>>2946482
Anon, it already has a 100pF cap across its single op-amp. Another 10 wouldn't do much.

>>2946677
A problem you may run into, is that the 100pF is already a significant enough impedance to influence the frequency response of the circuit. At 20kHz 100pF is 80 kiloohms, so using a 1nF capacitor will almost certainly change the tone by rounding off the sharper edges.

But I can't really see why it would oscillate, there shouldn't be any positive feedback possible, and the phase delay in the feedback loop is simple and small. Assuming it's built as designed, and worked as it was designed. Maybe the high impedance power rail is causing feedback, in which case you could try adding a bunch of bulk capacitance, or reducing the series resistor. I see some schematics with a 100R and some with a 47R, but it seems you've got a 47R so that likely isn't the source of the issue.

I guess I'd just clean the board thoroughly for any flux residue or little bits of swarf or solder bridging pads. If you have a scope you can check the inverting and non-inverting inputs to see what's going on to some extent, I guess.