grid* voltage, which is
the
input, via the miller capacitance.


Its no use talking about diodes. We have a triode to deal with.


Just wondering BTW if there are such things as diodes with
screens?


You could build one.
Just build a tetrode without a control grid.
But its easier to just use a pentode and connect the grid to the
cathode,
and the "screen" then can be used as a control grid for what will
be a high plate resistance triode with low U, low Gm, and
pretty useless, except some folks have done just this to great
effect,
Tim DeParavicini designed an SE pentode amp with screen drive, and
lotsa
NFB,
and got good reviews.


Anyway, how can you say your nfb is internal when it requires an
external load? Without a load it disappears.


The FB is internal, and it depends on the signal changes at the
anode
to get any FB.

Its the same as any amp, short the output, and there is no FB.
No sound either.


And conversely, if I use an effectively zero input impedance at
the
grid, then there can be no feedback to it, and the triode will
still
behave like a triode. Your so-called nfb will not disappear then.
Only the miller effect will disappear.


Well, with a grounded grid in a normall biased tube, what you have
is a
diode.
The only thing that changes Ia is the change of Ea.
By keeping G1 at various fixed selected values of voltage and by
forcing
the anode to change its voltage,
and measuring the current,
we get the anode curves for the data sheets.

Diodes have the same curves as triodes, but with a diode, there is
only
one curve,
since there is no grid.

Pentodes on the other hand act like there is very little
electrostatic
feedback,
but let's not complicate the discussion any more than it is.



cheers, Ian




Surf my web pages at, http://users.rcn.com/jbyrns/
  #30   Report Post  
Patrick Turner
 
Posts: n/a
Default



Ian Iveson wrote:

"Patrick Turner" wrote

[below]


I don't believe you are trying to understand, Patrick. As Dylan put
it, you try to hide what you don't know to begin with. Glad to see
Positively 4th Street is on his latest "best of" cd.

Oh, and before I forget...not a knife, but a razor. Occam's, I
think. Can't remember where it fits in though...

One more time, two points.

1***Mathematically, any system can be contrived to be a feedback
system. That means everything, including every part of an amplifier,
including the chassis and the shelf it sits on, including the sun
and the moon and the stars and all the fishes in the sea.


I don't agree that **anything** can be contrived as a feedback system.
Being a realist, the only thing that can be contrived to be a feedback
system
is one which actualy is such a system, and can be seen to be so when the
concept
is examined without needing any maths to describe or proove it.

Its like the model of a tube.
We start with a voltage generator with a pair of zero output impedance
terminals
and operated by an input voltage so the output is u x Vin.
Then we have a series R to act as the Ra, and voila, a model for any
tube.
So with a pentode, u = 3,000, so for one volt in, we get 3,000 v out at
this gene,
and then this is is applied to 1M of plate resistance before being
applied to the load of say 50k,
we end up with 142v at the 50k, and we have a tube gain of 142.
Now everyone knows that 3,000v doesn't actually occur in our little
signal pentode, say a 6AU6,
but the concptual model I have described here allows us to draw a
complete curcuit in
using "equivalent models" to allow the everyday workings to be described
better on the back of
an envelope as something real, and drawn up, so we understand.
The same simple approach can be taken with a 300B.

Normally, the model of a triode is also a u.Vin generator, and Ra = the
plate resistance we observe,
but we couls also describe it as a current generator with gm x Vin, and
a loop of feedback to give us
our low triode plate resistance.

I suggest one has to think outside the square and allow the imagination
to work a bit to understand tubes,
mosfets, and transistors.

Its been done for about 90 years now.

There was a guy called Cathdode Ray, who wrote volumes in magazines of
the 1950s and 1960s,
and I suggest you dig around to find samples of what he wrote.
Its all in language any diyer can understand, and none of it is in what
we would now call
geek spiel, and he doesn't befuddle anyone with huge equations that
nobody can follow.

I have a couple of his books.

But only when you have read some more will you be able to comprehend
what I am trying to say.




2***For a system to be a feedback control system in the usual sense,
as in "12dB of nfb", there must be summation of input and feedback
control signals at the input. If a triode is driven with an
effectively zero input impedance to the grid, there can be no
feedback of control signal to the grid.


But beyond the grid, there is the applied voltage field inside the tube,

and from the anode there is also an applied voltage field.
The two fields sum, and the resulting field is what the electron stream
reacts to.

If one removes the field applied by the anode, by placing a screen grid
in there
you only have the applied voltage field of the grid, so gain is much
higher, also thd is higher,
because there is no FB from the anode.

