[Ian Iveson]

Whilst searching for a 6550 for my circlotron Mk7, I came
across this

http://www.normankoren.com/Audio/FeedbackFidelity.html

Norman, being a hard-core reproductionist, defected to
photography and took a lot of very accurate pictures that
you can see on his main pages if you like that kind of
thing. His other valve stuff is worth a read.

Ian

[Ian Thompson-Bell]

Ian Iveson wrote:
Whilst searching for a 6550 for my circlotron Mk7, I came
across this

http://www.normankoren.com/Audio/FeedbackFidelity.html

Norman, being a hard-core reproductionist, defected to
photography and took a lot of very accurate pictures that
you can see on his main pages if you like that kind of
thing. His other valve stuff is worth a read.

Ian



I know this site well. A pity he took up photography as he was doing
some good tube work.

Cheers

Ian

[Patrick Turner]

Ian Thompson-Bell wrote:

Ian Iveson wrote:
Whilst searching for a 6550 for my circlotron Mk7, I came
across this

http://www.normankoren.com/Audio/FeedbackFidelity.html

Norman, being a hard-core reproductionist, defected to
photography and took a lot of very accurate pictures that
you can see on his main pages if you like that kind of
thing. His other valve stuff is worth a read.

Ian



I know this site well. A pity he took up photography as he was doing
some good tube work.

Cheers

Ian

I quote all of Norman's talk and my comments are marked with ***.


Introduction


I've seen negative feedback attacked, reviled, denounced, and defamed
for so long I can no longer stand idly by. Feedback may be as stylish as
a ‘63 Dodge Dart,
but it’s a dear friend who’s brought beauty and joy to my life. It may
not be a saint, but we don’t expect Marilyn Monroe to be Mother Theresa,
do we? It’s just
misunderstood— often as much by its supporters as by its detractors. In
the ten years I’ve been designing and building amplifiers I’ve learned
how to bring out its
best while respecting its limitations. The time has come to share that
knowledge.

In his January 1998 Stereophile article "A Future Without Feedback ,"
Martin Colloms asserts that "measurements do not fully describe sound
quality," and goes on
to suggest that "corrective feedback is fundamentally unmusical." He
poses the question, "has anyone explored the implications of negative
feedback for reproduced
sound quality in the absolute sense?" What follows is the results of
such an exploration. We shall describe new measurements that provide
improved insight into the
origins of audio quality, and we shall use these measurements to
determine when and where to apply feedback to best advantage. Although
we will use vacuum tube
circuits as examples, all observations apply equally well to
solid-state. We shall also revisit traditional measurements. One of
them— harmonic distortion at rated
power output— has an unexpected correlation with sound quality.

Sometimes it seems that so much has been written about feedback’s
harmful effects that it’s easy to forget its benefits. Why do engineers
love it? Improved
frequency response, reduced harmonic distortion, better gain control,
increased input impedance (in many circuits), and decreased output
impedance (in circuits with
voltage feedback). So why do audiophiles hate it? Harsh, gritty, grainy,
glaring sound. Reason enough.

Now for the BIG question: Are these qualities intrinsic to negative
feedback or do they arise from its improper application? From my
experience it’s mostly the
latter. Mostly. There are a few places where feedback runs into
unavoidable problems, and it shouldn’t be a big surprise that one of
them is the single-ended vacuum
tube power amplifier. How can we know this? We have an instrument for
looking deep into the heart of amplifiers— an instrument that may be
within your reach this
very moment.

***If only he could be brief, and get on with it!

Computer modeling


The idea of using the computer— the ultimate digital machine— to design
old-fashioned vacuum tube circuits may seem more than a little
incongruous. So it may be,
but in the computer we have powerful tool that wasn’t available in the
glory days of Dynaco, McIntosh, and Marantz; an affordable tool for
anyone interested in
high-end audio, and usable by anyone with modest engineering skill. More
precisely, our tool is a computer program called SPICE— acronym for
Simulation
Program with Integrated Circuit Emphasis, originally developed at the
University of California Berkeley.

SPICE is widely used in industry to prove integrated circuit designs
before they are cast in silicon, where fixing errors is far more costly
than in concrete. There are
several commercial versions of SPICE, all of which start with the
Berkeley algorithms and add user-friendly front-ends and outputs.
Probably the best-known of
them is PSpice from Cadence Design Systems
(http://www.orcad.com/Product/Analog/analog.asp). A free evaluation
version that can simulate up to fifty parts is
available on CD ROM or can be downloaded from the web. Fifty parts won’t
get you far with semiconductors, but it’s sufficient for the design of
surprisingly
sophisticated vacuum tube circuits. Full versions of PSpice are very
expensive. Another excellent program is Electronics Workbench from
Interactive Image
Technologies (1-800-263-5552; http://www.interactiv.com), which comes in
a $299 package (sometimes on sale) that can do some serious simulation.
Each of
these programs has its learning curve, and since I’ve taken the trouble
to learn PSpice, I’m stuck with it. I love it.

I’m not the only one who values SPICE. When I escaped from Silicon
Valley in 1985, I had a neighbor who was developing a version (HSpice)
in his garage. While
I was tinkering with tubes, he was quietly taking over the market for
large-scale integrated circuit simulation. In August 1997 I read that he
had sold his company—
Meta Software— for one hundred and sixty million dollars. (That was
before the dot com boom, when hardware still got some respect.) Can
there be a lesson here?
I suppose we tube lovers must be content to receive our reward in
heavenly sound. None of us will become another Bill Gates. Besides, the
only reproduced sound
he ever hears is digital.

