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In Reply to: Re: What happens to back EMF with a feedback amplifier? posted by morricab on August 17, 2006 at 09:45:44:
And you obviously haven't a clue about amplifiers, impedance, or elementary circuit theory.
Follow Ups:
"And you obviously haven't a clue about amplifiers, impedance, or elementary circuit theory"what have I said, specifically, that you find blatantly incorrect?
Ah but you see this is where you are very wrong. I am listening, a lot, and I am working my way backwards from the listening to the explanation. I am a scientist so I believe in cause and effect. I make observations on sound and then try to develop a hypothesis for why I hear what I hear.Start with listening, if you are familiar with how music SHOULD sound (unamplified of course) then it is not so difficult to spot gear that keeps good recordings in tact (not just sounding good but sounding realistic). Once you have identified such gear you start looking at measurements to see if they have similar patterns in distortion harmonics, distortion vs. power, distortion vs. frequency, frequency response, etc. etc. I think I have identified general trends that seem to apply to various circuit types and now I am in search of answers as to the cause of such behavior that gives good sound and bad sound.
I know enough electronics and engineering to look at the circuit by its elements and think about how the signal really flows through the circuit rather than a black box approach as soundmind stated "just look at the damping factor!" as if that suddenly explained everything. I am looking at the damping factor and asking a hard question as to why it drops suddenly around 1Khz (give or take) with a simultaneous rise in distortion.
Tom Danley and John Curl are both reasonable and willing to discuss a bit (Tom's I think is probably right on) without Dan's or Soundmind's sneering. Dan's refuge is name calling (at least soundmind refrains from this low form of personal attack), not backing things up with data or hard discussion (soundmind again at least gives a somewhat hard discussion). I ask him for an explanation, give him examples of what I am talking about and wait for an explanation. I have my theories but I am not a trained electrical engineer like Dan. It would be nice to get some real analysis from him. None is forthcoming so what does that mean?
However; it is not enough to say that a high damping factor means low output impedance and that is that because we are dealing with what happens when the signal comes BACK into the amplifier. Then you have to look at the impedance of the elements to find where the signal can flow. My electrical engineer colleague here for sure knows much more than Soundmind about these things and I am beginning to think that Dan never even thought about it seriously. He considers it a major concern and flaw with feedback and when one really thinks about it it certainly seems so.
I wanted to hear a good counter argument from some people, hopefully a lively debate where engineers who think feedback is still a proper solution weigh in with facts not name calling. If the best they can do is that or "look at the damping factor" arguments then I guess it was a mistake to start this thread. The article Dan posted even questions the need for a really low damping factor with regard to frequency response.
What I have found on negative feedback is that most engineers consider it a standard tool and therefore universally good if applied "correctly" . The problem is that anytime an amp measures good and sounds bad then people say, "Oh the feedback wasn't done correctly". Based on Dan's comments, you would swear he is the only engineer making a commercial product who ever used feedback correctly as if he has a deeper understanding of what is behind feedback than all those engineers who have designed with it, used to believe in it, but have now changed their course because experience has showed them amps simply sound better with less and less of it.
Despite what you think, I never troll. Asking hard questions and trolling are two differnt things. I asked a hard question, which Tom Danley explained quite nicely and John Curl also finds important. I was sneered at by the two resisdent pit bulls who think anything other than the orthodox engineering they were taught in school is rubbish. They don't even view tubes as a viable amplification alternative. Tubes are a viable alternative, something that has been shown to my satisfation many times by more than competent engineers.
What they seem to fail to realize is that other, equally well trained engineers, realize that something interesting is going on that conventional amplifier design doesn't readily address.
"I am a scientist"If you are a scientist and you applied your scientific training to the understanding of the problem of sound recording and reproduction and the kind of efforts which have been made in that direction in the last few decades, you would know that the entire approach we use in light of what we know today is absurd and we only stick with it because it is easier to tweak what was inherited from more primitive times than to rethink the entire problem from scratch and come up with much more effective ideas. You would also know that there are no real scientists working on the problem, just tinkerers masquerading as scientists and engineers. When Dr. Floyd Toole researches what people like best, he is not investigating how to make more accurate sound systems, he is investigating how to make more money for his boss Sidney Harmon by doing market research.
The relative handful of engineers who even work on these problems have for the most part shut their brains off going through the daily grind of earning a living. Is it true that this equipment doesn't and can't do what it is proported to do? Well that's not just my opinion, it was the opinion of the editors and reviewers of TAS in a rare moment of candor.
You seem to be at the more loony end of the audiophile spectrum.If you take an instantaneous snapshot of any circuit network, the current/potential at any node is defined by the well proven theorems of Ohm, Kirchoff etc. Have been for centuries.
The anthropomorphisation of circuit elements and currents, claiming they have a predilection for going in a certain direction, have intentions and objectives, know where they are coming from and are going to etc. etc. is a dangeroud thing!
As is believing that audio circuits are somehow "special", delicate and subject to their own laws (?) of physics that no-one but the initiated inner circle are blessed to understand.
