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Old 27th September 2012, 05:40 PM   #121
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I would like to do a FFT Distortion analysis like the one I see on Paradise's thread.
Can anybody here nudge me in the right direction?

I had download a little while ago a little script with a block but it didn't seem to offer the same resutls other's were getting.

Thanks.
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Last edited by Stefanoo; 27th September 2012 at 05:44 PM.
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Old 27th September 2012, 05:44 PM   #122
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here you go....



Isn't it really beautiful? So far so good...see if what I have studied works in practice too...
It may be, but the initial offsets cancel, I would like to see both starting at an positive (or negative) offset now one starts at a positive and the other at a negative offset.
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Old 27th September 2012, 05:46 PM   #123
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It may be, but the initial offsets cancel, I would like to see both starting at an positive (or negative) offset now one starts at a positive and the other at a negative offset.
I tried all 4 combinations and I get the same exact result.
To this scheme it doesn't matter if either side is starting positive or negative or same offset, it will zero-it-out in common-mode.
Maybe I am missing something and I can try now to really mess things up trying to make the common-source different and see what happens then.


I will post shortly.
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Old 27th September 2012, 05:49 PM   #124
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Now if I change the source resistors to create a current unbalance as well as AC unbalance, the outputs slam to the rails that may suggest that any unbalance at the FE can't be controlled.
I don't know how much that works in practice.

Any thoughts?
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Old 27th September 2012, 06:11 PM   #125
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here is the flaw....


if the source resistance is made a little different, which could easily account for tolerance on the IDss of the Jfets, that is where the common mode behaviour of the servo pops up again.

I would also like to trim both source resistance to compensate for 2nd harmonic distortion due to asymmetry on the transconductance curve between p and n channels Jfets.
I am wondering if that same trimming can be used to correct the common mode offset and if the offset would stay down two zero once trimmed or it would wonder due to the thermal unstability of this Jfet's family i.e. not useful tempco available operating point?

Any advice would be surely very appreciated.
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Old 28th September 2012, 02:23 PM   #126
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I have created the prototype and tested the Servo.

Unfortunately servo isn't working so it is impossible to perform any kind of test and/or listening tests.

If anybody here can take it up a notch and throwing some possible solutions that can be tried, this would help moving this on...

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Old 28th September 2012, 02:36 PM   #127
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I have created the prototype and tested the Servo.

Unfortunately servo isn't working so it is impossible to perform any kind of test and/or listening tests.

If anybody here can take it up a notch and throwing some possible solutions that can be tried, this would help moving this on...

Maybe the easy way out is, use 2 servo's for now, when this functions then give it some more thought-time and update (to one servo or any other solution). I cannot spend much time on this (at the moment), so I will try to follow and comment (as I see needed/wanted or prudent) but that is all for the foreseeable time
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Old 28th September 2012, 02:52 PM   #128
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yeah that sounds reasonable to me too.
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Old 28th September 2012, 03:05 PM   #129
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I am not trying to be a pain, but would it be easier and perhaps more educating to start with simpler layouts, even balanced, and work up. This is a very complicated design and even with the help of many others, it may be too much of a first try. Often in working from simplr to more complex designs, you find /learn things that will help improve the final product anyway. Yo may even find the simpler version is preferred, as you will have listened along the way. Just a suggestion.
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Old 28th September 2012, 03:15 PM   #130
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No pain, don't worry!
The circuit is very simple if you look at it. Some of the circuitry is needed to keep the servo away from, the path and the rest purely to increase gain but doen't have any additive extra stage to it.
It almost can be seen with a 1 stage gain plus simple buffer.

The problem here is to design a working servo for balance operation.
I am not an expert of servos but if you have any suggestion on simpler topology we can start from I would be more than happy to consider it and start with that.
I like the approach bottom top anyway...
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