The impedance tied to the input grid is of little consequence, although
with triodes its best if the grid
is driven from a low impedance to avoid miller effects at too low an F.

(The control signal is a
voltage, so any feedback signal to be summed with it must also be a
voltage, but you can't sum voltages at the input if it is driven
with a zero impedance source.


You completely miss the point about the voltage field effects within the
triode.

Such a source is easy to
arrange...amps designed for A2 operation use them.) If triode
behaviour really results from feedback to the grid, then this would
defeat it. Yet it still behaves like a triode. Hence its behaviour
cannot be explained in terms of feedback to the input. It is
therefore not a feedback control system.


Ah, but greater minds than yours or mine suggest there **is**
NFB in a triode.
Before you doubt them publicly, at least try to see what they are
saying.
I suggest you read a few more old books.



But you know in your heart that John's estimate of nfb in a SET amp
is ludicrous. You are being silly, all three of you, and you know
it. Not for the first time either.


John is about dead right about the 14 dB NFB for a 3k load for 300B.
I said it was about 18 dB for 5k.
Applied FB depends on open loop gain, and gain varies with load.



A diode with a screen grid would make a CCS.


Still a diode though. current would only flow one way.

If you held the screen
at constant voltage and attached the load to the anode. And if the
screen of a tetrode were called the anode, and the anode were called
a collector, or sink, or somesuch, then ppl would not be so confused
about the tetrode or pentode, which is constructed by putting lots
of holes in the anode and adding another electrode behind it to
prevent the electricity gathering and molesting its characteristics.


I am not confused by the conventional terminology thus far contrived.

Patrick Turner.



cheers, Ian

The input signal is to the grid, referenced to ground, not to the
anode. Of course a change in Va effects the current, but it does
not
impinge on Vg normally except via the miller capacitance.


The grid remains a high impedance input port of what is a
differential
amp
if you consider the triode as a perfect pentode with an internal
loop of electrostatic NFB.
The electrostatic FB network works rather like a resistance shunt
FB
network between the
anode and grid, but its unseen and inside the tube, and works like
an
imaginary resistance divider,
but it has no real resistance, since if it did, you'd have input
currents, but we don't,
all we have is an input voltage field, and then the field from the
anode
sums with the applied grid field
to make a net field which controls the flow from the cathode.



A diode works the same way. The effect is between cathode and
anode,
essentially. It is modified by the grid in the case of the
triode,
but the anode voltage only effects the *grid* voltage, which is
the
input, via the miller capacitance.


Its no use talking about diodes. We have a triode to deal with.


Just wondering BTW if there are such things as diodes with
screens?


You could build one.
Just build a tetrode without a control grid.
But its easier to just use a pentode and connect the grid to the
cathode,
and the "screen" then can be used as a control grid for what will
be a high plate resistance triode with low U, low Gm, and
pretty useless, except some folks have done just this to great
effect,
Tim DeParavicini designed an SE pentode amp with screen drive, and
lotsa
NFB,
and got good reviews.


Anyway, how can you say your nfb is internal when it requires an
external load? Without a load it disappears.


The FB is internal, and it depends on the signal changes at the
anode
to get any FB.

Its the same as any amp, short the output, and there is no FB.
No sound either.


And conversely, if I use an effectively zero input impedance at
the
grid, then there can be no feedback to it, and the triode will
still
behave like a triode. Your so-called nfb will not disappear then.
Only the miller effect will disappear.


Well, with a grounded grid in a normall biased tube, what you have
is a
diode.
The only thing that changes Ia is the change of Ea.
By keeping G1 at various fixed selected values of voltage and by
forcing
the anode to change its voltage,
and measuring the current,
we get the anode curves for the data sheets.

Diodes have the same curves as triodes, but with a diode, there is
only
one curve,
since there is no grid.

Pentodes on the other hand act like there is very little
electrostatic
feedback,
but let's not complicate the discussion any more than it is.



cheers, Ian





  #32   Report Post  
John Byrns
 
Posts: n/a
Default

In article , Stewart Pinkerton
wrote:

On Wed, 15 Dec 2004 17:54:47 -0600, (John Byrns) wrote:

Hi Ian,

I have two problems with your view of "NFB".

First were is it written that in order to have "NFB" you must sum
voltages? I have seen analog "NFB" systems where the signals being summed
were represented by air pressure, so why not create "NFB" by summing
electric fields? In the triode the input and output voltages are
transduced to electric fields, and those electric fields are then combined
within the triode to create "NFB".

