What is wrong with negative feedback?

It is like a boob job. Too much and you are all over the place.

I don't believe it is fair to talk about damping factor without mention of speaker 'Q'.
A hi DF and a 'Q' of over say....1.3 will still produce sloppy bass as will a very low DF and a critically damped 'Q' of .707
Also, no mention has been made of Voltage source vs Current source amplification, and the speakers which are best suited for each.

'Current source' is what I call the Power Paradigm, as amps in that category try to make constant power, rather than constant voltage. Its the intention of the designer of the speaker that puts the speaker into the Power camp as opposed to the Voltage camp.

The pivotal issue herein is feedback: Voltage source amps tend to use feedback to create the voltage source aspects. A price is paid for this: odd ordered harmonics, which is responsible for brightness or hardness.

Nelson Pass' 1st Watt amps are an example of a 'current source' (Power Paradigm), just like many low powered SETs.

The Power Paradigm vs Voltage Paradigm is really what we are talking about here, the same is true of tubes vs transistors and the importance of amp/speaker matching:
http://www.atma-sphere.com/papers/paradigm_paper2.html

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.pdf
There 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.

Kirkus, I do have a problem with this:

And then there's the source degeneration resistor R4 . . . this is feedback exactly like R2, no? Why is it somehow more okay?

Degeneration occurs in real time against the signal and so is not part of this argument. It is different from loop feedback in that regard and that is why it is 'somehow more okay'.

Further, Nelson has succeeded in building wide-bandwidth amplifiers wherein the passband is unaffected by the addition of feedback, much like our amplifiers are. So the -6 db slope issue does not play into this. Now I have mentioned this before but I see in your responses that you always go back to the rolloff issue. I concede your point that that regard, but don't see it as relevant- it applies to opamps and similar circuits of the type you have described. However I should point out that it is those circuits that do enhance odd orders, so if not my explanation than what is it?

I have avoided the proof in the pudding aspect of all this, but at some point it will come to bear on this in a big way if Nelson's and my explanations are not to be accepted by you, I am hoping you will explain what the phenomena really is, since your explanations so far have not addressed that.

As to Chaos, an initial comment: we are really, seriously, **NOT** talking about *anything* with the word 'quantum' in it! As far as audio goes, use of the word 'quantum' is the nutbag identifier, IMO/IME :) Seriously. Chaos theory OTOH is a science of complex systems, wherein a simple set of rules governs what seems a complex behavior, often with unexpected results.

FWIW, in any field of endeavor, when Choas theory is applied, there is usually a howl of protest from the establishment. That is, until said establishment realizes the actual implications. The result has been improved weather forecasting, improved aircraft efficiency, improved hydraulic pumps, improved genetics, improved disease control, improved exhaust and combustion and now I am suggesting that it can improve audio reproduction as well.

So, to Chaos:

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.

condition 1 is satisfied, as the signal and gain conditions are always different. Even a digital source can't be assumed to be 100% *exactly* repeatable, humidity in the room can affect the way the loudspeakers behave, which will affect the way the amp responds. Keep in mind that we are talking about a wide range of amplifiers here.

There is a great example of how water dripping from a tap is an example of a Chaotic system. People walking in other parts of the building, variations in water pressure, temperature and actually a huge variety of other issues all come into play. The same is true of an audio signal, there are all sorts of variations that affect it as audiophiles are only too keenly aware: line voltage, noise on the line, noise in the environment, warmup of the amp, break-in considerations, interaction of cables, corrosion of components (such as inside semiconductors and inside switches) and connections; this is a list that knows no end!

condition 2 is satisfied by the fact that the bifurcation that arise are not consistent. The problem is the only means we have of analyzing distortion is through steady-state waveforms, which tell us nothing about the dynamic state of the amp.

condition 3 is satisfied by the strange attractor, which is quite dense. I refer you to Norman Crowhurst on that one. His writings may be old, but it would be foolhardy indeed to cast them aside by using the logical fallacy known as 'guilt by association'.

Frankly, given the research I have done, I suspect that Crowhurst is spot on. Occam's Razor suggests that when his writings and Chaos agree on so many points (only a few of which have been touched on here), the simple explanation is that he is probably right. The very complex explanation is that he is wrong, but it just turns out that in spite of that, things behave the way he and Chaos say they do but for entirely different reasons. I think the point of this is that there is a frontier here; I find the idea that we know everything already is arrogance and nothing more.