Voltage Controlled Looping Envelope Generator

This project is a development of my earlier VCADSR envelope generator. Whilst that chip had both GATE and TRIGGER inputs, this chip replaces the TRIGGER input with a MODE CV that allows selection of normal ADSR, Gated looping, or full LFO-style Looping modes.

Standard ADSR mode generates the shape you expect, and allows CV control of envelope depth and overall time.
Gated looping mode loops while the Gate is high. When the Gate goes low, the envelope goes to the Release stage.
LFO Looping mode loops all the time, acting like a highly versatile LFO.

The LOOPENV chip keeps the other features of the original VCADSR, like the LEVEL CV which controls the output level and can act as an ‘envelope depth’ control, and the TIME CV which shortens the overall time of the whole envelope. The TIME CV makes an excellent modulation input for either envelope modulation or LFO frequency modulation.

Full details are in the datasheet below, but the envelope times range from 1mS through to 10Secs, in 4 even decades, and all control voltages run from 0-5V.

There is also a digital input which selects either a ‘traditional’ exponential envelope shape, or a early-digital-era linear envelope.

Pinout Diagram


Version 1B – a bug fix

After publishing this code, a small bug was discovered whereby the Gated Loop mode wouldn’t exit to the release stage correctly. Version 1B corrects this.

More details

Looping Envelope elsewhere on the web

Rick Holt of Frequency Central developed a DIY PCB and panel in Eurorack format for this project, described on the Frequency Central Loop/Env generator DIY page.

Obsolete! This chip has been superseded by the EnvGen8

The new chip combines the features of the VCADSR7B and the features of LoopEnv chip too, has improved resolution, and also no longer requires a crystal. It’s an improvement in pretty much every way! Head over to the next generation Druid chips page and read all about it!.

7 thoughts on “Voltage Controlled Looping Envelope Generator

  1. Hey Tom, another chip to get really excited about for me! I just wanted to get a little bit of clarification. I want to use this chip in a guitar context to use the envelope to control the “mix” knob between a guitars dry signal and an effects loop and the way I envision it is that I could use it as a standard envelope follower, but also with a momentary footswitched gate —— ie. attack is triggered by the switch press and the release is triggered by the switch release. I’m also curious about the looping envelope mode and what that sounds like…


  2. Hi,
    Can this IC be used with Roland Juno-60 hardware envelopes IR3R01 as a replacement ?

    Best Regards

    1. It can’t be used as a direct drop-in replacement, but it might be possible to use the chip to make a daughterboard module that would work as a replacement. The two chips certainly seem to be similar in several respects, although I found finding details on the IR3R01 isn’t that easy.

    1. They’re similar in that I think they both use 5V CVs, but the pin-outs are completely different, and the LoopEnv needs a filter after it’s PWM output (you should use the ENVGEN8 to get around this problem – it has a full DAC output). But the output on any of the Druid envelopes is 0-5V, not 0-10V, and there’s no requirement for a 10V reference. Finally, I don’t know how similar the time range would be – the original code wasn’t aiming to emulate this chip, although it’s probably broadly similar in a “msecs to few seconds” sense. So there are significant differences.


  3. Hi Tom,
    As the most of us are not aware of Electronics, is it possible to design a board for using the LOOPENV 1B Envelope Generator as a direct replacement for the IR3R01 ?

    Thank you

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