Bob Cordell Interview: Negative Feedback - Page 2 - diyAudio
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Old 18th January 2007, 09:47 AM   #11
ingrast is offline ingrast  Uruguay
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Quote:
Originally posted by forr
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.

I am not aware of the papers you mention, but it has been widely shown and can be easily simulated, that significant nonlinearities in the forward path, give rise iteratively under close loop conditions to large amounts of low level spurious components. Some people use to refer to a "veil" probably in connectˇon with this.

This nonwithstanding, it is obvious that if spurious products are really low, i.e. below 100 dB or better dynamic range, chances are they will neither be of consequence.
If a sufficiently high open loop gain can be attained within practical stability constraints to achieve this level of performance, then the native nonlinearities can be considered to be adequately tamed, and may be this is what your mentioned papers made reference to.

Rodolfo
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Old 18th January 2007, 11:44 AM   #12
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Quote:
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.
This is very likely if this high global feedback poweramp is made with bad output stage. The problematic section is output stage, but the troubleshooting is made in differential pair+VAS, building huge OL gain in these front stage. It's like a car having problem in the tyres, but you keep fixing the steering wheel. Not shooting the real problem.
Better to have good output stage first (like using classA or using EC) and then using suitable gain front stage than building bad output stage with enormous front end gain.
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Old 19th January 2007, 01:48 PM   #13
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Quote:
Originally posted by Bob Cordell



... 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.
High open loop bandwidth often means also well-defined open loop bandwidth and well-defined pole frequencies, which is helpful in reducing dynamic distortion mechanisms, PIM included.

About VAS loading:
D. Self uses darlington output stage, so output stage input impedance is probably around 50k-100kOhm with 8ohm load and good Hfe trasistors. This impedance is as we all know terribly non-linear with reflecting crossover mechanisms, capacitive and modulated by speaker impedance, right?
Now I use tripple darlington output stage based on high Hfe transistors (2n5401/2n5551, 2sa1837/2sc4793, 2sa1943/2sc5200), so input impedance of output stage is around 10Mohm probably with all those nonlinear mechanisms, but is paralleled with 67kohm most linear electrical device around.
Which one would you prefer, input impedance being of comparable value?

regards
Adam
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Old 19th January 2007, 07:48 PM   #14
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Quote:
Originally posted by darkfenriz


High open loop bandwidth often means also well-defined open loop bandwidth and well-defined pole frequencies, which is helpful in reducing dynamic distortion mechanisms, PIM included.

About VAS loading:
D. Self uses darlington output stage, so output stage input impedance is probably around 50k-100kOhm with 8ohm load and good Hfe trasistors. This impedance is as we all know terribly non-linear with reflecting crossover mechanisms, capacitive and modulated by speaker impedance, right?
Now I use tripple darlington output stage based on high Hfe transistors (2n5401/2n5551, 2sa1837/2sc4793, 2sa1943/2sc5200), so input impedance of output stage is around 10Mohm probably with all those nonlinear mechanisms, but is paralleled with 67kohm most linear electrical device around.
Which one would you prefer, input impedance being of comparable value?

regards
Adam
It has been shown that having wide open loop bandwidth does not help PIM and related nonlinearities. It all comes out in the wash. I know this may seem non-intuitive. The less-well-defined pole has movement that has less effect on the result. A pole at 100 Hz that moves around 10% will have about the same effect as a pole at 10 kHz that moves around 0.1%. Putting in a shunt resistance that moves the pole from 100 Hz to 10 kHz and at the same time reduces pole uncertainty from 10% to 0.1% has no net effect. The underlying effect causing pole shift, e.g., nonlinear capacitance, has not changed, and its effect on propogation delay through the circuit has not changed by adding shunt resistance. SPICE it, you'll see.

A simple Darlinton output stage is completely inadequate for reasons of the nonlinear resistance you cite, at the very least. With a triple, this issue pretty much goes away. I'd go with the triple, with a VAS that is not artifically loaded by a resistor. The addition of that resistor will never decrease distortion; it will only increase it.

Bob
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Old 19th January 2007, 08:09 PM   #15
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Actually, I once found a measureable example of adding a bandwidth widening resistor and having LOWER overall distortion. I attibute it to lowering the drive inpedance to the transistor follower output stage.
For the record, Bob's opinions directly contradict the opinions of Matti Otala and Charles Hansen. I'll stick with Matti, as I have done in the past, on these issues.
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Old 19th January 2007, 10:31 PM   #16
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Same example has been shown by MikeB on this forum. I also SPICED the fact, that sometimes adding resistive loading to VAS decreases distortion by several percent or so, not to mention, that it sounds good to me.
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Old 19th January 2007, 10:33 PM   #17
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Quote:
Originally posted by john curl
Actually, I once found a measureable example of adding a bandwidth widening resistor and having LOWER overall distortion. I attibute it to lowering the drive inpedance to the transistor follower output stage.
For the record, Bob's opinions directly contradict the opinions of Matti Otala and Charles Hansen. I'll stick with Matti, as I have done in the past, on these issues.
Clearly Matti Otala did audio a great disservice with his wilful obscurantism.
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Old 20th January 2007, 12:16 AM   #18
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Quote:
Originally posted by john curl
Actually, I once found a measureable example of adding a bandwidth widening resistor and having LOWER overall distortion. I attibute it to lowering the drive inpedance to the transistor follower output stage.
For the record, Bob's opinions directly contradict the opinions of Matti Otala and Charles Hansen. I'll stick with Matti, as I have done in the past, on these issues.

Yes, John, my opinions do directly contradict those of Matti Otala. He misled a thousand audio engineers. Whether it was willful on his part, or due to a lack of experimental and scientific discipline, or something else, we may never know.

However, that does not mean that his work was without value. He deserves credit for casting a spotlight on some things that really needed examination, even if his proposed solutions were completely misguided. The technical discussions and arguments that took place as a result of his work did have worthwhile outcomes. He also deserves credit for at least putting forth a scientific postulation for his theories, and for proposing a measurement technique for measuring the effects he claimed to be curing (e.g., TIM, DIM). However, his very own objective measurement techniques were what provided the basis for disproving his "cures" for the problem.

It is very easy, using Matti's very own DIM test, to show that, given the same amount of HF NFB above 20 kHz, a wide open loop bandwidth does not improve slew rate and does not reduce measured TIM.

What he did, which was his own un-doing, is much more than we get from some audio folks today who postulate a bunch of pseudo-scientific cr@p and don't back it up with any means of proving it one way or the other. Half the oil in our community is snake oil; the hard part is distinguishing which is which.

I know this area is one in which we will continue to respectfully agree to disagree.

Cheers,
Bob
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Old 20th January 2007, 01:20 AM   #19
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Who cares about more slew rate, IF you have virtually no TIM, already? THEN, other factors 'might' become more important.
Bob, you are stuck on 'TIM' and ignoring 'DIM' which Matti had already coined as early as 1976, when I worked with him.
I, too, did NOT like his ignoring slew rate as a number, and his insistence (in the late '70's) that high open loop bandwidth appeared to be necessary, but Walt Jung's and my work on op amps, both as IC's and discrete, tend to go in this direction, when listening to the results.
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Old 20th January 2007, 01:54 AM   #20
Nixie is offline Nixie  Canada
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Quote:
Originally posted by john curl
when listening to the results.
And I trust the results are valid since the comparisons were done with blind testing?
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