Landgraff Dynamic Overdrive Squealing Issue

[Ofek Deitch]

Hi guys,
Just finished to build a Landgraff OD on my Proto-Board, and as soon as I fired it up, it started squealing - as the title says.
I noticed that the Volume knob still works - it sets the squealing level, and also the gain knob changes the pitch a bit.

I'm using this schematic:



I have no idea why this is happening, so any help would be greatly appreciated!
Ofek

[Focalized]

Does it only squeal with the drive up?

I get squeal with my Son Of Clay Jones overdrive. Similar drive with that 1M pot which I think might be part of the cause. I only get it at the last 20% of the drive pot. Which is then affected by the tone and volume.

Would like to know about a fix too. Maybe reducing the 1M down some with a resistor until the squeal disappears?

[mistahead]

There's a feedback loop in there somewhere, maybe a solder bridge or too low a resistance in the opamp feedback network? Does the tone knob change the squeal?

Chasing a similar problem introduced when I housed up my tube caster, I'm shorting something in my on-socket p2p build and @#$%ed up and concealed it when mounting it lol.

Hmmm feedback network there - is that where your diodes and switch are? Still learning

[GibsonGM]

Neat design. You have input signal coupling back from an output to an input.   Feedback loop.    This COULD be related to lead dress, and you may need to use shielded wire for some wiring runs to stop the coupling.

All electrical components have a capacitance associated with them...wiring, opamps, transistors, etc....these capacitances are able to bleed signal between the pins of an IC, the legs of a transistor (Miller effect), and between amplifying elements and wiring runs.    With high-gain circuits, you're pushing the devices very hard, so there is a lot of stray stuff floating around which will readily couple itself into your wiring ('path of least resistance').   Think of holding a guitar next to a very distorted amp, and the feedback squeals that result - this is just about what you most likely have going on!

Try getting it to *work* a bit (pass some signal), then find where to set the knob to get the squeal...provided you have no wiring mistakes (most common issue), you can probably find a way to "dress" the wiring by moving it around gently to where the squeal stops.   You can also try audio probing to maybe locate the area it's happening in....you can remove the 1K between the 2 opamp sections and see if the output of A1 is squealing...etc etc.   Altho they may be working together to give you such a lively shriek, ha ha...

[kappafleo]

pretty old topic, but my 2 cents:
I had the exact same issue in the esact same circuit, in the same circumstances (pot positions and such)
500pf instead of 51pf cap in the feedback loop solved the problem.
I guess it is due to the heavy modification of the gain pot and following res (10k) while maintaining the same cap value, resulting in a poor low pass filter.
Hardly noticeable tone darkening.
Every other part in the clipping stage prooved to be completely ininfluent.

[TGP39]

Thanks for that info Kap....I'm building one of these next weekend. Much appreciated.

Steve.