Feedback

Do you guys like or dislike feedback in amplifiers?

I know it is a question that . . . . -_- sigh

But still, what do you think?

I think we can't live without feedback, so what is ur feedback?

Lost'n Found wrote:

> Do you guys like or dislike feedback in amplifiers?
>
> I know it is a question that . . . . -_- sigh
>
> But still, what do you think?
>
> I think we can't live without feedback, so what is ur feedback?

Say one forgets to mow the lawns.

Wife says, "lawns are looking a bit ragged dear"

Youse start thinkin, "Hmm, no sex tonight if I don't mow the lawns..."

So you go mow the lawns.

The natural order if for man to not mow lawns as often as the audience
around him wishes, and this is his mistake.

The error is pointed out by someone close to him, and action is taken to

correct the mistake and mow the fukkin lawn.

The lawns which are the signal now looks clean and tidy;
the audience is happy.

However such error corrections in electronic NFB are so fast, and happen

with such rapidity that the corrections of errors occur while errors are
made,
and it as if the wife is intimately in contact with the man's brain and
has him
cutting each blade of grass shorter while it grows longer,
so the apearance of the lawn is always beautiful.

The negativity of such feedback may seem indeed to be horridly negative,

nobody likes to be henpecked that much, but in ampifiers that's what
occurs,
only ithe reality is that the mistakes the amp makes are fed back in
opposite phase to oppose their own creation
as they are created so less of a mistake occurs.

Under such circumstances, and considering that no correction system is
perfect
because it includes the forward path of the misbehaving amp
then there are mistakes in this mistake fixing process and it results in

some ""second order"" harmonic products being formed which under some
circumstances
make the amp sound worse than if no NFB was applied.

But where the distortion is less than 10%, the bandwidth without NFB is
adequate,
phase shift low, and applied NFB
about over 14dB, the reduction of THD is going to sound better than had
nothing been done
and no FB applied.

There are many books on the subject of NFB.
Have you read any?

Patrick Turner.

Lost'n Found wrote

> Do you guys like or dislike feedback in amplifiers?
>
> I know it is a question that . . . . -_- sigh
>
> But still, what do you think?
>
> I think we can't live without feedback, so what is ur feedback?

Ah, well, erm...we seem to like arguing about it, anyway.

Firstly, on the definition of feedback. Every real dynamic system
contains feedback in the sense of regulation due to load, or dynamic
equilibrium. This sense is trivial for our purposes.

Engineers have a particular meaning, however, which comes from and is
precisely defined by control system theory. One part of this
definition is the "canonical system diagram", which represents a
system as a forward function, whose output is summed with (or
subtracted from) the system input, and whose input is that sum. There
is no function in the feedback path. The implication is that the
forward and feedback paths must be separate and distinct. If there is
in reality only one path from input to output, then the analysis
becomes trivial, and there is no point in considering it as a system
with feedback.

Feedback in the trivial sense cannot be a bad thing because it is in
everything. Hence if it is bad, there is no such thing as good, in
which case bad becomes as trivial as the feedback: it doesn't exist.

Feedback in the engineering sense is a bad thing, IMHO. Often,
however, it is less bad than its absence in a particular real circuit.
Think of it like medicine. Never a good thing, but better than being
sick.

Bad because it is a complication. Because it is problematic and the
problems require solutions at the expense of further complication.
Because the complication is not euphonic and so, because the solutions
are never perfect, the inevitable faults aren't musical.

The issue is strongly linked to another. Some believe that domestic
audio systems should be excluded from the category of musical
instrument, on the grounds that their function is merely to reproduce.
It follows from this premise that performance can be measured in terms
of deviations from some original music that happened somewhere else,
or at a different time, or both. Performance measured by such criteria
is always improved by any reduction in the sum of deviations.
Perfection is guaranteed if every deviation is zero, and hence the sum
of deviations is also zero. Such perfection cannot be achieved without
feedback, and in reality so far feedback has been found necessary to
get even close.

Several problems arise from that view. Such perfection is not actually
possible: deviation is never quite zero. On the simple face of this,
there is no agreed method of combining various deviations, or kinds of
distortion, into a single measure. Hence there is no single measure of
quality. As a simple example, if I can reduce 2H distortion by 20dB at
the expense of creating 3dB of extra 7H and a spot of crossover
distortion, is that an improvement? Or a matter of taste...perhaps
even statistically average taste? Second, rather less simply, it is
close to self-evident to say that the idea of reproducing the sound of
a symphony orchestra or a rock concert in my room is impossible. Two
speakers in this room is just never going to do that. Now, it is
possible to see this in terms of error, as an engineer might, and end
up with another matter of taste.

I believe there is a serious philosophical question behind all this. I
don't actually *want* a reproduction. Music is music...it is
indubitably here in my room...my system is making actual real music.
Judging it by the sum of differences is just not appropriate. Fidelity
is not the same thing as precision. I want my music to be coherent *in
its own right*, to have its own spirit and life. I want to hear the
music the musicians play, not what they measure. That is not a matter
of taste, but one that begs philosophical interpretation.

The same question arises in a different form if I ask whether there is
a single, perfectly right and proper way of playing, say, Beethoven's
5th? Obviously not, so is every performance, regardless of any measure
of quality, of equal merit? Obviously not. How do we define fidelity?
How do we judge performance?

