Understanding A Guitar Pedal Circuit (MXR Envelope Filter) | Electronics Forum (Circuits, Projects and Microcontrollers)
The Quackmire: A simpler vactrol LP envelope filter for 1590A
Guitar Pedal X - GPX Blog - Drunk Beaver's 8th Monthly Pedal Drop is the Odessa Envelope Filter - based on MXR's MX-120 Envelope Filter with a few circuit enhancements
Dirtbox Layouts: DOD Envelope Filter 440 Reissue
A Simple LDR Envelope Filter that is just delightful | Circuit Salad