An instrument buffer: the jack J20's tip on GTR_IN (a pickup that wants at least 1 MΩ), 2.2 MΩ to the reference VREF (10 kΩ / 10 kΩ from +5V, 22 µF), U20 an op-amp follower on +5V with 100 nF; its output coupled through C22 1 µF (film) onto BUF_OUT with R24 100 kΩ to ground – an AC run expects the follower flat at 1 kHz and the coupling's corner, 1.6 Hz, below 10 Hz.
Why: Every pedal starts with a buffer at the instrument input. It holds the pickup's load, a reference inside the op-amp's input range, a bypass that keeps the signal's level, and the corners of the signal path in an AC run.
