A typical GNFB amp will have 60-80dB. The Zero GNFB is by definition 0dB.
Would the optimum set equal to both, and design a very linear amp then add say 30-40dB. Would that be a middle ground for all the protagonists here?
Would the optimum set equal to both, and design a very linear amp then add say 30-40dB. Would that be a middle ground for all the protagonists here?
Yes, and is only recent that it has given a name by Cordell: SGD - Spectral Growth Distortion. I like that name.
I have posted a couple of graphs illustrating it a couple of days ago but I guess often people are allergic to graphs. And to be fair, some of the technical documentation like those on my website you so kindly linked to only make sense if you have some education, self-taught or not, in electronic engineering and math. If you don't have that and don't want to educate yourself, you never find out how clear and concise this all has been established, and you continue to hold on to erroneous ideas thinking these are just 'dogma'.
Edit: the application of feedback, its advantages and disadvantages, can be a pretty complex and highly technical issue. So if you are not comfortable with the technical side, but instead rely on what you think it is, or what you read or heard from non-technical persons, you shouldn't be surprised that much of what you think is plainly wrong.
And that happens to all of us. I am pretty sure that I hold some opinions about a field that I am not trained in, that are wrong. And I am being told regularly ;-)
After all, for every complex and difficult problem there is a simple and easy to understand wrong answer ;-)
Jan
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Hugh, I don't think you can give an across-the-board number for all cases. It depends on a lot of factors. And what would it mean to be 'optimal'?
Logically, once you are over the 'hump' after which all high-order distortion start to decrease, you probably would want to go as high as you can get away with, stability-wise, because that gives you the most linear amp.
But if you are in the audio business, there are other factors to consider than just the amp linearity; for example, you can have the world's most linear amp but if no-one buys it because it clips horribly, the point is moot.
Jan
I've tried that in numerous experiments over the years... I think that amount of feedback also depends of topology. In some cases, more feedback gives less measured distortion, as expected, but to me sounded compressed.
Btw I think it's pointless to discus this, it's like you enter the church with upside down cross in your hand.. 🙃
Many branches of science are dogma based, but it's blasphemy to even mention something like that round here, one could end up called stupid from "golden members" cult.
Discuss it all you like, without measurements it's a waste of time unless you are just looking for good copy.
I'm inclined to agree if you take it to the individual extreme. Didn't Toole try to make the correlation with most people, which makes most sense? Afterall our hearing mechanisms have more similarities than differences.
Picking an arbitrary value of feedback isn't going to help. A parameter that would place confines on the amount of feedback introduced would be preferable. Currently we quote 20-20K specs and ignore out of band performance. People are going to say I am not a bat, why would I care? From an engineering standpoint you do care because out of band behavior indicates when feedback stops becoming useful.
Many amps fall off a cliff distortionwsise out of band. If the amps specification were to include distortion up to its cut off frequency, then the approach to the application of feedback changes.
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Forgot to mention: the amount of global feedback typically varies with frequency. It is the difference between open loop and closed loop gain, and open loop gain typically falls at 6dB/oct or more above 1kHz (YMMV of course).
So you may have 40-60dB of NFB at 1kHz but at 20kHz it normally is (much) less.
Jan
Traderbam, you replied to my: You can't correlate good measurement with good sound, because every bodies idea of good sound is different." and called it dogma.
No dogma. I can easily proof that the idea of good sound varies among listeners. You can too, as you are aware.
Jan
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In this public forum, I'm interested in challenging peoples' erroneous assumptions rather than giving away answers.
Excellent music reproduction is a hard engineering problem. I claim that as a very experienced engineer. It is hard because the electrical goals are unclear, the attention to details is crucial and it is vital to reconsider a lot of common assumptions and dogma. There are also a lot of audio prophets of bad information and muddled thinking who profit from publishing it; it makes wince each time someone quotes one of these people.
We tend to be lazy and stubborn. It's comfortable. E.g. It is easier to take on maximizing GNFB based on dogma and faith than to do the hard trial and error work to understand its full consequences. So hard that sometimes the builder blames the listener rather than their creation.
One can hardly characterize Putzeys' use of feedback and his recommendations for its use as "based on dogma". Unlike most audio designers, he puts the math and logic out there. I find it laughable that no one has tried to debate his position by arguing with his technical rationale and the math he has presented to support it.
The proof is in the pudding- his designs have evolved and become closer to a wire with gain. They have had great commercial success and have received excellent critical reviews. By any measure, it would seem he has found a successful approach. As most rationale people understand, like food, music preferences are subjective. That's all fine- buy what you like, but if you want to argue about objective criteria, stick to the quantitative facts.
The proof is in the pudding- his designs have evolved and become closer to a wire with gain. They have had great commercial success and have received excellent critical reviews. By any measure, it would seem he has found a successful approach. As most rationale people understand, like food, music preferences are subjective. That's all fine- buy what you like, but if you want to argue about objective criteria, stick to the quantitative facts.
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Hugh's not likely to get much out of you then, it's a funny old game, I suppose DIY means do it yourself, it's not about sharing at all which seems to be contrary to the purpose of this site. Why are you here exactly, to "correct" people, is that all?
People seem to keep forgetting the plain obvious - the job of the musician and sound/recording engineer is to make "good sound", the job of the amplifier is to reproduce it faithfully enough. All it can do is fail at that task - and if it fails by more than the limits of human audio perception, it can be heard to fail at that task.
If you want to be your own sound-enginee, that's fine, just do it with an effects processor / preamp of some sort, leave the amplifier the task of transparently increasing the signal power in the band of interest.
Though why you would want to do this other than to compensate for room and speaker shortcomings or a shonky recording is a mystery to me, unless your DJing/sampling etc.
[ And yes, a guitar amplifier is part of a musical instrument, nothing to do with sound reproduction at all ]