Oscillation with four JFETs

[Jussi]

Hey,

I have this circuit on my breadboard. If I add fourth gain stage (which is colored in red) I start getting oscillation all over the distortion. Adjusting the 100k pot or guitar's volume pot will change the oscillation sound. What could be the problem?

[Keppy]

To quote R.G., "With enough gain, everything oscillates." No insulation is perfect, so some of your signal by one path or another makes it back to the earlier stages and gets reamplified.

Also, gain stages are multiplicative. Say each stage has a gain of 10, which isn't all that much. Well, 10 x 10 x 10 x 10 = 10,000. That's a lot.

You might have a build problem, but you might also just have too much gain. It's tough to know on the breadboard, because these problems tend to be worse in that environment. If you build it, careful layout and shielded wire to the jacks might help, but it's not a certainty.

[jymaze]

Not sure that is the answer, but a glaring thing missing on that schematic is the polarizing resistors from the gates of the fets to the ground (or whatever positive voltage if you want so). Once these resistors are put in, then you can even put some pF range cap in parallel to get rid of some RF that could induce oscillation.

[defaced]

In cascaded gain stage circuits, it's standard practice to divide down the signal between stages. Simply inserting 100k pots like you have between the first and second stage will allow you to play with their settings. 

[J0K3RX]

In cascaded gain stage circuits, it's standard practice to divide down the signal between stages. Simply inserting 100k pots like you have between the first and second stage will allow you to play with their settings. 

Agreed... You have nothing but open road between stages... 

[Lurco]

Add a fifth stage, so the output signal will be out of phase, and thus will not interact too bad with the input signal once it comes close to it at the bypass switch!

[Jussi]

Thanks for the tips! I managed to get enough gain with three JFETs.

[J0K3RX]

Thanks for the tips! I managed to get enough gain with three JFETs.

cool! Any sound clips?

[Jussi]

Thanks for the tips! I managed to get enough gain with three JFETs.

cool! Any sound clips?

Yea I got some recorded with my cell phone:

To clean channel, sounds like crap http://soundcloud.com/666saatana/sounds-from-friday-morning (sounds like it needs a little bias adjust)
To blackstar ht-5r OD channel with a little gain: http://soundcloud.com/666saatana/jfet-preamp-2

I want it to sound good on clean channel too because I won't use this as a pedal - I will use it as a preamp for a 0,5 watt amplifier which works with 18 volts.

[Rob Strand]

The 4 stages will promote oscillation because it has more gain (and possible the output is in-phase with the input.)

Try
- Use shield cable for input and output wiring.  With the shield grounded.
- Keep the input wires away from the output wires.   (Don't run them side-by-side)
- Keep input wires away from output wires near the foot switch
- You could try adding a cap across one of the 25k resistors start with say 47pF, then double it until the oscillation is gone.
- A cap on the power rail might also help (100nF to 10uF).  Probably not but it doesn't hurt trying.