Madbean Kingslayer Squeal

[toddvirgil]

 I'm hoping someone can chime in and help me out here.

I've built a bunch of the madbean kingslayers with no problem. However the last four I've built have this crazy squeal when all the controls are at max. It's more of a high-pitched whistle.

Schematic: http://www.madbeanpedals.com/projects/Kingslayer/Kingslayer.pdf

Here's what I've learned so far.

1. The squeal goes away if I turn any of the controls down to about 3o'clock (75%).
2. it doesn't matter whether the OD clipping is on and it doesn't matter which signal clipping I'm using.
3. I've tested with opamps from known working circuit but still get the squeal.
4. I've tried removing and replacing the zener, neither affected the squeal.
5. I get a squeal on all of the opamp outputs on the tlc274
6. If I remove the 5n6 at C15 the squeal goes away (but effectively flattens out the Tone control)
7. If I replace the tone control with a 5K it wont squeal when pegged (no surprise there).
8. With a 10k pot on the tone, I get squeal on pin 1, but pin 2 and 3 are clean.

Any ideas what I should try next?

Thanks in advance!

[mth5044]

Check to make sure your charge pump is the SCPA and not the CPA type.

[toddvirgil]

Check to make sure your charge pump is the SCPA and not the CPA type.

Yep, it is. Like I said, I've built a bunch with no problem, but for some reason this latest batch is going nuts. I've checked with IC's from a known working build.

[ggedamed]

Have you tried putting an 100n capacitor directly on the supply pins of IC1?

[toddvirgil]

Have you tried putting an 100n capacitor directly on the supply pins of IC1?

No, I haven't -- will do -- what would that accomplish exactly (from a technical standpoint)? Would the intention be to filter out frequencies coming from the chargepump?

[IvIark]

Brian mentioned that you can get some oscillation because he opted for a 1M for R11, so it may be worth reducing the value and seeing if it sorts the problem.

[midwayfair]

Brian mentioned that you can get some oscillation because he opted for a 1M for R11, so it may be worth reducing the value and seeing if it sorts the problem.

That's a different sort of oscillation. When you lift the diodes and turn up the gain, about midway it freaks out the (third, I think) op amp and turns the pedal into a tremolo. It scared me the first time it happened ...

[toddvirgil]

Brian mentioned that you can get some oscillation because he opted for a 1M for R11, so it may be worth reducing the value and seeing if it sorts the problem.

Yeah, that's a different issue. I talked to Brian and he recommended swapping the 1k8 in the tone control out to a 6k8 or 10k since the treble is pretty intense/unusable that way anyway. the 10k did the trick, squeal gone. Still a mystery as to what was causing it.

[ggedamed]

Have you tried putting an 100n capacitor directly on the supply pins of IC1?

No, I haven't -- will do -- what would that accomplish exactly (from a technical standpoint)? Would the intention be to filter out frequencies coming from the chargepump?

They're called decoupling capacitors and their job is to provide a short path to ground for the AC traveling through the supply rails. You have some info here or here. Oh, and right here, from the two guys I'm always following - R.G. and PRR.