Re: NFB101 part 3
- From: "Ian Iveson" <IanIveson.home@xxxxxxxxxxxxxxxx>
- Date: Tue, 3 Jun 2008 16:36:46 +0100
flipper wrote:
In a CF the feedback is taken from directly across
the
load and is
therefore shunt derived. Remember shunt derived
feedback
lowers output
impedance which is why a CF has a low output
impedance.
RDH4 does not agree with what you are saying.
The difference is easily misinterpreted. An unbypassed
cathode resistor provides current-derived feedback if
the
output is from the anode, but not if it is from the
cathode.
At least I think that's what these folk mean. I think it
is
simpler, if the feedback is a voltage, to describe where
it
comes from in each case, rather than rely on a general
term
that is easily misunderstood.
That's because RDH4 is old, apparently , but modern
texts
will specify
both ends just as he says.
The feedback in a standard Williamson or a cathode
follower
is known as series voltage negative feedback.
RDH4 is apparently speaking of the input side only but
look back at my
headphone amp thread. There are two ways to get a
signal
from the
output end. One is what most would likely consider
'normal': across
the load. That's shunt derived
The other would be to put a current sense R under the
load
and feed
that signal back just as the other was. That's series
derived.
RDH4 doesn't provide any means to describe the two but
they will
behave quite differently.
Which is where I rather gave up on that thread. You are
trying to have your cake and eat it with that headphone
amp.
I have no idea what the hell you're talking about.
Well, so it seemed then.
You should try actually saying something other than broad
generalizations ending with "It should be possible to find
the data."
On the one hand, you argue that headphones are
near-as-dammit resistive loads such that a 200ohm series
resistance is not an impediment in relation to 32ohm
phones,
You're the one who 'argued'
It was you who argued the case for the series resistance.
When asked why I used a series resistor on the 'PC
Speaker' amp I said
I had seen it done before and upon further research to
'refine' the
value I discovered there's an official IEC specification
(61938) for
headphone interfaces.
Which has sod all to do with fidelity. You also attempted to
justify the high output impedance of your design on other
grounds related to the characteristics of headphones.
Btw, it's 120 ohm, not 200.
Thanks.
You wanted to 'argue' with the IEC and made
generalizations, ending
with "It should be possible to find the data." Well, did
you find it?
A long time ago, thanks. I was encouraging you to do the
same, since it is you who is designing the amp.
As for your 'argument' that "In reality, unless you are
designing
headphones, you don't need to know how they work. We just
need
performance data," that's what I did. I used the IEC
'data' for the
interface.
Now you are being deliberately silly.
and on the other that series- and shunt-derived feedback
behave quite differently.
They do. How the device under control 'behaves' depends on
the device.
Not always, as I have pointed out.
How can you have it both ways? If
feedback is from a resistive load, there is no difference
at
all between series- and shunt-derived.
In the first place, I never said headphones were a
'resistive load',
that was your leap.
Not how I remember it, but if they are not resistive then
how come it's OK to drive them with a comparitively high
source resistance?
And, secondly, the nature of headphones has
nothing to do with the characteristics of the 4 basic
feedback
topologies: series-shunt, series-series, shunt-series, and
shunt-shunt.
To the extent that they are resistive, there is no
difference between series- and shunt-derived feedback.
Maybe you don't care if series-series feedback increases
the effective
output impedance if you're going into a grid but maybe you
care a lot
if it's going into a reactive load. But, in either case,
it's useful
to know what the feedback does so you can make an
intelligent
decision.
That can be done by inspection in any particular case.
Deciding whether it falls into this or that conventional
category is an extra complication that a novice doesn't need
to face.
A generalised distinction between series/current-derived
and
shunt/voltage-derived feedback is a contrivance best
avoided.
Along with how they behave differently too I suppose.
There's an awful lot of universities that will need to
scrap their
E.E. texts to meet your criteria.
There's a whole lot of universities these days that don't
deserve the title. That aside, I rather guessed that it
must
be standard fare on EE courses...
Did the gratuitous poke in the eye of un-named
'universities' boost
your ego?
Why should it? Neither was it gratuitous: I was making the
point that following the practice of universities may not be
the best choice. If I named all the universities here in the
UK not worthy of the title, it would be a long list of no
value to anyone here. We used to have universities,
polytechnics and colleges offering, more or less
respectively, academia, practical analytical skills, and
training. Some years ago, polytechnics were given university
charters, and more recently many colleges have also begun
calling themselves universities.
Consequently many large employers of graduates have adopted
their own entrance tests and, more widely many degrees are
discounted because they have been confered by institutions
of ill repute.
it smacks of received
wisdom ill-applied.
Your comment smacks of more self aggrandizing.
More? It was a jab at you, the rampant EEs, and other
self-proclaimed experts here who keep being horrid for no
good reason. There is nothing grand about independent
thought, which is commonly derided and admittedly on
occasion a hostage to fortune.
IIRC Ian is specifically not writing for
that audience, so doesn't need to complicate matters with
unnecessary and apparently misleading conventions
contrived
presumably for some wider historical purpose. You are not
alone in the field, I realise that: Morgan Jones, for
example, also makes the distinction. I just said I think
it's best avoided, and I meant in this context.
Since you "guessed that it must be standard fare on EE
courses" you
apparently haven't read any and perhaps you should before
declaring
universal wisdom on what is "ill-applied."
Possibly in other contexts too. I've got this book on SMPS
design that makes the same distinction, equally
confusingly
because by current-derived it means from sensing switch
current, whereas by voltage-derived it means from sensing
load voltage. That's two differences, so the single
distinction made by the jargon isn't enough. I had to leaf
through looking for a circuit diagram before I could be
sure
what the text was on about.
Maybe if you read some of those EE texts you wouldn't be
so confused.
Cheap enough I suppose, but you're still missing my point.
I don't need an EE degree, so why should I? That's another
point I'm making.
The word 'shunt' does not appear, because the NFB does
not
try to shunt the input signal.
The word shunt doesn't appear because they ignore how
the
signal is
derived.
Don't they say where it comes from? Isn't that enough?
They say where it goes too. Isn't that enough? So why
mention shunt
or series on that end?.
It's a simpler, more straightforward distinction at that
end.
It's the same 'straightforward distinction' on both ends.
No, as I have intimated above with the SMPS example. I could
have used the example of an unbypassed cathode resistor used
in an anode follower. The input is unambiguous as the
destination of feedback, but the source of the feedback is
not always the output. This means that the rules you apply
to your categories of feedback must be used with particular
caution with respect to its derivation.
I don't particularly want to champion RDH4 though,
I gave RDH4 another look and while I didn't do an
extensive
investigation for consistency it seems they tend to use
the terms
'current feedback' and 'voltage feedback' for the modern
shunt/series
'derived' (from the output) terminology and series/shunt
for the
input. However, RDH4 also throws in a couple of 'if then,
unless, but
not, or else' kind of caveats into their 'what they do'
table so the
modern texts are more 'universal' and consistent.
It's the ifs and buts that make received wisdom potentially
dangerous. I doubt RDH4 was wrong, so perhaps the only
difference is that EE conventions have evolved towards
greater cohesion, so the caveats may be in another chapter
of whatever book you read. Perhaps it's that cohesion that
has led to its current insularity.
and
if I'd been history I would have tried not to create such
awkward categories, or attached to them such misleading
jargon.
In particular, it contributes nothing to understanding for
the purpose of designing and building valve audio
equipment.
Just a thought but perhaps 50 years worth of experts. e.g.
designers,
authors, universities, et al, know something about it
you've missed.
Eh? How does knowing more about it make them more easily
misled? Or help them to understand more how someone outside
their world might be misled by the careless bandying about
of EE rules and conventions?
I'm right in the middle of Ian's target market, you aren't.
The customer is always right, and you're not.
Ian
.
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- Re: NFB101 part 3
- From: Ian Iveson
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