When it seems just right, just now. It is a single feeling, not a sum
of differences. It is unlikely to come from a system with lots of
feedback, which is aimed at the sum of differences. Some report it is
possible, more or less just about nearly, with none. Or very little.
You'll know when, or not.

cheers, Ian










"in message . ..
>

Lost'n Found wrote:

> Do you guys like or dislike feedback in amplifiers?

It's not really a case of liking or disliking it.

You can't make a very linear amplifier without using it. Case closed.

A LOT of nonsense is talked about feedback, mostly by the uninformed /
foolish / gullible / wannabe crowd.

Graham

"Lost'n Found" > said:

>Do you guys like or dislike feedback in amplifiers?
>
>I know it is a question that . . . . -_- sigh
>
>But still, what do you think?
>
>I think we can't live without feedback, so what is ur feedback?


Every amplifier uses feedback, but us tubies usually only use the
"local feedback" variety.

A cathode resistor is local feedback, and some even argue that a
triode has inherent feedback.

So it's not a matter of liking or not liking, it;s just there.

Now loop feedback, that's something different.
If possible, I try to avoid it in my tube amps, usually because they
apparently don't need it.
But when they do, I don't hesitate to use it with care.

I know of no (commercial) transistor amp that can work without loop
feedback.

--
"Due knot trussed yore spell chequer two fined awl miss steaks."

Sander deWaal wrote:

> A cathode resistor is local feedback, and some even argue that a
> triode has inherent feedback.

Strictly speaking that's an *unbypassed* cathode resistor but I know
what you're driving at.

<snip >

> I know of no (commercial) transistor amp that can work without loop
> feedback.

It could be done though. It's simply not conventional to do so.

Graham

In article >,
Eeyore > wrote:

> Sander deWaal wrote:
>
> > A cathode resistor is local feedback, and some even argue that a
> > triode has inherent feedback.
>
> Strictly speaking that's an *unbypassed* cathode resistor but I know
> what you're driving at.
>
> <snip >
>
> > I know of no (commercial) transistor amp that can work without loop
> > feedback.
>
> It could be done though. It's simply not conventional to do so.

Wasn't it done about a year ago right here in this group, although it
wasn't commercial which I assume is the reason for the disclaimer.


Regards,

John Byrns

John Byrns > said:


>> > A cathode resistor is local feedback, and some even argue that a
>> > triode has inherent feedback.


>> Strictly speaking that's an *unbypassed* cathode resistor but I know
>> what you're driving at.


That's what I meant, sorry.
It's njust not a habit of mine to bypass cathodes ;-)


>> > I know of no (commercial) transistor amp that can work without loop
>> > feedback.


>> It could be done though. It's simply not conventional to do so.


>Wasn't it done about a year ago right here in this group, although it
>wasn't commercial which I assume is the reason for the disclaimer.


Yup. I've tried it with BJTs, and even my hybrid (MOSFET out) amps
don't use global feedback for AC (there is a DC servo loop, though).

But commercially, I have not seen it.
Densen claimed they did it, but it turned out there *was* global
feedback in there..........

But we're digressing into silicon again, gentlemen, some readers will
punish us for that ;-)

--
"Due knot trussed yore spell chequer two fined awl miss steaks."

John Byrns wrote:

> Eeyore > wrote:
> > Sander deWaal wrote:
> >
> > > A cathode resistor is local feedback, and some even argue that a
> > > triode has inherent feedback.
> >
> > Strictly speaking that's an *unbypassed* cathode resistor but I know
> > what you're driving at.
> >
> > <snip >
> >
> > > I know of no (commercial) transistor amp that can work without loop
> > > feedback.
> >
> > It could be done though. It's simply not conventional to do so.
>
> Wasn't it done about a year ago right here in this group, although it
> wasn't commercial which I assume is the reason for the disclaimer.

I'm not sure. It's entirely possible I made the same claim a year ago
though.

I know exactly how to do it. It's almost trivial in fact. I guess it might
actually be commercially viable given the crazy claims being made about
feedback.

Graham

Eeyore wrote:
> John Byrns wrote:
>
> > Eeyore > wrote:
> > > Sander deWaal wrote:
> > > > I know of no (commercial) transistor amp that can work without loop
> > > > feedback.
> > >
> > > It could be done though. It's simply not conventional to do so.
> >
> > Wasn't it done about a year ago right here in this group, although it
> > wasn't commercial which I assume is the reason for the disclaimer.
>
> I'm not sure. It's entirely possible I made the same claim a year ago
> though.

You're tripping over your ego again, Poopie, thinking everything
everyone says relates to you. But Mr Byrns is referring to an
outstanding solid state amplifier designed by that great post office
engineer Stewart Pinkerton.

> I know exactly how to do it. It's almost trivial in fact.

Go for it, Longears. We'll agree the spec and then you can draw,
develop and publish the design, running a thread here all the time with
an account of how you're going. Be a way for you to regain some
respect.

I guess the first parameter should be ZNFB, though we might have to be
a bit flexible about that.

>I guess it might
> actually be commercially viable given the crazy claims being made about
> feedback.

Forget a commercial deal. That's too tough a test to set; designing the
thing to a ZNFB parameter is a tough enough nut to crack.

> Graham

Andre Jute
Visit Jute on Amps at http://members.lycos.co.uk/fiultra/
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