Bob explained that in an earlier post:
There is too much of that going around lately, even (or especially) among "Big Box" designers. Fix your open loop design first, not cover your mistakes with NFB. Hear enough lousy amps designed in that manner, and everyone assumes it's the NFB, not the misuse of NFB, that's the cause of the problem.
The problem with the "audio crowd" isn't that they're stupid: it's that they know many things that just ain't so.
gootee,
change is a fundamental signal property. The signal changes constantly and suddenly. Transient distortion is created when amplifier components cannot handle the rate of change of the signal; an uncontrolled burst of energy containing of intermodulation products and noise, completely unrelated to the signal, that`s why it sounds awful. Another cause of transient distortion is insufficiencies and disturbances in the power supply. And switching transient - the worst kind of all.
change is a fundamental signal property. The signal changes constantly and suddenly. Transient distortion is created when amplifier components cannot handle the rate of change of the signal; an uncontrolled burst of energy containing of intermodulation products and noise, completely unrelated to the signal, that`s why it sounds awful. Another cause of transient distortion is insufficiencies and disturbances in the power supply. And switching transient - the worst kind of all.
My goodness, how can so much misinformation and ill-informed opinion be packed into so few words.
Clearly you are either just trolling or you are trying to make a joke. On both counts you have not done a good job.
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Dull; poor 3d imaging, poor attack in the mid-low area.
I don’t know exactly but my ears don’t lie. I suppose at the time capacitors are conducting, the mains has to charge the cap's AND feeding the amplifier. Even this is a small part in time, it has influence. IMO there must be a balance between mains current speed and buffer capacity/ESR. Better would be to reduce ripple with a choke like used in tube amp's.
Hi Hugh,
I agree that people may prefer amps with low or no feedback, this is subjective and everybody is free in his tastes and choices.
I completely disagrees when one tries to support these opinions with an unfunded technical argument such that high NFB amps should be unable to correctly reproduce transients. It's an evident point that people having such claims never demonstrate them with objective facts.
Regards.
People do try to find correlation between measured parameters and listening expertise, by introducing non-standard metrics, like Rnonlin.
What is also has been demonstrated, that simple combination of THD and IMD has NO correlation with listening results.
These newly proposed metrics are not easy to calculate, so they are not popular. Moreover, they play against interests of many mass producing companies.
A perfect example of a lousy amp with feedback used as a bandaid. Why not use as an example a Marantz 9 or a Futterman? Both highly prized tube amps with basically good design and performance, both with lots of feedback. Hmmm....
Gootee,
what a wonderful pair of posts (16 & 17).
Well argued, unemotional, simple statement of your beliefs and explanation of why you hold those beliefs.
I will make no comment on the 100% accuracy of every statement (I am not competent in all you areas you discuss), because that accuracy does not affect the gist of your argument. Having stated where you come from it is up to protagonists to argue for or against any single statement. Let's see them do so equally well. I look forward to seeing further discussion on some or all of the points you have raised.
what a wonderful pair of posts (16 & 17).
Well argued, unemotional, simple statement of your beliefs and explanation of why you hold those beliefs.
I will make no comment on the 100% accuracy of every statement (I am not competent in all you areas you discuss), because that accuracy does not affect the gist of your argument. Having stated where you come from it is up to protagonists to argue for or against any single statement. Let's see them do so equally well. I look forward to seeing further discussion on some or all of the points you have raised.
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no mention of the chin cup !
That does not matter, what you are showing is that human existence relies on feedback. Every step we take, every kiss we make.
Insufficient feedback can be worse than no feedback. Unless the amp is almost perfect (which SET is not), 8dB is probably insufficient. Therefore I am not surprised that people prefer no feedback for these amps. This is not evidence against feedback, but evidence against insufficient feedback (or possibly evidence against SET!).AKSA said:
There are people who think that if some feedback is good, more feedback must be better. Not necessarily true. Other people think that if some feedback is bad, all feedback must be bad. Not true either. In both cases, people are trying to 'design' by blindly using overly simplistic old wives tales heard from a bloke they met on the internet instead of actually trying to understand feedback, its advantages and limitations. Then they spread their ignorance around, so influencing others.
