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Old 30th September 2011, 08:34 PM   #301
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Juma,
Can't figure it out. With source grounded, Vgs is at 0 and Jfet is fully on, passing full Idss current. This current drops a voltage across Rs which causes the Vgs to change. With this change the current drops somewhat due to the change until we reach equilibrium. With source tied to the negative rail, the Jfet should still be full on, but what detmines Vdrop across Rs. Can't be the same, because with Jfets with Idss of 10mA, we would be dropping 3.3V. That would turn the Jfet off.
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Old 30th September 2011, 09:39 PM   #302
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Quote:
Originally Posted by buzzforb View Post
Juma,
...With source grounded....
What schematic are you talking about ?
In sch. from post #42 JFET sources are not grounded.
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Old 30th September 2011, 09:43 PM   #303
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I was talking about standard configuration where sources are grounded vs the preamp version where the source is tied to negative rail.
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Old 30th September 2011, 10:33 PM   #304
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Quote:
Originally Posted by buzzforb View Post
Juma,
....With source tied to the negative rail, the Jfet should still be full on, but what detmines Vdrop across Rs. Can't be the same, because with Jfets with Idss of 10mA, we would be dropping 3.3V. That would turn the Jfet off.
All wrong. I can't find the way to explain it more clearly than I already did - please build one channel and measure it, then read my previous explanations once more and it will dawn to you eventually...
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Old 1st October 2011, 01:36 PM   #305
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Perhaps when connected to negative rail R16 and R28 form current source and set current through Jfet.
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Old 1st October 2011, 06:09 PM   #306
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R16 and R28 form a voltage divider that sets appropriate DC potential on Source. C3 excludes R28 from AC path (C3 establishes ground path for AC signal).
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Old 2nd October 2011, 07:10 PM   #307
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Hi everyone, just finished my headamp(preamp), the one from post 42. Very happy with the sound, and a BIG improvement over my Asus soundcard. Bias at 400mA and heatsink around 45 degree celsius. Powered with two transformers, CRCRC and 7815/7915 regulators. I guess i need new headphones though.
Thank you Juma!
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Old 2nd October 2011, 07:22 PM   #308
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Is degeneration a function of the AC signal passing through Rs and dynamically altering the gain or does the signal affect the dc bias and therfore the operating parameters of the Fet?
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Old 2nd October 2011, 10:16 PM   #309
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Cathode/Source/Emitter degeneration is often considered as a form of negative feedback and yes, it's used to control the gain and the working regime of the active device.
There's a lot of stuff written about it here in forum as well as in the general electronics literature - search button and google will provide you with abundant info about it...


Nice job holminator, glad you like it
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Old 19th October 2011, 04:41 AM   #310
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Default regulators

Quote:
Originally Posted by holminator View Post
Hi everyone, just finished my headamp(preamp), the one from post 42. Very happy with the sound, and a BIG improvement over my Asus soundcard. Bias at 400mA and heatsink around 45 degree celsius. Powered with two transformers, CRCRC and 7815/7915 regulators. I guess i need new headphones though.
Thank you Juma!
Congratulations holminator.Very nice job.I think using a better regulator instead of 7815/7915 would be better.For example lm317/337 types.Thanks
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