What I have tried to convey here in the last two descriptive paragraphs is a straight forward, understandable explanation for certain audio problems. I realize that some will find it too 'simple', others may find it too 'incomplete', but it should give those of you with some technical knowledge an idea of what and why designers like Matti Otala and I made designs the way we did, and still do, to some extent.
It must be remembered that 35 years ago, high F(t) output devices were very expensive and delicate. Our BEST bipolar transistor output pair had an F(t) of only 4MHz. Yet, we both achieved 100V/us with different topologies. We had no need to copy each other.
Today, most output devices are 10 times faster or more. Think how easy it is, today, compared to back then.
It must be remembered that 35 years ago, high F(t) output devices were very expensive and delicate. Our BEST bipolar transistor output pair had an F(t) of only 4MHz. Yet, we both achieved 100V/us with different topologies. We had no need to copy each other.
Today, most output devices are 10 times faster or more. Think how easy it is, today, compared to back then.
Trevor White said:
Yes I agree but according to the anti-feedback brigade any feedback is too much feedback whilst others think that there is a moderate amount of feedback which yields the optimum performance but cannot explain why this is so.
I used the example of the Halcro amplifiers to dispel the myth that too much feedback always results in sub optimum performance from an amplifier
regards
Trev
Understood. But the fact that *some brigade* has some opinion doesn't make it a fact.
When you look around you, you can realise within a few 100 milliseconds that there are very good amps without feedback and very good amps with feedback. So if someone then preaches that feedback is by definition bad he/she is clearly at odds with reality.
That hasn't stopped the *brigades* but everyone is entitled to his own opinion of course.
Jan Didden
PB2 said:
Hi Pete,
Yes, it is quite possible that any nonlinear capacitance in the amplifier could modulate the open loop and/or closed loop bandwidth and cause some PIM. For example, suppose you have a VAS that is not cascoded and is not fed by an emitter follower. The Miller compensation capacitor is connected conventionally from collector to base of the VAS. Cm is in parallel with the collector base capacitance of the VAS transistor, and the altter varies with Vcb and thus signal voltage. This will cause the gain crossover frequency to move back and forth with signal, resulting in some PIM. Similar effects can occur in amplifiers without any negative feedback. I suppose it is also possible for some PIM to be generated by the variation in ft with current of the output transistors.
Cheers,
Bob
john curl said:
Hi John,
I'm pretty sure that indeed you have never measured PIM, as it takes specialized equipment to measure it. I have measured it and documented the results. PIM does indeed exist, as I showed in my measurements. It does appear to be fairly small, probably less than 10ns for most any decently designed amplifier, be it with or without NFB. In the very modest NFB amplifier I built to neasure it, PIM was about 3ns. To be honest, I don't know how audible 3ns of PIM would be.
However, the key finding is that with NFB, large amounts of NFB accompanied by low open loop bandwidth do not increase PIM. An amplifier with 20 kHz open loop bandwidth will not have less PIM than an amplifier with 1 kHz open loop bandwidth all else remaining the same, including closed loop bandwidth.
Moreover, my experiments showed that PIM exists in amplifiers with no NFB at all, and that the application of NFB actually reduced total PIM in the example that I looked at (YMMV).
Cheers,
Bob
john curl said:
I have to agree with John in regard to differences between global and local negative feedback. One can indeed get into more trouble with global NFB than with local NFB most of the time. Local NFB over more than one stage, particularly in some CFP arrangements, can, of course, get one into trouble with parasitic oscillations in some cases. Global NFB from the output of an amplifier to the input, encompassing the output stage and the variations of the load, takes a bit more skill to do right. However, when applied properly, it performs well.
It is notable that Charles Hanson has a much more stringent philosophy toward negative feedback. I believe that he eschews ANY negative feedback, be it local or global, with the exception of emitter degeneration.
Cheers,
Bob
Bob Cordell said:
Bob, I think you really needed to agree with John on something
The amount of trouble that NFB can bring has nothing to do with the NFB itself, but with the properties of the base amp. NFB is NFB is NFB.BTW, emitter degeneration is feedback as well, and it can be easily proved. While emitter degeneration increases the linearity and the stability at low frequencies , you can in principle find an amount of emitter degeneration that can make a real world common emitter stage unstable (definitely not in the audio frequencies range, though).
Bob Cordell said:
I have to agree with you Bob that philosophy, i.e. love of wisdom, is a very good thing, but it is incompatible with cookbook recipies, about what is always good in audio, what is always bad in audio.
However, cookbook approach helps to answer questions like, "Why you Sir have chosen such components for your wonderful legendary amplifier?", but it does not help to understand why certain components in certain places in certain regimes found to be preferrable. But what is worse, it does a harm rejecting something very useful that when used improperly caused bad effect.
john curl said:
I don't think confusion is the issue. Where the confusion stems from is the conclusion that is automatically drawn by you, that low bandwidth high global feedback, as per your definition of it must always result in a compromised design compared to a circuit that exclusively relies only on local feedback.
I have yet to see your analysis (and anyone else's) as to why this is so.
And like I said previously, you need to explain why a high global feedback amplifier such as the Halcro always gets the seal of approval from Stereophile in terms of both its objective and subjective performances.
Until you can do this then you have presented an unsound argument about high global feedback amplifiers, that has an incorrect conclusion that has been illogically drawn from a number of false premises.
I've found nothing crazy in Nelson's designs and in his reasoning. He is smart and knowledgeable designer. He teaches his disciples to think and make own decisions instead of blindly following recipes.
Edit: John, can you stop please lead us to off-topic judging personalities? You've started it from calling people to teach Syn08 and myself some elementary definitions that are not always right.
Edit: John, can you stop please lead us to off-topic judging personalities? You've started it from calling people to teach Syn08 and myself some elementary definitions that are not always right.
john curl said:
Just curious, why is it always Dick Sequerra to be called by Scott?
http://www.diyaudio.com/forums/showthread.php?postid=1535815#post1535815
http://www.diyaudio.com/forums/showthread.php?postid=1537333#post1537333
http://www.diyaudio.com/forums/showthread.php?postid=1856506#post1856506
Scott, do you owe to that guy?
