I bought a Mosky XP Booster pedal (one-knob clean boost) with the idea of hacking into the pot. and wiring a jack in it that I could plug an expression pedal into so I could run it as a volume-pedal / booster with a remote expression pedal.
I do like a linear expression pedal for my purposes more than an audio taper.
Does anyone know what value expression-pedal would be optimal, or even better how I could add an expression pedal that would make the range go 0-to-75%?
Mosky say it's based on the Xotic EP booster, if so, thread and schematic: viewtopic.php?t=7881 The Boost pot is a 10kC that bypasses the Q1 source resistor (in series with a cap), so that would require a 10k pot in your expression pedal. Wire the pedal just like the Boost pot, with lugs 1 & 2 to ground, lug 3 tied back to the Q1 source bypass caps. You'll want a switch to select either the internal Boost pot or your external expression pedal, else they'll be in parallel and won't give you much range.
If you're using a 10kB expression pedal, add a resistor in series, somewhere around 500 to 700 ohms. That will limit the 10kB to a 95% value at full rotation, which matches a 10kC Boost pot that's limited to 75% rotation. The fully counterclockwise direction will not be noticeably different, although the taper will be different.
