What is wrong with negative feedback?

06-25-10: Jamesgarvin
... "Seems to me the only way to make that determination is to take two identical amps, one with negative feedback and one without, and play them with the same source through the same speakers. I've not heard of anyone who has done that exercise, much less anyone who has heard it."

Hi Jamesgarvin;
Interesting. However, they could not be identical amps, even supposing that they only differed regarding feedback, but I assume you meant it ideally.
Practically speaking, we can, of course, choose one of each type from a number of respected designers and begin there.
When you referred to playing them with the same source, I'd like to reference "source" as recorded source.
Using acoustic instruments as an example:
If we are present in the recording space during the performance and we agree that the sonics are excellent, we'll have to verify that all components: mics, mic pre-amps, console or summing equipment, power-amps,
are non-feedback - as well, the same devices with feedback, We'd have to start there.
If this could be accomplished, we'd still have to consider
Class A, A/B, D, discrete/chips, SS/tube, etc.
We still haven't discussed what reference speaker system.
Considering all of the above, its amazing the amount of effort that drives the talented designers and enthusiastic lovers of audio and music.
For those concerned, having a very satisfying audio system is natural treasure.
I'm glad to see those efforts displayed here.

Right. We were careful to not allow the amplifiers to clip during these tests. However it really did not matter which amp we used or in what combination. The higher efficiency of the speakers meant that amps were not working as hard and so were more relaxed.

That was 12 years ago. We've made good progress reducing distortion since then, so the amps have continued to get more relaxed.

"Now of course you can imagine that one might do that with a more efficient speaker because it is possible, but really the limiting factor with the less efficient speakers was that the loudness cues made it uncomfortable to turn it up the volume any higher."

Hmm, I'm under the impression amp clipping generally becomes more of an issue at higher volumes with less efficient speakers?

I'm assuming that you run these with one of your tube amps, which would minimize loudness cues when distorting? All amps distort more at higher levels, correct? The question is in what way and to what extent the distortion is audible. For a given amp, this would occur sooner at lower volume with a less efficient speaker, all other factors aside, right?

Hi Bob, the system I have at home uses a pair of Classic Audio Reproduction T-3s with dual 15" woofers and field coil drivers in the midrange and tweeters. They are about 97 db.

When we first got them I noticed that phenomena of loudness cues: we were playing them a lot louder than the previous speakers, which were 89 db. Now of course you can imagine that one might do that with a more efficient speaker because it is possible, but really the limiting factor with the less efficient speakers was that the loudness cues made it uncomfortable to turn it up the volume any higher. The CALs in the system allowed the amps to make less power/less loudness cues so we were playing them louder very naturally and effortlessly.

Hi Atmasphere, Thanks for the advice. I have several "system busters" Lps. Certainly Daphos is high on the list along with Carmen suite on RCA and a few others that have very big dynamics.

I am curious as to your system, what does it consist of? There must be some nice OTL amps, but what else do you use? I dont believe I have ever saw references to your system speakers etc. Could you post a few pictures of your system here on the 'Gon. Although I would expect that it is constantly changing. Is this system in your house or at work?

Thanks,
Bob

If you get the original Vertigo press there is a lot of bass and bass impact! If you try to play that anywhere near 105 or 110 db, which is justified by the material, most systems just can't do it- too much energy required. The trick it to be able to play it that loud without it **sounding** loud, IOW the only sense of volume should come from the LP itself, not the system.

Atmasphere said "My system can play so loud that prolonged exposure can result in damage."

I assume you mean damage to your ears, not your system. I agree 115db is too loud as we do not want to damage our ears. Also I think that for audiophiles in general that level is very difficult to obtain. IMHO it is an unrealistic goal.

But you say, you play the Black Sabbath loud, so...

The Black Sabbath Lp you said you play loudly then is it louder than the Verdi or is the Verdi LP the LP that you play loudest? In other words, is the Verdi LP a test lp for "you want to hear loud?" then this is the loudest we go?

I am always on the lookout for LPs that streatch my systems limits, it sounds as if thats a great test LP for a systems ability to play loud. Though I dont get anywhere near 115db!

Bob

Acoustat6, that's true. My system can play so loud that prolonged exposure can result in damage. But playing rock as loud as it is often performed has a bugaboo: most rock recordings are anything but live! Quite often the guitar amps have only 15 watts in the studio, so who is to know how loud such a recording is to be played.

