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Old 30th July 2012, 05:31 PM   #6431
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You run the servo into a very low impedance. I hope that does not bring trouble. in your gain stage i do not 100% understand the biasing of the second stage. Why R16, R16 ?
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Old 30th July 2012, 05:36 PM   #6432
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I was just trying to use a current source now.
I can't make it work and I don't know why.

I attach the ASC file, maybe I am connecting something wrong and I don't realize.
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File Type: txt Library.txt (2.9 KB, 8 views)
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Old 30th July 2012, 05:36 PM   #6433
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Quote:
Originally Posted by Stefanoo View Post
Joachim,

I am still working on simulation for servo gain stage and buffer.
Does the servo look good?
In this case it is not on the signal path, but I am not sure it could work in practive.
It simulates just fine and with an input offset it acts and zeros the output out.

Would you mind comment it?
Thanks,
I would make R14/30 of a higher value, also I would split the input of the servo into 2 500K's and one 1uF to ground (adding a lp-filter in front of the servo). Also I would add an lp-filter on the output of the servo, this will eliminate any noise that may come from the servo.
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Last edited by FdW; 30th July 2012 at 05:50 PM.
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Old 30th July 2012, 05:40 PM   #6434
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Small update.

Got home from work tonight and She was listening to FiFi

This does not happens very often... first time in last 4 or 5 months...
It may be just nothing but I was prety plesed with it.
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Old 30th July 2012, 05:40 PM   #6435
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Quote:
Originally Posted by Joachim Gerhard View Post
You run the servo into a very low impedance. I hope that does not bring trouble. in your gain stage i do not 100% understand the biasing of the second stage. Why R16, R16 ?
That is a simple way that I find it very easy to implement, to degenerate the second stage and at the same time set the current bias it, which will simply be the voltage drop on the first stage resistor minus the conduction voltage of the fet divided by the second stage's resistor.

In one of previous prototypes I was running the gainstage wihtout the current feedback but only with a local feedback on the second stage from output to the gate of the fet.
That worked fine except from a power dissipation standpoint, which pushes me now to try the gainstage with a current feedback as shown in the circuit here.
Nevertheles the way I will bring the feedback will have to be determined whether will be current global feedbak or local degeneration based on listening tests. Either way I have to solve the issue with the servo which seems to be pretty significant since I am not very skilled and especially when it comes to servos.
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Last edited by Stefanoo; 30th July 2012 at 05:43 PM.
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Old 30th July 2012, 06:30 PM   #6436
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Why then make a high Gm stage and then degenerate it ?
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Old 30th July 2012, 06:38 PM   #6437
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Good Point!
I do it so that I can have a lower noise overall and maybe I am wrong, but using the 4 Jefet in parallel on the first and second stage I can manage to lower noise floor by 6dB, which is pretty significant.
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Old 30th July 2012, 06:44 PM   #6438
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The noise advantage always comes from the first stage. If you degenerate it you loose S/N.
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Old 30th July 2012, 06:46 PM   #6439
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Compared to the low value of R3, R4 it is not much though.
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Old 30th July 2012, 06:51 PM   #6440
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yes, this is correct.
Nevertheless, in this case by having an higher gm I can decrease the degeneration of first and second stage, thus increasing current and then lower the feedback current up to the limit of current capaciblity for the ouptu stage (in this case the second stage who carries the current for the feedback).
Although this is a good method to lower noise and output impedence, it is probably not ideal from a sonical standpoint and that probably circles back to your main observation.
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