That's why I think you were trolling: not looking to understand, but just for a fight.
"The anthropomorphisation of circuit elements and currents, claiming they have a predilection for going in a certain direction, have intentions and objectives, know where they are coming from and are going to etc. etc. is a dangeroud thing!"I did nothing of the sort. I merely stated that the back signal will take the path of least resistance and you know what, I was right! How do I know this, Otala et al. that's how. I now have AES papers from 1978 and 1980 that show that he phenomenon is real and potentially quite stongly affecting the sound. Shall I quote their findings to you?
Here is the abstract:"The possibility of dynamic intermodulation distortion at the amplifier-loudspeaker interface is discussed. THis distortion is produced in amplifiers using high values of negative feedback, and having moderate or high open-loop output impedance in comparison with the loudspeaker impedance. The mechanism is intermodulation between the signal and its delayed versions, generated by the loudspeaker and propagated in the feedback loop. Experimental measurements showing the probability of considerable distortion are described."
Here is the Summary:
"The analysis and the measurements show that:at its various mechanical resonant frequencies, the loudspeaker may feed back to the amplifier much of the energy it received.
The loudspeaker may also act as a signal generator due to mechanical excitation of its voice coil by cone break-up, sustained oscillation of moving parts, delayed responses, etc.
provided that the power amplifier has an OPEN-LOOP output impedance in excess of a few ohms, and simultaneously substantial overall feedback, these backward energies are not subject to intrinsic damping at the amplifier output, but a corresponding feedback signal is generated within the amplifier, in trying to counteract the loudspeaker-generated signal and keep the output voltage constant.
The signal in the forward path of the amplifier thus consists of two components, the original input signal and the loudspeaker reaction signal, both of the same order of magnitude.
These two signals my interact in the nonlinearities of the amplifier, generating intermodulation products between the two. Loudspeaker nonlinearity is here to be considered equivalent to any open-loop nonlinearity of the amplifier, since it will effectively be situated inside the feedback loop, unless the open-loop output impedance of the amplifier is considerably lower than the specified load impedance.
this distortion, here termed interface intermodulation, IIM, will be most prominent at low frequencies wehre the loudspeaker-generated signal is at its greatest.
The susceptibility of the amplifier to IIM distortion can be measured bz using a modified difference-tone method, where on of the signals is injected to the input, and one to the output of the amplifier. Maximally, the latter signal should equal in power tthe maximum output power of the amplifier, to create a conservative worst-case fr this effect."
So, I am not making this stuff up as you can see.
Here is the conclusions from the 1980 paper:
"It has been demonstrated that the loudspeaker can considerably alter the nature and composition fo the internal singals of an amplifier. Porvided internal nonlinearities of any kind exist within the amplifier (my note: all amps are somewhat nonlinear so this is a give), the loudspeaker thus has a capability to change the amplifier distortion properties. The effect is strongest at the low frequencies, where the loudspeaker reactive properties are most outspoken.
The susceptibility of an amplifier to this loudspeaker effect is different for different amplifier topologies. The most important parameter in this respect is the amplifier open-loop impedance Zol. A low value of Zol prevents the loudspeaker to have any remarkable effects, but if Zol by virtue of the circuit topology is large, the loudspeaker generated reaction signal inside the amplifier may be of the same order as teh original input signal.
The effect described does not normally increase the amplifier distortion considerably. Instead, it has the capability to change the amplifier spectra, and its audible character."
So I guess I am not making things up in my own fantasy world afterall, Cliff. I got the research of Otala to back me up on this one.
"As is believing that audio circuits are somehow "special", delicate and subject to their own laws (?) of physics that no-one but the initiated inner circle are blessed to understand."Who said anything about that? However, if feedback is involved it is clear that the OPEN LOOP output impedcance of the amp has a strong effect on how much back EMF goes through the feedback loop (as does the amount of feedback used) and thus how much effect there is on the sound. So in that sense the topology IS very important.
Just because I bother to read this stuff and you don't doesn't give you the right to call me a troll I am nothing of the sort.
s
seems this thread drew the sneering beasts out of the woodwork, eh?
d
Look in a mirror. Since you've been sneering at all of us for years, how could we conclude anything else from someone like you who posts that the acme of recorded sound was the 78 RPM shellac phonograph record? Some of us are not nearly as stupid as you and your overly inflated ego would like to believe.
Then it is a troll.
We keep telling where you are incorrect and you keep challenging know engineering facts that are readily available in texts. You obviously are not interested in the engineering facts because they don't suit your political agenda.
As they say in Polish: Toughski Shitski.
Engineering is a meritocracy, which is something you refuse to acknowledge. You obviously feel the need to pervert it with lies, half truths and what ever you feel like making up as you go along.
d.b.
Cliff,Remember when you said these words to me? "You are beginning to look very foolish and way out of your depth." It appears as if those words are coming back to haunt you now.
Even your fellow Objectivists are telling you, "Stick to chemicals."
Just about everyone, but you thinks morricab's question is a valid one, while you think it's troll.Your credibility just keeps sinking & sinking.
Thetubeguy1954
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