I do a lot of live sound for small venues, and 'student' & 'indie' club nights - some of the techinques for getting a good sound can differ from the norm, used in larger theatres and the like.
A club I regularly engineer in has a ridiculously small stage, which can make seperation/feedback avoidance tricky. Here's some thoughts:
Know your mics' characteristics & polar patterns. Make sure their null points are pointing at the monitor wedges, for max. feedback rejection.
Sounds rudimentary? How often do you see an SM58 on a boom, with the mic horizontal in front of the singer's mouth? An SM58 is cardioid and has its null at 180 degrees - if horizontal, the null cant possibly be facing the monitor! A horizontal mic will work better with a Beta 57 or Beta 58 - these are hypercardioid, and have nulls at 120 & 240 degrees, or threrabouts.
Educate the singers - I have a chat with them beforehand & tell them "Keep the tail (cable) of the mic pointing towards the monitor - it's less likely to feed back, and I can give you loads more monitoring level". Normally, they appreciate this and trust you a whole lot more.
Mixing desk: Start with the LPF (sub filter) switched in on all channels. Only switch it out again if the instrument actually needs reinforcement down below 100Hz. This leads to a cleaner sound overall, and conserves some amp power...
Use compressors on channels to prevent level overshoots; avoid using makeup gain, as this only increases the chances of feedback.
Cut, rather than boost with EQ. I normally start with cutting a bit of the lowest band, to compensate excessive proximity effect. Following that, a sweep through the lower-mid band to kill any resonances or boxiness... increases clarity no end.
Avoid using loads of reverb. It generally doesn't help, and it can be suprising how good a 'dry' (or dry-ish) mix can sound, especially in a small venue.
That's all for now - hard to add much as so many good tips have been given already.
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