I would like to direct you to an article written by Nelson Pass that is on his website, the one about distortion. I think you will see right away what the issue is, he, like myself, tends to work with empirical measurement rather than simulation. Spice is great for a lot of things but I regard it as inaccurate when subjected to the real world . . .
I take it this is the article to which you're referring?
http://www.passlabs.com/pdfs/articles/distortion_and_feedback.pdfThere are numerous problems with this paper -- namely, Pass (in his Fig. 9 test circuit) doesn't analyze the likely difference in the bandwidth between the forward path and the feedback path, as a result of the high output impedance of the circuit coupled with the mosfet's input capacitance. Second, he didn't necessarily keep the drain load constant with or without the feedback in place, which may affect circuit linearity. And then there's the source degeneration resistor R4 . . . this is feedback exactly like R2, no? Why is it somehow more okay? And then there's the drop in noise floor that could reveal higher-order harmonics that were there before feedback. No offense to Nelson Pass, I like him and his work, but this paper definately shouldn't be considered cannon.
I've been looking at what Chaos Theory has to say about negative feedback. What I have been seeing is that Chaos Theory describes an audio amplifier with feedback as a chaotic system with stable areas of performance. The problem here is that we are dealing with non-repetitive signals, but for our tests we use sine and square waves. The behavior of an amp with feedback with repetitive input signals is your stable area of operation; when non-repetitive signals are used the amplifier can become chaotic, particularly at higher powers but can do it at any power level.
I have two huge problems with this argument . . . first is that an audio amplifier does NOT qualify as a chaotic system, and second, a thorough classical analysis of an amplifier provides excellent correlation with both measured and subjective listening data. In audio, there's only one good reason to jump straight to "quantum" or "chaos" explanations . . . and that is to obfuscate the presence of misunderstandings of traditional electrical theory.
On Chaos Theory . . . please re-read in the link you provided the three required properties for a system to be considered chaotic:
In common usage, "chaos" means "a state of disorder",[19] but the adjective "chaotic" is defined more precisely in chaos theory. Although there is no universally accepted mathematical definition of chaos, a commonly used definition says that, for a dynamical system to be classified as chaotic, it must have the following properties:[20]
1.it must be sensitive to initial conditions,
2.it must be topologically mixing, and
3.its periodic orbits must be dense.
An amplifier certainly is NOT sensitive to initial conditions, this refers to the STIMULUS condition, NOT circuit operating parameters. It may be topologically mixing to a small degree if one considers the possibility of intermodulation with uncorrelated noise. But its periodic orbits are anything but dense, and feedback reduces the density of those orbits, which is why it reduces noise and distortion. Even considering an unstable oscillation-prone feedback amplifier . . . the oscillation state itself corresponds to the least amount of density in its periodic orbit. (Density of periodic orbit has NOTHING to do with the complexity of the input signal).
OTOH the ultrasonic behavior of an amplifier often says a lot about how it sounds. I am sure you have encountered that!
Oh, absolutely, especially if feedback is involved . . . but Chaos Theory isn't necessary or appropriate to analyze why this is so. Classical filter theory shows that for the most accurate in-band transient response, the transition-band behavior should correlate to a minimum-phase (first-order) slope, or a Bessel function. So as I said before . . . an idiosyncratic rolloff slope, coupled with rising THD vs. rising frequency (due to limited open-loop bandwidth, as I explained in my previous posts) . . . is more than enough to explain pretty much all of the negative subjective opinions of negative feedback.
That is to say . . . harsh and strident sound? Poor imaging? Fatiguing to listen? Artificial, mechanical, and non-musical? Yes, these impressions fit perfectly with measured data of many amplifiers that use lots of feedback, and also with many that don't. And in my experience, that measured data points clearly to innumerable other mechanisms that can be clearly linked to the problem.
