Next in my DIY modular synth build, I’ll be selecting the envelope generators. As mentioned in my previous post about VCAs, and envelope generator, or EG, provides the control signal which is typically applied to volume (via a VCA) or a filter cutoff (via a VCF).

Being a voltage however, this envelope can be applied to pretty much anything, hence why modular is so powerful. For example, if you start re-triggering the envelope or it has self-looping ability, it effectively becomes a LFO (low frequency oscillator). Given the distinct wave shape, you could then add or subtract with other control voltages or audio to provide some interesting results.

So my minimum specification is four envelope generators for the two VCAs and two VCFs, maintaining my two standard signal paths. At least two must output ADSR (attack, decay, sustain, release), and the other two can be a simpler AD or AR. Self-looping / re-triggering is also preferred. Also, my VCAs have linear response so it’s desirable to have both linear and exponential envelopes to provide some standard and snappy curves.

I also have the following selection criteria:

• The modules must be skiff friendly, i.e. not that deep. So no modules where the circuit board is perpendicular to the front panel.
• No kits that cost over $100, unless they can provide multiple outputs. Being able to purchase PCBs without any panels is preferable.
• Given the envelopes are control signals, video demos aren’t as important.

The list and initial assessment

I’ve compiled a long list of modules for consideration (in no particular order). I’ll perform an initial quick assessment in order to cut this down.

What made it:

• AI Synthesis AI003 Looping ADSR Env Gen – A nice basic ADSR, it provides looping and manual trigger. It doesn’t have the fastest attack (20ms) but does provide some longer times. A good low cost option for EGs 3 and 4 in my setup. It has only two ICs and a simple build.
• Befaco ADSR – A nice classic ADSR, with four sliders (which I’ve always loved for ADSR but may be hard to source), linear and exponential response with mix between, ~3ms to 10s timing, and unique gate outs for each stage. The full kit is expensive but if selected I’ll see if I can obtain PCBs only.
• Fonitronik TH CMOS ADSR – Another basic ADSR with loop mode, it fires off a pulse to re-trigger which is neat. I’m shortlisting this although it is a rather large panel for it’s feature set.
• Hexinverter Galilean Moons – Provides dual digital AR and ASR with looping (LFO) mode. Both linear and exponential curves, it also has VCAs built in and some other features. Around US$50 for the PCBs and ICs.
• L-1 VC ADSR – Quite a well specified unit, with two full ADSRs, linear/exponential modes for attack and decay/release, voltage control per segment, gate output for sustain, 0.1ms-10s attack and 0.5ms to 10s decay and release, positive and negative outputs. This more than does the job for the two main ADSRs. Final cost may be a consideration.
• NLC ADSR312 ADSR – Very basic ADSR. Potentially a cheap option if I need to increase my number of EGs.

What didn’t:

• Befaco Rampage – Multi-functional, it performs AR, ASR and ADSR, but is also an oscillator, LFO, slope detector, low pass filter and too many other things to mention.  This module looks really great but I’m going to reject it due to complexity. I want the common functions of the synth to be simple to use (even if this is slightly “un-modular” of me). Owning some multi-function modules they aren’t as accessible are more dedicated units. I may consider this again when I’m looking utilities modules as it may tick a lot of boxes.
• 4ms PEG Pingable Envelope Generator – The PEG kit is a different type of module, where the timing of it’s AD/AR generators is based on pings (i.e. clocks). The cost of the kit however is over AU$200, and although it has great possibilities I’d prefer something cheaper and simpler for this particular project.
• GMSN! Pure ADSR – Website is still down so this prohibits me ordering when ready.
• Erica Synths Polivoks ADSR – Linear ADSR with looping and AD mode, and only 27 euros for the PCB, ICs and transistors. I really like this but it has a perpendicular PCB.
• Frequency Central Loop Env Gen – Another classic ADSR with desirable features; linear / exponential models, self-looping with timing control, different polarity outputs. However at over AU$100 it starts to get expensive for multiples, and you cannot buy the PCBs separately*. So unfortunately this is out.
  * I’ve been informed that you can buy without the panels directly from them.
• Ian Fritz / Elby 4X4 AD/AR Generator – This unit provides a single AD/AR with a triggered pulse output, and can also be looped. Not a traditional EG and is on three PCBs. Seems like a lot of build for not much flexibility.
• MST Dual AD/ASR – Not ADSR but has two outputs which cycle for ping-pong effects, inverted outputs and some other useful features. However at over AU$200 for the kit it’s not worth considered for this project.
• NLC Triad Triple EG – This is a basic triple AD/AR EG. Looking at the images, the PCB doesn’t align parallel to the panel, and the pins on the LEDs look bent to compensate. Therefore this doesn’t look worth it.
• Synthesis Technology MOTM-800 ADSR Envelope Generator – Basic ADSR with positive and negative outputs. PCB is perpendicular so unfortunately this is out.

Envelope Generators 1 & 2 Selection

For the first two EGs I want an ADSR with looping, linear / exponential modes, and preferably some other features such as re-trigger or inverted outputs.

The only module that actually meets this criteria is the L-1 VC ADSR. In fact if anything it’s over specified, but in Eurorack you can never have enough options.

L-1 Dual VC ADSR

By the time you take out the cost of the front panels, it’s only around US$60 for the PCB, SMD parts soldered and some tempcos, which is fantastic. Looking at the board most of the work is in the SMDs so the pre-soldering of these is a nice option. One curiosity if you look through the pictures is that the ICs don’t have sockets, and appear soldered directly to the PCB; not sure why this is.

Envelope Generators 3 & 4 Selection

There’s a lot more options for EGs 3 & 4:

• AI Synthesis AI003 Looping ADSR Env Gen
• Befaco ADSR
• Fonitronik TH CMOS ADSR
• Hexinverter Galilean Moons
• NLC ADSR312 ADSR

Given EGs 1 & 2 are quite well specified, these should be simple, low cost, reliable and not too wide. The width crosses the Fonitronik of  the list.

I really like the mix between linear and exponential curves on the Befaco, but for some reason they don’t sell the PCBs and the kit is over 100 euros, so this misses out.

The NLC knobs look tightly spaced (AS and DR knobs), and given the other product had a weirdly angled PCB, I’m not confident enough to go with this.

That leaves the Hexinverter Galilean Moons and the AI Synthesis AI003.

The AI003 is basic, whilst the Galilean Moons is more complex and includes 2 VCAs. A wise man once said you can never have too many VCAs! The AI is US$15 per PCB, so US$30, where as the Moons has two EGs and VCAs for US$50 (PCBs and ICs). However the Mouser cart is AU$106.

So I’m going to do something I haven’t done before in this build, and call it a draw! What does that mean? Basically, I’m going to hold off selecting EGs 3 & 4 until I’ve selected more modules. I think this will provide me with a better idea of what I need and budget.

In my next post I’ll be selecting the filters! In the mean time here’s what my synthesizer is looking like with it’s native panels.

DIY Modular Synthesizer Preview