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Distortion spectrum vs feedback
Distortion spectrum vs feedback
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Old 20th June 2011, 04:39 PM   #21
jan.didden is offline jan.didden  Europe
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It's a Baxandall graph, not Linsley-Hood as SY noted.
The graph is a result of feeding back a signal through a square-law device. Any harmonics in the feedback get again distorted by the square law, go through the feedback again, get distorted etc etc. So initially, when feeding back, you get a whole slew of additional harmonics before the feedback gets so powerfull that they all get supressed.
In your case, if you don't have the square law, if your amp is already reasonably linear, the feedback products don't get distorted again so much because your amp is reasonably linear.
The original Baxandall graph was for a single non-degenerated FET device which, as we know, has an almost perfectly square-law transfer curve.

BTW a pure square-law would be like Vout = A*Vin^2. After that, it's just mathematics

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Last edited by jan.didden; 20th June 2011 at 04:40 PM. Reason: sp
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Old 20th June 2011, 05:36 PM   #22
knutn is offline knutn  Norway
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Quote:
Originally Posted by artosalo View Post
I see that the main information and value of my experiment - at least to me - is that there is no reason to avoid small GNFB levels (3...6 dB) at tube amplifiers.
This will not generate or increase high order harmonic levels as has been believed as a fact.
You are right (but remember: you have a phase splitter with a lot of local feedback). However it was not my intension to make this an academic discussion. I also find your experiment interesting.
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Old 20th June 2011, 05:38 PM   #23
knutn is offline knutn  Norway
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Originally Posted by SY View Post
No, your result is not false, as I said before, it's accurate. The Baxandall calculations (WW, Dec 1978, figure 7) so beloved of feedback-phobes, only apply to pure square law devices. Tubes are not, so even if you didn't have local feedback, you still wouldn't get the results predicted by Baxandall. And that's OK, you're not trying to disprove his calculations, you are demonstrating that for real world amps, small amounts of feedback are useful and don't always increase their higher order harmonics (at least out to as far as you checked). That's a useful demonstration!
Agreed.
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Old 20th June 2011, 05:42 PM   #24
artosalo is offline artosalo  Finland
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Quote:
You are right (but remember: you have a phase splitter with a lot of local feedback).
I can not see how the high local feedback at the phase splitter could have effect to whole amplifier.

My explanation to the results I got is that tubes (I used and how those are biased) form a very linear amplifier chain and therefor differs very much from square-law components like fets and transistors.
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Old 20th June 2011, 05:56 PM   #25
Wavebourn is offline Wavebourn  United States
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Distortion spectrum vs feedback
I always prefer nested feedbacks.
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Old 20th June 2011, 06:41 PM   #26
costis_n is offline costis_n  Greece
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Basically, I am impressed that you chose to use all metal tubes, which are considered "lesser". Well done!
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Old 20th June 2011, 07:08 PM   #27
john curl is offline john curl  United States
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I am concerned with the coupling caps. Ceramic, by any chance?
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Old 20th June 2011, 07:10 PM   #28
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by artosalo View Post
I can not see how the high local feedback at the phase splitter could have effect to whole amplifier.[snip]
The phase splitter is one of the stages that together determine the transfer curve of the amp. If you make one very linear, the whole amp gets a bit more linear.

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Old 20th June 2011, 07:59 PM   #29
Bob Richards is offline Bob Richards  United States
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Global Negative feedback looks great on paper, with a 1kHZ test signal. In the real world there are many variables; topology, device choice, local or degenerative feedbacks, slewrates and miller effects of each device, etc. The increased higher order harmonics that I saw with increased GFB may be due to the limitations of the feedback loop in trying to deal with supersonic frequency energies generated by the non perfect front end differential amplifier (or equiv.) (I.M. products included), or the rolloff caused by the output devices, thereby undoing the corrective efforts at those higher frequencies. It may be a combination of several things. You never want to ask a feedback loop to correct something it can't correct, for whatever reason, or it may well create some new distortions; spurious oscillation, I.M. products, slewing, overdrive/clipping of internal stages, or excessive smoke and funny popping sounds. Oh, was that your $5000 speaker? Sorry.
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Old 20th June 2011, 08:10 PM   #30
jan.didden is offline jan.didden  Europe
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Well I do agree with you but you make it sound as if it is some kind of black magic. It isn't. It is all fully understood for many decades and competent designers know how to design a feedback amp to stay clear of the problems that could pop up if you don't follow the well-known rules.

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