All about it
The MM312 Envelope Generator is a classic all-discrete ADSR envelope generator ideal for sequencing or manual gating.
Yes, there is no voltage control over response times or levels - but its controls have a musical, natural-feeling response that rewards hands-on real-time tweaking.
Also, unlike the AR generators with simple on-off gating that litter the modern modular landscape, its independent dual trigger and gate inputs can add something different to generative patches by allowing it to respond to the intersection of independent clock sources. In a nutshell, the envelope requires a rising edge from one of the trigger inputs and a voltage above 2V at either (or both) gate input to start, and can be retriggered once the envelope has passed the Attack phase.
The two trigger inputs are diode-mixed together, as are the two gate inputs, meaning that a steady high input to one trigger input can be used to inhibit the other (effectively a logical XOR) and the gate will be active if either of the gate inputs are active (a logical OR).
The output is sent via a 1k protection resistor to an internal 4-way patchbay. There is also a bonus 4-way patchbay which can be connected to give 6 outputs in total, or can be used to distribute other signals in your system.
The original circuit betrays its kinship with the ARP2600 by being quite fast - starting with its minimum 3 millisecond rise and fall times. Like the 2600's envelopes its maximum settings can still be a bit abrupt for evolving textures, so we have added a switch which can slow the response ranges by a factor of 5.
We have added a blinkenlight to the Manual Gate pushbutton to show output status. It uses a low-current high-efficiency LED and a constant-current (and temperature compensated!) driver circuit to minimise any bleedthrough to supply or ground rails.
One thing to note is that the end point of the envelope generator is a very slightly negative voltage-this is intrinsic to the output buffer design and would have been intended as a way of preventing signal bleed in VCAs built with 1970s components which may have not been perfectly "nulled".
The original, being designed for a closed system, expected 10V gates and fast-rising triggers which are hard to find in modern Eurorack synths, as well as diode-coupled square wave sources which could be passively "OR-mixed". We have made some minor tweaks to the gate input buffer to reduce the gate input threshold down to about 2V, and have put low-drop Schottky diodes on all gate and trigger inputs to enable passive OR-mixing with any source.
Last but definitely not least, the output of the gate input buffer has been normalised via protection resistor to the TRIG 1 jack, which means that a +5V gate will fire the whole system if you crave "standard" behaviour. Indeed, a +/-5V triangle wave of 1Hz or faster into a gate jack should suffice to start the envelope this way. Plugging a jack into TRIG 1 will defeat the normalisation and return you to the original's reliance on both a gate and trigger to fire.
+5V: Not required
Eurorack standard, 10HP width
38mm depth below mounting rails with connected power cable
In advanced prototyping - release date may depend on SMT respin of existing design. Probable release mid 2017
Watch this space