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J201 - fizzy and dropping out

Started by johnnyfi, February 21, 2011, 06:00:22 PM

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johnnyfi

February 21, 2011, 06:00:22 PM
I have been building a few different OD ckts with both MOSFETs and JFETs lately.  I have used BS170s a lot in the past, but I wanted to try the infamous J201 to compare.  I've tried many biasing techniques, mu-amp style, having the J201 drive a MOSFET, having a MOSFET drive a J201, etc.  In all cases, the J201 ckts sound very fizzy and weak.  The last mu-amp setup I tried sounded pretty decent except it had a distinct dropout or sag in level when hitting low chords.  Any quick ideas?

petemoore

#1
February 21, 2011, 06:41:26 PM
  Mu amp/whatever it's driving having impedance mismatch ?
  J201 having characteristic that makes it not want to bias within the constrains of 9V supply ?
  Try different J201 or different Jfet ? Raise the power supply so the J201 isn't 'clamped' between 0-9vdc.
  Also see Runoff Groove Fetzer article, GEO Jfet article, to get a better handle on how to bias a Jfet gainstage.
Convention creates following, following creates convention.

johnnyfi

#2
February 21, 2011, 08:19:33 PM
I tried a bunch of J201s.  It's not a bad part.  I know the impedance might be an issue.  The mu-amp with J201s doesn't have much drive current.  It doesn't even drive my tone stack very well.  I'll try reading the other articles.  Thanks.

petemoore

#3
February 22, 2011, 06:27:47 AM
The mu-amp with J201s doesn't have much drive current.  It doesn't even drive my tone stack very well. 
  What is impedance mismatch, final answer.
Convention creates following, following creates convention.

DougH

#4
February 22, 2011, 07:43:15 AM
You will have to "custom bias" the J201. Use a meter and adjust the drain or source resistor until you get ~4.5v on the drain. Do some searches here for threads on JFETs. This has been covered ad nauseum for years, believe me...
"I can explain it to you, but I can't understand it for you."

johnnyfi

#5
February 22, 2011, 08:51:24 AM
Forgive me. I haven't figured out how to post a schematic. I  know this would be a lot easier if I did.  Help on that might be useful too  :D

Until then, I am trying to cascade JFET stages, and my tone stack is a Baxandall.  So, I guess I have 2 different impedances to drive:  the following stage and the tone stack.

I was trying to avoid having trimmers all over my board too, but I am resigned to put them in if it will make this work. 

And thanks for the suggestion on the Fetzer website. It was helpful in seeing the thought process of putting the JFETs together.
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