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Old 3rd December 2012, 06:38 PM   #7881
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Here we go :My version of the G = 1000 low noise measurement amp (for Ikoflexer).
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Old 3rd December 2012, 06:39 PM   #7882
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Here about noise in J-Fets :More FET noise measurements (for EUVL)
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Old 3rd December 2012, 06:43 PM   #7883
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One other thing :
Sampler mentioned that distortion goes down and bandwidth goes up when the output capacitance of the CSS is low. I though that this can be ameliorated by putting a resistor in series with the CSS like 1kOhm. It worked in my Tube RIAA.
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Old 3rd December 2012, 07:13 PM   #7884
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When either I or FwD or whoever else volunteers such as sample have a safe model up and running (maybe it would be better having a couple of people checking since results seem to vary by changing simulator) we could try all these different solutions to try to lower noise and improve DC.

At now, I am still not 100% sure that the circuit could work properly and we should also definitely at least try to correct for thermal drift if possible.
I would also like to check if this is an inverting circuit or not as it could be another potential problem.

All in all, the project sounds very promising to me.
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Last edited by Stefanoo; 3rd December 2012 at 07:18 PM.
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Old 3rd December 2012, 07:26 PM   #7885
FdW is offline FdW  Netherlands
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Quote:
Originally Posted by Stefanoo View Post
when either I or FwD have a model up and running we could try all these different solutions.
I am still not 100% sure that the circuit could work properly and we should also definitely at least try to correct for thermal drift if possible.
I would also like to check if this is an inverting circuit or not as it could be another potential problem.

All in all, the project sounds very promising to me.
Stefano, do not expect such a solution from me, it is not my 'thing'/expertise to do so and I will not have the time (in any near future) to learn that skill. The reason that I did this was just as a silly-time-to-consume exercise and I wanted to get some insight how an input stage like this operates. More like solving an interesting puzzle. There is a contradiction (somewhere in there) as I did it and I cannot, but that is how I function

In all things that I ever designed and or build there where never any fetís (of any kind (other than for simple current sources with jfet's)), that needs to change and that is why I took the puzzle (there is no game in looking at stuff that works ). Oh yes I did an amplifier using the ĎLazy Catí BIGTís, and yes there are a few hexfets (I love those) in the PSU, and finally (even before commercial hexfetís existed) I did a 100W amplifier using hexfetís (mmmÖ thatís more than 20 years ago) (yet again some contradicting information ).
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Old 3rd December 2012, 07:43 PM   #7886
sampler is offline sampler  Lithuania
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Quote:
Originally Posted by Joachim Gerhard View Post
One other thing :
Sampler mentioned that distortion goes down and bandwidth goes up when the output capacitance of the CSS is low. I though that this can be ameliorated by putting a resistor in series with the CSS like 1kOhm. It worked in my Tube RIAA.
For sure, both ccs and cascoding bjt parasitics should be as low as possible. Maybe I should elaborate more about schematic in post #7843. I'm sure JG knows this stuff, but in essence gyrator is a ccs with "natural" servo that keeps stable DC operating point. It's plain simple, divider R6/R8 senses output voltage and adjusts Q13 gate voltage increasing/decreasing current. C2 sets corner frq. ("authority of servo"). I have been using this gyrator with IRF9610's in my preamp for loading ECC88 for a very long time now, and it sounds superb. Some experimenting might be needed for finding practical values of R6/R8 and C2, but this should work with IRF9610 here too, just more thd.
I don't want to repeat my self, but I posted a link in #7824 to a thread, were similar circuit was discussed and nit picket to smallest details, I recommend it once more for everybody fighting with basic concept in LTspice. There is great input from Scott, JC, Syn09 and others... JG too ! Although I don't agree with verdict that whole concept of two ccs's fighting with each other which one sinks or sources more current is flowed in essence, but it doesn't make life easer for sure. I will make proto tomorow, now I'am obligated to go and watch new episode of Walking Dead
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Old 3rd December 2012, 07:58 PM   #7887
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Quote:
Originally Posted by sampler View Post
For sure, both ccs and cascoding bjt parasitics should be as low as possible. Maybe I should elaborate more about schematic in post #7843. I'm sure JG knows this stuff, but in essence gyrator is a ccs with "natural" servo that keeps stable DC operating point. It's plain simple, divider R6/R8 senses output voltage and adjusts Q13 gate voltage increasing/decreasing current. C2 sets corner frq. ("authority of servo"). I have been using this gyrator with IRF9610's in my preamp for loading ECC88 for a very long time now, and it sounds superb. Some experimenting might be needed for finding practical values of R6/R8 and C2, but this should work with IRF9610 here too, just more thd.
I don't want to repeat my self, but I posted a link in #7824 to a thread, were similar circuit was discussed and nit picket to smallest details, I recommend it once more for everybody fighting with basic concept in LTspice. There is great input from Scott, JC, Syn09 and others... JG too ! Although I don't agree with verdict that whole concept of two ccs's fighting with each other which one sinks or sources more current is flowed in essence, but it doesn't make life easer for sure. I will make proto tomorow, now I'am obligated to go and watch new episode of Walking Dead
Interesting.
Would you mind elaborate the circuit with IRF9610? Apparetly circuit doesn't seem to operate in my simulator.
Also, if you don't mind where did you get the model for the gyrator?
I have never used this device before.
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Old 3rd December 2012, 08:53 PM   #7888
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When you use the IRF instead of the Zetex the major difference will boil down to the Ugs where you get the same current, output capacitance not considered which is higher in the IRF parts. Look at the curve of Ugs against idle. One problem with steep mosfets is that tiny changes in Ugs make big changes in idle.
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Old 3rd December 2012, 08:58 PM   #7889
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Going from -5V to -6V nearly triples the current ( this is for IRF9610 ) so tight control of Vgs is mandatory.
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File Type: png IRF.png (58.2 KB, 116 views)
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Old 3rd December 2012, 08:59 PM   #7890
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Well, it is more like doubled, anyway.
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