***But wait a sec, you don't have to be Bill Gates to hear "digital
musiic". We all hear it routinely
when we listen to anything with a digitised source somewhere, so 99% of
ppl hear the effects of digital
processing whether they like it or not. 1% listen to vinyl only,
so they get pure analog, but there are plenty of vinyls that would have
sonded far better
had they never been recorded.


Many readers may wonder how well digital simulation can unlock the
secrets of analog electronics. In my experience it can do so
astonishingly well. I never cease to
be amazed by how closely measurements match SPICE simulations.



***Well since measurements are so accurate, and are the Real Thing, then
why simulate at all?

Models and measurements


No matter how strongly you believe that measurements don’t or can’t
correlate with sound quality, you must agree that electrical signals
inside circuits obey the laws
of physics. If a computer program has accurate enough device models, it
will simulate the signals with precision. If you examine the details of
those signals with
enough care, you may begin to find patterns that shed light on sound
quality.

What is wrong with conventional measurements? Two things. The first is
that most of them are made in frequency domain. The real world happens
in time domain.
Frequency domain measurements are derived from a mathematical construct
called the Fourier transform, which is defined for linear systems. When
a system
becomes seriously nonlinear— as an amplifier does when it saturates—
frequency domain measurements their meaning.

***???????????????

Time domain measurements, such as
pictures of clipped sine waves, are needed to tell the real story. The
one time domain measurement frequently seen in equipment reviews is the
10kHz square wave.
This measurement is usually made with a small signal— far from
saturation— and provides the same information as the frequency response
curve. SPICE produces
output in both time and frequency domain.

The second problem is that conventional measurements are taken only at
an amplifier’s external connections: A signal is fed into the input
terminals and measured at
the output terminals. What happens inside the circuit can make the
difference between sonic mediocrity and distinction. With SPICE, you can
probe deep inside of
circuits. I’ve made measurements that would be difficult, expensive, and
time-consuming with hardware instruments; measurements rarely if ever
seen in equipment
reviews; measurements that correlate much more closely with sound
quality than such old standbys as harmonic distortion and frequency
response. We’ve known
for a long time they didn’t hold the secrets.

***The "inside the amp signal" is called the "error signal" because it
contains an an opposite phase of the distortion
at the output which goes on to cancel the open loop distortion and
reduce this to the lower
amount we see when NFB is connected.

***Many people hace a real good look at this signal especially with a
square wave to see just how the amp copes
with trying to rid itself of its distortion.

A program’s performance is only as good as its models— sets of equations
that simulate device behavior— and SPICE does not have built-in models
for vacuum
tubes. External models must be added. For many years tubes were modeled
by the Langmuir-Childs law [1,2], which represents a tube as a
voltage-controlled
current source whose current is proportional to the three-halves power
of the voltage on the controlling elements. This model approximates tube
performance fairly
well in the middle of the operating range, but fails miserably near
cutoff, a region particularly critical to the performance of class AB
push-pull amplifiers. It works
well for calculating frequency response but not for distortion.

*** the non-linear cut off transfer curve doesn't matter much because
all audiophiles using
well made tube amps operate them with enough bias current they rarely
ever move out of class A,
so nothing cuts off.


A new set of models, accurate enough to match experimental tube behavior
in all critical regions, has recently been published [3] and applied to
the design of a
modified Dynaco PAS preamplifier [4] with stunning sonic results. We
shall use the old and new PAS line amplifiers (figures 1 and 2) as
examples of problems and
solutions related to feedback.

To you non-technical readers, I offer an apology. Feedback cannot be
discussed intelligently without getting somewhat technical. I shall try
to keep this exposition as
readable as possible— There will be no heavy formulas, and you may
safely skip over circuit descriptions and references to resistors and
capacitors.

*** Gee, so just how does NFB get explained without a sketch or two or
some wave forms.
The reader needs to be invited to
see actually what happens with NFB in an amp.
Formulas come later, when concepts are understood,
and somebody wants to build something.



Before we proceed, a few definitions are needed: There are two types of
negative feedback, or degenerative feedback as it is sometimes called:
local and global.

Local feedback is connected within or around a single gain stage; global
feedback is connected around several gain stages, usually from the
amplifier’s output to its
input. Local feedback is generally regarded as benign, and with this
view I concur. The amount of feedback, expressed in decibels (dB), is
the ratio of the gains
without and with feedback (the open and closed-loop gains): 6dB is a
factor of 2 in voltage (4 in power); 20dB is a factor of 10 in voltage
(100 in power), etc.

The trouble with feedback


Despite its advantages, negative feedback can degrade amplifier sound
quality in three ways: First, it can lead to instabilities that appear
as response peaks or even
oscillations at an amplifier’s frequency extremes. Second, it can
increase susceptibility to RF interference. Third, it makes clipping
more abrupt: This rarely affects
preamplifiers, which have plenty of headroom, but it is always a concern
in power amplifiers. With proper design, the first two problems can be
eliminated and the
third can be controlled. We shall examine each of them closely with the
help of SPICE.