Second, what is wrong with doing the summing within the tube, why must the
summing be external? Taking your reasoning to the extreme, virtually no
audio amplifiers that exist today use "NFB", because the summing is done
internally to the amplifier. By your reasoning, for an amplifier to have
"NFB" the summing must be done in a circuit external to the amplifier
housing, for example by a rat's nest of resistors hanging off the input
and output terminals.


OK, can we just simplify things here?

If it's done by a triode, then it's not NFB and therefore, by divine
intervention, it sounds musical and alive. If it's done by a
transistor (boo! Hisss!) then it's by definition NFB, and makes music
harsh and unnatural. Does that sum up your position?


No, my only claim is that triodes incorporate an internal negative
feedback mechanism. I have not made any comments in these threads, that I
am aware of, about the relative merits of tube and transistor circuits
from musical perspective.

Basically, you just change the rules as you go along, so that triodes
come out ahead - regardless of reality.


Which rules have I changed? You may not agree with me that triodes
incorporate an internal negative feedback loop, but that doesn't mean I
have changed any rules, we just don't agree on the rules as they relate to
the operation of triodes, I might just as well accuse you of changing the
rules regardless of reality.


Regards,

John Byrns


Surf my web pages at,
http://users.rcn.com/jbyrns/
  #33   Report Post  
Ian Iveson
 
Posts: n/a
Default

"John Byrns" wrote in message
...

Hi Ian,

I have two problems with your view of "NFB".


More than two, in my view.

First were is it written that in order to have "NFB" you must sum
voltages?


The control input to a triode is a voltage.

I have seen analog "NFB" systems where the signals being summed
were represented by air pressure, so why not create "NFB" by
summing
electric fields? In the triode the input and output voltages are
transduced to electric fields, and those electric fields are then
combined
within the triode to create "NFB".


The control input of a triode is not air pressure, nor an
electrostatic field. You cannot sum a voltage with an air pressure,
nor with an electrostatic field, nor indeed can you sum an air
pressure with an electrostatic field.

Second, what is wrong with doing the summing within the tube, why
must the
summing be external?


I have not said that it is necessary for it to be external. I have
argued that none of the effects you speak of are internal as you
have claimed, but depend in part on external elements. I was arguing
that your claim that it is internal is false.

Taking your reasoning to the extreme, virtually no
audio amplifiers that exist today use "NFB", because the summing
is done
internally to the amplifier. By your reasoning, for an amplifier
to have
"NFB" the summing must be done in a circuit external to the
amplifier
housing, for example by a rat's nest of resistors hanging off the
input
and output terminals.


No. That is your lack of understanding, not my reasoning.

If anyone is still following this thread and wondering about the
truth of the matter, the best thing you could do would be to go to
your local college or university library, and find a recommended
text on feedback control systems. Read the introduction, look
through the chapter headings. You will almost certainly want to buy
it if you think you can handle the maths.

Because it is generic to all control system engineering, it will
allow you to map your knowledge and experience of systems in general
onto audio amplifier design. So if you are an experienced plumber,
for example, you can find out that ringing or oscillation in water
pipes happens for the same reason as it does in amplifiers, and can
be cured or designed out using the same principles. So an educated
builder could understand valve amplifiers through his experience of
building. Then he just has to learn about the particular components.
Real engineering is the same everywhere. Everything you know and
feel about the world applies to audio amplifiers.

Also, you will find out if you don't already know that mainstream
audio engineering is Fourier-fetishist. You may wonder what happened
to Laplace.

cheers, Ian

cheers, Ian


  #34   Report Post  
Ian Iveson
 
Posts: n/a
Default

"Patrick Turner" wrote

Ah, but greater minds than yours or mine suggest there **is**
NFB in a triode.


Maybe greater than your mind. If any of them claim what you are
claiming, their minds are definitely not greater than mine. There is
dynamic equilibrium in a triode, that some may loosely, maliciously,
dishonestly, or foolishly call NFB. I have accepted all along that
this loose definition has currency, as you will see if you read my
posts.

But it is not feedback in the sense used by audio engineers. It is
not feedback in the sense used when you say that an amplifier has
12dB of NFB. A feedback control system must by definition, and quite
categorically, have a summing node at the input that combines the
input signal with feedback from the output. It is allowable (and
often necessary, as in most global nfb applications in amplifiers)
for the output signal to be transformed before being applied to
summing node. We tend to trim its bandwidth, and we may transform
current to voltage (if current is considered the output and voltage
is the input this is necessary because the feedback signal must
summable with the input signal). But whatever we do with it, it must
be summed with the input signal at the input.

