Doepfer A-142-1

This decay-only envelope generator is great for TB-303 style filter sweeps, and absolutely essential for percussive sounds. As the decay time can be affected by a CV input, you can, for instance, make the note's velocity dictate the length of certain aspects of its sound. If you want to wire up your own drum patches, it's time to stock up on A-142s.

The only downside is that it doesn't pay attention to the gate signal on the system bus, but that's what the A-185-1's extra gate outputs are for.