*** But you don't need spice to examine such matters, just built the
amp, measure and trim it.




Instability


Negative feedback operates by subtracting a portion of an amplifier’s
output signal from its input. This is quite straightforward for the
middle frequencies where an
amplifier’s open-loop gain is relatively flat. Bode’s theorem tells us
that there is very little phase shift in this region. But things can get
ugly at the frequency extremes.
Every RC (resistor-capacitor) network that contributes to an amplifier’s
rolloff adds up to 6dB per octave to the rolloff and up to 90 degrees to
the phase shift. Not
all of these RC networks are obvious in the schematic: Many involve
stray capacitances within the tubes. If the total phase shift exceeds
180 degrees at any
frequency where the loop gain (A/G-1, where A is the open-loop gain and
G is the closed-loop gain) is greater than one, the amplifier will
oscillate [5]. If it merely
approaches 180 degrees, a peak will appear in the frequency response
curve that corresponds to ringing in the time-domain. This will
definitely degrade sound
quality.

***It depends where the peaks are. If they are within the audio band in
an appallingly made amp,
then the F response will be upset. But when they appear well outside the
AF band, there is little
audible effect because there is little signal content below 20Hz and
above 20kHz.



To make matters worse, capacitance in shunt with the load increases the
phase shift.



*** To be sure that there will be no oscillations due to added phase
shift of a capacitive load,
we trim gain and phase shift in the OLG. Then any value of C can be
connected,
and the amp won't oscillate. It won't provide full MF power up to 50kHz
either, but then it doesn't have to.

Interconnect cables have around 20pF per foot, and electrostatic
loudspeakers
are nothing more than big honking capacitors-- as large as 2
microfarads-- that know how to move.

***ESL can have a lot more than 2uF in the reflected C of their panels.
And they can have 2uF just in the C of the step up tranny if its
designed badly, as many are.
But a series R in the form of added R of say 1.5ohms or allowing the
winding resistances to be high
will provide a load never lower than the R. The R damps the circuit. The
leakage inductances of the
step up trannies provide some series L.
So saying ESL are "big honking capacitors" is not quite a correct thing
to ever say.




So load capacitance is always present. An amplifier that
performs nicely with a purely resistive load (widely used in equipment
reviews) may misbehave in the real world. Amplifiers have a property
called phase margin—
the difference between the maximum phase shift and 180 degrees— that
indicates how well they can tolerate capacitive loads. A qualitative
estimate of phase margin
may be obtained by measuring the response with capacitance in shunt with
the load. This is particularly easy with SPICE.

*** And easier and faster to examine with an amp and CRO....

To keep phase shift under control, a single RC network must dominate the
rolloff. For global feedback loops, this usually involves adding a
capacitor to the circuit.
This is accomplished in the original PAS line amplifier (fig. 1) with
33pF capacitor CLFB connected in parallel with feedback resistor RLFB,
and in the new design
(fig. 2), with 7pF capacitor C3M in the input circuit. Although these
capacitors reduce the high frequency cutoff (-3dB point), it is still
around 100kHz in both
circuits— well beyond the limits of human hearing. Most well-designed
preamplifiers achieve good frequency response and stability at the same
time— but this is not
always the case for power amplifiers with output transformers.


***I have ommitted the section on PAS line level amps using bucketfulls
of NFB and having tone controls.


*** Its difficult to comment here because we cannot reproduce
schematics.

*** See my webpages for far simpler ways of doing things.



The Parts Connection Catalog & Resource Guide (1-800-769-0747;
http://www.sonicfrontiers.com/tpc) contains an excellent collection of
schematic diagrams of
classic tube preamplifiers and power amplifiers. Nearly every amplifier
with feedback— and that’s almost all of them— uses the traditional
compensation technique
with the capacitor in parallel with the feedback resistor. In all
fairness, this technique is easy to apply and provides good stability
with minimum frequency response
loss. The new technique, Miller compensation in the input stage,
requires two capacitors (C3M and C3C in fig. 2) to achieve good
frequency response extension.

*** This Miller C should not be needed, but the additional
C of then 7Pf x tube gain is in shunt with 27k as local shunt NFB.
I prefer to use a gain stepping network.
But I don't use 12AX7 with low current and limit NFB in preamps to
simple shunt NFB
around 1 satge only.

With the proper component values, difficult to obtain without SPICE, the
new technique has improved stability, i.e., less of a response peak with
capacitance in
shunt with the load. Compensating a feedback loop at the input stage
also reduces response peaks at intermediate gain stages that can
occasionally cause such
problems as unexpected saturation.

***The more common idea used to reduce HF OLG gain is to have a HPF
at the amp input, and not more C from V1 to grid.

In part 2 we discuss feedback in power amplifiers.


*** Rather than plough through part 2
I'll only voice my casual opinions after a brief read.

I don't like the modified Dynaco MkIII revised schematic.

IMHO, 12AX7s don't belong in any power amp.
The NFB is in two loops from the OPT sec which is shown as a winding
with CT.
Its applied as balanced NFB to the cathodes of the two driver 12AU7
triodes.
Some C = 680pF between cathodes reduces FB but tends to force the 12AU7
triodes to work more like
an LTP at HF.
6CG7 or a pair of EL84 would be better trodes to use.
There is added miller C = 180pF in shunt with 10k series for added
Miller C
to reduce OLG gain of 12AU7 drivers.
I am not so sure of any benefits.

The OPT load is a nominal 4.3k a-a only.

It'd be better at 8k.

Patrick Turner.

[keithr]

"Patrick Turner" wrote in message
...


snip

Now for the BIG question: Are these qualities intrinsic to negative
feedback or do they arise from its improper application? From my
experience it's mostly the
latter. Mostly. There are a few places where feedback runs into
unavoidable problems, and it shouldn't be a big surprise that one of
them is the single-ended vacuum
tube power amplifier. How can we know this? We have an instrument for
looking deep into the heart of amplifiers- an instrument that may be
within your reach this
very moment.