I have a white-label Vertigo press of Black Sabbath's 2nd LP (Paranoid), which is an amazing recording and one that can bring most systems to their knees in a heartbeat. You play it loud, but even that one is hard to tell how loud it should be played.

I play in a rock band, and recently we did a memorial show where we were the only band on the bill that was not metal. The club we were playing in was a metal club. It was on that night that I discovered that metal bands don't play all that loud. The most powerful guitar amp we saw that night only made 25 watts. They rely on the PA.

So- how loud is that supposed to be? Rock is always tough because there is no good answer for it.

Atmosphere said "06-25-10: Atmasphere
Acoustat6, yes, sometimes I do. Most music does not demand it but some does: The Verdi Requiem, on the Soria box set (RCA) is a good example of vinyl being put the limits, and stereos too. If you play the quiet spots at the correct volume, the peaks will be at 115db."

Hi Ralph, If you listen to the Verdi Requiem at peaks to 115db (which is an impressive level for any system) then, and since we all know that rock is supposed to be even louder than an orchestra, how loud would you listen to Cream live or other "loud" rock?

Because you said, "IOW, a proper stereo should lack loudness cues, such that you can approach the same volumes in your room that the real live music could."

Then just how loud do you listen to rock on your system to approach what a live rock concert does. Its gotta be louder than a recording of Verdis Requiem!

Bob

The loudness perceived compared to actual SPL was most dumbfounding at first especially in that teh IcePower amp power spec is 4X what I had before (500w/ch into 8 ohm compared to 120w/ch prior). Its almost like the additional power is utilized to flush out the music, kind of like blowing up a ballon, as the volume goes up yet the perceived loudness does not increase so much.

This sounds like there is less odd-ordered harmonic generation.

"Mapman - I also observed that listening at low volumes with Icepower is very good. I know that part of the problem is our hearing but I enjoy very good and clean sound (with very articulate bass) at low levels without any loudness correction. I cannot explain it. Do you experience the same? "

Yes, bass is very articulate at all volumes. That and the loudness behavior I described above are perhaps the two biggest differences I hear on my system with icepower compared to prior SS amps.

The loudness perceived compared to actual SPL was most dumbfounding at first especially in that teh IcePower amp power spec is 4X what I had before (500w/ch into 8 ohm compared to 120w/ch prior). Its almost like the additional power is utilized to flush out the music, kind of like blowing up a ballon, as the volume goes up yet the perceived loudness does not increase so much.

Kijanki, one other thing, with my particular speakers, the high current and efective doubling of power into 4 ohms helps produce a natural balance in the bass which also helps at low volume whereas with some amps, the bass is relatively leaner. Toss in teh articulation in the bass provided at all levels then and you have quite a winning combo that not only delivers a detailed low end, but also allows higher frequency details to come through to boot. Very satisfying to listen to! Articulate, detailed and "musical". Very hard for me find fault at present, which is a very nice place to be!

Unsound - I remember Ar_t posting on the subject of switching amps. I don't know about TacT now. At the time I bough my Icepower based amp I read opinions on Icepower and Hypex. Both configurations (Full Bridge vs Half Bridge) have good implementation for instance Hypex based Channel Island amps.

Mapman - I also observed that listening at low volumes with Icepower is very good. I know that part of the problem is our hearing but I enjoy very good and clean sound (with very articulate bass) at low levels without any loudness correction. I cannot explain it. Do you experience the same?

Acoustat6, yes, sometimes I do. Most music does not demand it but some does: The Verdi Requiem, on the Soria box set (RCA) is a good example of vinyl being put the limits, and stereos too. If you play the quiet spots at the correct volume, the peaks will be at 115db.

So- to answer another question- 115 db peaks occur front row center, equivalent to where the microphones are usually placed. When you are in the orchestra, you don't get the same volume exposure that the audience does unless you are in the percussion section.

Some of the class D amps do not have feedback as well as some transistor amps as I have mentioned. Some of the class D processes allow for time manipulation, which is a fancy way of saying that they have a way around this issue. I don't doubt that this may be part of why class D amps are challenging the traditional transistor art.

