Thank you very much indeed. This is a very important point for me.
So the zero feedback statement i guess can only be applied to tube amp/preamp.
Anyway for now it is more than enough. I was very confused about this issue of zero NFB.
Thanks again, gino
A close approach to zero feedback is the classic textbook BJT grounded emitter amplifier: 2 resistor potential divider to set the base voltage, bypassed emitter resistor, output from collector resistor. It will sound horrible with anything but the tiniest signal, as it can be shown that the % second harmonic distortion is equal to the peak input voltage. So 5mV RMS in gives 7% distortion out (plus 3rd, 4th etc. at much lower levels).ginetto61 said:
ginetto61
Mr. Pass does not support religion following.
He freely provides his circuits to the diyers and discuss the technical issues with people that understand them.
There are a lot of his articles where he explains the design route, the technical pros and cons.
The circuits are simple to implement but there is a lot of technical background that needs to be worked-out and understood by the implementer.
One of these is the distinction between the terms global negative feedback and local feedback.
There is no voodoo and there is no claim of ‘best creation’ in that stable.
George
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Thanks another important point.
So not interesting for an audio circuit at all.
Ok ... no more question. I will leave the zero NFB concept to others.
Thanks again, gino
Hello and this i like very much
But i was reading here ...
https://www.passdiy.com/gallery/preamplifiers/bride-of-zen
i concluded, wrongly, that the BOZ is a no feedback design.
I did not understand correctly the words. It has indeed local but no global NFB !
Now i have understood that no zero local feedback circuits are usable for audio.
So i have made my day ... not a small discovery for me indeed
Do you mean that he was referring to zero global negative feedback ?
okkei ... fine ! I understand now better.
There is local but not global NFB.
Anyway the schematic looks quite basic to me.
This supports the statement that a circuit must not be necessarily complex to sound pretty good, when correctly designed and built of course.
And this also i like a lot.
Thanks a lot, gino
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Hi i have no problem with the term "feedback".
What i was clearly missing is the difference between local and global, a fundamental difference i guess.
When it is agreed that local feedback is employed everywhere i do not see any reason to discuss it. It is just necessary for an amp to work decently.
Global feedback, maybe, is a different story.
Speaking about the "leap of faith" i read often some claims like "the best amp i have ever heard" (i am not referring to the BOZ of course).
As there are no objective way to assess this, i think, one has to believe or not to the writer.
With many people saying that their creations are the best the confusion is big. And there is no easy way to sort it out.
Another consideration about "textbook" design.
I remember an old amp from Harman Kardon, the Citation 12 here depicted
The schematic i read is very basic and similar to one present in the output bjts datasheet (i do not know if this can be qualified as a textbook design). And it is said to sound very good.
I wonder how much a fine tuning (i.e. parts selection) of this very basic design can improve its already nice sound. I guess a lot indeed.
Of course someone will tell me that this design is obsolete.
Some parts are without any doubt ... when i see those open trimmers for bias regulation those are obsolete for sure.
But the design ? is it really so obsolete ?
Thanks a again for the valuable help.
Kind regards, gino
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"fine tuning" by " parts selection" implies that either the parts specified were incorrectly specified, OR the parts orignally supplied were not to spec.
And fine tuning isn't really fine tuning unless you objectively measure performance. And no, objective doesn't have to include meters and scopes and the like, but it does have to include dispassionate measurement suited to the question at hand.
Two or more friends crowded around a DIY effort by one of them (probably beer in hand) and commenting on the perceived sound quality resulting from a newly inserted capacitor doesn't meet this criteria.
And fine tuning isn't really fine tuning unless you objectively measure performance. And no, objective doesn't have to include meters and scopes and the like, but it does have to include dispassionate measurement suited to the question at hand.
Two or more friends crowded around a DIY effort by one of them (probably beer in hand) and commenting on the perceived sound quality resulting from a newly inserted capacitor doesn't meet this criteria.
No, not a fundamental difference at all. That is a common fallback position for those who at first claim that degeneration is not feedback, then have to reluctantly admit that it is feedback so they start to talk about two different types of feedback.ginetto61 said:
Feedback is feedback. The issue is not whether it is local or global (both terms which are hard to define exactly) but where does it come from and where is it injected and how much phase shift etc. is included in the loop, which can be roughly summed up by 'how many stages are in the loop?'.
One stage is usually fine, and could be called local feedback. Two stages are usually fine, but can be marginal. Three stages and you have to start thinking seriously about loop stability and compensation. Four stages and you have to be very clever to get it to work at all.
When does feedback become 'global'? Strictly only when it is taken from the output point and applied to the input point. The inverting opamp is an example. But what if the opamp is embedded in a circuit - does the 'global' feedback seen by the opamp become 'local' feedback for the circuit as a whole? What about the non-inverting opamp (or some valve amps) where the feedback comes from the output but is not applied to the input but an alternative input - is that 'global'?
No, the difference between 'local' and 'global' feedback is not fundamental - it is not even rigourously definable!
Hi and thanks for the reply.
I do not think i have to link to one of the endless parts comparison mabe by listening on capacitors mostly but even on resistors.
They rank capacitors for sound quality even if they are all correctly specified.
I understand that you are skeptical about this kind of test.
It could be just marketing i agree.
Sorry this i do not understand. How can you perform measurements with meters and scopes and the like ?
I agree that measurements are objective. That is sure. Almost.
Agreed. Note reliable at all.
So it seems to me that all boils down to measurements. Fine.
I like this approach.
Thanks and regards,
gino
My strong advice to the OP is to go and do some serious reading and experimentation on circuit theory. Once you know how circuits work you are in a position to make your own judgements on component issues. Before then you just have to decide who to believe, yet with no valid criteria to make that decision. Once you understand circuits you will begin to realise just how much popular nonsense is spouted about audio circuits by self- and journalist-proclaimed 'experts'.
Hi and thanks for the explanation. I think i have learned some things.
First feedback is everywhere, in different amount of course.
Second the more the stages the more the technical challenges.
Given that all the circuits employ some amount of feedback i derive that it is not a bad thing per se.
I think i will stick with some proven unit because if audio design is a very complex discipline. I think i will play a little with the BOZ soon.
There is alway the power supply issue. I would like to try the TL783.
Thanks again and regards,
gino
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