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Nelson Pass 3rd November 2008 06:16 PM

Distortion and Negative Feedback
I wrote another article:

Later today it will also be up on


jan.didden 3rd November 2008 07:06 PM

Nelson Pass' distortion tutorial
mod note: 2 threads merged into 1 :cop:

Nelson Pass has a quite interesting tutorial about amplifier distortion in 6moons here:

Recommended reading.

Jan Didden

Nelson Pass 3rd November 2008 07:33 PM

Flattery will get you anywhere. Sorry we didn't find the time
to yak at BAF, twice now.


myhrrhleine 3rd November 2008 08:26 PM

nice piece :)

Nelson Pass 3rd November 2008 08:47 PM

Thank you. I know you as a member who does
not compliment casually.


SCD 3rd November 2008 09:27 PM

Very interesting piece.
Nfb is a lot like salt. A little can enhance, a lot can kill.

I also believe that a low piece count is the way to go. Now I guess fewer stages also makes sense.

I am of the opinion that for the most part, errors add together they rarely cancel each other out. Every piece in the chain has an error factor associated with it.

Thanks for the thought provoking article.
I sense there is more to come

Nelson Pass 3rd November 2008 09:33 PM

I was left with a lot on the table at the end, but I wasn't
up to "War and Peace", so I'll save the leftovers for snacks.


Zen Mod 3rd November 2008 09:40 PM


Originally posted by Nelson Pass
I was left with a lot on the table at the end, but I wasn't
up to "War and Peace", so I'll save the leftovers for snacks.



that's MY Papa ......


GRollins 3rd November 2008 09:55 PM

First worthwhile thing I've read in a month of Sundays. Thanks.
One thing that I've always bounced back and forth in my mind is the idea that both the mathematics and real life measurements indicate different curves for the different gain devices. Trivial observation, right? Maybe, maybe not. Suppose you've got an all-bipolar circuit. It might then be argued that the harmonic content of the bipolars might have a--for lack of a better word--optimum feedback ratio. But that ratio might be different for other gain devices. Okay, still no revelation...after all tube gear (as tube lovers like myself have pointed out for years) gets by with far lower feedback and sounds as good or better than equivalent solid state pieces.
But...and I may not have set my thesis up properly; you'll have to look at this in just the right way to see what I'm trying to say...
What if you've got a hybrid piece, meaning not just tube/solid state, but FET/bipolar or bipolar/MOSFET or the like, and you find that things just won't gel? What if the bipolars need X amount of feedback, but the MOSFETs would really do a better job with less (or more)? The 'conflict' (again, I'm struggling to pick the right word here) between the competing requirements of the two or more kinds of gain devices might produce a piece that's neither fish nor fowl, as the expression goes.
Yes, it's possible to combine gain devices (e.g. Lovoltech power JFET cascoded with another device so as to flatten the curve) to produce something greater than the sum of its parts, but unless done intentionally at each and every stage of the circuit, you're in danger of producing an anti-synergy.
One possible solution is no feedback at all. This may or may not fly, depending on any number of other factors. Another would be local feedback loops. For example, if you're building a tube/solid state amplifier, run a feedback loop around the front end (presumed to be the tubed part) only, then another around the back of the amp (presumed solid state) so as to allow each class of device its optimum feedback. Yes, there are high-feedback tube pieces out there, but the predominant strategy is low feedback for tubes. The converse is true for solid state.
This line of thought echoes a point I was trying to make in a series of posts wherein Srajan Ebaen and I had differing views on hybrid circuits. He had just reviewed (with high praise) a tube front/MOSFET back end amp (something by Thorens?). While acknowledging that I hadn't heard the amp, I noted that there has never been a hybrid that was an unqualified, runaway success in the market. Does that, [I]ipso facto[/] mean the circuit sucks? No, of course not. And maybe this one amp that he was so impressed with really is the Holy Grail. And maybe not. I still haven't heard it and can only say that I have no opinion.
One thing I said at the time was that I run a tube preamp with (mostly) solid state amps. It sounds good. But one of the more subtle points is that each chassis, by definition, contains its own feedback loop, which we will assume for the sake of argument is optimized for its sort of gain device.


P.S.: And, yes, those who are familiar with tubes might easily begin thinking that triodes and pentodes need to be treated differently in terms of feedback.

leapcat 4th November 2008 03:51 AM

Would a fast amp lower the Im distortion of a complex

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