Phil wrote:
Andre Jute wrote:
I've read your arguments, Phil. Their subtext is methods to make NFB
more usable, less damaging to reproduced sound. That is a legitimate
outlook for Otala, who has to fit into an existing industrial
environment where NFB is an ineradicable part of that dominant
electronics cost-accounting system in which the explicit purpose of NFB
is to make cheap parts tolerable.
But that has buggerall to do with me as a DIYer. I can afford to take
the far simpler, more fundamental course of making my amps so superior
in design and components right from the start -- that I can do without
NFB and without the damage that NFB does. So I design amps without NFB,
period. I shortcut the problem of NFB and eliminate it before it
arises. My amps are ultra-silent without NFB; they do not need NFB for
any purpose whatsoever. Since one of my ZNFB amps is capable of 80W
when in its PSE mode, more than enough to drive electrostatic panels to
power-rivetter volume, I take the view that any designer who requires
NFB to make his hi-fi amps work has either permitted cost-accountants
to bully him, is an impressionable fashion victim, or is too thick to
put his mind in gear, tick one or more boxes.
The rest is interesting speculation but not of such consuming interest
to me that I will spend a morning setting up bench experiments to prove
the details of something I already know: that NFB smears the sound. A
complete summary of my view on NFB can be found here
http://members.lycos.co.uk/fiultra/K...dre%20Jute.htm
and here
http://members.lycos.co.uk/fiultra/K...dre%20Jute.htm
I haven't changed my mind one jot or tittle over the recent discussion.
I have heard absolutely nothing that proves a contrary case. That is
not to say you may not be right, that the psycho-acoustic effect which
gives Class A1 ZNFB (or very low NFB) amps their distinct superiority
is a subliminal reaction to the HF phase- smearing in NFB amps that you
say Otala posits. I just haven't seen any proof yet, and know that,
when I do see such proof, I will consider it of intellectual interest
-- and continue to build the sort of ZNFB amps I have always built, in
which NFB is excluded for its amplitude smearing at frequencies
starting below 100c/s, so that HF phase smearing, if it exists in NFB
tube amps, is no danger to my sound.
Be fair now, you didn't ask, "Why should I build high feedback pentode
amps,"
But I do build pentode amps, if without the "high feedback". The best
amp I ever designed, my Type 114 "Triple Threat" is a PP EL34 with a
pentode/ultralinear/triode switch and NFB tunable from zero to about
6dB in the latest iteration but up to 20dB in some early versions. Of
course, it is my "best" amp only when operated in the trioded ZNFB
mode, but the choice is in the hands of the builder.
you asked, "Why do do [no feedback] triode amps sound better?" I
merely attempted to give you an answer. You believe that triodes have an
internal feedback mechanism, and you wondered why they (still) sound
better.
My point is a little more subtle. I see an effect which looks like NFB
happening between the terminals of a triode; I'm happy to call it
internal or natural NFB until someone else offers an explanation that
justifies giving it another name; no one offers such an explanation,
merely negative objections to naming it NFB; until they pull their
finger from their arse and say something positive (and more cogent)
than Patrick, who *does* have a case, I shall not change my mind just
because they're "agin". You should also understand that some of these
clowns are agin not for any good reason but only because it is *me*
saying so; Pasternack, for instance, and the usual sockpuppets of the
Magnequest Scum, whose disruptive presence on RAT I dispensed with
wholesale a few years ago ("Jute is wrong even if he is right, isn't
he?" one of them, Ron Bales, complained pitifully!). Pasternack, for
instance, has been repeatedly caught out lying on professional matters
"in my zeal to flame Andre" -- which is his own sickening excuse in his
own words.
The basic core of my response/answer is that (1) negative
feedback transforms relatively benign amplitude distortion into much
less musical phase distortion, and (2) either triodes do *not* have this
distortion mechanism, or it occurs at such staggeringly high frequencies
that triodes can "get away with it," since the amount of phase
distortion produced decreases as the high frequency limit increases.
