Bob Cordell Interview: Negative Feedback - diyAudio
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Old 17th January 2007, 02:04 AM   #1
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Mr. Cordell, is there any advantage or disadvantage to have the same amount of feedback for low frequencies (like50hz) and the high frequencies (like 20khz)?
Usually this means to make the OL curve as flat as possible from low to high frequencies. A trick to do this is to put heavy local feedback (emitor resistor) in transistors from differential pair upwards.
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Old 17th January 2007, 02:42 AM   #2
jcx is offline jcx  United States
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covered somewhat earlier in this thread

try ~ p35 +/-
http://www.diyaudio.com/forums/showt...64#post1052464

although touched on before and after for a few pages at a time

this thread is long enough, do we really need to go in circles?
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Old 17th January 2007, 03:06 AM   #3
Nixie is offline Nixie  Canada
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Originally posted by jcx
do we really need to go in circles?
Welcome to diyaudio!

Actually, this forum would benefit greatly from the ability to search within specific threads.
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Old 17th January 2007, 10:30 PM   #4
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Originally posted by lumanauw
Mr. Cordell, is there any advantage or disadvantage to have the same amount of feedback for low frequencies (like50hz) and the high frequencies (like 20khz)?
Usually this means to make the OL curve as flat as possible from low to high frequencies. A trick to do this is to put heavy local feedback (emitor resistor) in transistors from differential pair upwards.

There are some who advocate wide open-loop bandwidth, and there are also those who advocate low or no negative feedback. The amount of feedback that you can have at 20 kHz is essentially limited by stability considerations. With a typical 6 dB/octave open-loop gain roll off, an amplifier with 20 dB of NFB at 20 kHz will have to be stable out to a gain crossover frequency of 200 kHz. An amplifier with 40 dB of NFB at 20 kHz will have to be stable out to a gain crossover frequency of 2 MHz, which is more difficult to achieve. Most non-low-feedback solid state amplifier designs have between 20 dB and 40 dB of NFB at 20 kHz.

Once you establish how much NFB you have at 20 kHz, then whether you have more feedback or about the same amount of feedback at lower frequencies is a matter largely of philosophy: i.e., whether or not you want to deprive the lower frequencies of the greater amount of NFB that is naturally available in order to get a high open-loop bandwidth or not.

Much confusion originated here with Otala's mistaken assertion that one needed high open loop bandwidth to achieve low TIM (e.g., high slew rate). All else remaining equal, high open loop bandwidth has nothing to do with achievable slew rate or lower HF distortion.

So it all ends up being the philosophical choice of the designer, just like the choice of a designer as to whether he wants a no-feedback design, a design with modest feedback, or a design with high feedback. Since if there is feedback, it is often a function of frequency, one must be clear about what they mean when they say high or low feedback. Are they talking about NFB at 20 kHz, or are they talking about NFB at 1 kHz.

Bob
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Old 18th January 2007, 12:12 AM   #5
ingrast is offline ingrast  Uruguay
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Bob:

I cannot completely agree with some points in your previous post.

While I recognize some designers are willing to choose how much feedback to apply, this does not really make sense "per se" unless qualified by some other type of consideration.

What we are seeking to improve is overall performance, and this is the bottom line, no matter the means employed.

If a certain design strives for the largest possible gain so as to apply the maximum amount of correction, but in doing so introduces system nonlinearities that eat up most of that resource, it will certainly perform worse than an alternative design where native linearity is carefully adressed, something that may imply a tradeoff in gain and thus in available negative feedback.

My point is the amount of negative feedback in isolation is not an issue in itself, but should be the result of an optimum balance between native nonlinearity - unavoidable and frequently worse with higher open loop gain - and correction.

A further consideration is that large amounts of negative feedback applied to a questionable basic amplifier, unavoidably leads to a compound forest of multiple low level IM products, the type of syndrome frequently attributed to amplifiers that measure very well in single tone tests yet perform very bad at auditioning as compared with other designs of inferior measured performance. This is particularly true with the first solid state designs of 20-30 years back.