However the artifacts I am talking about also occur at much lower levels. BTW, it is odd ordered harmonics that allow SETs to have the 'amazing dynamics' that are so often associated with them despite their low power levels. When an SET is at low power, it makes no odd orders at all, but when you push it over about 50% of full power or so, they begin to show up. Since it is the transients that take all the power, that is where the loudness cues are now occurring too. As a result, an SET with low power might seem to play so loud that 85 or 90db seems like a lot. This is the effect of artificial enhancement of the natural loudness cues.

As a result I am careful about using the word 'dynamics' because so often when I hear audiophiles use the term they are really talking about 'distortion'.

If you can't be in the room with musical peaks at 100 db (if its uncomfortable or sounds too loud), then its very likely that the odd orders are being enhanced.

Kijanki, I could be wrong about this, and perhaps it doesn't matter, but, I don't think Ar_t 's amps were ice amps.
Have the TacT amps changed since then?

When I first heard the IcePower amps in my setup, the difference in the nature of teh sound from anything I had heard prior was most startling. It was most apparent in that major volume adjustments seemed to have comparably little effect on volume, though the SPLs were clearly increasing as expected.

My observation of how the IcePower amp sounds as volume is increased seems consistent to me with what Ralph describes as the benefit of not using feedback and loudness cues. Most really good systems I have heard over the years tend to behave this way more or less I believe.

At the same time, Icepower applies negative fedback clearly. So I have to conclude that the IcePower amp has achieved a lot of teh benefits that Ralph attributes to no feedback in its specific implementation.

If so, another reason why Icepower/Class D can be considered a major innovation in home audio along with the other advantages.

Unsound - Feedback in Icepower amps is called Multivariable because it consists of two feedbacks controling voltage and time.

I suspect that feedback in class D is less evil for few reasons:

Amp cannot become unstable (oscillate) since it is already oscillating.

Response can be very fast limited only by Mosfet's max current and resistance (sort of Hysteretic converter)

Voltage feedback is shallow because duty cycle is more linear than transistor characteristic(less feedback required) and also because "time feedback" already corrected most of nonlinearities. I suspect that voltage feedback is helping to deal with load regulation. Early class D amps (Tact) had no voltage feedback at all and were sensitive to load conditions.

Momentary saturation of output stage (charge trapped at the junction) that happens in bad cases of class AB is irrelevant since time (duty cycle) and not the voltage is analog quantity.

A few years ago, we had a regular and good contributor (be nice if he came back) Ar t an amp designer, suggested that in his typical ss designs he would try to keep NF to a minimum, but in his switching amps though there was much more NF, it didn't seem to be an issue. I wonder if it was due to the NF in the switching amps occurring at a faster rate?

Jamesgarvin I had a pair of Manley ref 440/200 monos that had adjustable slopes and feedback to custom taylor the sound;there were noticable differences but I did not consider it annoying just different.

"These amps are high damping factor which matches well to the OHMs, so I suspect there is NF applied, but not sure"

Mapman - Not only NFB is applied but it is mulitiple (two) called Mulivariable Enhanced Cascade Control. It is in the Karsten Nielsen doctorate work available on the web (Icepower). Feedback doesn't have to be deep since class D amp has inherently low output impedance even without feedback. Speaker is always connected to +Vs and GND and only direction changes (very low resistance Mosfet Bridge). Output impedance increases with frequency but DF is still about 1000 at 1kHz.

Ralph Atmosphere said, " A typical orchestra can do 115db with ease, yet many audiophiles will not turn up the volume past 95 db because its 'too damn loud' or they get 'turn that @#$% down!' from their S.O. This mostly due to artificial loudness cues which are totally coming from distorted odd ordered harmonics, and by that I mean only 100ths of a percent."

I ask do you listen to your system at this level?

Bob

The Bel Canto ref1000m2 Icepower Class D amps I am using on my larger full range OHM Walsh omnis seem to be able to go as loud as desired without an edge on most good recordings.

These amps are high damping factor which matches well to the OHMs, so I suspect there is NF applied, but not sure. If loudness cues are more present as a result, I'm not sure that it is significant enough to matter.

I also use Triangle and Dynaudio monitors on teh same system. Monitors in general will not fare as well with orchestral music at high volumes (at least not without a sub of some sort to offload much of the work), so I can say that the speakers are definitely a factor as well.

