Just to reinforce something that Dan said, make sure all your mains earths are connected.
Some suicidal guitarists disconnect the mains earth of their amps in a futile attempt to
reduce hum. It's not unheard of for similarly stupid people to do the same with mixers.
Tha mains earth is there to protect you and should NEVER be disconnected. EVER.
should prevent the smell of burning flesh when stepping up to sing your backing vocals.
There's nothing we can't face... Except for bunnies