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Old 10th June 2011, 07:22 PM   #1861
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Quote:
Originally Posted by michaelkiwanuka View Post
True! There is no such thing as a zero feedback amplifier.
Really? I'm listening to a very nice amplifier with no loop and no
degeneration. Where's the feedback in that?

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Old 10th June 2011, 07:54 PM   #1862
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Quote:
Originally Posted by Nelson Pass View Post
Really? I'm listening to a very nice amplifier with no loop and no
degeneration. Where's the feedback in that?

Really, even including what can be internal to the semiconductors? Does the amplifier have voltage gain?


ES
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Old 10th June 2011, 08:23 PM   #1863
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Quote:
Originally Posted by AB5NI View Post
I'll be sure to pick it up next paycheck, Bob. Can always use another good tech book for the library.



Great info for me, Bob. I guess you could say that, in a perfect world, there would be absolutely no reason for negative feedback, and everything would be stable as hell without the use of it. Actually, I can see this happening in the not-too-distant future, where nano-tech graphene chips are monitoring linearity, with circuitry making minute adjustments to make absolutely sure things stay that way, but that's a ways off yet . Well, now that i think about it, they'd probably have to use minute levels of negative feedback to stabilize the signal. lmao


BTW, are u guys fired up about graphene, and have u considered the ramifications of it's use at audio? I can only imagine what a great amp is going to sound like in the next few years .


Nice typing at ya, Bob, and best regards...

Randy
Hi Randy,

I think most things in nature have some sort of negative feedback if you look closely enough in the right way. It is certainly true that the process of driving a car involves negative feedback. I would hate to be on the road with someone who was driving open-loop.

Come to think of it, when someone is texting they are driving open-loop. We know what happens to some of them.

With respect to graphene, each new technology brings with it its own promises and its own challenges. When decent power BJTs came along we thought that SS amps would always win, hands down. Then we found out otherwise - they had their own set of problems.

Cheers,
Bob
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Old 10th June 2011, 09:22 PM   #1864
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Quote:
Originally Posted by simon7000 View Post
Really, even including what can be internal to the semiconductors? Does the amplifier have voltage gain?
The argument that there is no such thing as no-feedback contributes very
little. Perhaps the patent office really should have denied Black's patent
based on the idea that nature has already imposed loop feedback and
degeneration internal to gain devices.

While I'm at it, I will repeat my assertion that loop feedback and degeneration
are two different things. They share some common effects, but each has
unique attributes.

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Old 10th June 2011, 09:29 PM   #1865
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Quote:
Originally Posted by Nelson Pass View Post
.... I will repeat my assertion that loop feedback and degeneration
are two different things. They share some common effects, ....
Even instability (under certain conditions)
__________________
Een volk dat voor tirannen zwicht, zal meer dan lijf en
goed verliezen dan dooft het licht…(H.M. van Randwijk)
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Old 10th June 2011, 09:35 PM   #1866
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Quote:
Originally Posted by Nelson Pass View Post
The argument that there is no such thing as no-feedback contributes very
little. Perhaps the patent office really should have denied Black's patent
based on the idea that nature has already imposed loop feedback and
degeneration internal to gain devices.

While I'm at it, I will repeat my assertion that loop feedback and degeneration
are two different things. They share some common effects, but each has
unique attributes.

Black did an interview where he explained the difficulty in getting the patent. It seems up until then everyone knew that feedback always resulted in oscillation!

No I haven't followed your definitions of feedback. The start of the reasoning was that it is impossible to build an amplifier without feedback. If you define that as a loop then the answer is a trivial of course you can and there are lots of examples. I thought you might actually have come up with a clever way of avoiding all versions.
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Old 10th June 2011, 09:50 PM   #1867
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Quote:
Originally Posted by Nelson Pass View Post
The argument that there is no such thing as no-feedback contributes very
little. Perhaps the patent office really should have denied Black's patent
based on the idea that nature has already imposed loop feedback and
degeneration internal to gain devices.

