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Old 4th February 2012, 06:18 PM   #1811
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Quote:
Originally Posted by janneman View Post
The more feedback, the more transparent the amp. The more feedback, the more the amp starts to look like a wire with gain.
...when you carefully dose input signal and load so it does not screw up any of active/passive elements of the amp that are in common feedback loop.

That's why steady sinewave is good to measure and report how close the amp is to the wire with gain.
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Old 4th February 2012, 06:24 PM   #1812
a.wayne is offline a.wayne  United States
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Ok jan,

Now hand me dem pliers, OK, err , argh .. OK turn this knob a bit .. whaa la 120db feedback

12 g wire...with gain ?
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Old 4th February 2012, 06:26 PM   #1813
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Not in my experience.
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Old 4th February 2012, 06:37 PM   #1814
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Quote:
Originally Posted by bcarso View Post
In still earlier days, using tube opamps, the range was typically +/- 100V Those folks also paid a lot of attention to dielectric absorption.
Here is opamp:

Click the image to open in full size.
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Old 4th February 2012, 07:09 PM   #1815
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Quote:
Originally Posted by Wavebourn View Post
...when you carefully dose input signal and load so it does not screw up any of active/passive elements of the amp that are in common feedback loop. [snip]
In other words, good commonsense engineering that we all learned, ehh Wave?

jan
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Old 4th February 2012, 07:33 PM   #1816
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Quote:
Originally Posted by Wavebourn View Post
Here is opamp:

Click the image to open in full size.
But that one has mechanical choppers!
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Old 4th February 2012, 07:50 PM   #1817
dvv is online now dvv  Serbia
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Quote:
Originally Posted by janneman View Post
The more feedback, the more transparent the amp. The more feedback, the more the amp starts to look like a wire with gain.

jan
Perhaps you are right, but that is quite the opposite of my experience, As I know it, amplifiers which use a lot of feedback are prone to be harsh and rather dead.

Amplifiers which use zero (overall) feedback tend to sound sort of unfinished to me, as if there's something missing.

As a general rule, it is my view that the best can be expected of amplifiers which use between 20 and 30 dB of overall feedback.

But of course, these are not rules carved in stone, I have heard exceptions to the above general outlines.
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Old 4th February 2012, 07:54 PM   #1818
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Quote:
Originally Posted by janneman View Post
In other words, good commonsense engineering that we all learned, ehh Wave?
Common sense, particular sense, all have to be considered. However, if you draw straight lines like in the article you can come to one conclusions, if you draw lines for every subsystem in the system you can get to another conclusion. The axiom is still valid, "The system made from optimal subsystems can not be optimal". From another hand, each subsystem have to be optimized to perform functions that are used in order to get the resulting function. But this is no way proof that the axiom is not valid.

Like in my example, class A and class C output amps had properties defined by feedbacks such a way their transfer functions are smoothly glued together working in parallel on the load. Then, errors of "glue" were minimized by overall feedback.
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Old 4th February 2012, 07:58 PM   #1819
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Quote:
Originally Posted by dvv View Post
Perhaps you are right, but that is quite the opposite of my experience, As I know it, amplifiers which use a lot of feedback are prone to be harsh and rather dead.

Amplifiers which use zero (overall) feedback tend to sound sort of unfinished to me, as if there's something missing.

As a general rule, it is my view that the best can be expected of amplifiers which use between 20 and 30 dB of overall feedback.

But of course, these are not rules carved in stone, I have heard exceptions to the above general outlines.
Yes. Feedback alone is never a panacea. One thing we tend to forget is whatever we do to the feedback loop, the amp itself always works open loop! In a sense, the only thing (global) feedback can do is manipulate the effective input signal such that the output is as perfect as possible a replica of the input signal.
So you must be aware of what it is that will be thrown at the amp itself, and make sure it doesn't overload or clip internally and doesn't slew limit.
Taking any old amp and wrapping it in a feedback loop doesn't gurantee good sound.
But that has been done for almost 3/4 of a century so isn't very exciting anymore.

jan
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Old 4th February 2012, 08:00 PM   #1820
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Would phase margin be a better indicator than the amount of feedback?
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