Feedback is probably the biggest fear for any live mixing engineer (there's a song in there), and here are 4 tips on helping you eliminate feedback before it happens and get you fired.
1. Make sure that you have gates on your toms and that they are set correctly, this will get rid of that nasty bottom-end feedback.
2. Using high-pass filters (or low-cut if you prefer) on the vocal microphones. You can filter everything below 90Hz and sometimes go as high as 160Hz if it's a big sound system.
3. Before turning up the gain because you aren't hearing a specific instrument, first check if the microphone hasn't accidentally been moved. this happens a lot with loud and energetic live acts - specifically the guitarists! ;-)
4. Watch out when using overly bright reverbs - a lot of sound engineers like using bright reverbs on vocals as it cuts through the mix without having to drench the vocals in reverb, but sometimes the actual high frequencies in the reverb can be what's triggering the feedback.
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