Rodolfo
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Old 18th January 2007, 12:52 AM   #6
Nixie is offline Nixie  Canada
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Originally posted by ingrast
What we are seeking to improve is overall performance
But that has not even been properly defined here. I remind you there is yet to be formulated a metric that correlates perfectly with blind testing results of perception of distortion. Thus, every time you make a comparison, such as when you use the word "worse" later in your post, you have some benchmark in mind which, by not being made explicit, is not questioned as to its validity.
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Old 18th January 2007, 03:12 AM   #7
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Originally posted by ingrast
Bob:

I cannot completely agree with some points in your previous post.

While I recognize some designers are willing to choose how much feedback to apply, this does not really make sense "per se" unless qualified by some other type of consideration.

What we are seeking to improve is overall performance, and this is the bottom line, no matter the means employed.

If a certain design strives for the largest possible gain so as to apply the maximum amount of correction, but in doing so introduces system nonlinearities that eat up most of that resource, it will certainly perform worse than an alternative design where native linearity is carefully adressed, something that may imply a tradeoff in gain and thus in available negative feedback.

My point is the amount of negative feedback in isolation is not an issue in itself, but should be the result of an optimum balance between native nonlinearity - unavoidable and frequently worse with higher open loop gain - and correction.

A further consideration is that large amounts of negative feedback applied to a questionable basic amplifier, unavoidably leads to a compound forest of multiple low level IM products, the type of syndrome frequently attributed to amplifiers that measure very well in single tone tests yet perform very bad at auditioning as compared with other designs of inferior measured performance. This is particularly true with the first solid state designs of 20-30 years back.

Rodolfo

I don't think you are disagreeing with any of my points, but rather elaborating on them or adding the usual necessary caveats. Of course we should not sacrifice open loop linearity just to increase NFB. Unfortunately, in some cases the opposite is true; in some misguided designs, people load the VAS in order to get wide open-loop bandwidth, and in so doing make the VAS work harder, actually resulting in more open loop nonlinearity. It is generally a misconception when people think that one somehow has to strain to get open loop gain; it is not hard to get with great linearity. Indeed, in my amplifiers, I start with an open loop amplifier that is more linear before I apply NFB than many no-feedback practitioners do. I think you are preaching to the choir.

Cheers,
Bob
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Old 18th January 2007, 10:22 AM   #8
ingrast is offline ingrast  Uruguay
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Quote:
Originally posted by Bob Cordell



I don't think you are disagreeing with any of my points, ....
Apologies Bob, you are right and I may have been careless in my wording. What struck me was your:

Quote:
.....
Once you establish how much NFB you have at 20 kHz, then whether you have more feedback or about the same amount of feedback at lower frequencies is a matter largely of philosophy....
Which I interpreted as if it were a relevant designer's choice by itself to select such or such amount of feedback without reference to other contextual variables.

On second reading it is apparent that was not what you meant.

Rodolfo
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Old 18th January 2007, 10:29 AM   #9
forr is offline forr  France
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Ingrast
---large amounts of negative feedback applied to a questionable basic amplifier, unavoidably leads to a compound forest of multiple low level IM products---

This seems to be in disagreement with the Baxandall papers on amplifier design.
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Old 18th January 2007, 10:32 AM   #10
ingrast is offline ingrast  Uruguay
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Quote:
Originally posted by Nixie

But that has not even been properly defined here. I remind you there is yet to be formulated a metric that correlates perfectly with blind testing results of perception of distortion. Thus, every time you make a comparison, such as when you use the word "worse" later in your post, you have some benchmark in mind which, by not being made explicit, is not questioned as to its validity.

While I agree there is not a widely **accepted** - and I want to remark the "accepted" part - objective metrics, it is also true only objective metrics can be used as unquestionable comparison references, in the strict sense that nobody will question that 2+2=4.

These objective metrics are what I implicitely refer to as performance results, or better - worse evaluation results.

If subjective evaluation is allowed to figure in, then there is no hope of reaching consensus, for it involves personal preferences, expectatives, experience, natural aptitudes and so forth.

Rodolfo
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