I don't need to go that sophisticated. By far the most popular of my
loudspeaker designs (of the published ones; an expensive licensed speak
sells amazingly well in the Far East considering the wretchedly high
price dictated by the cost of the drivers) is an economy fullranger
using a guitar driver, in most installations that I know of with the
tweeter disconnected or, on my advice, never fitted.
http://members.lycos.co.uk/fiultra/K...Impresario.jpg
It has been years since I have been impressed with bandwidth as measure
of audio goodness, and I don't just mean excessive bandwidth, I mean
the upper end of what is commonly called the "audio spectrum". Most
people can't hear it even when they're young. Most people into hi-fi
are over middle age; they're lucky if they can hear past 12kHz. In any
event, Top C on a piano is a frequency of 4186c/s; over 70% of the
energy of that note (over half the decibel value referred to the
fundamental's energy) will be in the 2nd and 3rd harmonics, which takes
you just past 12kHz. (Try an experiment: filter out everything below
the 3rd harmonic of top C, 12558c/s, and listen to what remains. It is
horrid, just white noise.) By the way, the same argument of harmonic
weight in the production of sound applies to the bass end and well into
the midbass: an open G string on a violin played with medium intensity
has only 0.1 per cent of its energy in the fundamental -- which is
196c/s; this is the basis for my thesis that the human ear
reconstitutes the fundamental from the dominant harmonics, where the
energy is. The upshot is that the lower extension of the so-called
"audio range" is another engineering chimera that has nothing
whatsoever to do with music and everything to do with a bunch of
arrogant engineers sitting in a room feeding of each other's
testosterone while setting standards that should instead have been set
by psychologists or musicologists or, in fact, anyone but engineers. In
practice, it means that you can roll speakers off quite high, where the
bass will "measure" modestly but sound very, very clean, and they will
sound better over the long term than those one-note boof-boff big-bass
abominations so beloved of "audiophiles" and "engineers" alike. Peter
Walker didn't build his ESL63 any bigger than 45c/s bass (the last pair
I measured was 8dB down at 32c/s) because to the ear even a 16c/s organ
note, if on the recording at all, will sound startlingly precise
because all the energy are in the first two harmonics above the
fundamental.
More, drivers with a natural high frequency mechanism, like the classic
Lowther driver, can sound screechy in the treble precisely because it
is not a fundamentally natural noise
http://members.lycos.co.uk/fiultra/K...20T91HWAF3.jpg
Lowthers with the whizzer tweaked by the insertion of cotton wool
behind it or by stiffening with C37, always sound more "natural" even
as the measurements start looking less impressive. (I prefer the
waterfall anyway, but only after I have run it through a smoothing
program because the small glitches are a distraction to the eye but not
the ear.)
And I proposed a test, which may not work anyway, and which apparently
only I have any interest in performing! However, I believe you when you
say you already know that feedback smears the sound somehow, and don't
need a test to "justify" your decisions, which is a stand I do respect.
Hang on a minute. I have demonstrated smearing in the frequency band
I'm interested in. I've done it again and again and again,
theoretically by mathematics, by measurement with instruments,
graphically, and by placebo listening tests (what the pretentious call
ABX).
http://members.lycos.co.uk/fiultra/K...dre%20Jute.htm
http://members.lycos.co.uk/fiultra/K...dre%20Jute.htm
I have no problem standing by my taste when taste is the question (for
about ten years I was the most widely read music critic in the world
with a column syndicated to 9.2m readers every week) but when science
has an answer I am as keen as the next man (and clearly keener than the
diplomaed quarterwits on RAT) to find the correct answer by the proper
scientific method.
