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Old 22nd March 2012, 06:36 PM   #101
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[QUOTE=DF96;2955986] This is the reason why the amplifier should be as linear as possible open-loop, before you close the loop.

QUOTE]

I came across a non linear issue created by a faulty tube in a diff circuit: the global feedback loop did a perfect rescue job. This implies that your issue of non linearity before closing the loop is questionable as global feedback loop is a very powerful correcting tool.
Another example is the small signal pentode front end in a 3-4 stage power amp. The screen g2 volts can be varied, drastically effecting the stage distortion; then apply global neg feedback, and the distortion drops regardless of where g2 is set !

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Old 22nd March 2012, 06:45 PM   #102
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Quote:
Originally Posted by DF96 View Post
Feedback works properly with a phase shift. Two ways to see this:
1. do the maths
2. trust those who can do the maths when they tell you it works
Is the maths done with music or with a sine wave?
For instance using the maths the PSRR of an SRPP is -6dB, but in reality it isn't, is it?

Richwalters:
I think then that the inverted correction signal is not affected in the same way as the original - perhaps because it's the inverse!

So if NFB works perfectly and corrects all of the non-linearities in an amplifier, how is it that all amplifiers have their own sonic signature?

Or do they all really sound the same and we're just chasing our tails?
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Old 22nd March 2012, 06:52 PM   #103
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by Globulator View Post

So if NFB works perfectly and corrects all of the non-linearities in an amplifier, how is it that all amplifiers have their own sonic signature?

Or do they all really sound the same and we're just chasing our tails?
LOL..

Nice one..we know they all sound the same..its just that some sound more like the same than others...

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Old 22nd March 2012, 07:05 PM   #104
DF96 is offline DF96  England
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No feedback loop can do a "perfect rescue job". My statement that the amp should be as linear as possible before closing the loop is true. If you can halve the open loop distortion, then the extra distortion added by feedback will be only a quarter of what it would otherwise be. Of course it is possible to develop an amp which is so pathological (e.g. non-monotonic in Vout vs. Vin) that feedback can't help, which is why it is usually stipulated that a quasi-linear approximation is used - this excludes pathological horrors but includes most real amps.

The maths is not done with music or a sine wave but an arbitrary signal. Distortion is anything in the output which is not in the input.

I have never measured the PSRR of an SRPP, but in the usual balanced configuration with both cathode resistors unbypassed it will be around -6dB. If the low cathode is bypassed then it will be better, maybe -9dB. Are you saying that valves and resistors do not obey circuit theory? If your measurement differs markedly from these figures then there is something wrong with your measurement technique.
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Old 22nd March 2012, 07:41 PM   #105
M Gregg is offline M Gregg  United Kingdom
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I don't agree or disagree with this,

However here is another take on the same feedback argument:

Go down to Negative Feedback

Lenard Audio - Education - Valve Amps

And so all amps sound the same except for phase shift caused by component distortion?

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M. Gregg
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Old 22nd March 2012, 07:56 PM   #106
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Quote:
Originally Posted by M Gregg View Post
However here is another take on the same feedback argument:

Go down to Negative Feedback

Lenard Audio - Education - Valve Amps
I hate to be blunt, but this part of this webpage is a bunch of stuff that fell outta the end of a horse. Most of the rest is not bad at all. Just my opinion, of course.

Thanks,
Chris
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Old 22nd March 2012, 08:00 PM   #107
M Gregg is offline M Gregg  United Kingdom
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Originally Posted by Chris Hornbeck View Post
I hate to be blunt, but this part of this webpage is a bunch of stuff that fell outta the end of a horse. Most of the rest is not bad at all. Just my opinion, of course.

Thanks,
Chris
Yes LOL nothing like food for discussion.

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Old 22nd March 2012, 08:02 PM   #108
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Quote:
Originally Posted by Globulator View Post
Has anyone done any papers on NFB through a non-linear amplifier? The phrase 'reasonably linear' worries me as it implies a 'not notice' factor whereas I am after the best solution, not the usual boringly mediocre ones .
The earliest that I've heard of was by Baxandall and the latest is by Bruno Putzeys in Linear Audio Vol.1.

"Reasonably linear" means that we can use a simplified equation. The exact equation isn't simple, but it does exist - feedback is still completely predictable.

All good fortune,
Chris
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Old 22nd March 2012, 08:15 PM   #109
DF96 is offline DF96  England
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Quote:
Originally Posted by M Gregg
However here is another take on the same feedback argument:

Go down to Negative Feedback

Lenard Audio - Education - Valve Amps
The early stuff is not too bad (only a few errors) but his ideas about NFB are complete nonsense. His claim that 12dB is about the max that can be used would be surprising to the Mullard designers of the 5-20 which uses around 30dB of feedback.

What is it about feedback that seems to create such ignorance and nonsense?
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Old 22nd March 2012, 08:24 PM   #110
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Quote:
Originally Posted by Chris Hornbeck View Post
The exact equation isn't simple, but it does exist - feedback is still completely predictable.
Yes, if you have complete mathematical models of all componsnts. I want one!
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