Works but annoying hum

Basically I have to attest to the audio switcher that operation. All devices are connected via RCA with the Umschlater. Therefore adapter jack to RCA or jack / RCA are necessary in most cases.

The device works mechanically with pressure switches on the front. In the first operations they are still a little stiff, over time they have but to operate with normal pressure.

The headphone jack works! Also flawless.

In notebook (Toshiba) everything works great.

The captioned hum (ground loop) occurred with me when I have my Mac connected to the device. Without the switch, it does not exist - in other words the direct connection of the Mac to the Logitech boxes. Exchange of cables and inputs did not change. The use of different circuits brought no change.

I'm just not an electronics / electrical engineers and can make therefore no further information on the actual cause of the problem.

Since I have just bought the switcher for the Mac, now rests with me a piece of "electronic waste". At a purchase price of around 10 euros, the return does not make sense ...