Each of the 3.5mm sockets has a GND terminal and as soon as a connection is made to external equipment a GND loop may occur.

If the rails are connected to GND/Earth or not is not that important (though they are finally also connected to GND/Earth via the
front panels as soon as modules have been installed).

Best wishes
Dieter Doepfer

> Hello Dieter
>
> On 13.06.16 09:34 , yahoo@... 1 wrote:
> > The hum is probably caused by a ground loop. Due to safety reasons all metal parts of the A-100 cases are connected GND
> AND to earth
> > shield of the mains connector.
>
> Scott wrote
> "No the rails are not connected to ground. This is the way Doepfer built
> the box and I was not advised to change it."
>
> These are LC9 cases. Are those cable in a different way
>
>
> Florian