First let me say that I know these were designed for bass but I find them very good on guitar also. These are highly sought after. Bringing top money on eBay. Stock they are really great tone shaping pedals but are rather noisy. This mod reduces noise and adds clarity to an already good circuit. With this mod we replace the cheaper noisy op-amp chips with low noise higher quality chips.
Lower quality capacitors are also replaced reducing noise even more and also adding a more focused Hi-Fi tone to the circuit. Noise is substantially reduced and the signal is passed through preserving the original tone of your guitar. You’ll be shocked at the improvement this mod will make on this.
I am unsure if there are different versions of this circuit so before you order make sure your pcb looks like this.