Digisound 80 - Dual EG modification
One of the nice features of the Digisound 80 Dual EG module is its 3 envelope modes: Auto, Normal and Damped. This functionality was optional and the module I acquired was built to only operate in Normal mode. So of course I had to add the extra components to enable the other 2 modes.
Unfortunately the addition of this extra circuitry creates a problem when operating in Normal mode. This problem is documented in the build instructions and it states that 'some users' have experienced gating problems when gating the unit with less than 15V. I would think this is more likely 'all users'! Since I wanted to interconnect my Digisound 80 modules with MOTM and other modules I needed the thing to trigger properly even with 5V gates.
The build instructions do suggest a sort of fix for this problem, but with this fix the module will only trigger with gate voltages above 9V - no good for interconnection with many other modular systems.
So here's the mod ...
The Gate in sockets of each EG section need to be connected to J1 and J5. J2 and J6 need to be connected to the common terminals of the Mode Select switches (where the Gate in sockets were connected to). And 15V and 0V power needs to be supplied to J3 and J4.
This modification has 3 benefits:
(a) The EGs are now stable when triggered by anything from 3V to 15V.
(b) The EGs are now properly protected against negative gate signals - the germanium diodes D1 in the original circuit are only really effective protection when driven by a module with a fairly high output impedance: driving negative voltages from a module with low output impedance would very likely damage D1 and probably the CEM3310 and/or LM358.
(c) The input impedance is now significantly higher at about 30K. The original circuit input impedance is about 4K7 which can cause significant loading of the driving module, especially if more than 1 EG is driven from the same output.
The 2 drill holes are spaced so the board may be mounted on the edge of the Digisound 80 PCB with a couple of M3 mounting pillars: