I just bought a project essential usb turntable (300 euros) and I have a technical problem:

The turntable has a built-in preamp, I use it connected to a line input of the mixer: everything works, but I noticed the classic hum of the ground wire unconnected.

My mixer has no phono input at all, so it does not have the classic “Ground” screw. I tried to loose a screw of the mixer and connect ground wire to it, nothing changes and the buzz remains. Someone recommended to connect the ground wire to the ground pin of the electrical plug, but i dont think is a good idea.

So, I really dont know where to connect ground wire of turntable to get rid of the hum: any advices?

Spec of turntable:
http://www.project-audio.com/main.php?prod=essential2phonousb&cat=turntables&lang=en