2x Decay

Sometimes you don’t need a full ADSR, you just need a simple decay. For example, when creating percussive sounds, stabby chords, or short notes. That’s exactly where this circuit comes in handy.

Because it only requires a small number of components (nothing fancy: just resistors, capacitors, a diode, and a couple of classic TL074 or TL072 op-amps) it feels like a a perfect first project if you want to build a very simple envelope generator.

This project is originally based on a design from Bartone Musical Circuits, a graat website filled with a lot of different DIY modules. It’s definitely worth exploring ! There’s a lot of great material, including detailed PDF documentation explaining each part of the circuit.

This is the second time I’ve built a module based on this design. The first one was a quadruple decay generator, closer to the original. I added an attenuator with a buffered output for each decay, as I prefer placing attenuation on the emitter/output stage. It's my module, I do what I want ! :)

For this module, I decided to pack two decay generators together and wire them to the same gate input. This way, a single trigger can generate two envelopes. This is perfect for generating synced envelope for both a VCA and a VCF simultaneously.

By experimenting I also replaced some values from the original :

• I changed the 10µF capacitor by a 2.2µF one, which results in a snappier, more immediate response.
• to maintain a similar maximum decay time, I increased the potentiometer value from 100k to 500k, and chose a logarithmic potentiometer, which feels also more natural.
• I also changed the 220Ω resistor to 1kΩ. Since this resistor defines the minimum decay time, the original value felt too short for my needs.

I also included an attenuator for each decay output. If you want to simplify the build, this stage can be omitted by removing the final op-amp, potentiometer, and associated resistor.

Component List:

1x TL074
1x TL072
2x 100kΩ potentiometer
2x 500kΩ log potentiometer
10x 1kΩ resistor
4x 100kΩ potentiometer
2x 10nF capacitor
2x 2.2µF capacitor
4x 100nF capacitor
2x 10µF capacitor
2x 1N4148 diode
2x LED
3x Jack socket

I probably could have reduced the component count by merging both decay paths after the gate-to-trigger stage (the first op-amp in the circuit). Keeping them separate could be useful later if I decide to give each decay its own input.