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Old 19th February 2007, 07:58 PM   #1
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Default What is nested feedback, how it realy works and some examples...

There`s not so many threads about nested feedback round here.

There are some examples of such a design like AKSA`s "Lifeforce" and Greg BAll`s "SKA" but the schematics are not available. Everyone saying that these amps sound wonderfull.

ANyone have schematic or example?

It would be nice of someone to tell how it realy works and wath`s the trick.

Cheers!
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Old 19th February 2007, 08:04 PM   #2
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I didnt ment Aksa`s and Greg`s schematics (I know that only kit buyers have them and I respect that).

I meant some other design exaples of amp with nested feedback? Anyone round here designed such a amp?
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Old 19th February 2007, 08:05 PM   #3
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This should get you started:

http://www.sx.ac.uk/ese/research/aud...20feedback.pdf

http://www.personal.reading.ac.uk/~s.../pof/pof20.pdf

NDF is quite complex, and the Cherry paper is difficult to read.
NDF gives you a bit more feedback gain at the freq extremes and so measurably improves linearity there.
Whether it is audible of course depends on a LOT of other factors.
Due to the complexitity in stabilizing each FB loop and the limited advantages it never really caught on.

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Old 19th February 2007, 08:09 PM   #4
Duo is offline Duo  Canada
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Nested feedback implies certain feedback loops (AKA: local feedback) in an amplifier of some sort that also has global negative feedback.

An example of nested feedback in an amp using global feedback would be a voltage gain stage using an emitter/source/cathode resistance to degenerate the gain of the stage.

Another possibility would be degeneration resistances in the legs of a differential pair. Resistors would be added to the emitter/source/cathode of each side of a long/short tailed pair to degenerate the active devices and reduce distortion.

The major concept here is to reduce the inherent distortion of each important part of the amplifier separately so that the reliance on global feedback is not so heavy. This is important for reducing the dynamic distortions like intermodulation distortion and others.

Hopefully that clears a bit up. Others will have more on this.
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Old 19th February 2007, 08:17 PM   #5
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Quote:
Originally posted by Duo
[snip]An example of nested feedback in an amp using global feedback would be a voltage gain stage using an emitter/source/cathode resistance to degenerate the gain of the stage.

Another possibility would be degeneration resistances in the legs of a differential pair. Resistors would be added to the emitter/source/cathode of each side of a long/short tailed pair to degenerate the active devices and reduce distortion.

[snip]

Not correct. Please review the references.

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Old 19th February 2007, 08:22 PM   #6
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My bad on the terminology, my examples are not technically nested. They are local.
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Old 20th February 2007, 12:14 AM   #7
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Local feedback loops are “nested” in the sense of being enclosed in the global feedback loop

if “local feedback” is defined as feedback around single stage then of course only a single level of “nesting” is possible, however I know of no rule that “local” is restricted to only one stage - but rather is just any feedback loop that isn’t global

some common audio amp circuits can be seen as more levels of nesting:

emitter degeneration of a VAS with a conventional Miller C is “inside” both the Miller loop and the Global loop for 2 levels of nesting, as would be either input or output unity gain followers buffering the VAS Q

Sziklai pair/triple output stages also add one or two more feedback nesting levels beyond the one level of nesting that results from unity gain follower output stage being considered a local feedback itself enclosed in the global loop

these examples are not regularly structured and extensible as is the Cherry scheme but I think it is still fair to call them examples of nested feedback
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Old 20th February 2007, 12:19 AM   #8
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JCX: I agree. This is what I had based my first post on. I understand that the nomenclature sways a little, but from the literal meaning of the word 'nested' it is plausible.
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Old 20th February 2007, 07:00 AM   #9
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You are all correct, I suspect.

Nested fb is only intuitively understood not to include local feedback around gain elements. Nested refers more to two fb loops, one global from output to input, and another across two stages or more inside the global loop.

Jan is right. These designs are much more difficult to stabilise, and there are Zout issues because the global fb is diminished by the internal, nested fb. But the advantages are increased resolution, flatter power bandwidth, and higher tolerance to reactive loads.

Cheers,

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Old 20th February 2007, 07:05 AM   #10
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Quote:
Originally posted by AKSA
You are all correct, I suspect.

Local is only intuitively understood not to include local feedback around gain elements. Nested refers more to two fb loops, one global from output to input, and another across two stages or more inside the global loop.

Jan is right. These designs are much more difficult to stabilise, and there are Zout issues because the global fb is diminished by the internal, nested fb. But the advantages are increased resolution and higher tolerance to reactive loads.

Cheers,

Hugh

Yes. Nested feedback as developed by Cherry is a very specific nest of feedback loops, starting all at the output and enclosing progressively more stages until the most outer loop goes back to the input stage as normal global feedback.

The aim is to tailor the feedback gain freq response in such a way that the max feedback is possibly where it is most needed: at the high freq end of the bandwidth.

I would really recommend to read the Cherry paper (I'l see if i can find it) before commenting on it.

Jan Didden
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File Type: pdf page 1 from cherry pa improver.pdf (70.6 KB, 327 views)
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