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. Commented 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? Commented 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. Commented Apr 11, 2017 at 5:35
• @RobhercKV5ROB I didn't have a multimeter in the studio. Commented 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? Commented Apr 11, 2017 at 13:05

1 Answer

Answering to my own question after working a bit on it.

I created a cable that connects the mains earth to the audio ground of a jack. To be safe I removed the fuse in the (UK) mains plug and completely isolated the live and ground from the plug. There is no way that a wrong connection can happen, even if the cable is shorted. Also I completely isolated the tip of the audio plug to avoid other wrong connections.

This cable correctly ground audio equipment and I realised that this is safe. This is particularly useful if using equipment with lifted ground (audio players, laptop etc), when you need to ensure that at least one path to the earth is established.