***If only he could be brief, and get on with it!

come on Patrick, you aren't exactly known for your berevity.

snip

So load capacitance is always present. An amplifier that
performs nicely with a purely resistive load (widely used in equipment
reviews) may misbehave in the real world. Amplifiers have a property
called phase margin-
the difference between the maximum phase shift and 180 degrees- that
indicates how well they can tolerate capacitive loads. A qualitative
estimate of phase margin
may be obtained by measuring the response with capacitance in shunt with
the load. This is particularly easy with SPICE.

*** And easier and faster to examine with an amp and CRO....

True, but it is a lot quicker to set up a simulator than to build an amp,
and a hell of a lot quicker to change the circuit around in the simulator.
A free simulator is also a lot cheaper than a scope if you don't have one.

Keith

[Arny Krueger]

"Ian Thompson-Bell" wrote in
message
Ian Iveson wrote:
Whilst searching for a 6550 for my circlotron Mk7, I came
across this

http://www.normankoren.com/Audio/FeedbackFidelity.html

Norman, being a hard-core reproductionist, defected to
photography and took a lot of very accurate pictures that
you can see on his main pages if you like that kind of
thing. His other valve stuff is worth a read.

Ian



I know this site well. A pity he took up photography as
he was doing some good tube work.

Looks like he saw the errors of his ways and pulled away from the tubed
weirdness.

[Ian Thompson-Bell]

Patrick Turner wrote:

***But wait a sec, you don't have to be Bill Gates to hear "digital
musiic". We all hear it routinely
when we listen to anything with a digitised source somewhere, so 99% of
ppl hear the effects of digital
processing whether they like it or not. 1% listen to vinyl only,
so they get pure analog, but there are plenty of vinyls that would have
sonded far better
had they never been recorded.



Don't forget us tape heads who listen mostly to analogue tape - we must
make another 1% at least!!!

Cheers

Ian

[Ian Thompson-Bell]

Arny Krueger wrote:
"Ian Thompson-Bell" wrote in
message
Ian Iveson wrote:
Whilst searching for a 6550 for my circlotron Mk7, I came
across this

http://www.normankoren.com/Audio/FeedbackFidelity.html

Norman, being a hard-core reproductionist, defected to
photography and took a lot of very accurate pictures that
you can see on his main pages if you like that kind of
thing. His other valve stuff is worth a read.

Ian


I know this site well. A pity he took up photography as
he was doing some good tube work.

Looks like he saw the errors of his ways and pulled away from the tubed
weirdness.



Are you suggesting the world of photography is LESS weird???

Cheers

Ian

[Iain Churches[_2_]]

"Ian Thompson-Bell" wrote in message
...
Arny Krueger wrote:
"Ian Thompson-Bell" wrote in
message
Ian Iveson wrote:
Whilst searching for a 6550 for my circlotron Mk7, I came
across this

http://www.normankoren.com/Audio/FeedbackFidelity.html

Norman, being a hard-core reproductionist, defected to
photography and took a lot of very accurate pictures that
you can see on his main pages if you like that kind of
thing. His other valve stuff is worth a read.

Ian


I know this site well. A pity he took up photography as
he was doing some good tube work.

Looks like he saw the errors of his ways and pulled away from the tubed
weirdness.

Are you suggesting the world of photography is LESS weird???


An old friend of mine Iain McMillain, who took the photo
used on the sleeve of the Beatles album "Abbey Road" told me
once that he took up photography because he couldn't solder!

Iain

[Ian Thompson-Bell]

Iain Churches wrote:
"Ian Thompson-Bell" wrote in message
...
Arny Krueger wrote:
"Ian Thompson-Bell" wrote in
message
Ian Iveson wrote:
Whilst searching for a 6550 for my circlotron Mk7, I came
across this

http://www.normankoren.com/Audio/FeedbackFidelity.html

Norman, being a hard-core reproductionist, defected to
photography and took a lot of very accurate pictures that
you can see on his main pages if you like that kind of
thing. His other valve stuff is worth a read.

Ian


I know this site well. A pity he took up photography as
he was doing some good tube work.
Looks like he saw the errors of his ways and pulled away from the tubed
weirdness.
Are you suggesting the world of photography is LESS weird???


An old friend of mine Iain McMillain, who took the photo
used on the sleeve of the Beatles album "Abbey Road" told me
once that he took up photography because he couldn't solder!

Iain



But he was presumably quite happy being closeted in a small, dark room
with nowt but a red light, some trays of chemicals and a pair of rubber
gloves???

Cheers

Ian

[keithr]

"Ian Thompson-Bell" wrote in message
...
Iain Churches wrote:
"Ian Thompson-Bell" wrote in message
...
Arny Krueger wrote:
"Ian Thompson-Bell" wrote in
message
Ian Iveson wrote:
Whilst searching for a 6550 for my circlotron Mk7, I came
across this

http://www.normankoren.com/Audio/FeedbackFidelity.html

Norman, being a hard-core reproductionist, defected to
photography and took a lot of very accurate pictures that
you can see on his main pages if you like that kind of
thing. His other valve stuff is worth a read.

Ian


I know this site well. A pity he took up photography as
he was doing some good tube work.
Looks like he saw the errors of his ways and pulled away from the tubed
weirdness.
Are you suggesting the world of photography is LESS weird???


