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In Reply to: Thanks for the info.... posted by Charles Hansen on August 01, 2003 at 21:24:59:
I've heard Atmaspheres and I've heard Hurricanes, but I will let the reviewers make the public pronouncements about their qualities. Trust me, doing so is a no-win situation for a mfr.Output impedance is a real issue with tube amps, even those with feedback, because the feedback is not always fully operative due to load reactance. The definitive solution is to use a great many output tubes.
Charlie, I pulled the feedback from the EL34 version of my V8 some three years ago. Its got a bigger soundstage and is more vivid in the midrange, but I can't say as I like it any better overall. I am having some luck, believe it or not, with 6550 beam tubes using *local* feedback. This converts the horizontal plate curves to triode-like diagonal curves. These 6550 amps don't use any global feedback. They were used to drive the Avalon Opus at the recent HE2003.
But I have not given up on feedback. I find it a facinating challenge. The reason I think it can work is because it is used throughout the human body. Even the ear uses it. It is what protects the inner ear from overload. The touch of a finger on a piano key is a servo operation through and through. There is even compensation by integration. So I believe feedback can work. It's just difficult.
Follow Ups:
Hi,> I am having some luck, believe it or not, with 6550 beam tubes using
> *local* feedbackVery interesting. The pentode with local feedback could actually
be a more linear triode than a triode. How much local feedback is
this?I also note the switch from el34s to 6550s.
The amount varies with tube type. We can use whatever is needed to bring the sensitivity of the tube down to the level of its triode connection. This way, they both have the same drive requirement.As for linearity vs the triode connection, that again depends on the tube type, as well as the amount of feedback appied. It can be better, yes. Esp if the pentode is made to operate halfway to a CF.
you might be right, but I'm not convinced. For example, in the ear feedback is used only for relatively long-term level changes. It is not trying to operate on each individual part of a waveform as happens in a feedback amplifier.The example of a piano player also doesn't sway me. It would seem to me that short time scale eye-to-hand feedback is used when *learning* to play the piano, but a skilled pianist only uses feedback in a longer time scale sense. In fact, it seems likely that the musical differences between a beginner and a skilled player are actually due to feedback. The beginner has to find each note tentatively, using a clumsy series of feedback controlled iterations to move the fingers, while the skilled player instead uses muscle entrainment to play each note confidently.
If this is true (I don't know if it is or not), then the analog as applied to amplifiers would be to use feedback for long time scale functions, such as DC servos or thermal feedback (in solid-state amps), but avoid using feedback to control the audio signal on a moment-by-moment basis.
Interesting stuff, and I don't claim to have definitive answers here (though I do have strong suspicions). However, I do like your approach of trying to make amplifiers mimic living beings!
Best regards,
Charles Hansen
The idea originates at MIT circa 1942:"Thus Wiener and Bigelow discovered the closed loop of information necessary to correct any action--the negative feedback loop--and they generalised this discovery in terms of the human organism....Their purpose was to approach the study of living organisms from the viewpoint of a servomechanisms engineer and, conversely, to consider servomechanisms with the experience of the physiologist." http://pespmc1.vub.ac.be/CYBSHIST.html
"The beginner has to find each note tentatively, using a clumsy series of feedback controlled iterations to move the fingers, while the skilled player instead uses muscle entrainment to play each note confidently."
Muscle entrainment is an enhancement of the overall feedback process. The skilled player uses *conscious feedback* to make micro-corrections in real-time while depending on his muscle memory to walk (less consciously) through the notes. When a tennis player returns a serve standing on one leg there is more to it than muscle memory. When a golfer corrects his forearm rotation in mid-release there is more to it than muscle memory. When a cat swats an escaping bird in mid-flight there is more to it than muscle memory.
Here is a quote from "Consciousness as a Feedback Interface":
http://www.innerworlds.50megs.com/consciousness.htm
"The speeds at which neuromagnetic signals are propagated, together with their capacity to recruit/alter multiple modalities suggests that the underlying mechanism has been selected to make instant choices on which specific portions to recruit in order to facilitate the behaviors acted out of the State, and to do so quickly....When it comes to response to threats, or sighting prey, the evolutionary advantages are obvious."
I still don't see any examples of feedback in living organisms extending to frequencies above a few tens of Hz. Think about a cat attacking a bird. In mid-spring he will have to adjust his attack as the bird takes flight. Human reaction times (at least in relation to driving) are usually quoted as around 0.25 to 0.5 seconds. Even if we give the cat a full order of magnitude advantage (just to be *really* safe), we are still only talking about a maximum of 20 to 40 Hz.So that seems interesting. Unless you can find some example of reaction times (feedback) much, much faster than 50 mS, it would seem that feedback as used in living organisms is used only *below* the audio range. Maybe that's the principle that should be adopted for audio electronics as well...
Best regards,
Charles Hansen
True, we know that neurochemical response times are limited by the transmission time across the synaptic gap to the order of .5 to 2mS. By comparison, the propagation of action potentials is much faster. Look again at:http://www.innerworlds.50megs.com/consciousness.htm
"...an action potential can travel a full centimeter (a couple of orders of magnitude larger than a synaptic gap) in about 1.3 msec. The brain's electrical responses, therefore, happen orders of magnitude more quickly than do it's chemical ones (10)."