Learn about the algebra of feedback, and how it affects distortion. Learn about stability and plots of loop gain on the complex plane. Remember that real life is never quite as simple as mathematical models, but not wholly different either. Then you will have something sensible to say about feedback.
There was once in interesting article in EW+WW stating that you either have no or a lot of feedback. If you only have a small amount of feedback you may actually degrade your amp.
Furthermore one has to keep in mind that a feedback factor that is increasing with decreasing frequency may lead to shaping the THD spectrum in an unfavourable manner.
Single-ended class-A amps have one definite advantage that many more complicated amps don't have: They have a transfer function that might not be that linear but it has always the same monotony. And it gets closer to linear with decreasing signal level. That might be the reason why some people prefer these on a subjective basis.
I'd say that feedback is best
1) applied in large quantities
2) used on amps that don't have really crappy open-loop linearity
3) used on amps that don't have crappy open-loop frequency response.
Re point 3): Preferably the open-loop gain should be constant over most of the audio range.
Regards
Charles
Furthermore one has to keep in mind that a feedback factor that is increasing with decreasing frequency may lead to shaping the THD spectrum in an unfavourable manner.
Single-ended class-A amps have one definite advantage that many more complicated amps don't have: They have a transfer function that might not be that linear but it has always the same monotony. And it gets closer to linear with decreasing signal level. That might be the reason why some people prefer these on a subjective basis.
I'd say that feedback is best
1) applied in large quantities
2) used on amps that don't have really crappy open-loop linearity
3) used on amps that don't have crappy open-loop frequency response.
Re point 3): Preferably the open-loop gain should be constant over most of the audio range.
Regards
Charles
1) Feedback is best applied in appropriate quantities. Too much leads to instability. Too little can be worse than none.
2) agreed
3) not necessarily true, but feedback can create an inversion effect as you say: if the open loop amp is an integrator, then the remaining distortion after feedback is smaller but also differentiated so may have sharper spikes.
2) agreed
3) not necessarily true, but feedback can create an inversion effect as you say: if the open loop amp is an integrator, then the remaining distortion after feedback is smaller but also differentiated so may have sharper spikes.
Feedback is best applied in appropriate quantities. Too much leads to instability. Too little can be worse than none.
"Too much" applies only for some circuits.
There are now numerous examples of op-amps and amps using huge quantities of NFB (lets's say at least 40 dB at 10 kHz) without suffering any trace of instability.
"Too much" applies only for some circuits.
There are now numerous examples of op-amps and amps using huge quantities of NFB (lets's say at least 40 dB at 10 kHz) without suffering any trace of instability.
No, "too much" applies to almost all circuits. Most circuits have a point where HF phase shifts accumulate to 180 degrees. Too much feedback then would cause oscillation. "Unity gain stable" op amps are still subject to this rule, as you can always use a transformer to apply enough feedback to push the closed-loop gain below unity. About the only exception would be a circuit with such low power gain that you can't actually apply enough feedback. Such a circuit would be of limited use.
What is your opinion about this Open Loop Gain diagram, it could use 40dB of NFB up to 20kHz?
dado
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People forgot "nature" here. Just like what we eat, drink, breathe, ..
That tube amp, with less feedback will more natural it is. It is not just feedback or transient issue, because tubes are fast.
There are few people that care about how their amplifier move their loudspeaker, and how their loudspeaker move the air, and how their ear inform it to brain and what brain detects for ears dynamic response.
May be there will be interesting to try with laptop or cell phone loudspeaker.
They normally driven by classD. Just make a simple small horn from paper as cone horn, attach it close to loudspeaker and hear the sound differences. Horn also help to make a little more natural sounds.
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