Before you doubt them publicly, at least try to see what they are
saying.
I suggest you read a few more old books.


I know precisely what they are saying. You have misunderstood most
of them. They are using the loose meaning of feedback, not the more
precise meaning used by control system engineers for the purpose of
designing, for example, audio amplifiers. If they used the term
"dynamic equilibrium", then perhaps they would be less likely to be
misunderstood by wanabees.

I don't agree that **anything** can be contrived as a feedback
system.
Being a realist, the only thing that can be contrived to be a
feedback
system
is one which actually is such a system, and can be seen to be so
when the
concept
is examined without needing any maths to describe or proove it.


Everything *can* be if you make an effort, but I have counselled
against that tendency if you check (search for "cows" if you can't
find the place). It is up to an engineer to decide what form of
analysis is most useful and convenient in each case. In the case of
the triode, it is neither useful nor convenient. If it were to be
described correctly as a feedback control system, it would not be as
you have done.

It is you and your current crew who go on about imaginary devices,
and daft theories that you have no understanding of or use for.

But only when you have read some more will you be able to
comprehend
what I am trying to say.


I understand what you are saying, cheeky boy. Don't expect me to
understand what you are trying to say. You are possibly trying to
make sense, but I wouldn't bet on that.

Fresh students make your mistake more commonly than any other. Only
difference is, you aren't learning. While you have been nursing your
misapprehensions for the last several years, I have taught the truth
to hundreds of people who will go on to design stuff that you may
rely on in your old age.

Answer to John, and indeed most of my posts, deals with rest of your
questions [below]. Can't comment on all the imaginary stuff in your
head that you wrote about. [also below] It's your head, do what you
want with it.

Oh, that leaves this...

A diode with a screen grid would make a CCS.


Still a diode though. current would only flow one way.

er, yes. Wouldn't be constant if it didn't, would it? Your carping
has become pathological I fear.

cheers Ian


Its like the model of a tube.
We start with a voltage generator with a pair of zero output
impedance
terminals
and operated by an input voltage so the output is u x Vin.
Then we have a series R to act as the Ra, and voila, a model for
any
tube.
So with a pentode, u = 3,000, so for one volt in, we get 3,000 v
out at
this gene,
and then this is is applied to 1M of plate resistance before being
applied to the load of say 50k,
we end up with 142v at the 50k, and we have a tube gain of 142.
Now everyone knows that 3,000v doesn't actually occur in our
little
signal pentode, say a 6AU6,
but the concptual model I have described here allows us to draw a
complete curcuit in
using "equivalent models" to allow the everyday workings to be
described
better on the back of
an envelope as something real, and drawn up, so we understand.
The same simple approach can be taken with a 300B.

Normally, the model of a triode is also a u.Vin generator, and Ra
= the
plate resistance we observe,
but we couls also describe it as a current generator with gm x
Vin, and
a loop of feedback to give us
our low triode plate resistance.

I suggest one has to think outside the square and allow the
imagination
to work a bit to understand tubes,
mosfets, and transistors.

Its been done for about 90 years now.

There was a guy called Cathdode Ray, who wrote volumes in
magazines of
the 1950s and 1960s,
and I suggest you dig around to find samples of what he wrote.
Its all in language any diyer can understand, and none of it is in
what
we would now call
geek spiel, and he doesn't befuddle anyone with huge equations
that
nobody can follow.

I have a couple of his books.



2***For a system to be a feedback control system in the usual
sense,
as in "12dB of nfb", there must be summation of input and
feedback
control signals at the input. If a triode is driven with an
effectively zero input impedance to the grid, there can be no
feedback of control signal to the grid.


But beyond the grid, there is the applied voltage field inside the
tube,

and from the anode there is also an applied voltage field.
The two fields sum, and the resulting field is what the electron
stream
reacts to.

If one removes the field applied by the anode, by placing a screen
grid
in there
you only have the applied voltage field of the grid, so gain is
much
higher, also thd is higher,
because there is no FB from the anode.

The impedance tied to the input grid is of little consequence,
although
with triodes its best if the grid
is driven from a low impedance to avoid miller effects at too low
an F.

(The control signal is a
voltage, so any feedback signal to be summed with it must also be
a
voltage, but you can't sum voltages at the input if it is driven
with a zero impedance source.


You completely miss the point about the voltage field effects
within the
triode.

