BSIABII Squeals After Dr Boogey

[railhead]

I had something weird happen earlier tonight: when I put my BSIABII pedal after my Dr Boogey, when I turn on the BSIABII, I get a constant, high-pitched whine/squeal. Both pedals work fine on their own, and both sound just as they should -- but the BSIABII doesn't like going after the Dr Boogey.

I'm using J201s in both.

THoughts? Ideas?

[John Lyons]

Which Dr boogey circuit do you have?
Scaled tone stack with 100K output impedance (gaussmarkov) or Electritabs 1M impedance?

john

[mike_a]

I think it's simply a matter of too much gain.
Both of these pedals together form a circuit with too much gain, and it goes over the edge of stability - causing oscillations.

Try lowering the gain in both of them.
This can be done by swapping JFETs for lower gain types.
it's usually the first ones (at the entrance).

[railhead]

Which Dr boogey circuit do you have?
Scaled tone stack with 100K output impedance (gaussmarkov) or Electritabs 1M impedance?

john

Scaled down

[railhead]

I think it's simply a matter of too much gain.
Both of these pedals together form a circuit with too much gain, and it goes over the edge of stability - causing oscillations.

Try lowering the gain in both of them.
This can be done by swapping JFETs for lower gain types.
it's usually the first ones (at the entrance).

Something like using an MPF102 in stage 1 for both pedals?

[mike_a]

Something like using an MPF102 in stage 1 for both pedals?

yep, something like that 

[railhead]

Having never used them, though I've read people using them in both builds, will they affect the overall gain of the circuit?

Also, I should make sure everyone knows I'm not talking about running both effects at the same time. Going guitar > Boogey > BSIABII, with the Boogey off and the BSIABII on, I get the whine.

[mike_a]

Having never used them, though I've read people using them in both builds, will they affect the overall gain of the circuit?

Also, I should make sure everyone knows I'm not talking about running both effects at the same time. Going guitar > Boogey > BSIABII, with the Boogey off and the BSIABII on, I get the whine.

oh   

Well, that's a different story....
I thought you where running them together, and that was the source of the whine.
In this case, I have a feeling it has something to do with input/output impedance mismatches causing the oscillations.

Try putting a buffer between them.  You can try something like clean-boost (pick any) set to unity gain.
or you can perf or breadboard a simple source/emitter follower - just for the experiment.

[frokost]

Is the input on the Boogie grounded when bypassed? If not, it may oscillate and somehow make it's way into the signal and get amplified by the BSIAB... perhaps.

[railhead]

Here's how I wire switches:



Should I bridge lugs 1 and 6?

[frokost]

That might be it! Make a temporary connection between the circuit input and ground. If the squeel disappears, use this wiring method instead: http://www.diystompboxes.com/pedals/images/3PDTbwiring.jpg

EDIT: Or, the one I always use - "offboard wiring 5" on the Tonepad offboard wiring project: http://www.tonepad.com/project.asp?id=35

[railhead]

I like my current layout because it fits so nicely with the setup of the guts. Is there any reason I couldn't just use my original version with the 1-6 bridge (assuming it works)?

[dschwartz]

yes..try that... is the same thing

[John Lyons]

With the bottom off the box and using a test clip just ground the circuit input of the boogie and see if the Oscillation goes away.
Grounded input switching is the way to go for high gain and touchy circuits.

john

[railhead]

THAT WAS IT!

Man, I've been building pedals without that ground for AGES and never had any issues. I think I'll make that a normal connection now, though, for all my effects.

That said, the cause of this was what -- just leaking from not grounding well enough?

[John Lyons]

Sort of.
Basically the circuit is still fuctioning, the input is open and there is nothing connected to it and the circuit oscillating to some extent.
The same way as if you had the pedal output connected but no input jack in the socket. Even though the output jack is not connected to the circuit output there is still some signal bleeding through..or a possbility of it doing so. Wire cross, signals run next to each other and parasitic elements are at work.
The DB is really a high gain circuit and With FETs there is still something a little too high gain or maybe just impedance issues between FETs compared to tube triodes.
By grounding the input (in bypass) you kill the signal before it gets amplified at all.

john

[frokost]

Glad to hear it worked out!

I thought that might be it. I remember debugging a doctor a while ago. I noticed that I didn't actually have to touch anything with the audio probe - it picked up signal just by being close to the circuit. If that circuit starts to oscillate... better ground that input.

[earthtonesaudio]

One little side benefit of grounding the effect input while bypassed: you can omit the input pull-down resistor for better input impedance.