Is the 115dB at a mid-hall seat, or is that the level on the stage? Apparent sound levels in a concert hall are far more influenced by the indirect sound level than in the home environment. This is due to the enormous difference in volume between a concert hall and a normal audiophile listening room (say 200-300 cubic meters vs. 20,000-30,000 cubic meters). In a home environment you listen to predominately direct sound that has nor been tonally shaped (highs rolled off) and homogenized by a concert hall volume of space. Hence it sounds louder than the exact same measured SPL in a concert hall. It really has nothing to do with amplifier design.

"Its been my contention that negative feedback is a major culprit, literally violating one of the most fundamental rules of human hearing: how we detect the volume of a sound."

Maybe. Maybe not. I've got a Music Reference RM-200II, and there are some compact discs (never records, it seems) in which I can only turn up so loud before my ears bleed, and others I can turn up without any irritation.

Then you factor in the speakers, and their ability to generate clean sound at the levels you are talking about.

Seems to me the only way to make that determination is to take two identical amps, one with negative feedback and one without, and play them with the same source through the same speakers. I've not heard of anyone who has done that exercise, much less anyone who has heard it.

Hard to disagree with Ralph's last post in my opinion.

That is true. However I have found the if given the option, people prefer a system that it not bright and harsh, given otherwise that there is no lack of detail, bandwidth and with no tonal aberration.

IOW, a proper stereo should lack loudness cues, such that you can approach the same volumes in your room that the real live music could. A typical orchestra can do 115db with ease, yet many audiophiles will not turn up the volume past 95 db because its 'too damn loud' or they get 'turn that @#$% down!' from their S.O. This mostly due to artificial loudness cues which are totally coming from distorted odd ordered harmonics, and by that I mean only 100ths of a percent.

Its been my contention that negative feedback is a major culprit, literally violating one of the most fundamental rules of human hearing: how we detect the volume of a sound.

Atmasphere,

I suspect it would be hard to set up any meaningful comparison of an amp alone in that an amp cannot produce music alone without speakers. The results could vary widely depending on speakers used, personal preferences, and all the usual culprits that result in different strokes for different folks.

My gut feel is that as you say, its all in the implementation and there are many ways to make tasty soup.

The tech talk is over my head too, but I'll say that the best amp I've heard of late that uses feedback is the Music Reference RM-10 MkII.

That might actually be simpler- the proof in the pudding. Kirkus and I have had plenty of exchanges in the past and I have always respected his demeanor, but at the same time I do not think there is anything that I could say that would change his mind- he clearly knows his book larnen'.

OTOH, I'm pretty sure that I'm likely to stay put too. If I had not spent so much time doing this, I might be easier to convince.

So maybe, we choose the amps from the two camps that we think are the best examples of feedback and no feedback.

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.

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.

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.

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.

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.

'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 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.

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

The math is fine right up the 2nd to last paragraph where an assumption is made that is incorrect. It matters a lot what the output section topology is. An excellent example is the difference between a triode gain stage and a cathode follower using the same triode. The CF will be found to have a lower output impedance, according to Rp/1+mu where Rp is the plate resistance and mu is the 'voltage gain' of the tube.

So if we take the example given we get 17.5/3 = 5.767 (the mu of a 6AS7G is 2), which is for a simple CF circuit. For a Circlotron, which is a CF variant, the formula is similar, the 1 is replaced by a 2 as above so we get 17.5/4 = 4.375

In these cases it is assumed there is no feedback.

Ralph, this is the wrong place to ask, but would you care to answer the math as proposed by:

http://www.transcendentsound.com/amplifier_output_impedance.htm

Just trying to get a grasp on the "how". The other controversial article is not related to this thread.

Fascinating relating electronics to chaos theory. Maybe we can use it to predict long term digital format forecasts.

I'm not sure if DF>100 makes much sense since inductor in series with the woofer has resistance in order of 0.08ohm.

8/0.08=100

Thanks Ralph good information.

"There are no known speakers requiring a damping factor of over 20"

I am not aware of any speakers that publish damping requirement specs. What are some?

I am aware that different designs benefit differently from varios damping factors, but not that vendors specify damping requirements for their speakers in that determining which combos sound good is often a largely subjective determination?