Such a source is easy to
arrange...amps designed for A2 operation use them.) If triode
behaviour really results from feedback to the grid, then this
would
defeat it. Yet it still behaves like a triode. Hence its
behaviour
cannot be explained in terms of feedback to the input. It is
therefore not a feedback control system.



But you know in your heart that John's estimate of nfb in a SET
amp
is ludicrous. You are being silly, all three of you, and you know
it. Not for the first time either.


John is about dead right about the 14 dB NFB for a 3k load for
300B.
I said it was about 18 dB for 5k.
Applied FB depends on open loop gain, and gain varies with load.




If you held the screen
at constant voltage and attached the load to the anode. And if
the
screen of a tetrode were called the anode, and the anode were
called
a collector, or sink, or somesuch, then ppl would not be so
confused
about the tetrode or pentode, which is constructed by putting
lots
of holes in the anode and adding another electrode behind it to
prevent the electricity gathering and molesting its
characteristics.


I am not confused by the conventional terminology thus far
contrived.



  #35   Report Post  
Patrick Turner
 
Posts: n/a
Default



Ian Iveson wrote:

"Patrick Turner" wrote

Ah, but greater minds than yours or mine suggest there **is**
NFB in a triode.


Maybe greater than your mind. If any of them claim what you are
claiming, their minds are definitely not greater than mine. There is
dynamic equilibrium in a triode, that some may loosely, maliciously,
dishonestly, or foolishly call NFB. I have accepted all along that
this loose definition has currency, as you will see if you read my
posts.

But it is not feedback in the sense used by audio engineers. It is
not feedback in the sense used when you say that an amplifier has
12dB of NFB. A feedback control system must by definition, and quite
categorically, have a summing node at the input that combines the
input signal with feedback from the output. It is allowable (and
often necessary, as in most global nfb applications in amplifiers)
for the output signal to be transformed before being applied to
summing node. We tend to trim its bandwidth, and we may transform
current to voltage (if current is considered the output and voltage
is the input this is necessary because the feedback signal must
summable with the input signal). But whatever we do with it, it must
be summed with the input signal at the input.


The grid input signal is a voltage, and there is an electric field
extending out some distance from where
the voltage is applied. The intensity of the field diminishes with
distance.
The same goes for the field that is applied by the voltage at the anode.

The electron stream is at some distance from both the grid and anode,
and the field experienced where the eectrons are
is the sum of the applied fields from both grid and anode.

I suggest you read a little more about how triodes work, and leave your
red herrings in the can.




Before you doubt them publicly, at least try to see what they are
saying.
I suggest you read a few more old books.


I know precisely what they are saying. You have misunderstood most
of them. They are using the loose meaning of feedback, not the more
precise meaning used by control system engineers for the purpose of
designing, for example, audio amplifiers. If they used the term
"dynamic equilibrium", then perhaps they would be less likely to be
misunderstood by wanabees.

I don't agree that **anything** can be contrived as a feedback
system.
Being a realist, the only thing that can be contrived to be a
feedback
system
is one which actually is such a system, and can be seen to be so
when the
concept
is examined without needing any maths to describe or proove it.


Everything *can* be if you make an effort, but I have counselled
against that tendency if you check (search for "cows" if you can't
find the place). It is up to an engineer to decide what form of
analysis is most useful and convenient in each case. In the case of
the triode, it is neither useful nor convenient. If it were to be
described correctly as a feedback control system, it would not be as
you have done.

It is you and your current crew who go on about imaginary devices,
and daft theories that you have no understanding of or use for.


We have much more use of our models and theories than you suggest.
We would maintain we understand things better than you do.

I suggest you read more, and learn more before you try to convince us
there
isn't any NFB is a triode.
You are not succeeding to convince me.




But only when you have read some more will you be able to
comprehend
what I am trying to say.


I understand what you are saying, cheeky boy. Don't expect me to
understand what you are trying to say. You are possibly trying to
make sense, but I wouldn't bet on that.


To end all the doubt you have, read more, and stay cool, and quiet
until you have read all the experts in the field.
I am not an expert.
But those who preceeded me sure are, and you'd wanna be somewhat careful
before you
say Child and the others are BS artists.

Most ppl don't give a **** about whether there is NFB in a triode or
not, they just hook 'em up
and listen to them, and the simple u, Ra, and gm data plus curves is all
they ever
think about or need if they build an amp.

But there is more to it for those of us who like to think a little more
deeply....