For a while there, all of us had to face serious criticisms for saying
that in our own experience, cables do sound different,
Sure, I believe you can hear cables, under two very specific
conditions. Again, my conclusion is based on a scientific test. I flew
to a different country and in a warehouse spliced huge reels of cable
until I had the several of different construction of a length that an
engineer (a proper one, not one of the local clowns) calculated for me
should be audible by the laws of physics. They were, barely. The other
condition under which cables are audible also answers to the laws of
physics: when components in the audio chain are mismatched, the
resistance and capacitance on a particular cable can make the
combination sound better or worse, hence "cable is audible". I
personally use Cardas golden section multi-diameter cable because the
concept appeals to me: my Impresario speak is designed on Phi, the
formulaic base of the golden section.
http://members.lycos.co.uk/fiultra/K...Impresario.jpg
and it really is
a mark of character to stand up and say something that is true, but
"officially" stupid.
Man, you got enough problems already. You really don't want my
character as well: I was sent into exile from my motherland for saying
the statistical basis of apartheid was flawed (everyone could see it
only twenty years after I said it), and later twice hunted by assassins
sent by the apartheid government for a couple of my books.
http://members.lycos.co.uk/fiultra/THE%20WRITER'S%20HOUSE.html
As an economist I was academically haunted by being the only monetarist
in keynesian countries in a keynesian age, as a psychologist in a
Freudian age thought very odd, most unreliable, a loose cannon on deck
for saying that Freud was a literary giant but didn't know **** about
human character. Today, of course, everyone knows I am right and was
right then. That was nothing to the filth heaped on me for saying the
Common Agricultural Policy of the European Community was a wasteful
abomination, or pointing out that the case against DDT was and is
unproven, or that "global warming" is an official lie (I've been saying
it since the 1960s when I had a running joke in a satarical newspaper
column about the missing hole in the ozone layer).
If only the "official" beliefs didn't so often turn
out to be the ones that are actually stupid, we could all be mindless
sheep, and be better off for it! ;-)
Nah, I like being "the most dangerous class", as Lenin called
reformers. Sure, the price has been high, but the alternative would
have been dull.
However, there are sometimes good reasons for more fully understanding
something. If we assume for a moment that my analysis -- which is
basically my attempt to guess at the rest of what Otala was saying,
since I have yet to see the full text -- is correct, then several
interesting things follow. First, since a threshold below which we
cannot hear phase distortion realistically *must* exist
There is no *must* about any threshhold. We're discussing the known
preference for Class A1 triode sound by an important niche of
knowledgeable audiophiles -- despite the known fact that it measures
worse than the alternatives on quite a few parameters. Even if those
parameters are at present driven to ludicrous lengths by the engineers,
it is clear that the highest level of triodes -- DHT, ZNFB, SE or Class
A1 trioded ZNFB PP pentodes -- will never come within a magnitude of
current technology, at least not on the meter. The preference must be
explained by some subliminal effect. The problem with subliminal
effects is that they recede with experience, that is, that they're
educable; for instance, the limit at which distortion now becomes
audible is lower than it was when Olsen first studied the subject 70
years ago.
-- and again,
for Patrick's benefit, this is phase-smearing, and *not* the simple
phase shifting which feedback does correct -- we should be able to add
some feedback and get "all gain, no pain."
You're still on someone else's agenda, trying to make NFB usable. My
amps started sounding brilliant the day I rejected NFB on principle.
Patrick said that he added 6
dB to lower output Z, and it sounded fantastic, as opposed to a 20 dB
version of the same amp.
Hang on a minute. Patrick said that he found that 6dB of NFB didn't
degrade the sound audibly -- a matter of taste for him and his client
which we must permit them to enjoy in peace; I shall just say I tend to
believe that 6dB is very likely a common audibility limit. Patrick also
made the point that the NFB lowers the output impedance, on which he
puts a high priority *because he builds amps for sale, often to be used
with unknown speakers". Anyone who can design his speakers first can
also design them so that output impedance and the consequent need for
NFB is less; amps can be designed to be speaker-friendly without NFB as
long as you know from the beginning that NFB will be excluded.