An old friend of mine Iain McMillain, who took the photo
used on the sleeve of the Beatles album "Abbey Road" told me
once that he took up photography because he couldn't solder!

Iain



But he was presumably quite happy being closeted in a small, dark room
with nowt but a red light, some trays of chemicals and a pair of rubber
gloves???

Cheers

Ian

Ah those were the days, the smell of the stop bath and fixer, but I have to
admit, in my old age, Photoshop is a whole lot more convenient, and , in the
long run, cheaper.

Keith

[Patrick Turner]

keithr wrote:

"Patrick Turner" wrote in message
...


snip

Now for the BIG question: Are these qualities intrinsic to negative
feedback or do they arise from its improper application? From my
experience it's mostly the
latter. Mostly. There are a few places where feedback runs into
unavoidable problems, and it shouldn't be a big surprise that one of
them is the single-ended vacuum
tube power amplifier. How can we know this? We have an instrument for
looking deep into the heart of amplifiers- an instrument that may be
within your reach this
very moment.

***If only he could be brief, and get on with it!

come on Patrick, you aren't exactly known for your berevity.

Indeed, because when I explain something, I EXPLAIN, OK.

snip

So load capacitance is always present. An amplifier that
performs nicely with a purely resistive load (widely used in equipment
reviews) may misbehave in the real world. Amplifiers have a property
called phase margin-
the difference between the maximum phase shift and 180 degrees- that
indicates how well they can tolerate capacitive loads. A qualitative
estimate of phase margin
may be obtained by measuring the response with capacitance in shunt with
the load. This is particularly easy with SPICE.

*** And easier and faster to examine with an amp and CRO....

True, but it is a lot quicker to set up a simulator than to build an amp,
and a hell of a lot quicker to change the circuit around in the simulator.
A free simulator is also a lot cheaper than a scope if you don't have one.

If I took up art, and began painting pictures of Nicole, and perhaps
Kylie,
I would NOT use a ****ing simulator.

I get the paint, the canvas, the brushes, a nice lounge for the girls,
and I paint, right?

Or I travel out into the hills and set up for some real brush strokes in
the Great Outdoors, and paint.

No simulations.

I am hopelessly and wilfully, and utterly addicted reality.

Patrick Turner.

Keith

[Patrick Turner]

Ian Thompson-Bell wrote:

Patrick Turner wrote:

***But wait a sec, you don't have to be Bill Gates to hear "digital
musiic". We all hear it routinely
when we listen to anything with a digitised source somewhere, so 99% of
ppl hear the effects of digital
processing whether they like it or not. 1% listen to vinyl only,
so they get pure analog, but there are plenty of vinyls that would have
sonded far better
had they never been recorded.



Don't forget us tape heads who listen mostly to analogue tape - we must
make another 1% at least!!!

So 98% are into mainly digital.

Even vinyls and tape source often has content that has been digitised
somewhere in the chain.

If you live in the past and am sure there have never been any digits
poked up your analog, then indeed you are lucky, because analog when its
goog is GOOD.

Patrick Turner.


Cheers

Ian

[keithr]

"Patrick Turner" wrote in message
...


keithr wrote:

"Patrick Turner" wrote in message
...


snip

Now for the BIG question: Are these qualities intrinsic to negative
feedback or do they arise from its improper application? From my
experience it's mostly the
latter. Mostly. There are a few places where feedback runs into
unavoidable problems, and it shouldn't be a big surprise that one of
them is the single-ended vacuum
tube power amplifier. How can we know this? We have an instrument for
looking deep into the heart of amplifiers- an instrument that may be
within your reach this
very moment.

***If only he could be brief, and get on with it!

come on Patrick, you aren't exactly known for your berevity.

Indeed, because when I explain something, I EXPLAIN, OK.

I rather think tha that is what the guy was trying to do.

snip

So load capacitance is always present. An amplifier that
performs nicely with a purely resistive load (widely used in equipment
reviews) may misbehave in the real world. Amplifiers have a property
called phase margin-
the difference between the maximum phase shift and 180 degrees- that
indicates how well they can tolerate capacitive loads. A qualitative
estimate of phase margin
may be obtained by measuring the response with capacitance in shunt
with
the load. This is particularly easy with SPICE.

*** And easier and faster to examine with an amp and CRO....

True, but it is a lot quicker to set up a simulator than to build an amp,
and a hell of a lot quicker to change the circuit around in the
simulator.
A free simulator is also a lot cheaper than a scope if you don't have
one.

If I took up art, and began painting pictures of Nicole, and perhaps
Kylie,
I would NOT use a ****ing simulator.

You ought to see someone about this hangup about Kylie and Nickole. But a
Kylie or Nikole ****ing simulator - would that the inflatable?

I get the paint, the canvas, the brushes, a nice lounge for the girls,
and I paint, right?

Or I travel out into the hills and set up for some real brush strokes in
the Great Outdoors, and paint.

No simulations.

So what do you think about photography then? That would be seen by some as a
painting simulator.


I am hopelessly and wilfully, and utterly addicted reality.

You've obviously never worked in the world of commercial electronic design
where the job has to be done quickly and accurately. You don't want to stop
a production line because after tinkering you find that R16 should be a 4K7
not a 1K8, there you use any tool that you can.

