Weird oscillation with 2 Overdrives

[drolo]

So I was building myself a double overdrive combining something like an OD820 and somthing like a BSIAB. The OD820 is first in the chain. Both circuits can be bypassed individually.
I had been testing it and they sounded great either on their own or combined.

Then... I actually plugged my guitar directly into the pedal. Until then I had been playing through a looper that plugged into the peal, but when plugging the guitar directly into it I get a huge high pitched oscillation when both circuits are engaged. The pitch of the squeal varies with almost all the knobs of the 2 pedals. Also the guitar knobs affect the squeal pitch.

As soon as I insert some buffer between the guitar and the pedal all is fine.

This can happen, but what I find the weirdest is that the first thing in my OD820 circuit is a regular textbook opamp buffer (unlike the actual OD820 which is a fet buffer) ...

I just don't get this ... maybe someone has an idea of what could be causing this, and why the onboard buffer does not prevent the oscillation like an external one does

[midwayfair]

Let me guess ... do you have an order switcher toggle?

[drolo]

nope

[playpunk]

Last night I was playing my od-820 and used my Kingslayer after it and got some motorboating with the gain on the KS up. Both had 1044 scpa charge pumps, so it could have been something with that.

[midwayfair]

Ah.

In any case, you have a ton of gain, and some tiny amount of positive feedback that's being amplified. You can try to fix it with good lead dressing and shielded cable. A buffer always on at the input of the circuit (close to the input jack!) MAY solve it, or it may not since it's still inside the same enclosure. Depending on some other things, you could also probably solve it with a low-pass filter at the circuit inputs, but that's not necessarily desirable for obvious reasons.

[drolo]

I think it's the layout. The input trace from the input jack was running parallel on top of a trace at the end of the BSIAB tone control. I routed the input now with a differnet wire (shielded) and the remaining unconnected trace seemed to have an effect on the oscillation. I grounded it and it helped a bit. It behaves a bit better now. Just can't go into extremes when using both at once, which is ok. What I really find odd is that the oscillation would not occur when another buffer was between the guitar and the pedal no matter what gain settings ... when there is a buffer on board .. perhaps some impedance thing ... oh well ..

[helos]

Let me guess ... do you have an order switcher toggle?

I have the same issue, and I do have an order switcher toggle... is that switch the cause?