> Ah! So it's actually partly due to the fact that I use the square
> wave as a gate, and I can't get a square wave from the ADSR

It depends what you call a square wave. Even the rectangle output of an LFO
or VCO has a limited rise/fall time (i.e. the voltage jump takes always a
limited time of some microseconds, it's never really zero). An ADSR with
shortest time settings (e.g. an A-140 with attack and release set to zero
and switch to high range) may generate a signal close to an ideal rectangle.
But then it's no longer something you would call an envelope because the
rise and fall times are very short. In this case you could use also the gate
signal that controls the ADSR.

Best wishes
Dieter Doepfer