[Patrick Turner]

keithr wrote:

"Patrick Turner" wrote in message
...


keithr wrote:

"Patrick Turner" wrote in message
...


snip

Now for the BIG question: Are these qualities intrinsic to negative
feedback or do they arise from its improper application? From my
experience it's mostly the
latter. Mostly. There are a few places where feedback runs into
unavoidable problems, and it shouldn't be a big surprise that one of
them is the single-ended vacuum
tube power amplifier. How can we know this? We have an instrument for
looking deep into the heart of amplifiers- an instrument that may be
within your reach this
very moment.

***If only he could be brief, and get on with it!

come on Patrick, you aren't exactly known for your berevity.

Indeed, because when I explain something, I EXPLAIN, OK.

I rather think tha that is what the guy was trying to do.

Trying he was.

snip

So load capacitance is always present. An amplifier that
performs nicely with a purely resistive load (widely used in equipment
reviews) may misbehave in the real world. Amplifiers have a property
called phase margin-
the difference between the maximum phase shift and 180 degrees- that
indicates how well they can tolerate capacitive loads. A qualitative
estimate of phase margin
may be obtained by measuring the response with capacitance in shunt
with
the load. This is particularly easy with SPICE.

*** And easier and faster to examine with an amp and CRO....

True, but it is a lot quicker to set up a simulator than to build an amp,
and a hell of a lot quicker to change the circuit around in the
simulator.
A free simulator is also a lot cheaper than a scope if you don't have
one.

If I took up art, and began painting pictures of Nicole, and perhaps
Kylie,
I would NOT use a ****ing simulator.

You ought to see someone about this hangup about Kylie and Nickole. But a
Kylie or Nikole ****ing simulator - would that the inflatable?

I might be tempted to **** a well designed simulator. Maybe its better
than a real sheila.

As you age, many things can become better than the real thing.

In 20 years, nobody will want to care about me at all when i might need
some care.
A robot that simulated a sheila might be just what nearly every old man
might need,
and it may become available, and affordable, and extend the time you
can stay in your own home before you die.

I finally had to tell one gal of 57, after an enormous amount of time
wasted wooing her
that I'd rather ride a bicycle for 4 hours than ride her for 4 hours.

At least the bicycle would get me somewhere.

The alternative is to spend 4 hours painting a picture of her.

Nobody would buy it though, so this option is out.

I wouldn't even be thinking of a simulator at any time.



I get the paint, the canvas, the brushes, a nice lounge for the girls,
and I paint, right?

Or I travel out into the hills and set up for some real brush strokes in
the Great Outdoors, and paint.

No simulations.

So what do you think about photography then? That would be seen by some as a
painting simulator.


I am hopelessly and wilfully, and utterly addicted reality.

You've obviously never worked in the world of commercial electronic design
where the job has to be done quickly and accurately. You don't want to stop
a production line because after tinkering you find that R16 should be a 4K7
not a 1K8, there you use any tool that you can.

Yeah, but I need to be sure the 4k7 will work and the best way is to
try it and see, when the item is simple, like a tube amp.

An SS amp can be horribly complex. many other things are as well,
and sure, if design was all i did I would have a simulator program.

But I don't need one for tube amps.

I don't even want one.

Patrick Turner.

[Ian Iveson]

Patrick Turner wrote:

Ignoring irrelevant sniping:

***The "inside the amp signal" is called the "error
signal" because it
contains an an opposite phase of the distortion
at the output which goes on to cancel the open loop
distortion and
reduce this to the lower
amount we see when NFB is connected.


Er...eh?

An error signal is called an error signal because it is
derived by subtracting the output from the input. The
difference between the two is the error, obviously.

Compensating a feedback loop at the input stage
also reduces response peaks at intermediate gain stages
that can
occasionally cause such
problems as unexpected saturation.

***The more common idea used to reduce HF OLG gain is to
have a HPF
at the amp input, and not more C from V1 to grid.

Perhaps you should read more carefully and with a more
constructive attitude. If you learn about feedback
(simulation would help), you should be able to see a
*fundamental* difference between the two.

Ian

[Ian Iveson]

"Ian Thompson-Bell" wrote in
message ...
Ian Iveson wrote:
Whilst searching for a 6550 for my circlotron Mk7, I came
across this

http://www.normankoren.com/Audio/FeedbackFidelity.html

Norman, being a hard-core reproductionist, defected to
photography and took a lot of very accurate pictures that
you can see on his main pages if you like that kind of
thing. His other valve stuff is worth a read.

Ian

I know this site well. A pity he took up photography as he
was doing some good tube work.


Yes, and his power amp is good representative of a
particular breed. I've cited it a couple of times here, and
remember a discussion about the merit of using one of the
four output valves as a reference for the servo bias. The
concensus was that an independent reference should be used,
on the grounds that, otherwise, if the reference valve fails
it might take the others with it. I wasn't convinced and
still aren't.

On the subject of feedback, I think he tells the common
story quite well. I found myself wanting to insert, at a
particular point in his narrative, the rule:

"Loop gain may be as large as the ratio of the two most
dominant time constants"

(Quoted from Morgan Jones "Valve Amplifiers"), because there
is an easy and satisfying bit of maths that makes it
obvious.

While I have Morgan in front of me, you just have to laugh
at his dry wit sometimes. Feedback "is a powerful weapon
cabable of oppressing anything."

Still with Morgan, here is a passage tinged with that wit,
and worthy of note for some:

"Many practical amplifiers, having exhausted the two
possible methods of achieving stability described (above),
resort to manoeuvering the amplitude response independently
of phase response using step networks. Traditionally, these
were adjusted on test, but if we are forced to use this
method, computer aided AC analysis is an excellent tool for
observing the effects of changes..."

Ian

[Iain Churches[_2_]]

"Ian Thompson-Bell" wrote in message
...
Iain Churches wrote:
"Ian Thompson-Bell" wrote in message
...
Arny Krueger wrote:
"Ian Thompson-Bell" wrote in
message
Ian Iveson wrote:
Whilst searching for a 6550 for my circlotron Mk7, I came
across this

http://www.normankoren.com/Audio/FeedbackFidelity.html

Norman, being a hard-core reproductionist, defected to
photography and took a lot of very accurate pictures that
you can see on his main pages if you like that kind of
thing. His other valve stuff is worth a read.

Ian


I know this site well. A pity he took up photography as
he was doing some good tube work.
Looks like he saw the errors of his ways and pulled away from the tubed
weirdness.
Are you suggesting the world of photography is LESS weird???


An old friend of mine Iain McMillain, who took the photo
used on the sleeve of the Beatles album "Abbey Road" told me
once that he took up photography because he couldn't solder!

But he was presumably quite happy being closeted in a small, dark room
with nowt but a red light, some trays of chemicals and a pair of rubber
gloves???


Indeed. Makes tube audio seem very mundane:-)

[Patrick Turner]

Ian Iveson wrote:

Patrick Turner wrote:

Ignoring irrelevant sniping:

***The "inside the amp signal" is called the "error
signal" because it
contains an an opposite phase of the distortion
at the output which goes on to cancel the open loop
distortion and
reduce this to the lower
amount we see when NFB is connected.

Er...eh?

An error signal is called an error signal because it is
derived by subtracting the output from the input. The
difference between the two is the error, obviously.


You really have no clue do you?

What actually happens in the real world needs to be considered.

In any amp with NFB connected there IS some distortion at the output.

Consider most tube amps with global series voltage negative feedback.

Let us consider a typical tube amp example where Open Loop Gain, OLG =
Vo / Vin = 20V / 0.2V = 100.

Let us suppose there is a gain reduction of 1/6 times due to the use of
the NFB and that
Closed Loop Gain, CLG, gain which is enclosed by a loop of FB, = 16.

So to make 20V at the output, we need 20V / 16 applied at the input, =
1.25V.

Therefore the feedback voltage is 1.25V - 0.2V = 1.05V.

The fraction of the output fed back = 1.05 / 20 = 0.05 and is called ß.

The output signal of 20V is reduced by a resistance divider to 1.05V and
applied to the input tube's second input point, the cathode.

Let us suppose there is 0.2% THD at the output when NFB is applied, ie,
CLG THD = 0.2%.

This amounts to 20V x 0.2/100 = 0.04V.

The resistance divider reduces this THD from 0.04V to 0.04 x ß = 0.002V.

There is NO distortion in the input grid signal or 1.25V.

At the cathode, there is an undistorted signal of 1.05V PLUS a sample of
the
amp's distortion = 0.002V.

The distortion is amplified by the OLG of 100 to become 0.2V at the
output.

But we must remember phase relationships, so must nominate input signal
phase at 0 degrees and being +.
The amp is arranged so the output signal is the same phase as input
phase, so its phase is also +.
But any signal aplied to the input triode cathode will produce a phase
that is opposite to the grid made signal,
so output from the cathode is 180 degrees different and -.

So the grid input is a "non-invering input".
The cathode is an "inverting input".

So, we have +1.25V at input grid, and this makes +20V at output.
There is +0.04V of THD at the output, reduced to +0.002V by R divider
and applied to cathode.

This applied signal becomes -0.2V at the output.

This seems to be an incomprehensible situation because we MEASURED
+0.04V of THD at the output.

How the hell can we say that -0.2V of THD exists where we know there is
+0.04V??????????????????????????????????????????

We have not taken into account the distortion without NFB applied.

The 0.04V THD we measured is the DIFFERENCE between the OLG THD and the
CLG THD.

So if the difference = +0.04V, and we can prove we have -0.2V of
inverted THD at the output,
then there must be also a non inverted THD signal present at well.
This signal = +.04V - ( -0.2V ) = +0.24V.

IE, we have +0.04V THD measured residual or difference at the output
plus -0.2V THD "correction signal", so the
distortion without NFB produced by the amp must have been +0.24V.

0.24V of THD existant within 20V of signal = 100 x 0.24/20 % = 1.2%
without NFB

The OLG THD has been reduced by 1/6 times, the same amount as the amount
of OLG gain reduction
due to applied NFB.



Every amp or amp stage is subject to the same behaviour if it has any
FB.

And my explanation is the simplest way for the Ian Ivesons of this world
to understand.

My method allows anyone to work out what the OLG THD might be without
measuring it
or disconnecting the loop NFB resistances.

In an SS amp, OLG might be 20,000, and CLG = 20, same as our tube amp.

If the THD with NFB = 0.002%, and ß = 0.04, then what must the OLG
distortion have been?

If you have followed my reasoning, you'll be able to work it out.

If you simulate it, because you are ****in lazy, and can't calculate or
can't/won't measure anything,
then the answers should be the same.

If you have an amp you can measure, what do you measure?


The above reasoning of mine is somewhat simplistic though.

The amplified sample of distortion is also distorted by the slightly
mon-linear OLG character
of the amp, so other harmonic products are generated and "sent around
the loop".
These products are also reduced by the NFB action depending on the phase
response of the amp,
ie, its OLG F response profile.

Not only that, as VO increases the OLG THD% becomes larger and there are
IMD products
generated which become significant.

