My “Cute Little Eurorack” is shown elsewhere here on my blog. It was OK, but needed a few simple features just for testing modules during development: Signal mixer, VCA and a fixed-ADSR-envelope generator. I made it, and you can see it below. When placed in the mini-rack it is connected to the builtin amplifier and loudspeaker, but it can also be used in the standard Eurorack.
The 3-input mixer consists of three potentiometers with a common sum-amplifier. Very simple.
Very simple “VCAs” are coupled directly in series with each potentiometer. You can hardly call them VCAs, since they consist of homemade VACTROLs, which act more like a “soft” gate. One VACTROL is coupled directly to GATE on the bus, another is coupled to frontpanel input, and the last is coupled to an envelope generator.
The envelope generator is more interesting. It is built around only one HEF40098 hex inverter, and one darlington transistor as active elements. It delivers a fine ADSR-signal, as you can see in the top picture. The four parameters can be controlled this way: Three 1 Mohm potentiometers for A, D and R-timing and one 10 kohm potentiometer for S-sustain-level. In the prototype shown, the timing is with fixed resistors, and level is a trimpot, since this is just for testing. The whole circuit is very small, as you can see on the picture below:
The envelope generator-principles are showed here below. It works so well that I will use it in other and more demanding applications:
This is how the VACTROL reacts to the envelope…. not very good, but still OK for testing.
The complete diagram can be seen here: 3-input, mix-gate-ADSR