# Safely build an earth to audio ground cable

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?

• The strings are indeed connected to the audio ground, but the battery operated preamp probably doesn't and causes the problem but I can't change this preamp. Apr 10, 2017 at 18:18
• If you touch one terminal of a multimeter to the mixer's audio ground, and one to the ground pin of a properly grounded extension cord, power strip, or power outlet, what readings (AC & DC) do you get? Apr 10, 2017 at 19:31
• Sounds like Efield from the mains, coupling into the guitar pickup, then that charge tries to find a return path, any return path. Current level? 4" by 4" pickup metal region, 1 meter from some mains; C = E0*Er*Area/Distance = 9e-12 * 0.01m^2/1meter ~~ 1e-13 Farad. With clean mains at 220 volts RMS at 50Hz, dV/dt ~~ 80,000 volts/second. Multiply C * dV/dT to get 80 e+3 * 1e-11 = 80e-8 amps or 0.8uA needing a path. Apr 11, 2017 at 5:35
• @RobhercKV5ROB I didn't have a multimeter in the studio. Apr 11, 2017 at 13:03
• @analogsystemsrf so is this normal? I guess that if I use a battery operated device I will always need a earth connection in any point of the path, right? Apr 11, 2017 at 13:05