Rleff, that is mostly right; damping factor is the ratio of load impedance vs that of the amplifier driving it, and can be increased by adding negative feedback. Some amps achieve a high damping factor with zero feedback, the Ayre is an example of that.

The question is whether high damping is desirable. There are no known speakers requiring a damping factor of over 20, and there are some that are better off if the damping factor is between 0.1:1 and 2:1.

This is very much a part of the equipment matching conversation!

Ralph is there a relationship that exists between negative feedback and damping in relation to how the amp(s) react to the what a speaker reflects back to the amp? So when you see a amp with high damping specs it means a large amount of feedback is being used;would that be right or wrong?

Thanks Kirkus for your response. I tend to go with Norman Crowhurst rather than Baxandall. However I've been researching this issue myself for some years and while I regard ignorance of the past as foolhardy, I also try to keep an open mind.

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- it is quite good for economizing the design side though.

In a nutshell Nelson encountered some odd orders in his study, that in order to eliminate them, he figured levels of feedback that are in excess of 50 db, requiring increased gain, which means more distortion, so more feedback...

OTOH these distortion levels are absent in zero feedback amplifiers of proper design. You can count on one hand the number of transistor amps that meet that description (Nelson's is one of them and no surprise that his amps get high accolades).

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.

Distortion is known as bifurcation in Chaos Theory; what we see in an amplifier with feedback is the bifurcation elements do indeed behave as Crowhurst predicts, and interestingly enough and apparently not coincidence, the formula he shows for feedback in an amplifier are startlingly similar to the formulas used in classic Chaotic systems. He goes so far in his books to actually show an example of the strange attractor that models amplifier-with-feedback behaviour, years before Chaos Theory was established.

Nelson Pass, while not mentioning Choas, does point to a tell-tell aspect of chaotic behavior, that of having to add more and more feedback to get rid of the higher odd orders (with attendant greater amounts of gain required to do so).

This is very similar to the way noise behaves in digital circuits, due to Cantor Dust and is the reason we use parity bits in all digital communications. When IBM was first studying the problem of noise in digital circuits, they were trying to make the signals bigger to overcome the noise, which Chaos showed was not going to work. The parity bit was the solution- IOW don't try to fight the Cantor Dust.

In a similar way, its telling us the same thing about feedback. IOW, negative feedback is a **destabilizing** aspect of amplifier design. Amps without feedback are inherently stable. I have seen this borne out in practice: some amps with feedback oscillate just by the use of certain speaker cables, but there is no zero feedback amp that will do that. More importantly, Chaos supports Crowhurst with regards to bifurcation and predicts harmonic and inharmonic generation in the way that Crowhurst specified. In fact it appears that we are not altering the energy of the bifurcation- we are taking the energy and spreading it out over frequency. Some of these frequencies are well past the band-pass of many amps, so in a way we are getting rid of the energy to a certain degree, OTOH the ultrasonic behavior of an amplifier often says a lot about how it sounds. I am sure you have encountered that!

It is a fascinating study. If you are not familier with Chaos Theory you can start at http://en.wikipedia.org/wiki/Chaos_theory

Kirkus: Nice post. Accurate and true to practice. One point of interest - distortion is a consideration for all type of signal processing. I spent decades designing signal acuisition and processing circuits for medical research applications. Negative feedback is employed in every discrete application that I saw or worked with - clinical as well as research. Although an amateur radio operator for decades and an electrical engineer by education and occupation for decades, I never encountered the "bad bad negative feedback" argument until I started looking around audiophile websites. Kind of wonder where the better theory is, huh? Thanks for the information as to the origin of this myth in audiophile circles. Also nice cite to Self's book

The issue I see is that if you have a wideband amplifier, and I do, the problem is that the squarewave response looks nothing like you described: it has a lot more in common with the input. It might be kind of strange to think about a tube amp that can do justice to a 10KHz squarewave but that is what I am talking about.

Thought I'd address this as well . . . I do applaud that you build amplifiers with wide bandwidth, and especially applaud that you're up front about some of the side-effects of your design approach, namely certain speaker incompatability from high output impedance, and poor power efficiency. These are quite reasonable choices for a niche product in an enthusiast market.

