The sockets used by Doepfer aren't standard 3.5mm mono sockets (where no
real standard exists), but special sockets, where the ground contact is
more raised to the place, where on a stereo(=headphone) jack the "ring"
contact is. This is no problem with mono jack connectors, but can cause
unpredictable results with stereo jacks - especially as the 3.5mm or
1/8" jack/socket system is not really normed.

Florian