If you can increase the speed of the amp (decrease the propagation delay) theoretically you could speed it up to the point that the odd-ordered enhancement is pushed well outside of the audio band.
Our amps are also pretty fast- 600V/usec is a typical risetime, and we only have one stage of gain. But IME this is still not fast enough, so we resort to other means of getting rid of distortion: class A operation coupled with fully differential balanced operation, which cancels even ordered harmonics not just at the output, but throughout the amplifier. This leaves us with the 3rd harmonic, which is controlled by using only one stage of gain. Odd ordered harmonics are exacerbated by noise problems in the ground and the power supply, so we use star grounding (a lot easier since most of the grounds are balanced) and separate power supplies for the driver and output sections, which also reduces IM distortion.
Distortion has the property of masking detail in addition to adding loudness cues, so if you can get rid of distortion you get greater transparency and greater smoothness at the same time, provided your techniques for getting rid of distortion don't enhance the 5th, 7th and 9th harmonics. IOW real reductions in distortion have real, immediate sonic benefits that anyone can hear: extreme detail accompanied by smoothness are the hallmarks to look for. |
Loop feedback in any form is supposed to reduce distortion. It is questionable whether it increases bandwidth, and in some models (see the link I provided) it reduces 'output impedance'. You'll see why I use the quotes if you look at the link. The *big* problem is that loop feedback, in the process of doing all this stuff, exacts a penalty. This comes from the fact that any circuit that can amplify is doing so at speeds that are easily measured on rather pedestrian test equipment. This time is called Propagation Delay- the time it takes for the signal to propagate from input to output. Now feedback is created by taking the some of the signal from the output, and applying it to an earlier portion of the circuit, which has a propagation delay. So you can see that the feedback signal is arriving ever so slightly too late to do its job right. The fact that it is too late causes the amplifier to become less stable as frequency increases, and there can be inharmonic noise created at the point where the feedback is returned. This causes feedback to inject a low level harmonic distortion noise floor composed of harmonics up as high as the 81st harmonic into the output of the amp, and it has two audible artifacts. The ear uses naturally-occurring odd-ordered harmonics to figure out how loud the sounds are. They are the 5th, 7th and 9th harmonics and they get enhanced (distorted) by feedback by a small amount. However, because these are loudness cues to the human ear this small amount **is easily audible** and audiophiles use the terms 'hard' 'harsh', 'bright', 'brittle', 'chalky', 'clinical' and so on to describe this distortion. Keep in mind that this is the case when the distortion of these harmonics may only be 100ths of a percent!! This is why two amps can measure the same frequency response on the bench but one will be bright and the other is not. The 2nd problem is that the harmonic noise floor, through another hearing principle called 'masking', will block the ear's natural ability to hear into the noise floor of the playback system (the ear can hear 20 db into a natural noise floor like tape hiss or the wind blowing). Any information below the noise floor is not heard by the ear or not detected as easily. Since ambient soundstage information exists at low level, one of the more obvious effects of feedback is to foreshorten the soundstage depth and width. Amplifiers in particular that use no feedback tend to have a different voltage response in dealing with the loudspeaker and the designer of the speaker has to accommodate this behavior. IMO, a speaker that requires an amplifier with feedback, due to the issues above, will never sound like real music. Speakers that *are* friendly to zero feedback amps at least have a chance. see http://www.atma-sphere.com/papers/paradigm_paper2.html for more information |
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, my technique for measuring propagation delay is simple: compare the input to output while using a squarewave source. Observe the difference in time between the rising input waveform and the rising output waveform. That's the delay time. I have yet to see an amplifier where I could not see that on the 'scope. Since negative feedback only exists if the open-loop (feedback-free) gain is above unity, and since the open-loop response falls off at 6dB/octave . . . the input/output phase response must be 90 degrees or less. So if we're going to talk about "transit time", how would you define that? It really seems to me that something is glossed over here. In this model phase and time become the same, and is inadequate to explain the behavior of an amplifier that has wide (+200KHz) open loop bandwidth. In such amplifiers the model below falls apart: Since we know that comparing the phase at the input the output will give us 90 degrees, the "transit time" at 100KHz will be 2500 nanoseconds. At 200KHz, it will be 1250 nanoseconds. At 20KHz, it will be 25000 nanoseconds. So it seems that talking about "transit time", or "propegation delay"[sic], or "delayed feedback", or whatever . . . is a wholly inadequate way of understanding what's going on. Rather, classical Control Theory uses phase relationships to analyze feedback. Propagation Delay does not alter with frequency anywhere near the audio band, and at those frequencies the delay time is easily measurable. In fact, we can see that at low frequencies feedback works pretty well, but as frequency increases, the feedback is progressively inadequate due to the fixed propagation delay of the circuit having a larger effect as the waveform time decreases. This introduces a time-domain distortion- ringing and odd-ordered harmonic enhancement. It is this phenomena that requires networks in many amplifier designs to prevent negative feedback from becoming positive feedback due to the phase at very high frequencies that are out-of-band but can cause the amp to go into oscillation if not addressed. The model you are proposing relies on propagation time being mutable, which it certainly is not. I'm with Spectron on this one. Sounds to me like control theory is being misapplied here. |
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. |
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? |
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! |
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. |
'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 |
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. |
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. |
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. |
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. |
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. |
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. |
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. |
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. |
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. |