But in order to understand the theoretical basis for the proper application of negative feedback, you must understand the phase response of the amplifier in the frequency range(s) where the response rolls off - the stability of a feedback amplifier is inexoribly linked to its transition-band behavor. This is true no matter how extended the open-loop bandwidth may be . . . if it rolls off in an idiosyncratic manner, there will be instabilities if global feedback is applied.

Also, we can definately agree that there are many amplifiers out there using global negative feedback, that do indeed exhibit high-order distortion products and poor transient response (ringing). The point of my previous post is that these high-order products are virtually always present before the feedback is applied, and it's extremely common in many amplifiers for the feedback only to be effective at reducing the lower harmonics. Since a conventional three-stage solid-state Class B bipolar amplifier remains the poster child for global negative feedback (and higher-order distortion products) I think it makes the best example of why this is NOT caused by the feedback itself.

For this type of amp, all of the voltage gain is provided by the second stage, a transresistance amplifier . . . which can provide extraordanary amounts of gain with extremely good linearity. Its drawbacks are that it's very sensitive to loading, and the exact amount of gain you get is determined by the transistor's beta (the most variable characteristic of a bipolar transistor). But both this voltage amplifier stage and the differential input stage that preceeds it (if properly designed) will deliver extremely low distortion even without any global negative feedback.

Rather, virtually all the distortion comes from the output stage . . . in the real world, this is further exacerbated by the fact that thermal bias control is frequently inaccurate, the large half-wave currents drawn by the output stage can crosstalk into other parts of the circuit . . . it's also tough to keep nonlinear drive currents away from the preceeding voltage amp. So suffice it to say that there are lots of all kinds of distortion products being produced, of both low and high order harmonics, before feedback is ever applied. On top of it, the output stage is by far the slowest and most bandwidth-limited, with a rather unpredictable multi-order rolloff slope.

Now for the feedback. In order to have good stability, we need to have the open-loop gain and rolloff, and consequently the phase-shift, be predictable as frequency increases . . . this is done by applying freqency-dependent local feedback around the voltage amplifier in the form of the Miller compensation capacitor, reducing the gain at the rate of a tidy single-order slope as frequency increases . . . thus keeping the phase margin with feedback at 90 degrees.

So the open-loop response of a conventional solid-state amplifier, with compensation, is NOT wideband . . . its rolloff starts very much in the audioband, maybe at 200Hz or so? It's tough to measure and calculate, because the actual value is beta-dependent, and the low-frequency gain is super-high an difficult to measure. But as frequency rises and local Miller-capacitor feedback takes over, the open-loop gain becomes both lower and more predictable. And since the amplifier will have a flat closed-loop gain to well outside the audioband, what's happening is that as the frequency increases, the amount of global negative feedback actually decreases.

And when we look such an amplifier on the test bench, we might notice that for mid-band distortion, there are virtually no lower-order distortion products, but there are some higher-order harmonics. We also notice that the THD percentage rises with increasing frequency. But this is NOT a result of the feedback creating high-order products from lower-order harmonics . . . the distortion is all coming from the output stage and exists with or without feedback. What's actually going on is that for the higher harmonics and frequencies there's TOO LITTLE feedback to get rid of the distorion, because the compensation capacitor is causing the open-loop gain to fall at 6dB/octave. Also, the feedback has the benefit of lowering the noise floor, which can cause previously undetected/inaudible higher-order distortions to be uncovered.

The problem with solid-state feedback amplifiers in the 1970s was twofold: first, the power semiconductors of the day were SO slow that any form of compensation had to be pretty heavy-handed just to keep it from oscillating. And second, there were so many high-order distortion products from other aspects of the circuit that what little higher-frequency feedback was left had no chance of getting rid of it. The feedback was simply the big flashlight shining into the dark basement . . . and likewise it isn't the flashlight's fault when rats are discovered.

It has been known since the mid-1950s that loop feedback enhances odd ordered harmonics and there were cautions expressed that long ago about excess use of Global negative feedback due to this problem . . . How do you square that reality against what you have stated?

Atmasphere, this is not reality, it is rather a myth -- there are two soruces that I am aware of. First is Norman Crowhurst's 1957 AES paper on feedback in amplifiers (where he refers to "regenerative distortion"), and the second is from Peter Baxandall's 1978 series of articles in Wireless World (where he discusses the theoritecal possibility of an amplifier with only second-harmonic distortion generating higher orders through intermodulation with feedback). While I greatly respect both authors and recommend especially the Baxandall works for reading, this particular theory simply doesn't hold up in practice.

