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In Reply to: Well, yahbut... posted by Charles Hansen on August 02, 2003 at 11:57:30:
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.
Follow Ups:
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. ;-)
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. ;-)"
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.)
"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.
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TV sucks!Keep us posted on your progress and conclusions!
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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