I have a mixer that use a two poles connection to the mains (double shielded so it doesn't need earth).
If I connect my guitar (with the signal boosted by a battery operated amp) to the mixer I'll get the typical 60Hz noise. My guitar is grounded to my body but the battery operated preamp might lift the ground.
If I touch the mixer audio ground (touching the metal of any audio connector) while I play the guitar I solve the problem.
I also noticed that if at the same time I connect to the mixer other instruments that are not battery operated and that have their power supply connected to the earth I don't need to touch the mixer.
So I concluded that I need at least one connection to earth in my signal circuit. But using those additional instruments sounds like an overkill.
I was wondering whether I could build a cable that connects the mains earth to the audio ground of the mixer. Is it safe? Basically I would disconnect the live and ground of a normal male mains cable, and connect the other side to the ground of an audio cable. Is it safe?