I have been using a DBX DrivePro, which is (all in 1 rack space):
a.) Feedback eliminator
b.) Electronic crossover
c.) multi-band compressor
d.) sub-harmonic synthesizer
e.) graphic eq/realtime analyzer (aka "Acoustic consultant in a box")
If feedback is a problem, there are many causes:
a.) Room acoustics
b.) Speaker/mic placement
c.) mic, speaker, and other electronic nonlinearities.
The RTA in the drive rack is fully automatic and will straighten out any problems in the linearity of your PA as well as compensate for most problems in the room acoustics. It will do a better job of setting the eq than any human can and it will do it in about a minute.
This alone will cut your chances of feedback way down. From my experience, doing ANY EQ without tuning the room is pointless.
The feedback eliminator wizard will then walk you through turning up the microphones. As you reach feedback, the device will recognize the frequency and insert a notch filter at that frequency that can be as narrow as 1/80th of an octave. This is virtually inaudible compared with the butchery that occurs with hand tweeking even the finest graphic EQ. As you continue to raise the levels, the device will set up to 12 filters as new feedback modes are reached. These filters can either be permanent or floating, or any combination of the two. Floating filters will catch new feedback modes that show up when:
a.) the audience fills up the room
b.) the lead singer starts stage diving without taking off her headset mic
c.) drunken slam dancers knock down your PA.
You can make things louder by turning up the PA or you can make them SEEM louder by playing with people's heads. Any half-sober roadie can do the former which, of course increases the chances of feedback. DBX's sub-harmonic synthesizer does the later. By synthesizing the natural harmonics of the lo frequencies and adding them subtly back into the mix, the audience will be fooled into thinking you have a much bigger PA--all without increasing the gain.
The Drive Rack may seem pricey, but when you consider all the gear it replaces, all the problems it solves, and its bonehead simplicity, it is quite the bargain. I won't spec a PA without one, and I keep one in my traveling kit in case I have to mix on an unfamiliar rig. I have even been known to drag it out in a studio if I am suspicious of someone's monitors. (i.e. if I notice a graphic eq in the monitor chain).
It is also much easier to use than the description above would suggest. While it does suffer from cellphone syndrome (you have to punch your way through a bunch of menus on a tiny little display) the built-in wizards walk you through the process and the logic in the box really does all the heavy lifting. In fact, you may want to hunch over the console while it tunes the room and pretend to be twisting knobs and turning faders so no one finds out how easy it really is.
