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Negative Feedback

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#142
smoking-amp said:
Take a triode that has nice evenly spaced curves. It has gain, and we generally think of the electrostatic effect of the plate V on cathode emission as N Fdbk. But it doesn't generate re-entrant distortion. (does it?) You could draw a load line on it's curves with very little distortion. The Fdbk there is speed of light, without any device resonances to get in the way. So any kind of re-entrant effect is only a few pSec delayed. Any recirculating stuff is looking just like the original signal with a microscopic phase delay.

So, seems to me that the usual looped circuit N Fdbk successfully time aligns the Fdbk for subtraction purposes (using its predictor), but it does not prevent actual physical delay from causing re-entrant distortion. Or can it?

Can the re-entrant dist. corr. be brought into time alignment with the incoming signal generated distortion so they just sum to zero? The re-entrant stuff is inverted after all. Obviously this needs some thinking over and maybe some checking out.

Or using components that are vastly faster than the signal frequencies so there is no appreciable loop delay.

Suppose the distortion N Fdbk were just slightly enhanced so that it does completely cancel out the generated dist., would that help? (I'm thinking Error Correction technique, which can adjust the distortion down to "near" zero. Is that "near" null just the re-entrant stuff slipping by?) Could EC be adjusted just a little past the null point so that a small amount of (inverted) single pass dist. comes out, but no re-entrant crap?
Click to expand...
If you accept that a vacuum triode has internal feedback from the plate (I'd call that a tautology, but I'm often wrong) then you could argue that the rawest, least fedback "triode" is one where it operates into a short, leaving no signal voltage at the plate, maybe like a cascode, and so no internal feedback. Then a non-zero load on the plate generates the signal voltage from signal current. Because the larger the load the larger the signal voltage, this could be called positive feedback.

Simultaneously, the plate signal voltage does its internal (negative) feedback thing. Not sure if this is potentially useful to you or not. I'm lost.

All good fortune,
Chris
 
#144
I wouldn't propose solving all N Fdbk problems with this approach, but at least for me it has been quite illuminating. Like peering under the bonnet of a car engine and disassembling it, rather than just presenting thermodynamic equations. Standard N Fdbk doesn't consider transport time, so cannot explain Feldtkeller/Baxabdall type distortion. It IS clear to me now, and Baxandall presented measurements of it. The Triode was an anomaly for not having Feldtkeller/Baxandall type distortion, but it is now clear WHY not. I now have a deeper appreciation of Triodes. That then shows how ordinary N Fdbk could be fixed to avoid Feldt/Bax dist., but unfortunately it is not very practical in general (except to use high N Fdbk). But an individual pentode stage appears fixable and that is useful for local Fdbks in multi-stage amplifiers, which can then use global N Fdbk around them without fear of Feldt/Bax dist (due to low forward dist.). I'm quite pleased with the progress.
 
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#147
The increase in higher-order distortions when you add feedback is easy to show mathematically even in the absence of time delays. I understand a closed-form solution in the general case is impossible.

Again, I'm not sure what the point is here. Every now and then someone comes up with some new "theory" and makes a big fuss over it. First-cycle distortion, hello?

Here is how real engineers communicate about things like this (Nyquist stability criterion, for instance):

1671665379947.png


If you want my opinion
It doesn't mean a thing
If you haven't got that ability to swing

This thread is entertaining (sort of) but I'm done. Thanks, guys.
 
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#150
Feldtkeller/Farren/Baxandall/Putzeys was all calculated without considering propagation time and give correct results without it. But understanding the mechanism of feedback with recursion requires that we understand the causal chain of voltage (or current) comparison followed by integration. And this requires that we remember that the recursive events are time sequenced. And this requires time delays, however small.

Those who don't care about understanding the mechanism needn't trouble themselves.

All good fortune,
Chris
 
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#155
I was trying to give an example of a proper technical proof or derivation.
Click to expand...
OK. Do you expect DIY Audio members to be conversant with Eng. Systems 205 Nyquist stability proofs?
I wouldn't hire a thermodynamicist to fix my car. I want someone who understands the actual problem.

I was just passing thru the thread when I saw the interesting Feldtkeller/Baxandall..... problem for low Fdbk. I solved the issue to my satisfaction with a simple understandable model for everyone, without pulling out old moldy college textbooks which would NEVER have elucidated this particular problem. Pure detective work found this.
 
#156
Yes, I absolutely believe that if you want to have a productive discussion about feedback, you need to be conversant with control systems theory, or the equivalent.

Edit: I should say, if you want to have a productive discussion of the more nuanced aspects of feedback, you need this theory. Obviously, you can get the basic gist of it (gain equations, simplistic distortion reduction) with basic algebra. What you are talking about is advanced theory, and I see no evidence of a simple, understandable model. Just hand-waving.

Sorry, I hate to be a douche. TBH, I'm conflicted. I should just let it go.
 
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#157
OK, I'll keep it to myself in the future.
Not what I thought DIY Audio was about. I find the discussions and arguments stimulating and thought provoking. New ideas develop and either fly or don't fly. I like bad ideas too, interesting exercises to determine why they aren't working, sometimes a fix to them becomes apparent. Exciting sometimes. Crazy drive for instance, won't find that in any textbook, but it does fly beautifully. The oldies missed the grail. Tube Voltage Mirrors and Tube Current Mirrors, long lost. Active Ultralinear, tube emulation, inverted triode Fdbk, composite V and I amplifiers for tube veneering, class A amps without the heat, room temperature thermionic electron emitters, .... lost. gone. Erase all my threads and comments.
 
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#159
hpasternack said:
if you want to have a productive discussion of the more nuanced aspects of feedback
Click to expand...
To understand practical DIY design trade offs using a methodology targeted to the task it makes more sense to write a Spice model of an ideal gain stage with customization distortion profiles, add feedback and press the run button.
Incidentally, I once hired a masters of electrical engineering with a solid resume who just couldn't parse the concept of charging a capacitor with a resistor to test if it was still good. A more recent grad than me explained engineering schools are all about simulation these days. Is the academic curriculum still teaching calculus level design?
 
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