Engineering a live show presents a new list of problems. One of which is the problem of feedback. When bad microphone placement and reflective cement walls aren’t helping your mix there are a few things you can check and adjust. Make sure that your vocal microphones are job specific. In this case you will want a cardioid microphone for vocal performers. This type of microphone has a hot spot and a blind spot. You can aim the blind spot directly at the stage monitors. The cardioid pattern has plenty of coverage for a vocalist while ignoring loud sound levels from the stage speaker monitor in front of the vocalist. In some cases the band and vocalist will wear light weight headphones for monitors and these headsets also can house a small microphone. Usually dance oriented performances with singing will utilize this equipment configurations for live pop/dance performances so that the performers can entertain the audience hands free.
If you have this set up correctly then you should have more control over the levels. Check for any uni-directional microphones or other types of overly sensitive microphones that may be placed near other instruments. You can also make the performer aware that there are loudspeakers pointed at them during the performance. This will alert them that they can cause unwanted feedback during the performance so that they may take precautions. Now check your levels on your mixer for any tracks that may be clipping and reduce their respective volume sliders accordingly. Make sure the sum of all console tracks are below clipping on the master channel. Set all levels lower than needed at first.
Now check your amp racks. I usually set the amps at 70 to 80% for initial testing and then ramp them up as needed during a show. Try not to go past 95% on the amps. At this point you should be cooking nice with a fair amount of noise floor in the system. This is normal and after all we are only looking for feedback points. Have the program audio start or ask the band to play and make sure the levels are reasonable. If everything checks out you should be loud and clean with the power without causing any feedback. A lot of engineers miss the cardioid microphone trick if they don’t do their homework or if the band brings their own cheap microphones to the show.