Bob Cordell Interview: Negative Feedback - Page 53 - diyAudio
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Old 17th March 2007, 04:11 PM   #521
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Hi, Mikeks,

Quote:
Edmond's arrangement can only be said to constitute an "improvement" if the later is expressed as an increase in loop gain in the audio band without compromising stability.
Maybe not increasing total loop gain of the whole feedback system, but increasing the amount of gain (available for feedback to burn) in output stage. If this is right, output stage will be better.
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Old 17th March 2007, 04:13 PM   #522
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It does neither as far as i can tell...
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Old 17th March 2007, 05:07 PM   #523
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Quote:
Originally posted by lumanauw
Hi, Estuart,
Off course
Please submit them
I have 1 question. What happens if I don't use buffer before VAS (omitting Q2 in your graph, the input differential pair directly feds base of Q3)?
Hi lumanauw,

If Q2 is omitted, distortion of the VAS (2nd harmonic) itself raises to such a dominant level that improvements of the O/P stage are largely masked out.
Here some (simulated) figures:
THD at 10kHz and 30W
Traditional: 40ppm, w/o Q2: 120ppm
My trick: 6.2ppm. w/o Q2: 90ppm

Cheers

PS: I'll send you the schematic by e-mail, as it is too large to put it here.
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Old 17th March 2007, 05:08 PM   #524
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Hi, Estuart,

OK, my email is lumanauw@bdg.centrin.net.id

Back to post#515

Quote:
Funny. Indeed, based on same principle, except that feedback is led to a different point. Probably doesn’t work with BJTs in the O/P stage, but with MOSFETs it’s OK.
Why this method will not work with BJT output stage?
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Old 17th March 2007, 05:28 PM   #525
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Hi Andrew,

Low bias was used to study probs; years since I last did so.
Increase Iq to further reduce distortions (within reason).

The resistors linearise the phase response by loading a Miller connected C.dom and reducing output stage gain. Makes differential stage work harder and sounds better.

The split C.dom gives lower overall distortion and better load damping than using simple Miller C.dom which loads VAS more. Also this arrangement is not reactively tuned at a high audio frequency via a C-R connected to the output terminal and current phase shifting loudspeaker load.

However, there are so many ways for everyone to achieve desired results, with different choices relating to what individuals feel is most important.

Cheers ....... Graham.
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Old 17th March 2007, 05:45 PM   #526
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Quote:
Originally posted by Bob Cordell

Who originally came up with this form of compensation?
I think that this topology might benefit from connecting C2 to an emitter-follower (fast) buffered output of the VAS. This would isolate the VAS from currents flowing in R1. This emitter follower could just be the first guy in the output triple.
Bob
Hi Bob,

I fully agree with your comment. Connecting C2 to an emitter of a pre-driver does improve thd, but not much. However, as such arrangement might affect the stability of the inner loop, I didn’t dare to mention this possibility.

“Who originally came up with this form of compensation?” If you mean who first invented it, it seems to be Baxandall (according to Self). If you who mean who put it here, I must confess, I did.

Cheers,
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Old 17th March 2007, 05:50 PM   #527
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Quote:
Originally posted by lumanauw
Hi, Estuart,
OK, my email is lumanauw@bdg.centrin.net.id
Why this method will not work with BJT output stage?
Hi lumanauw,

I’ll send it tomorrow, OK?

Why this method will not work with BJT output stage? Stability problems, at least in my hands.


Cheers

BTW, my name is Edmond.
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Old 17th March 2007, 06:01 PM   #528
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Quote:
Originally posted by mikeks
Edmond, let's cut to the chase: what is major (global) loop transmission (gain) with your set up compared with ordinary Miller compensation?
http://www.diyaudio.com/forums/showt...75#post1160075

Hi Mike,
As I told you before (READ post 505) there is no difference. They are equal, just the same, on purpose. Did you get it now?
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Old 17th March 2007, 06:10 PM   #529
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Quote:
Originally posted by mikeks
It does neither as far as i can tell...
Don’t guess, run a sim!
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Old 17th March 2007, 08:58 PM   #530
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Quote:
Originally posted by estuart
They are equal, just the same, on purpose.
If minor and major loop gains remain unchanged, then what's the point?
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