Simple AR Gen is an old Attack-Release (or Attack-Decay) Eurorack module project.
Circuit Description
The circuit is based on charging and discharging a capacitor through two diodes installed in opposite directions.
Each diode conducts and charges or discharges the capacitor via its respective potentiometer.
The first op‑amp, configured as a non‑inverting amplifier with a gain of 2, boosts the incoming trigger or gate signal. This amplified signal is then fed to the diodes. When the signal is positive and exceeds the voltage across the capacitor plus the forward voltage drop of the attack diode, the diode conducts and further charges the capacitor.
Conversely, when the output of the first op‑amp drops below the voltage across the capacitor plus the forward voltage drop of the release diode, that diode conducts and the capacitor begins to discharge.
As the capacitor charges, the voltage across it increases; as it discharges, the voltage decreases.
Each potentiometer, wired as a variable resistor, controls the amount of current flowing into or out of the capacitor, thereby setting the rise and fall times of the envelope.
The capacitor voltage is buffered by the second op‑amp, configured as a voltage follower, and sent to the output jack.