Well, maybe this is completely true, but if we
*know* that it is true, then, for example, amps that use high-mu
transmitter tubes with positive grid drive and a bit of feedback to get
the Zout down begin to make sense,
Been there, done that. You're still on someone else's agenda, this time
"mo' powa' is betta powa'". It's bullcrap. Even at the lower end of the
same argument, a 211 doesn't sound nearly as good as an 845 when both
are built to give the same power, simply because the 211 must be driven
across the 0V bias line into A2 while the 845 stays strictly in A1.
It's another example of a subliminal effect on the ear when very little
difference shows on the measuring instrument.
especially when the feedback is used
in a two-stage configuration that does not include the output
transformer, meaning that it can have a *very* high upper frequency
limit (you don't need to "dumb down" a stage like you often need to do
with a three-stage to prevent oscillation), which limits the damage
feedback can do to sub-threshold levels. Many people report that they
LOVE the sound of these things, but an unjustified, in this case, bias
against *any* use of feedback could prevent us from even trying one.
No. This isn't a bias against NFB. This is a preference for staying in
Class A1.
Or, let's look at the home builder who wants to make a solid state amp
-- what the hell -- or at least one with a SS output stage.
Sander swears by his hybrid amps. I play SS amps often; the one I like
best is the Quad 405 Mk II, because it is so livable. You might check
Google for the threads when Stewart Pinkerton, a Poopie Stevenson type
clown but with a smidgin more class, challenged me to a design contest.
In theory he was supposed to design a silicon amp to beat my 300B SE
amp, or at least produce something that sounded close to it.
http://members.lycos.co.uk/fiultra/T...trafi-crct.jpg
http://members.lycos.co.uk/fiultra/T44bis-'Populaire'-crct.jpg
Even with months of help and coaching from John Byrns and Patrick
Turner, what Pinko produced was such a botch that not even he wanted to
build it. Bored with waiting for the interminable process to run its
fractious course, I designed my own simple SS amp and showed how it
could be developed to sound something like a good tube amp by driving
it into class A. I'm playing it right now. Thanks for reminding me to
plug it in and try it again; for the cost it can't be beat!
http://members.lycos.co.uk/fiultra/K...dre%20Jute.htm
http://members.lycos.co.uk/fiultra/K...dre%20Jute.htm
http://members.lycos.co.uk/fiultra/K...20mGBschem.jpg
http://members.lycos.co.uk/fiultra/K...%20mGBmatr.jpg
http://members.lycos.co.uk/fiultra/K...%20NoBleed.jpg
Basically, I'm saying that, if there is phase-smearing as you claim, I
also think you're right, that it will be inaudible. I am more
interested in what is audible, even if only subliminally. The rest is
of theoretical interest and my time is limited. Thanks for the
entertainment.
Andre Jute
More at
http://members.lycos.co.uk/fiultra/JUTE%20ON%20AMPS.htm
and
http://members.lycos.co.uk/fiultra/T...mp%20INDEX.htm
There are
basically two forms of feedback, the normal one, and the "active-error"
version described by J. R. MacDonald and others. The active-error
version only "corrects" the output when an actual error exists, whereas
the standard version has to correct the open-loop gain even when the
load is a steady resistance and the devices are behaving with perfect
linearity. If tests show more phase-smearing with the standard version
than with the active-error version, well, I know which version I would
want to use, or have in a new television. As a bit of a side note, with
better sounding SS output stages, maybe we can more easily hear the
advantage, assuming one exists, of using a tube to produce the error
signal (a tube doesn't have poor quality parasitic capacitances to
potentially mess up the low level information).
All of this may sound like something only of interest to home builders,
but at least some manufacturers actually would be happy to produce
noticeably better sounding products, if they could do so for about the
same money! If EE's in general become aware of the full characteristics
of feedback -- and if home builders start to do this, many EE's and high
end manufacturers will indeed follow, eventually -- then we might
actually see better products in cars and TV's. No, I'm not saying do
this so that we will get better products, but a good understanding of
what is needed to make better audio products does tend to help everyone,
sooner or later. Low output Z triodes are in fact the theoretically best
audio devices at this time, sound-wise, but they require an expensive,
heavy, big, high quality output transformer, and that will always limit
their use.
