Bob Cordell Interview: Error Correction - Page 3 - diyAudio
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Old 29th October 2006, 11:09 AM   #21
thanh is offline thanh  Viet Nam
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Thank mikeks!
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Old 29th October 2006, 08:41 PM   #22
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My experience says that the best (stability, linearity) output tripple darlington stage looks a bit different:
-the pre-drivers are biased by means of base-to-emitter resistor of drivers (150R or so)
-drivers have their emitters connected through a resistor in parallel with cap (I use 40R||33n).
-no base resistors
-output bjt's biasing a little bit higher than optimum, say, 100mA for 0R3 emitter resistors.
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Old 29th October 2006, 08:49 PM   #23
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As far as Bob's article about slew rate and feedback factor:
I have great respect to mr. Cordell, but here he seems to have missed the point.
Higher feedback DOES generate higher TIM, but this happened at MENTAL stage. The engineers were so delighted with feedback and its possibilities, especially in solid state OTL designs, that they missed some more important issues. Thanks godnees, Matti came and the "maximum feedback from minimum number of devices, who cares about anything as feedback will correct" thinking is gone.

Or not entirely gone...
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Old 29th October 2006, 09:05 PM   #24
mikeks is offline mikeks  United Kingdom
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Old 29th October 2006, 11:38 PM   #25
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Quote:
Originally posted by darkfenriz
the "maximum feedback from minimum number of devices, who cares about anything as feedback will correct" thinking is gone.
...

Quote:
Originally posted by darkfenriz
Or not entirely gone...
Quote:
Originally posted by mikeks
No
Ok, I have to agree with you Mike, there still are some, who think the way I mentioned above.
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Old 30th October 2006, 12:06 AM   #26
gerhard is offline gerhard  Germany
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Quote:
Originally posted by darkfenriz
As far as Bob's article about slew rate and feedback factor:
I have great respect to mr. Cordell, but here he seems to have missed the point.
Higher feedback DOES generate higher TIM, but this happened at MENTAL stage. The engineers were so delighted with feedback and its possibilities, especially in solid state OTL designs, that they missed some more important issues. Thanks godnees, Matti came and the "maximum feedback from minimum number of devices, who cares about anything as feedback will correct" thinking is gone.

Or not entirely gone...
NO, Cordell is absolutely right.

Applying feedback is the process of 1. having lots and lots of open loop gain and then 2. giving away most of it
to get other nice things: higher input impedance, lower output impedance, flatter gain, less distortion.

But the requirement is: You've got to have a lot of gain to give away in the first place.

If you have an incompetently designed differential+VAS stage that cannot support the current to charge its Cdom,
then you get maybe a 10V step at 10 KHz for a 1 V input step. This is an effective gain of 10 and that even without closing
the global feedback loop.

A factor of 10 does not qualify as "lots of excess gain" if you need that much already to reach the required output level.

Therefore, even if you draw a global feedback voltage divider: You don't get global feedback at all.

Otalas point of view is usually shorted to "global feedback is baaad"
while in reality, a slewrate-limited amplifier is punished for not having enough open loop gain dynamically
to support any global feedback.

regards, Gerhard
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Old 30th October 2006, 01:46 AM   #27
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If you read carefully, you'd notice I am not talking about technical side of Cordell's article, which is of course more than correct.
I was talking aboutM E N T A L (sorry, but only capital letters last time weren't enough) mechanisms.
There are many ways to get more 'dumb' gain, for example skip diff. pair emitter restistor, which lead to problems with slew rate. Robert's way of limiting gain does not change slew.
Look at schematics from 60's and early 70's and use a bit of imagination to see how someone had to say: "stop, feedback's not a cure-all".
Historically speaking, the desire of more and more feedback and carelessness of anything else led to slew problems.
Of course one can imagine high feedback amp of similar of even better slew rate than low feedback amp, but that's not really the point.
That's my view.
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Old 30th October 2006, 02:07 AM   #28
AKSA is offline AKSA  Australia
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Does anyone here know why feedback has such a bad rap?

Is there a reason its implementation seems sonically compromised?

I ask the question innocently......

Hugh
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Old 30th October 2006, 06:01 AM   #29
gerhard is offline gerhard  Germany
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Quote:
Originally posted by AKSA
Does anyone here know why feedback has such a bad rap?

Is there a reason its implementation seems sonically compromised?

I ask the question innocently......

Hugh
Don't know.

But when I see that 47 labs, makers of the original uncloned Gainclone
brag about their ultrashort feedback path (definitely not global!) and
measure the feedback path as 9 mm only, then I guess the reason
is not qualification.

They say it's a good thing that they need only 1000uF in the power supply for > 50 W,
and that this makes it sonically superior.
I suppose they want to make positively sure that their customers
are terminally braindead, so nobody wants his money back.


Gerhard
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Old 30th October 2006, 11:12 AM   #30
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Quote:
Originally posted by darkfenriz
If you read carefully, you'd notice I am not talking about technical side of Cordell's article, which is of course more than correct.
I was talking aboutM E N T A L (sorry, but only capital letters last time weren't enough) mechanisms.
There are many ways to get more 'dumb' gain, for example skip diff. pair emitter restistor, which lead to problems with slew rate. Robert's way of limiting gain does not change slew.
Look at schematics from 60's and early 70's and use a bit of imagination to see how someone had to say: "stop, feedback's not a cure-all".
Historically speaking, the desire of more and more feedback and carelessness of anything else led to slew problems.
Of course one can imagine high feedback amp of similar of even better slew rate than low feedback amp, but that's not really the point.
That's my view.
Well I'm still confused. Do you agree with Cordell or not? Is more feedback by definition bad or not? Do you agree with Otala that more feedback by definition gives TIM?

Jan Didden
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