What is wrong with negative feedback?

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.

Roxy Ah lets make this simple,feedback is touted as bad correct? Got that?
High damping factor is touted as good. correct?
What i said was these figures are often misunderstood.Meaning that they can be irrelevant to the actual sound of a component,IE low feedback or no feedback doesn't have to equal good sound,High damping doesn't have to equal better sound either,its the final sound that counts.

Ah, Atmasphere, thanks for your response . . . I think we're getting somewhere here. Let's start with:

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'.

The source degeneration and drain load resistor are indeed identical mechanisms, and both occur in "real time", it must because the same current flows through both resistors! (see Kirchoff's laws) Yes, they do behave differently, but this is simply because the output impedance is higher from the drain than from the source. In both cases, the bandwidth available for the negative feedback is defined by the gate capacitance of the mosfet, but when it's driven by the higher impedance of the drain, the rolloff of course starts sooner (higher impedance driving the same capacitance). So if you build two circuits with identical low-frequency gain, one with a capacitor-bypassed source resistor and a feedback ladder from the drain, and the other with only source degeneration, the amount of feedback available as the frequency rises is less from the former. THIS is why it is less linear, and has poorer phase margin, and is more likely to have a peak in its ultrasonic response before rolloff (less feedback makes its gain increase).

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.

I always go back to the rolloff issue, because analyzing relationships between open-loop and closed-loop bandwidth, a.k.a. rolloff, is the fundamental cornerstone of understanding how feedback works. And as far as I'm concerned, if one condemns the use of negative feedback, and hasn't gone through the process of figuring out where the poles and zeros in the response fall, and analyzing the phase margin . . . they simply haven't a leg to stand on.

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 am hoping you will explain what the phenomena really is, since your explanations so far have not addressed that.

I think I have, several times. They are the result of circuits that have the following:
-Nonlinear open-loop transfer functions that cause both low- and high-order distortion
-Topologies (i.e. differential, push-pull) that are more effective at cancelling even-order distortion products than odd-order
-Feedback (and hence closed-loop linearity) that decreases as frequency increases.
Put the three together, and you have a system that enhances higher-order, and odd-order distortion products. But the root cause is NOT the feedback.

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.

Well, I'm definately with you on the idea that the entire reproduction/perception chain can be thought of as a Chaotic system. But in order to be applicable, there needs to be a large volume of data that's both accurate, and seemingly uncorrelated . . . of which we must make sense. And the required function of an amplifier is pretty damn simple - this is what's meant by a lack of density in periodic orbits. Now if you have a large mixing console with a few hundred or so cold solder joints and dirty potentiometers, then we have a chaotic system . . . the various possibilties of output voltages from various sections of the console cover a dense cloud of results.

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.

Einstein's razor is frequently quoted to counter Occam's:

It can scarcely be denied that the supreme goal of all theory is to make the irreducible basic elements as simple and as few as possible without having to surrender the adequate representation of a single datum of experience.

To paraphrase - the best explanation is as simple as possible, but no simpler. And in discussing negative feedback in audio, I find it very unfortunate that the data resulting from a proper stability and bandwidth analysis are surrendered without representation . . . an alarming percentage of the time.

The source degeneration and drain load resistor are indeed identical mechanisms, and both occur in "real time", it must because the same current flows through both resistors! (see Kirchoff's laws) Yes, they do behave differently, but this is simply because the output impedance is higher from the drain than from the source. In both cases, the bandwidth available for the negative feedback is defined by the gate capacitance of the mosfet, but when it's driven by the higher impedance of the drain, the rolloff of course starts sooner (higher impedance driving the same capacitance).

There are a couple of problems I see with this. First, we have a capacitance involved, so there is a time issue associated with the related phase issues of that capacitance. If we have a series of these circuits together, we will be able to measure a delay time to it. So- where does it come from? It probably the circuit itself, ergo it has a delay time too.

And as far as I'm concerned, if one condemns the use of negative feedback, and hasn't gone through the process of figuring out where the poles and zeros in the response fall, and analyzing the phase margin . . . they simply haven't a leg to stand on.

And if one *did* go through that exercise, and still finds the feedback to be detrimental, what then?

I think I have, several times. They are the result of circuits that have the following:
-Nonlinear open-loop transfer functions that cause both low- and high-order distortion
-Topologies (i.e. differential, push-pull) that are more effective at cancelling even-order distortion products than odd-order
-Feedback (and hence closed-loop linearity) that decreases as frequency increases.
Put the three together, and you have a system that enhances higher-order, and odd-order distortion products. But the root cause is NOT the feedback.

This **sounds** like an argument for adding feedback to an SET, although I suspect that its not. But an SET can lack the issues above, yet still be degraded by the use of feedback. I myself use fully differential circuits and have to jump through a lot of hoops to prevent odd-ordered generation (we wind up with the 3rd but none of the higher orders) but otherwise our amps don't have the issues you present above either. Yet when feedback is added, increased odd ordered harmonic distortion can be measured (although its tricky as the increase is very slight; OTOH it does not take much as the human ear uses odd orders to gauge volume so tiny amounts are instantly audible).

Frankly, this last quote seems to indicate that feedback should not be used as the circuits that have these issues would seem like something to be avoided.

Of course a complete audio system has a Chaotic behavior. But amplifiers do too. If you look at the formula for feedback, its nearly identical to the feedback formula for classic chaotic models. IOW, we have a strange attractor that models the amplifier's behavior under feedback, we have the other conditions of classic chaotic systems: if it walks like a duck, quacks like a duck... BTW, 'dense orbit' refers to the strange attractor. If you look at a simple pendulum, it is a classic example of a chaotic system and we have been using them for mechanical timing mechanisms for several hundred years. It is often the utter simplicity of chaotic systems that is what throws people off- why they initially don't want to look at things as being Chaotic. BTW its important to understand that the term 'Chaotic' is not the same as the typical dictionary meaning.

You guys are in way over my head! I can sort of follow the tech talk but have no way to translate that into results regarding actual sound quality to expect via the various design approaches.

I'm curious to see the bottom line in the end ie what key points relating to feedback and sound quality you two agree and disagree on. Perhaps also if there are any amps most might be familiar with that are good representatives of how the different technical approaches sound.