Or, maybe someone just wants to write an article for AudioXpress about
the true nature of feedback, and how to best use it, if its use cannot
be avoided!
Phil
Andre Jute
Visit Jute on Amps at http://members.lycos.co.uk/fiultra/
"wonderfully well written and reasoned information
for the tube audio constructor"
John Broskie TubeCAD & GlassWare
"an unbelievably comprehensive web site
containing vital gems of wisdom"
Stuart Perry Hi-Fi News & Record Review
Phil wrote:
Andre Jute wrote:
In an effort to be agreeable, I tried hard to give you negative
feedback inside the tube as an explanation of the overwhelming
superiority of triodes (or trioded pentodes) for audio reproduction,
among other reasons because NFB is accessible to many who belong on RAT
and is a genetic deformity of the silicon scum whose only purpose on
RAT is dissension. NFB is what the silicon slime abuse to make their
inadequate components sound passable, and what even tubies inspired by
the age of sophisters and cost-accountants use to linearize pentodes.
NFB thus has a base level of familiarity which gives it a head start in
any black box model intended to explain something to diplomaed
quarterwits among the silicon slime as well as the better-educated
kibbitzers in my own camp.
But fine, you want to reject my explanation, then you must offer a
better reason to explain why triodes are such superior amplification
devices to anything else, so much more pleasing to the ear, so much
more accurate to the cultivated taste.
Despite my cracks about the metaphysics of tubes, there *has* to be an
electrical reason for the superiority of triodes. But sure, all kinds
of input is welcome.
Andre Jute
Andre, I think you are too quick to dismiss the idea that feedback
transforms amplitude distortions into phase-smearing, like Otala claimed
in his talk/paper (assuming a paper ever followed the talk!). When
Patrick was defending feedback, he mentioned that poorly designed amps
sound like crap when they have lots of feedback, but that if you fix
them up a bit, meaning get rid of much of their excessive nonlinearities
-- read, *amplitude* nonlinearities -- then adding feedback sounds okay.
Now, if we eliminate the idea that feedback produces random noise -- and
we *know* that it reduces amplitude non-linearities -- then the only
distortion mechanism left, I believe, is phase-shifting. However, I want
to describe the usual "constant 20 degrees phase lag at 40 KHz" as
"phase-shifting," and a dynamic, microsecond to microsecond shifting
back and forth of one frequency relative to another as "phase-smearing,"
or "time-smearing."
What Otala was saying is that applying feedback to circuits with lots of
open-loop distortions, which are (I believe) almost always amplitude
distortions, converts these distortions into a back and forth smearing
of the high frequencies relative to the low frequencies (and it may
smear both in terms of the delay through the amp). Nor can we assume
that this time-smearing is a simple function of the low frequency
amplitude, because it is probably proportional to the magnitude of the
distortions, as well as the LF amplitude: VERY non-musical. When enough
feedback is applied to badly designed amps, the amplitude distortions
become quite small, so why did Patrick find that they sounded much worse
than the same amp sounds when touched up enough to reduce the larger
open-loop amplitude distortions? As you say, *something* is wrong, and
if it isn't high amplitude distortions (it can't be), and if feedback
doesn't produce spurious noises, then the only thing left is exactly
what Otala said, time-smearing.
In essence, feedback *connects* two things that are normally separate in
an amp, namely amplitude distortions, and phase-smearing. It achieves a
balance between these two, a balance which is determined by the speed of
the amp, the amount of feedback, and the amount of amplitude distortion.
Contrary to what you say, Otala was *not* referring to TIM, and
transistors did not become so much faster after 1980 than the ones used
in his '73 article to make the problem he described go away. Yes, the
amps had to be designed well enough to avoid TIM, but that was not a
real problem even in '73 *if* you knew what you were doing. The problem
he described in '80 was quite different, and even high MHz tubes are
subject to it.
