Once the mic starts feeding back then you end up looping that and you end up with way too much feedback sound.
You'll need to reduce the gain on your mic so that it won't feedback either in the mains or monitors. Lots of people use the same setup with good results. You are going to have to do some experimenting to get the sound under control. There isn't much a sound man can do because you are sending him the signal from the looper and it's already feeding back from that point. If he uses eq or turns the channel down he'll also turn your guitar down and eq the guitar. Don't have the mic gain up so high that the mic is super sensitive and picks up everything. You just want the instrument (voice, acoustic instrument) to come through the mic.
Good luck.
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