Fresh students make your mistake more commonly than any other. Only
difference is, you aren't learning. While you have been nursing your
misapprehensions for the last several years, I have taught the truth
to hundreds of people who will go on to design stuff that you may
rely on in your old age.


Really, do go on old boy... jolly good show eh what....
haw haw there, got your honary degree in triodeology eh,
must be a super uni that gave yer that one....




Answer to John, and indeed most of my posts, deals with rest of your
questions [below]. Can't comment on all the imaginary stuff in your
head that you wrote about. [also below] It's your head, do what you
want with it.


Well if you cannot appreciate the power of imagination in others, when
its clearly explained,
I would say that like so many other men, you lack the ability for
empathy,
or even compassion, and your intellectual prowes is seriously
compromised
by a tendency to thugatious didacticisms.

Don't worry, most guys are unable to take a step outside themselves, and

look back at themselves from an imagined place, and then ask themselves
"who is that guy?" and " have I got this right? " and " If I'm wrong,
then why are they right?"'.

Try to weigh the arguments.....




Oh, that leaves this...

A diode with a screen grid would make a CCS.


Still a diode though. current would only flow one way.

er, yes. Wouldn't be constant if it didn't, would it? Your carping
has become pathological I fear.


I dunno; I don't care about the pathology involved here.

Patrick Turner.



cheers Ian


Its like the model of a tube.
We start with a voltage generator with a pair of zero output
impedance
terminals
and operated by an input voltage so the output is u x Vin.
Then we have a series R to act as the Ra, and voila, a model for
any
tube.
So with a pentode, u = 3,000, so for one volt in, we get 3,000 v
out at
this gene,
and then this is is applied to 1M of plate resistance before being
applied to the load of say 50k,
we end up with 142v at the 50k, and we have a tube gain of 142.
Now everyone knows that 3,000v doesn't actually occur in our
little
signal pentode, say a 6AU6,
but the concptual model I have described here allows us to draw a
complete curcuit in
using "equivalent models" to allow the everyday workings to be
described
better on the back of
an envelope as something real, and drawn up, so we understand.
The same simple approach can be taken with a 300B.

Normally, the model of a triode is also a u.Vin generator, and Ra
= the
plate resistance we observe,
but we couls also describe it as a current generator with gm x
Vin, and
a loop of feedback to give us
our low triode plate resistance.

I suggest one has to think outside the square and allow the
imagination
to work a bit to understand tubes,
mosfets, and transistors.

Its been done for about 90 years now.

There was a guy called Cathdode Ray, who wrote volumes in
magazines of
the 1950s and 1960s,
and I suggest you dig around to find samples of what he wrote.
Its all in language any diyer can understand, and none of it is in
what
we would now call
geek spiel, and he doesn't befuddle anyone with huge equations
that
nobody can follow.

I have a couple of his books.



2***For a system to be a feedback control system in the usual
sense,
as in "12dB of nfb", there must be summation of input and
feedback
control signals at the input. If a triode is driven with an
effectively zero input impedance to the grid, there can be no
feedback of control signal to the grid.


But beyond the grid, there is the applied voltage field inside the
tube,

and from the anode there is also an applied voltage field.
The two fields sum, and the resulting field is what the electron
stream
reacts to.

If one removes the field applied by the anode, by placing a screen
grid
in there
you only have the applied voltage field of the grid, so gain is
much
higher, also thd is higher,
because there is no FB from the anode.

The impedance tied to the input grid is of little consequence,
although
with triodes its best if the grid
is driven from a low impedance to avoid miller effects at too low
an F.

(The control signal is a
voltage, so any feedback signal to be summed with it must also be
a
voltage, but you can't sum voltages at the input if it is driven
with a zero impedance source.


You completely miss the point about the voltage field effects
within the
triode.

Such a source is easy to
arrange...amps designed for A2 operation use them.) If triode
behaviour really results from feedback to the grid, then this
would
defeat it. Yet it still behaves like a triode. Hence its
behaviour
cannot be explained in terms of feedback to the input. It is
therefore not a feedback control system.



But you know in your heart that John's estimate of nfb in a SET
amp
is ludicrous. You are being silly, all three of you, and you know
it. Not for the first time either.


John is about dead right about the 14 dB NFB for a 3k load for
300B.
I said it was about 18 dB for 5k.
Applied FB depends on open loop gain, and gain varies with load.