The interesting things, assuming that feedback problems are indeed
time-smearing (regardless of whether this comes from the conversion of
amplitude distortions), are one, a single tone will reveal nothing of
this, giving very low THD numbers, and two, multiple tones should show
something, although looking at it in the amplitude realm will only show
higher than expected IMD. There should be a fairly easy way to test to
see if this really produces time-smearing. In general, we put a 4 volt
60 Hz signal and a 10 mV 20 KHz signal into an amp with lots of
feedback, preferably using non-linear sections of the amplifying devices
(say, use two 12AX7 type triodes where the plate curves vary from widely
spaced to closely spaced, and use about 60 to 80 dB of feedback to get
the overall gain down to 1). Use a high pass filter to see only the 20
KHz signal, and use the 20 KHz signal from the generator to trigger the
'scope. If time-smearing exists, then the 20 KHz signal will appear
"fuzzy" on the scope, since it is being shifted back and forth. Repeat
without the 60 Hz signal to make sure the time-smearing isn't coming
from somewhere else, and then also divide the output from each tube down
so that you get the same amplitude output *without* using feedback, and
see if the time-smearing goes away. Finally, if possible test over a
region where the curves are fairly linear, to see if that also reduces
the time-smearing. If so, you have proof that feedback transformed the
amplitude distortion into time-smearing. I will try to do this, but I
have little time, less energy, and not the best test bench in the world,
so it may be a while! If you or someone else both can and wants to try
this, I suspect we will get an answer much faster than if I do it. Note
that we can use pentodes or transistors, too, but using solid state has
the disadvantage of possibly introducing other complications due to the
poor quality silicon parasitic capacitances. Also, I am suggesting the
use of very different magnitudes as well as frequencies for the two
signals, in case equal magnitudes somehow avoids this problem, or at
least masks it from this test. And again, contrary to what Patrick said,
the idea that *if* this were true, then by now someone would have
already tries it, and the results would have become widely know, is just
naive. The human race simply isn't that intelligent, at least not yet.
An interesting conclusion, assuming this feedback time-smear mechanism
exists, is that the output of a feedback amp does *not* match the input
when multiple signals exist! However, our normal tests cannot see this,
since they tend to focus on one frequency at a time. I believe that
Patrick, as well as Otala, said that using local feedback to achieve
good open loop linearity, combined with some global feedback, tends to
sound pretty good. This makes sense for two reasons, first because
although the local feedback will produce phase-smearing, it does so at
*very* high speed, which I believe directly reduces the amount of
smearing (common sense says that as the speed of devices approaches
infinity, feedback becomes "perfect"). Second, the result does contain
some mis-match between input and output, even given the high speed of
local feedback, but now the global feedback will not only reduce the
remaining "normal" amplitude distortions, it should also reduce the
time-smearing produced by the local feedback, since this time-smearing
will still produce an error signal for the global feedback. Of course,
this reduction in time-smearing will itself produce more time-smearing,
but it should be a case of 0.1 x 0.1 = 0.01, so the final degree of
time-smearing is reduced by "dividing up" the total feedback into local
plus global.
Finally, you saw the review of Otala that Phil Allison gave, and I think
you were as impressed by it as I was (although it looked bad to PA).
When a man who was as talented, knowledgeable, and honest as Otala
produces a PROOF that negative feedback *always* transforms the
amplitude distortions of the open loop into phase smears of the closed
loop, we should not dismiss it as simply a problem of "old devices," a
problem that a slight increase in speed can make go away, especially
when the discussion that led to his analysis (the Audio Critic BS
session) included a lot of talk about vacuum tubes, and how their speed
advantage made it easier to use them with feedback. We really should do
at least one or two tests before dismissing his conclusions. Again, I
will try to do so, but if you want to know anytime soon, you should
probably rely on someone else.
Phil