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Old 25th August 2010, 08:12 PM   #1
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Default Negative feedback and distortion

In this Wikipedia article:

Tube sound - Wikipedia, the free encyclopedia

it is said that negative feedback causes inter-modulation distortions but the references are unreachable.
Can someone give me some serious references that explain this phenomenon and maybe model it, etc. ?
I am curious if this type of distortion is the reason some audiophiles reject the circuits with negative feedback. Maybe they are onto something ?
Any comments and ideas are appreciated.
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Old 25th August 2010, 08:37 PM   #2
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Quote:
Originally Posted by fireworks View Post
In this Wikipedia article:

Tube sound - Wikipedia, the free encyclopedia

it is said that negative feedback causes inter-modulation distortions but the references are unreachable.
Can someone give me some serious references that explain this phenomenon and maybe model it, etc. ?
I am curious if this type of distortion is the reason some audiophiles reject the circuits with negative feedback. Maybe they are onto something ?
Any comments and ideas are appreciated.
This is a common mis-conception by some and based on a half-truth. A long time ago Baxandall showed that putting negative feedback around a JFET stage that was creating 10% 2nd harmonic distortion in the open loop then resulted in the creation of new, higher-order harmonic distortions, even though the negative feedback was reducing the original 2nd order distortion. This behavior essentially resulted because the "original" 2nd harmonic at the output of the amplifier was fed back to the input and given an opportunity to mix with the original signal in the second order nonlinearity, creating a 3rd order nonlinearity; and on and on...

Awhile back there was a thread here wherein it was shown that in real amplifiers there is higher order distortion to begin with, and that application of negative feedback more than about 15 dB caused a reduction in all harmonics.

Generally, a given nonlinear process will generate both harmonic distortion and intermodulation distortion. THD and IM are just different ways of looking at the symptoms of a nonlinearity. So the process described above also creates higher-order IM distortions if the amount of feedback is below the 15 dB point. BTW, local degeneration counts toward the 15 dB, so most of the time applying any additional global NFB will tend to reduce all orders of distortion.

The other thing that the Wiki might have been referring to was the creation of TIM, which is a whole 'nother can of worms that was beaten to death decades ago. The simple answer there is that TIM is created in a feedback amplifier that has not got enough slew rate. It is a distortion that is dependent on a signal's rate of change rather than its amplitude. There are many mechanisms in amplifiers that can create distortion that is a function of rate of change besides TIM, so even amplifiers that have no global negative feedback can create distortion that is indistinguishable from TIM.

Cheers,
Bob
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Old 25th August 2010, 10:49 PM   #3
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Quote:
Originally Posted by fireworks View Post
In this Wikipedia article:

Tube sound - Wikipedia, the free encyclopedia

it is said that negative feedback causes inter-modulation distortions but the references are unreachable.
Can someone give me some serious references that explain this phenomenon and maybe model it, etc. ?
I am curious if this type of distortion is the reason some audiophiles reject the circuits with negative feedback. Maybe they are onto something ?
Any comments and ideas are appreciated.
Professor W. Marshall Leach explains in great detail here: The Leach Amp - Background

I am cloning his amp , so I thought I had better re-read the material. As far as modeling it .... easy (LT spice). His "leach amp" uses less negative feedback than some but still achieves very low THD/TIM. With newer, faster devices negative feedback is NOT bad. The heck with the "audiophoolz".

BTW , I owned a tube amp and had a 200w class A ($7,000 genesis stealth) to compare to standard topologies and the only instance of the tube or class A superiority was in regard to clipping behavior (less harsh when pushed hard). At normal listening levels a DIY aksa 55 style or symasym was even superior in imaging/soundstage.

The class A $7,000 amp was nearly indistinguishable from the symasym , the latter running cool and quiet with a single pair MJL4302/4281, and the former heating up the room burning 400 watts of power among 8 MOSFETS. I guess a "phool" and his $$$ are soon parted.
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Last edited by ostripper; 25th August 2010 at 10:52 PM.
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Old 25th August 2010, 11:37 PM   #4
lineup is offline lineup  Sweden
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Quote:
Originally Posted by fireworks View Post
In this Wikipedia article:
-----
it is said that negative feedback causes ....
It is said.

There are so many things said, guessed and believed in audio.
Things that have no or very little truth in them.
We have had many topics in forum about 'Audio lies' and 'Audio myths'.
And still there are many to come ...

It is said.

And everyone of us should know not to believe in everything we hear is said.
People talk. In audio. And in normal life.
We should give things people say or write a proper examination.
Do any claims hold up to reality?
This is why it is good you ask here about this negative feedback thing.
Because here are people that know a great deal of what is true.
Bob Cordell is one good guy for give some info.

It is SAD.

It is SAD when lies, guessing and myths are published.
Especially when without any reservations.
To kill off such myths is almost impossible.
Once they start circulating they live on forever.
This is also why I have this signature, which tells it so well:
Quote:
One of the most striking differences between a cat and a lie
is that a cat has only nine lives. (Mark Twain)
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Old 26th August 2010, 12:33 AM   #5
ChrisA is offline ChrisA  United States
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I think this is one of the best explanations available on the Internet. He uses some math but I think in only one place more than high school algebra.