If you held the screen
at constant voltage and attached the load to the anode. And if
the
screen of a tetrode were called the anode, and the anode were
called
a collector, or sink, or somesuch, then ppl would not be so
confused
about the tetrode or pentode, which is constructed by putting
lots
of holes in the anode and adding another electrode behind it to
prevent the electricity gathering and molesting its
characteristics.


I am not confused by the conventional terminology thus far
contrived.




  #36   Report Post  
Stewart Pinkerton
 
Posts: n/a
Default

On Thu, 16 Dec 2004 15:00:59 -0600, (John Byrns) wrote:

Which rules have I changed? You may not agree with me that triodes
incorporate an internal negative feedback loop, but that doesn't mean I
have changed any rules, we just don't agree on the rules as they relate to
the operation of triodes, I might just as well accuse you of changing the
rules regardless of reality.


Apologies due, it was not *your* position with which I was in
conflict, but Ivesons. Which is unfortunate, since he and I are both
C&I engineers, but he doesn't seem to understand what goes on inside a
triode. I do agree that triodes have a large amount of internal
feedback (hence their low gain, and hence the pentode, which breaks
the loop). My position is that certain transistors, such as the Sanken
2SC2922/2SA1216 pair, are as linear as triodes when used correctly,
and can be employed to produce low parts count 'zero NFB' amps in
similar manner.

As previously discussed, the use of local degeneration does not in my
view count as loop feedback, as it does not to my knowledge result in
the kind of discordant intermodulation products previously noted to be
produced by global NFB loops under certain conditions. Hence, this
should not be a problem for the 'zero NFB' purist.Similarly, I don't
agree that cathode/emitter followers are the spawn of Satan, they seem
simply to be an excellent impedance transformer. Is there any
*logical* objection to them?

--

Stewart Pinkerton | Music is Art - Audio is Engineering
  #37   Report Post  
Stewart Pinkerton
 
Posts: n/a
Default

On Fri, 17 Dec 2004 01:52:17 GMT, "Ian Iveson"
wrote:

"Patrick Turner" wrote

Ah, but greater minds than yours or mine suggest there **is**
NFB in a triode.


Maybe greater than your mind. If any of them claim what you are
claiming, their minds are definitely not greater than mine. There is
dynamic equilibrium in a triode, that some may loosely, maliciously,
dishonestly, or foolishly call NFB. I have accepted all along that
this loose definition has currency, as you will see if you read my
posts.


If there is not an internal feedback loop in a triode, how is it that
breaking the loop with a screen increases the gain?

--

Stewart Pinkerton | Music is Art - Audio is Engineering
  #38   Report Post  
Chris Hornbeck
 
Posts: n/a
Default

On Fri, 17 Dec 2004 07:31:38 +0000 (UTC), Stewart Pinkerton
wrote:

Similarly, I don't
agree that cathode/emitter followers are the spawn of Satan, they seem
simply to be an excellent impedance transformer. Is there any
*logical* objection to them?


You've raised three separate interesting points, and I can only
comment on the last. Followers have a uniquely long recovery time
from overload (must be designed around), and at high frequencies,
an inductive input impedance. Maybe not a biggie-big issue in audio;
not sure.

If I had to take a stab at explaining the oft-expressed objections
to followers for audio use, I'd guess they originated in unrealistic
bandwidth expectations. Circuitry that we think about in audio
terms is still working just-fine-thank-you into the short waves.

Much like love lives and national foreign policy, it's the out-of-band
stuff that kills ya.

Chris Hornbeck
"If we gave your unit armor, we'd have to give
it to everybody" -Big Don
  #39   Report Post  
Patrick Turner
 
Posts: n/a
Default



Stewart Pinkerton wrote:

On Thu, 16 Dec 2004 15:00:59 -0600, (John Byrns) wrote:

Which rules have I changed? You may not agree with me that triodes
incorporate an internal negative feedback loop, but that doesn't mean I
have changed any rules, we just don't agree on the rules as they relate to
the operation of triodes, I might just as well accuse you of changing the
rules regardless of reality.


Apologies due, it was not *your* position with which I was in
conflict, but Ivesons. Which is unfortunate, since he and I are both
C&I engineers, but he doesn't seem to understand what goes on inside a
triode. I do agree that triodes have a large amount of internal
feedback (hence their low gain, and hence the pentode, which breaks
the loop). My position is that certain transistors, such as the Sanken
2SC2922/2SA1216 pair, are as linear as triodes when used correctly,
and can be employed to produce low parts count 'zero NFB' amps in
similar manner.


Well all you have to do is proove your position to be valid.

