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Author Topic: jfet emulator, buzzing - help me  (Read 3418 times)

reroe

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jfet emulator, buzzing - help me
« on: September 16, 2011, 06:30:52 PM »
i post to ask something about jfet emulation circuit.
many people says about tube to jfet conversion quality or difference but i'm not intending to discuss about it at this time.
 
i have a buzzing or oscillation problem.
i've just finished a carvin legacy jfet pedal (so called leegazzy in the web) and a laney gh100L preamp part(exactly same circuit with tube version)
and they have same phenomenon, that is, ~193hz buzzing  at high volume(or gain).  i write "193hz buzzing" because i think buzzing frequency is as same as with open g note and when i turn vol or gain pot, the pitch of oscillation changes
 
i googled to solve the problem and did something to fix it
0) trimpot biasing is around 4.5~4.7V for every JFET(i use 2sk117)
1) check ground scheme, jack isolation
2) 9V power (i use DC 9V 300mA adapter though) filtering with 100uf cap
3) add dropping resistor to gain stages like tube amp power supply chain
4) shorten gain and volume pot wiring
5) add 0.01uf (roll offcap?) to drain-gate, drain-gnd,gate-gnd
all of these above were not properly working.
 
could anyone suggest about my problem?
 
thanks.

Alexius II

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Re: jfet emulator, buzzing - help me
« Reply #1 on: September 17, 2011, 05:27:57 AM »
Have you tried it with a battery? (that would be the first thing I would do)
Also, please post some photos, so that we can see the wiring :tu:

In my experience with high gain pedals (or preamps),
the problem was often with proximity of in/out signal wires.

joecool85

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Re: jfet emulator, buzzing - help me
« Reply #2 on: September 20, 2011, 08:44:43 AM »
Have you tried it with a battery? (that would be the first thing I would do)
Also, please post some photos, so that we can see the wiring :tu:

In my experience with high gain pedals (or preamps),
the problem was often with proximity of in/out signal wires.

I've had issues with ground oscillation loops, something to keep in mind.  Make sure everything is properly grounded.
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J M Fahey

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Re: jfet emulator, buzzing - help me
« Reply #3 on: September 20, 2011, 11:56:26 AM »
Maybe you don't find this of any help ... now .... , but I remember my first amps were buzzy, oscillated, got all kinds of radios, worked as buried mine or gold detectors, you name it.
I used solid copper busbars, shielded everything, star grounding,  set all wiring parallel or at perfect 90º , Hiwatt style, all the tricks in the book, and still had problems.
Plus hiss.
Now I do not even use shielded wire , sometimes barely twist a couple wires ... and my amps do not hum , oscillate or hiss.
Maybe it can be called experience.
Nothing magic to it, just keeping doing it and things get better.
Oh well.
So, go on and step by step things get better  :tu:

mensur

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Re: jfet emulator, buzzing - help me
« Reply #4 on: September 20, 2011, 04:29:56 PM »
Maybe you don't find this of any help ... now .... , but I remember my first amps were buzzy, oscillated, got all kinds of radios, worked as buried mine or gold detectors, you name it.
I used solid copper busbars, shielded everything, star grounding,  set all wiring parallel or at perfect 90º , Hiwatt style, all the tricks in the book, and still had problems.
Plus hiss.
Now I do not even use shielded wire , sometimes barely twist a couple wires ... and my amps do not hum , oscillate or hiss.
Maybe it can be called experience.
Nothing magic to it, just keeping doing it and things get better.
Oh well.
So, go on and step by step things get better  :tu:

You're right, it is called experience :)
Just replace trimpots with fixed resistor values close the pots.
It's that simple :tu: