LEVEL MAINTENANCE If you have done all the preparatory setup work during a sound check, you should not have to ride the channel levels very much. The only levels you should need to change would be instrument solos, backup vocals and seldom-used channels that are kept shut off until needed. Lead vocals may need slight level adjustments if the vocalist has a habit of fading back. If they persist in doing this, a possible solution may be to turn that individual down through the monitors, a trick which should cause them to compensate by singing louder and/or getting closer to the mic (just remember to reward them by bringing their monitor level back up).
MONITOR LEVELS Channel monitor level settings will require some adjustments as well, and roughly the same ones as above. Keep in mind that pre-fader, pre-EQ monitor sends represent an independant mix. So, for example, when the harmonica player is ready to play, you will have to bring that mic channel's level and monitor level up - ditto for horn mic channels, acoustic guitar mic channels, etc. which would normally be left off until needed.
FATIGUE During the job, you will probably suffer from symptoms of hearing fatigue to some degree. Most common amoung these is loss of high-frequency sensitivity, although low-frequency hearing can be affected as well. Since there are no other symptoms such as discomfort or ringing in the ears, people often assume that the speaker system has lost some high end - that the horns or tweeters have somehow run out of "steam". Their next reaction is often to boost the high-frequency EQ or the HF gain on the crossover. This is NOT a good idea.
Too often there are customers who have just come in from somewhere else and their hearing is not fatigued. As a result, they hear an unpleasantly bright sound which, by then, is probably extra loud as well. Additionally, there is an increased risk of high-frequency feedback and, should you need to boost the FOH level significantly later on, the highs will be the first to run out of headroom and distort, possibly damaging horns and/or tweeters. You're better off to leave the EQ or crossover alone. Your hearing should normalize by the next day. Earplugs are a worthwhile investment.
DEALING WITH FEEDBACK Assuming everything is done right during the setup and sound check, feedback should not become a problem unless someone gets too close to a speaker with a mic. When that happens, you can probably see which mic it is and know which channel's monitor or PA level to turn down. If it's too close to a monitor, turn down the channel's monitor send level and if it's too close to the FOH speakers pull down the channel's level fader. However, if it's not immediately obvious which mic is responsible, try the following;
(A) If you have set the input channel gains high enough for there to be some clip light activity - see SETUP section 3 item (a) - the light on the channel which is feeding back should be brighter and on more steadilly than it was before. Scan the Clip LED's and turn down that channel's monitor send level. If it's a currently unused mic whose channel should be shut off, eg. a harmonica mic or acoustic guitar mic (did you do an "oops" and leave it on?), any clip light action at all would be an indicator. In this case turn down the monitor sends and pull down the channel fader. But, if you didn't set the gains high enough to use the clip lights, you'll have to make your best guess. Some "usual suspects" include:
(C) Now it is important to find a quick remedy which will let you get the main or monitor levels back up to where they should be. First, go to your main or monitor EQ. Pull down a few of the sliders slightly (-3dB) in the frequency range which your ears tell you is likely to be the right one. Now ease up the master. If the feedback starts again, lower the master a little, re-centre the EQ faders you just pulled down and try pulling down some other frequencies then bringing the master back up. Eventually, and hopefully soon, you'll have it under control, but now the main or monitor system frequency response has been altered and probably doesn't sound right. Try carefully pushing some of the EQ sliders back up towards centre position - you need to normalize the EQ as much as possible. DO NOT PULL DOWN ALL THE EQ FADERS AT ONCE. That would be about the same thing as lowering the mixer masters, only much more time-consuming and it might even cause new feedback problems later on.
(D) The problem remains that a mic and a speaker have decided to feed back. You still need to find these two culprits then re-position them or insert an EQ in that mic's channel in order to solve the problem properly. This can be done later, but it needs to be done.
(E) There may actually be situations where pulling down the main AND monitor masters fails to end the howling completely. Likely suspects would include feedback from a spare electric guitar and amp or an electric/acoustic guitar and amp waiting to be used and mistakenly left on.