Please set up a single transistor in common emmitter mode, ie, 8 ohm load in
the
collector circuit, idle current about 2.5 amps, grounded emitter.

The input is applied to the base from a very low impedance source.
with less than 0.001% thd.
This will require you to use a low thd amp with low Ro, because the base
input impedance is very low
Then measure the thd products and draw a graph of the level of each
harmonic product as it increases from 0.0 watts to
a maximum of about 10 watts.

Let us know how you get on.

But not before you have done the above experiment will any of us know if the
bjts you have chosen are anywhere near as linear as a triode, or even as linear

as a 6L6 set up SE, no FB, and in beam tetrode mode.





As previously discussed, the use of local degeneration does not in my
view count as loop feedback, as it does not to my knowledge result in
the kind of discordant intermodulation products previously noted to be
produced by global NFB loops under certain conditions.


But the emitter follower topology is a case of a high amount of NFB, and
its use completely negates the purpose of you quest, to proove that a bjt is as
naturally linear
as a triode.

If we were to accept you premise that EF topology is OK, then
we would then request that the 300B would also be allowed to have 30 dB of
applied global NFB around a second gain tube to make the playing field level
for the comparison.


Hence, this
should not be a problem for the 'zero NFB' purist.


It remains a real problem for everyone except you, and you cannot
accept that modern transistors are woefully non linear unless
large amounts of NFB are employed, and then there is the problem
of overcoming the naturally low base input impedance of the bjt.

Similarly, I don't
agree that cathode/emitter followers are the spawn of Satan, they seem
simply to be an excellent impedance transformer. Is there any
*logical* objection to them?


Some have no problem with follower stages. I don't have a problem with them
where I use them for the output of a preamp for example.
But in this case where you are floundering while you try to design a simple
bjt amp which is as simple as a 300B amp, follower topology is plainly
quite unacceptable.

But your aim and zeal is to proove bjts can be used as easily as a 300B and its
simple circuitry,
and with as little FB, zero feedback in fact, and still end up with a simple
amp, two devices max,
and have thd lower than 300B, and still have Rin above 10k, and able to be
powered from a
volume pot straight after a CD player..
So, unless you wish to be seen by all as the universe's greatest cheat, please
do not
keep insisting on the use of emitter follower in this case in this thread is
OK.

Patrick Turner.




--

Stewart Pinkerton | Music is Art - Audio is Engineering


  #40   Report Post  
Patrick Turner
 
Posts: n/a
Default



Chris Hornbeck wrote:

On Fri, 17 Dec 2004 07:31:38 +0000 (UTC), Stewart Pinkerton
wrote:

Similarly, I don't
agree that cathode/emitter followers are the spawn of Satan, they seem
simply to be an excellent impedance transformer. Is there any
*logical* objection to them?


You've raised three separate interesting points, and I can only
comment on the last. Followers have a uniquely long recovery time
from overload (must be designed around), and at high frequencies,
an inductive input impedance. Maybe not a biggie-big issue in audio;
not sure.


Followers are cases of a large amount of series voltage NFB.
A 12AX7 CF has an open loop gain reduction of say 75 times,
so there is 37 dB of local NFB, and so if thd was
0.1% with no FB, then in CF the thd is about 0.0027%.

If there is capacitance in the load of the follower, the voltage
at the cathode, or emitter or source will decline in voltage
as fast as current flow will allow.
So a 12AX7 follower will tend to have latch up when the load has
only 100 pF and the F is high enough, since the cathode current might
only be 1 mA.

But a larger triode with higher current will have the onset of slew rate Dn

occur at a higher F, and at a higher voltage threshold.

But nevertheless, McIntosh successfully use 12AX7 followers to drive
the output tubes in the Mc Intosh amps.

HF latch up in bjt emitter followe topology can
concievably result in the N and P devices in a complementary EF pair
being both turned on at the same time, and catastrophic currents flow
from rails through the bjts, and cause them to fail,
unless there are judiciously placed resistors to discharge the capacitance
involved.
Modern bjts have a higher Ft than types made 30 years ago, so latch up is
less of a problem than it used to be.

Patrick Turner.



If I had to take a stab at explaining the oft-expressed objections
to followers for audio use, I'd guess they originated in unrealistic
bandwidth expectations. Circuitry that we think about in audio
terms is still working just-fine-thank-you into the short waves.

Much like love lives and national foreign policy, it's the out-of-band
stuff that kills ya.

Chris Hornbeck
"If we gave your unit armor, we'd have to give
it to everybody" -Big Don


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