The short answer is that feedback poorly applied can cause some really bad problems and as most casual hobbyists don't understand feedback, it gets misapplied.
The true answer is "you have to work the numbers for your specific case".

Here is the link: http://www.angelfire.com/planet/funw...ok_CHAP-5.html

Worth reading two or three times I think. There are even exercises at the end so you can test yourself to see if you understand it.
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Old 26th August 2010, 03:36 AM   #6
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Hello

There is a good article about distortion and feedback, by Nelson Pass.

http://www.passlabs.com/pdfs/article...d_feedback.pdf

Bye

Gaetan
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Old 26th August 2010, 12:30 PM   #7
Orit is offline Orit  Italy
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This is the HiFi topic that "burns" the most between Spice'oholics and Audioholics, for sure, together with the eternal "tubes-solid state" debate.
Just my two cents: though being an electronics engineer, my knowledge is peanuts compared to the other posters here, BUT, have your own experience, try to listen to your favourite music with a well-designed low-feedback amplifier, no matter its age. If you, as me, end up with goose-skin and tears, you'll agree that some aspects of negative feedback still have to be investigated.
At this point, I seem to hear the reactions: you're just listening to pleasureable distortion, true hifi is a different topic, etc.
Please don't flame me, I love electronics and maths, and I see your points, but if I sit down and listen to music, it's for sheer Pleasure ...

Orit
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Old 26th August 2010, 12:32 PM   #8
DF96 is offline DF96  England
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The Nelson Pass article is quite good, but he seems to get a bit confused in his comments relating to Fig 9. He doesn't mention that R4 in the source also provides negative feedback, so he is actually comparing an amplifier stage with one type of feedback to a stage with two types of feedback.

Intermodulation distortion is caused by exactly the same mechanism as harmonic distortion - non-linearity in the transfer characteristic. TIM is a poor name, because slew-rate limiting causes harmonic distortion as well as intermodulation. Better to just call it slew-rate limiting.
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Old 26th August 2010, 12:44 PM   #9
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Quote:
Originally Posted by Orit View Post
This is the HiFi topic that "burns" the most between Spice'oholics and Audioholics, for sure, together with the eternal "tubes-solid state" debate.
Just my two cents: though being an electronics engineer, my knowledge is peanuts compared to the other posters here, BUT, have your own experience, try to listen to your favourite music with a well-designed low-feedback amplifier, no matter its age. If you, as me, end up with goose-skin and tears, you'll agree that some aspects of negative feedback still have to be investigated.
At this point, I seem to hear the reactions: you're just listening to pleasureable distortion, true hifi is a different topic, etc.
Please don't flame me, I love electronics and maths, and I see your points, but if I sit down and listen to music, it's for sheer Pleasure ...

Orit
Yes it's all personal, isn't it. I get goose-skin depending on the music I listen to, on ANY reasonable good amp .

jd
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Old 26th August 2010, 01:17 PM   #10
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Quote:
Originally Posted by Orit View Post
This is the HiFi topic that "burns" the most between Spice'oholics and Audioholics, for sure, together with the eternal "tubes-solid state" debate.
Just my two cents: though being an electronics engineer, my knowledge is peanuts compared to the other posters here, BUT, have your own experience, try to listen to your favourite music with a well-designed low-feedback amplifier, no matter its age. If you, as me, end up with goose-skin and tears, you'll agree that some aspects of negative feedback still have to be investigated.
At this point, I seem to hear the reactions: you're just listening to pleasureable distortion, true hifi is a different topic, etc.
Please don't flame me, I love electronics and maths, and I see your points, but if I sit down and listen to music, it's for sheer Pleasure ...

Orit
Hi Orit,

Thanks for posting. I'm not going to flame you because many of your points are reasonable. However, I do take issue with at least one thing that you implied which is a common mistake among many people on both sides of the fence - the fence in reality is not a bright line.

More specifically, it is wrong to group people as "Spiceaholics" or "Audioholics" or even "objectivists" vs "subjectivists". Many people make the mistake of classifying others as in one camp or the other. This one-dimensional black-and-white way of looking at the world is wrong. I, for one am all of the above, and many, many others are as well. To suggest that we cannot walk and chew gum at the same time is wrong.

I am a strong believer in using every tool available. I am a strong believer in engineering and measurement and simulation, to be sure. But I also listen a lot and do not dismiss the sonic issues raised by others.

It is easy to make a bad amplifier with or without feedback, and many have. That is not reason to paint either approach as bad with a broad brush.

There are still likely many things that we do not fully understand, or that we do not measure. Good engineering and careful subjective listening are by no means mutually exclusive. It is true that too many engineers have taken comfort in a traditional set of measurement and have proclaimed that others can't be hearing differences. That is unfortunate.

There are just as many others who are wrongly disparaging the engineering approach that many use to arrive at good sound in combination with listening.

We all know that at least half of the stuff that is peddled to the high end is put out there by charlatins trying to make a buck. The problem is that we usually don't have a good way of knowing which half. Similarly, there are many pseudo engineers out there peddling new designs and using fancy buz words. I certainly would not buy an amplifier from someone who merely said they SPICE'd it. SPICE is a tool, not a guarantee. You cannot build a house with only a screwdriver.

Cheers,
Bob
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