First of all, the point isn't about even-order distortion products becoming odd-order . . . it's about lower-order products becoming higher-order. In a push-pull topology (which cancels even-order products through another mechanism), that may be a supposition, but it's not part of the theory. Here's how it's supposed to work: If an amplifier has a strong second-harmonic distortion product, the addition of negative feedback causes the distortion and the fundamental to intermodulate and become third-harmonic . . . then the second and third intermodulate and become fifth-harmonic, the first and third become forth, etc. etc.

The counterpart to this is simply that negative feedback on the whole is so much better at eliminating distortion than generating it. I think Douglas Self put it concisely and eloquently in his book on power amplifier design (commenting on Baxandall, of which he is a huge admirer), so I'll quote him:

All active devices, in Class A or B (including FETs, which are often erroneously thought to be purely square-law), generate small amounts of high-order harmonics. Feedback could and would generate these from nothing, but in practice they are already there.

The vital point is that if enough NFB is applied, all the harmonics can be reduced to a lower level than without it. The extra harmonics generated, effectively by the distortion of a distortion, are at an extremely low level providing a reasonable NFB factor is used. This is a powerful argument against low feedback factors like 6dB, which are most likely to increase the weighted THD.

In addition, I've spent some time personally with the math behind this supposition, and done some SPICE simulation to back it up. The beauty of SPICE for this kind of application is that we can examine the feedback itself in its most pure, basic form, where it can exist without bandwidth limitations, stability problems, or any kind of loop "Propegation Delay".

First, I created a voltage stimulus with a controlled voltage source in series, to allow me to easily apply any amount of negative feedback, then another for open-loop gain. I then created another controlled voltage source, that adds a huge amount of pure second-harmonic distiortion (only 34dB below the fundamental!). No other distortion products exist, down to the FFT limits of about -200dB. I then applied various amounts of negative feedback, by changing the amount of open-loop gain. For a loop gain of 4 (12dB feedback), we see the 2nd harmonic drop to -52dB, a 3rd appear at -88dB, a 4th at -122dB, and a 5th at -155dB, and the 6th at -188dB.

So what does this mean? First, virtually any amplifier that's so ill-conceived as to have enough second-harmonic distortion as to be only 34db below the fundamental, will almost surely have some higher-order harmonics as well. But for even such an amplifier, adding just 12dB of feedback puts the third harmonic at -88dB, which will almost always be buried in the noise floor. And the rest (at <-120dB) will certainly always be undetectable and inaudible. But the improvement by knocking down the second harmonic to -52dB will be certainly be audible, and for the better. I think this supports Self's conclusion very nicely.

But there's another aspect of looking at this in SPICE -- these results exist in a world without any phase shift ("Propegation Delay") . . . meaning that they are equally valid for both local and global feedback! And the phase shift evident in real-world circuits can indeed introduce instability and transient-response problems (ringing), but it doesn't change the distortion-reducing effectiveness of feedback. So if you're truly worried about "regenerative distortion" . . . you'd better avoid all forms of local feedback as well. (Good luck with that!)

So again, if properly implemented, in the real world . . . negative feedback reduces ALL manners of distortion.

Coffeey,
What are you talking about, and what does it have to do with this thread?

Kirkus, I appreciate your input as always, and I am always interested in expanding my knowledge. I don't contest what you are saying, the problem is that it does not address my experience. I went to school too, FWIW.

The issue I see is that if you have a wideband amplifier, and I do, the problem is that the squarewave response looks nothing like you described: it has a lot more in common with the input. It might be kind of strange to think about a tube amp that can do justice to a 10KHz squarewave but that is what I am talking about.

So my test for delay time holds together with very little error from the means that you suggest. If we were dealing with an amplifier with the limited bandwidth product you describe I would be more inclined to agree, except that there is still one problem.

It has been known since the mid-1950s that loop feedback enhances odd ordered harmonics and there were cautions expressed that long ago about excess use of Global negative feedback due to this problem. In the last 55 years that has not really changed- you can add feedback to an otherwise functional amplifier and experience and measure this phenomena. It is as I laid out earlier in this thread.

How do you square that reality against what you have stated?