Unwanted oscillations on jfet guitar preamp/distortion

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ricothetroll

Well-known member
Joined
Jul 5, 2005
Messages
325
Location
Bruxelles
Hi !

I just built that preamp :

http://www.runoffgroove.com/englishchannel.html

It's a fet adaptation of the classic Vox AC30 Top Boost preamp, to be used as a distortion pedal.

My problem is that I have oscillations on the output. Its frequency is something between 2k and 5k, and changes while turning almost any pot, especially the input volume. The shape is some kind of sawteeth. When the input volume pot is between 10 o'clock and 1 o'clock, there's that strange sound :
http://www.wuala.com/ricothetroll/public/Eng_Chan_Oscillations.wav
(that sample has been recorded while turning the input pot from anti clockwise to fully clockwise)

Has anyone a clue of what could cause such noises ? About how to eradicate it ?

Thanx in advance !

Have a nice day.
 
Hi John,

Thanx for your answer ! I checked the power suply rail with oscilloscope but it's clean. Is it possible that the problem comes from there anyway ?

Here's my PCB layout :

http://www.wuala.com/ricothetroll/public/English%20Channel2.pdf (DC rail is highlighted in red and ground in light blue).

Oscillations appear under the first 100k trimpot (bottom left on the pcb).

Best regards.

Eric
 
You can try removing the 100p "bright cap" from the volume pot. It doesn't make much difference to my ears but might help tame the HF. Try adding 10-25pf caps in one or more places to kill more HF. I stuck mine on the volume pot wiper [to ground].
 
Hi ! Thanx a lot for your interest and answers !

It seems that the circuit is pretty sensible to noises because of its high gain... So I cabled the pots and jacks with the strict minimum of wire length. That improves already a lot, but I still have those oscillations running on the output !

I have now made another layout, where I can place the pots just on top of their conections on the pcb, and also trying to minimize the signal path. Input and output are now on opposite locations on the pcb. I'll also cable them with shielded cables.

After some signal tracing I noticed that the oscillations started after Q2.

Here is the new layout :
http://www.wuala.com/ricothetroll/public/English%20Channel3-1.pdf

Opinions are welcome !

To be continued...

ps : is that noise issue one of the reasons why most distortion pedals use clipping diodes instead of pushing transistors to their limit (less gain needed to saturate....) ?

Best regards;

Eric
 
ps : is that noise issue one of the reasons why most distortion pedals use clipping diodes instead of pushing transistors to their limit (less gain needed to saturate....) ?

Either way, most use a cap in NFB (usually) to quiet the HF. See the TS-808 and Fulltone 1970 for examples.

I think diodes are used mainly for their clipping characteristics.
 

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