While I'm at it, I will repeat my assertion that loop feedback and degeneration
are two different things. They share some common effects, but each has
unique attributes.

Hi Nelson,

I agree with you; I'm sure neither Black nor the patent office had local degeneration in mind. Its really all a matter of degree. Some rule out global NFB, some rule out local loops, some rule out DC servos - to each his own.

Personally, if I was going to market an amplifier that claimed to have no negative feedback, I would be OK with using emitter degeneration for sure.

However, for those who talk about re-entrant distortion (after Baxandall), it is important for them to understand that the re-entrant distortion mechanism exists just as much in emitter degeneration as it does in global feedback. The re-entrant distortion argument does not really hold in the real world anyway, so this argumemt is a bit academic.

Cheers,
Bob
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Old 10th June 2011, 10:32 PM   #1868
Zero D is offline Zero D  United Kingdom
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Default Re - Negative Feedback

Another great book, with Lots of info/data/circuits etc, is Douglas Self's - Audio power amplifier design handbook

According to him in there,

Chapter 2 - Page 44 - Negative Feedback in Power Amplifiers

Quote:
Some Common Misconceptions about Negative Feedback

All of the comments quoted below have appeared many times in the hi-? literature. All are wrong.

Negative feedback is a bad thing . Some audio commentators hold that, without qualification, negative feedback is a bad thing. This is of course completely untrue and based on no objective reality. Negative feedback is one of the fundamental concepts of electronics, and to avoid its use altogether is virtually impossible; apart from anything else, a small amount of local NFB exists in every common-emitter transistor because of the internal emitter resistance. I detect here distrust of good fortune; the uneasy feeling that if something apparently works brilliantly then there must be something wrong with it.
*

Susan Parker, yes SUSAN ! has designed this.

Zeus Power Amplifier

Quote:
A Zero Feedback Power Amplifier, for Audio and Other Applications.

There is no overall negative feedback. The only feedback mechanism is within Q1 and Q2 as they operate in voltage follower mode and regulate the voltage across the source / drain to match that of the gate ( less the semiconductor voltage drop ).
http://www.susan-parker.co.uk/zeus-about.htm

*

So it appears that feedback does exist, in one form or another, whatever might be stated, and/or claimed !
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Last edited by Zero D; 10th June 2011 at 10:35 PM.
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Old 10th June 2011, 11:48 PM   #1869
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Well, it's physically true that some infinitesimal portion of me is hanging out
in the Andromeda galaxy, but we should be able to ignore this and have a
practical discussion about my present location.

I choose to ignore the tiny resistance of the emitter connection in a
discussion of degeneration because it beings nothing to the table. Similarly
the "internal loop feedback" represented by the gain dependence on Vce,
(although I personally find that very interesting).

I appreciate that it annoys technical people when audiophiles
unreasonably dismiss the use of feedback. At the same time, I don't think
it's reasonable to try to annoy them back by insisting that everything has
feedback and that all forms of feedback are the same.

Surely we are bright enough here to recognize that the issue is more
complex than that.

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Old 11th June 2011, 02:19 AM   #1870
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Quote:
Originally Posted by Nelson Pass View Post
Well, it's physically true that some infinitesimal portion of me is hanging out
in the Andromeda galaxy, but we should be able to ignore this and have a
practical discussion about my present location.

I choose to ignore the tiny resistance of the emitter connection in a
discussion of degeneration because it beings nothing to the table. Similarly
the "internal loop feedback" represented by the gain dependence on Vce,
(although I personally find that very interesting).

I appreciate that it annoys technical people when audiophiles
unreasonably dismiss the use of feedback. At the same time, I don't think
it's reasonable to try to annoy them back by insisting that everything has
feedback and that all forms of feedback are the same.

Surely we are bright enough here to recognize that the issue is more
complex than that.

Hi Nelson,

Every incandescent lightbulb has negative feedback in it :-).

Just kidding.

BTW, I agree that it is silly of Self to characterize the dynamic emitter resistance of a BJT (1/gm) as negative feedback.

Cheers,
Bob
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