But that's just the tip of the iceberg:
"In what could turn out to be one of the most important discoveries in cognitive studies of our decade, it has been found that there are five million magnetite crystals per gram in the human brain (1). Interestingly, The meninges, (the membrane that envelops the brain), has twenty times that number. These ‘biomagnetite' crystals demonstrate two interesting features. The first is that their shapes do not occur in nature, suggesting that they were formed in the tissue, rather than being absorbed from outside. The other is that these crystals appear to be oriented so as to maximize their magnetic moment, which tends to give groups of these crystals the capacity to act as a system....This system, we speculate, is what makes the selection of which neural areas to recruit, so that States (of consciousness) can elicit the appropriate phenomenological, behavioral, and affective responses."
What does this mean in terms of feedback?
"Changes in state make changes in sensory and cognitive modalities, and they in turn, trigger changes in state. We can reasonably conclude that there is a feedback mechanism whereby each modality is connected to the others."
So we have all these different modalities connected simultaneously as a system linked by feedback loops. What is the upshot?
"Magnetic signals are propagated with much greater speeds than those of action potentials moving through neurons. Contemporary physics requires that magnetic signals be propagated at a significant fraction of the velocity of light, so that the entire brain could be exposed to a neuromagnetic signal in vanishingly small amounts of time....We might also conclude that neuromagnetic signaling is the context in which consciousness occurs."
Not known with certainty yet, so I can't claim an ironclad argument, BUT, there is no particular reason why feedback cannot operate predictably at high speeds. The GPS, for example, is a system of high-speed feedback controlling smart bombs.
The magnetic crystals would tend to explain a lot of things that don't otherwise add up about the nervous system. The comment that consciousness may be tied up here is very intriguing.However, I never said that "feedback cannot operate predictably at high speeds". Obviously it works very well, even in audio amplifiers (at least from a measurement standpoint). It's just that my experience is that non-feedback amps sound more "real" and "organic" than do feedback amps. Your hypothesis that using feedback is the "correct" approach because it is used in the human body is very interesting, but I'm not convinced yet. Of course, I could be wrong. ;-)
Thanks for the fascinating dialog!
Charles Hansen
Here's another crazy thought: the notion that quantum entanglement is a feedback system wherein two entangled photons communicate their location more or less instantly at whatever distance.http://www.mtnmath.com/whatth/node54.html
"Your hypothesis that using feedback is the "correct" approach because it is used in the human body is very interesting, but I'm not convinced yet."
No hypothesis, really, just a close-to-home example. All I know is that feedback around a linear stage has no penalty in terms of multiplication. The question is, how linear must those stages be to instantiate your "organic" amplifier.
"Of course, I could be wrong. ;-)"
Me too. ;-
Hello Scott,You talked about applying feedback around a linear stage. This raises two questions:
1) How linear does the stage have to be in order to "safely" apply feedback?
2) If the stage is linear enough to safely apply feedback, is feedback even then necessary (or beneficial)?
For instance our V-5x power amp has a bandwidth of 200 kHz, an output impedance of around 0.15 ohms, with distortion around 0.1% at 100 watts, 0.01% at 10 watts, and buried in the noise at 1 watt. This is all achieved without feedback. Sure, those numbers would be even "better" if we were to apply feedback, but I'm not sure there would be any point. (I would even hazard a guess that these numbers are comparable to a well designed tube amp such as yours after the application of feedback.)
Best regards,
Charles Hansen
"1) How linear does the stage have to be in order to "safely" apply feedback?"As a start we need to ensure that the feedback reduces all harmonics below the level of the open-loop amplifier. If this does not occur, then the device is too nonlinear, or there is not enough feedback.
"2) If the stage is linear enough to safely apply feedback, is feedback even then necessary (or beneficial)?"
Let me answer that with another question: Which devices and circuits are linear enough to not need feedback of some kind? The beauty of this question is that the answers apply to feedback as well as nonfeedback amplifiers. So both approaches are linked in this way, and both can be pursued in parallel.
"Sure, those numbers would be even "better" if we were to apply feedback, but I'm not sure there would be any point."
You said it yourself: you're not sure. Neither am I. There's only one way to find out, and that is to become an expert in feedback. But this is a long road, which may or may not be a dead end. According to jc, that's what it is, and he certainly qualifies as an expert. BUT, I remain curious, and I don't like watching TV.
TV sucks!Keep us posted on your progress and conclusions!
Cheers,
Charles Hansen
For instance our V-5x power amp has a bandwidth of 200 kHz, an output impedance of around 0.15 ohms, with distortion around 0.1% at 100 watts, 0.01% at 10 watts, and buried in the noise at 1 watt. This is all achieved without feedback.But, as has been pointed out previously, if you're using followers, you're using feedback. It's the feedback of the followers which is giving you an acceptably low output impedance by significantly reducing the output impedance of the previous stage. It's the feedback of the followers which is making the devices used for the follower behave far more linear than they would if they were configured as common emitter amplifiers rather than common collector amplifiers and thereby not drastically increasing the amplifier's overall distortion.
So while you can say you're achieving those numbers without global feedback (which isn't terribly difficult to do), you can't rightly say that you're achieving those numbers without any feedback because it's simply not true.
se
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