High frequent noise (RAT Pedal)

[pokus]

Hey there again! I tried to breadbord a RAT Pedal and it sounds quite well, but when turning the gain up it starts to give me that high frequent noise. It's like the sound you get when you hold your mic to the speaker (sorry don't know the english expression for that). I noticed that a couple of times when I was breadbording an overdrive circuit, but it generally didn't give me that noise so early when turning the gain pot up. The tone pot also determines how fast the hf noise starts. I know that the RAT has a extremely huge gain, so I guess the max gain will be set by the limit of the opamp. But what does the gain pot do then in the high range? Just shape the sound? And the most important question, how do I get rid of that unwanted noise in general, is this a particular sign for something I've done wrong?
The noise disappears when I replace the 100pF cap (C4) and put in a 2,2 nF. But that can't be the answer, because of the completely different roll off freq.
I did not build the output/buffer stage, so only placed that volume pot behind the tone section. Could that be a part of the problem? Thanks for any kind of help! 

[GGBB]

Oscillation. Most likely, this is because your are breadboarding. So the circuit is picking up a lot more noise than it would when inside a shielded enclosure. Try to shield it with a tin box or something. It can also be caused by output signal wires too close to input wires.

[rankot]

I had the same problem with the 2nd Rat I built, and I presume the cause was proximity of in/out traces on PCB, because I wanted to make clickless switching. It was solved by putting an 82k resistor in front of volume pot and replacing 100k volume pot with 20k. Not the best solution, but it worked.

I have also built a Voodoo, which is basically clone of a Rat with some different filtering, and it was also prone to oscillation. Finally fixed it with 100n cap across V+ and GND pins on LM308 and replacing 39 ohm resistor in series with gain pot with 1k5. Maybe it could work fine w/o that resistor, but it was the first thing I changed and later I was too lazy to remove it. There is still some noise, but I believe it is due to the fact that pedal is still not boxed.

You can see both PCBs here:

[rankot]

Huh, curiosity, curiosity...

I couldn't just let it go, so I removed 1k8 resistor and it seems that my Voodoo is working fine without it as well, so the real solution to oscillation stopping was 100n cap across V+ and GND. I usually put them in every pedal I make, but somehow I forgot it here.

On the other hand, RAT schematic shows 10n there, so maybe that make trouble sometimes? I will try my RAT with 100n cap.

[pokus]

Ok, thanks! So at least I know what my problem is and what it's called. I tried putting a 100nF cap between V+ and ground on my opamp, but the noise is still there. Where's the 10nF cap you mentioned?

[rankot]

It's marked as c10n on my PCB. You may try to add a resistor in series with volume pot.

[Jubz]

Hi. First post for me here

Sorry i cant give you an answer to the problem. I actually have the same : oscillation with a rat circuit on a breadboard. But i noticed something i m too noob to understand.

I play a switchable passive/active bass (g&l l1505). The active part is basically a big treble boost. When the treble boost is on the oscillation disappears and when i switch back to passive mode it shows again.

So ive concluded the treble content which  feed the gain stage is not the only responsible of the oscillation and, perhaps, the output impedance of what precedes the rat plays a big role. But i dont understand why. The signal sees an opamp gain stage right at the beginning of the circuit (lm308) so it has an high input impedance. I tried to put a buffer but it didnt have impact on oscillation. Even an opamp treble boost didnt do the trick (but ive kept it because i liked the sound)

Sorry for the approximative english

[thermionix]

Your English is very good Jubz.  Is this still on the breadboard?  You may have no problem at all once soldered and boxed up.

[rankot]

But why is there oscillation when using passive bass, and not when using active?

[thermionix]

When dealing with unwanted oscillation, pretty much everything will effect it.

[PRR]

> why is there oscillation when using passive bass, and not when using active?

A naked pickup has impedance over 10K.

Active boxes are usually 1K.

So it hints that his output is sneaking back AT the input jack, where the active pickup absorbs it better than the plain pickup.

[Jubz]

Thanks for the answer.

So if i understand the active preamp of my bass act as a barrier between the rat circuit and my pickup.

I didnt understand what happen because in fact the rat on my breadboard, after a few months of experimentations, doesnt have a lot in common with a rat circuit anymore. I ve added an opamp treble boost in front of the lm308 gain stages. The input impedance is already very high.

I made myself a test box a la beavis box connected to my breadboard and had the excellent idea of putting in and out very close...