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This circuit, designed as a bass scream, keeps generating a low noise about the same loudness as the guitar signal on the output.

Scream pedal shema

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    \$\begingroup\$ Have you assembled this on a bread board, as a PCB or something else? Show us a picture! :) \$\endgroup\$ – Jakob Halskov Jun 16 at 16:09
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    \$\begingroup\$ If the opamp is oscillating, you need to decouple its power pins with a bypass capacitor. \$\endgroup\$ – Dave Tweed Jun 16 at 16:12
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    \$\begingroup\$ Oooaoh, Dave, missed the missing decoupling caps! Also, Martin, really, debug this stage by stage: you can measure S1. Only if that is OK, you'd connect the second stage. Only if S2 is OK, the third and so on. Where exactly does it go wrong? \$\endgroup\$ – Marcus Müller Jun 16 at 16:15
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    \$\begingroup\$ Yes, between pins 8 and 4. Your first stage and second stage are both non-inverting, so if there's any inadvertent coupling between the two opamps via the power pins, the circuit could easily oscillate. This is one reason that inverting configurations tend to be preferred. \$\endgroup\$ – Dave Tweed Jun 16 at 16:21
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    \$\begingroup\$ @MarcusMüller: I see plenty of coupling capacitors, but no decoupling capacitors on the power supply. \$\endgroup\$ – Dave Tweed Jun 16 at 16:22
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You have a feedback through the wet-dry circuit. That's an oscillator. The dry signal should have a buffer amp before it's fed to the wet-dry balance pot. It very likely would stop the signal going from output back to the input.

Others have already said there's no decoupling capacitors in the DC supply inputs of the opamps. That is a common unwanted feedback mechanism (=through the power rails) and can cause oscillation.

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Here's one way to break that feedback path through the wet/dry mixer. I found this recently when reverse-engineering a powered speaker for a friend. Since the wiper of the pot is connected to ground, and the input to the opamp is held at "virtual ground", there is no signal path between the two inputs.

schematic

simulate this circuit – Schematic created using CircuitLab

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