It’s a running joke in the live sound world, but lead guitarists often suffer from “LG syndrome”, which involves turning the amp up throughout the soundcheck or gig, resulting in the singer not being heard. You will get mum, dad, managers, and punters all shouting “turn the singer up!” at you – when the issue is often too much guitar.
Somewhat obviously, all the mics onstage will pick up whatever’s in front of them, so if you’ve got a quiet singer, a loud drummer, and a guitarist who turns themselves up to match, you’re in trouble. The more you turn the vocal mic up, the more guitar and cymbals you get through the PA.
The solution: Turn the backline down! Often, the guitarist can’t hear themselves because their amp is on the floor and – news flash! – their ears are not in the back of their knees. Raising the amp on an amp stand, or (more rock ‘n’ roll) a flightcase will help. If that’s not possible, rocking the amp back so the speaker points to the guitarists’ ears may do the trick (amps like the Fender Twin have built-in supports for this reason).
Turning the amps so they’re angled towards the player and away from the audience/FOH, moving the lead singer so they’re further from the amp, or snare drum, or other offending stage noise, can all help keep onstage noise away from the mics – so when you raise the singer’s mic, you’re only raising the vocal.