But in most amps where OLG THD is low such as in the case of the above
tube amp with OLG THD = 1.2%,
the "second order" generation of additional harmonics are negligible.

If the same tube amp is operated mainly at an average of 2V output
instead of 20V,
the second order products are indeed utterly inaudible.

Therefore, my simplistic description and reasoning is valid ENOUGH.





Compensating a feedback loop at the input stage
also reduces response peaks at intermediate gain stages
that can
occasionally cause such
problems as unexpected saturation.

***The more common idea used to reduce HF OLG gain is to
have a HPF
at the amp input, and not more C from V1 to grid.

Perhaps you should read more carefully and with a more
constructive attitude. If you learn about feedback
(simulation would help), you should be able to see a
*fundamental* difference between the two.

I KNOW the difference, clot-head.

I have also tried the idea of local shunt FB to reduce the
OLG of the amp.

I know why I prefer my way.

Patrick Turner.

Ian

[Ian Iveson]

Patrick Turner wriggled, squirmed and obfuscated, to no
avail:

Ian Iveson wrote:

Patrick Turner wrote:

Ignoring irrelevant sniping:

***The "inside the amp signal" is called the "error
signal" because it
contains an an opposite phase of the distortion
at the output which goes on to cancel the open loop
distortion and
reduce this to the lower
amount we see when NFB is connected.

Er...eh?

An error signal is called an error signal because it is
derived by subtracting the output from the input. The
difference between the two is the error, obviously.

You really have no clue do you?

Don't you believe me?

What actually happens in the real world needs to be
considered.

Not necessary in order to know why an error signal is so
called. As for what an error signal may contain, which is
another matter, I dare say that Norman covers that, so I
don't know why you're batting on about it here.

You've been "explaining" this stuff for years and years and
none of your devotees are any the wiser, in case you haven't
noticed. Why do so many ppl feel the need to demonstrate to
the world that they know something about feedback?

[cut lengthy obfuscation]

Compensating a feedback loop at the input stage
also reduces response peaks at intermediate gain stages
that can
occasionally cause such
problems as unexpected saturation.

***The more common idea used to reduce HF OLG gain is
to
have a HPF
at the amp input, and not more C from V1 to grid.

Perhaps you should read more carefully and with a more
constructive attitude. If you learn about feedback
(simulation would help), you should be able to see a
*fundamental* difference between the two.

I KNOW the difference, clot-head.

It's obvious from what you wrote that you don't.

I have also tried the idea of local shunt FB to reduce the
OLG of the amp.

Very impressive. Wow.

I know why I prefer my way.

Because you can grope your way to a compromise, and because
you and your stupid customers are all half deaf.

Ian

[Patrick Turner]

Ian Iveson wrote:

Patrick Turner wriggled, squirmed and obfuscated, to no
avail:

Ian Iveson wrote:

Patrick Turner wrote:

Ignoring irrelevant sniping:

***The "inside the amp signal" is called the "error
signal" because it
contains an an opposite phase of the distortion
at the output which goes on to cancel the open loop
distortion and
reduce this to the lower
amount we see when NFB is connected.

Er...eh?

An error signal is called an error signal because it is
derived by subtracting the output from the input. The
difference between the two is the error, obviously.

You really have no clue do you?

Don't you believe me?

Very rarely do I ever believe you. This is often because you are so
obsure
and long winded about everything and anything. Your'e getting worse as
you drift towards the grave,
and I have less time myself to contemplate your bull****....

Just letting you know how many ppl here feel about you...

What actually happens in the real world needs to be
considered.

Not necessary in order to know why an error signal is so
called. As for what an error signal may contain, which is
another matter, I dare say that Norman covers that, so I
don't know why you're batting on about it here.

You've been "explaining" this stuff for years and years and
none of your devotees are any the wiser, in case you haven't
noticed. Why do so many ppl feel the need to demonstrate to
the world that they know something about feedback?

YOU are the only **** here who says such utter garbage.

About 450 ppl visit my website daily, to learn, to read, to assist
themselves towards sonic bliss using vacuum tubes.



[cut lengthy obfuscation]

Compensating a feedback loop at the input stage
also reduces response peaks at intermediate gain stages
that can
occasionally cause such
problems as unexpected saturation.

***The more common idea used to reduce HF OLG gain is
to
have a HPF
at the amp input, and not more C from V1 to grid.

Perhaps you should read more carefully and with a more
constructive attitude. If you learn about feedback
(simulation would help), you should be able to see a
*fundamental* difference between the two.

I KNOW the difference, clot-head.

It's obvious from what you wrote that you don't.

You really are a ****ing dumb ****.

This utter fuctard denounces what I have said but won't take the trouble
to
quote withinh ALL OF MY TEXT to proove I'm wrong.

So, apart from the appalling netiquette, he doesn't know how to debate
properly.

Why does he hang around? he's like a fart in a phone box.

Someone please open the door and let the smell outa here.

I have also tried the idea of local shunt FB to reduce the
OLG of the amp.

Very impressive. Wow.

I know why I prefer my way.

Because you can grope your way to a compromise, and because
you and your stupid customers are all half deaf.

Unfortunately, your experience in building and testing amplifiers and
repairing them
is probably only 1% of mine.

AFAIK, you have constructed 1 amplifier in the last 8 years.

AFAIK, Iveson has ZERO customers who treasure his amps above all others.

Does anyone else here want to take any notice of such an
unexperienced long winded obstinate, ignorant idiot as Iveson?

Patrick Turner.



Ian