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Old 23rd September 2009, 04:24 PM   #1481
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Now, PMA, I am curious whether adding the trimpot would gain an advantage, and also if balanced INPUT drive would balance it further, and remove the need for a trimpot?
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Old 23rd September 2009, 04:29 PM   #1482
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Quote:
Originally Posted by john curl View Post
I think that the question of perfect tolerance resistors or a trim, should be addressed by worst case analysis using a sensitivity matrix. Does Spice have such a matrix? Sorry, I know ECAP better in this situation.
Would you like to do the calculation, Jannman? Your 'guess' is as good as ours, at the moment.[snip].
I was thinking that even if there is a mismatch and say one half of the amp would have a few % more gain than the other half, that it would be unimportant. Since you mentioned a 'possible improvement' I thought I'd ask. I couldn't think of anything off the top of my head, I thought maybe you could.

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Old 23rd September 2009, 04:43 PM   #1483
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It is obvious from direct measurement, that 2nd harmonic drops when using balanced out operation, although not to zero, or unmeasurable. To get the best 2'nd harmonic cancellation, one would expect that uneven outputs would not be as effective in effecting harmonic cancellation. That is why the trimpot might be useful.

This design approach is what we like to call 'elegant' rather than 'sophisticated'. There are thousands of different designs, especially associated with IC's that use lots of cheap parts and design complexity.
We try for 'perfection' with really good parts, well matched, working together to make the most linear transfer function that we can make, especially avoiding higher order nonlinearities that can be generated by more complex schemes, such as tanh, etc.
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Old 23rd September 2009, 04:48 PM   #1484
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Originally Posted by Justcallmedad View Post
In fact is a schematic that I sent to courage before stopping to help people who wanted to clone it rather than learning.
I would appreciate if you or anyone else would stop associating me with the practise of cloning, because I have never build any of the circuits presented in this or any other BT thread. Take credit for any circuit you want if it feeds your ego, but please leave me out of it. Thank you!!
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Old 23rd September 2009, 04:52 PM   #1485
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Quote:
Originally Posted by john curl View Post
..............
We try for 'perfection' with really good parts, well matched, working together to make the most linear transfer function that we can make, especially avoiding higher order nonlinearities that can be generated by more complex schemes, such as tanh, etc.
Who is 'we' ?!
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Old 23rd September 2009, 04:55 PM   #1486
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Courage, good to see you here. I don't know about you in particular, but some have cloned the design to the best of their ability. In fact, most of my designs do tend to be cloned, sooner or later, it seems. That is why I resist putting up exact schematics of my designs. I think that this is what Justcallmedad was really saying, at least I hope so.
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Old 23rd September 2009, 05:29 PM   #1487
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Originally Posted by john curl View Post
It is obvious from direct measurement, that 2nd harmonic drops when using balanced out operation, although not to zero, or unmeasurable. To get the best 2'nd harmonic cancellation, one would expect that uneven outputs would not be as effective in effecting harmonic cancellation. That is why the trimpot might be useful.

[snip].
Perfect cancellation not only depends on perfect balance between two halves, but also that each half has exactly the same harmonic level and phase as the other. If you have perfect gain but unequal harmonics, it still doesn't 'perfect cancel' and then you're better of with some non-perfect balance. And I guess that would be reality: the two halves never have exactly the same level of harmonics, so best cancellation is with slightly UNbalanced outputs. With a pot, yes, to slightly UNbalance the system to get max harmonic cancellation. So it doesn't really matter whether R19 and R20 are matched, 1% should do.

So it seems that contrary to popular belief, best cancellation results from 'uneven outputs' although you can't predict how uneven.

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Old 23rd September 2009, 05:41 PM   #1488
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Depends on parts, not only input JFETs, but output MOSFETs as well. The circuit needs carefull component selection.
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Old 23rd September 2009, 05:47 PM   #1489
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Depends on parts, not only input JFETs, but output MOSFETs as well. The circuit needs carefull component selection.
Why?

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Old 23rd September 2009, 05:50 PM   #1490
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Because there is no global feedback to equalize components tolerances. I have the circuit in MC, so I can see what active part tolerances are resulting in.
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