6/17/2020: A discrepency was found in the review process for the schematic. This tremolo pedal prototypes two different VCO circuits: An LM13700, and a more cost-effective op amp solution. The op amp solution implements an audio grade op amp, and the circuit is designed to work from 9V - 0V.
The LM13700 circuit is designed to work from +15V to -15V. Biasing can correct scaling, but there needs to be an internal reference to tie the center tap. An adjustable regulator was added to compensate.
If the adjustable regulator contributes too much noise in the production of this board, it'll have to be scrapped.