So, I am a pastor, not a sound engineer. We were given a portable PA system, and I am trying to figure out why it is hooked up the way it is. The system consists of a Soundcraft 16-channel mixer, a Peavey EQ 31 and Peavey EQ 215, a Behringer CX 2310 crossover amp and a Peavey DPC 1000 amp. The main output from the mixer goes into the EQ 31, then into the Behringer amp, and then into one side of the Peavey amp. Since we are using basic pa speakers, with no subwoofer, is there really a point to the crossover amp being in the mix?
If your stage will vibrate due to sound system output (speakers on stage) and the mikes don't come with 200 hz cut built in (SM58 and SM57 have it), you might build up a resonant low frequency vibration that could blow up amps & speakers. If the Behringer crossover amp will roll those off and the mikes won't then using it might help.
If the soundcrafter mixer has a graphic equalizer built in, or a low cut function built in for the mike channels, then a second low rolloff would IMHO be over kill. Depends how floppy your stage is; some bars are terrible. Churches tend more to have brick back and side walls which cause audible howls, which blow up your ears instead of the amp. A graphic equalizer in the hands of a good sound man is the low cost solution for that. Else, rich churches buy a howl filter box with a computer in it, $$$$, which just kills the one frequency where feedback is building up.
If the soundcrafter mixer has a graphic equalizer built in, or a low cut function built in for the mike channels, then a second low rolloff would IMHO be over kill. Depends how floppy your stage is; some bars are terrible. Churches tend more to have brick back and side walls which cause audible howls, which blow up your ears instead of the amp. A graphic equalizer in the hands of a good sound man is the low cost solution for that. Else, rich churches buy a howl filter box with a computer in it, $$$$, which just kills the one frequency where feedback is building up.
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I can see a few things that aren't quite grounded in reality here, and I'd like to clear them up:
- SM58/7s start rolling off around 120Hz, although the proximity effect means they can pick up very low frequencies under close-mic'd conditions.
- You'll notice feedback occurring before it simply "blows up amps & speakers". Different stages resonate at different frequencies. Sometimes a "floppy" stage is better - that lowers the resonant frequency to below the passband of the PA system.
- It's best to apply channel processing to the channels that need it, than to put a 120Hz highpass filter on the desk output.
- Feedback will quite happily kill speakers as well as hurt ears. It's a sine-wave output which can quickly build in level until something clips.
- Rich churches have a sound engineer that knows what they're doing. Feedback destroyers aren't much use in live sound - move a mic a few inches, and the feedback frequency will change. If someone puts on a hat, you'll get a new set of feedback frequencies.
A feedback destroyer will also attempt to kill sustained notes, such as (intentional) feedback from an electric guitar.
To the OP,
If you're happy with a mono PA system, then I'd run it as follows:
Mixer main output into Ch1 on Behringer crossover. Set the crossover for 50Hz, and run the High output to Ch1 on the amplifier, and then to the main PA speakers.
Mixer monitor output to Ch2 on the Behringer crossover. Start with the crossover at 50Hz, take the High output to Ch2 on the amplifier, and then on to the stage monitor(s).
You might find that having the monitors receiving signals down to 50Hz means they're contributing quite a lot of "mud" on-stage. Taking the cutoff frequency up to 100Hz or even 150Hz will help to alleviate that.
I'm not sure which mixing desk you're using, but, if you can, switch on the highpass filters on any vocal channels. That will cut down on plosives. That's where the letter P or B makes the speakers go "boom" from the air movement at the mic.
I tend to cut down on the bass (and lower midrange) on all vocal mics to counteract the lower-midrange buildup from the proximity effect. 6-10dB off in the 150Hz region is usually enough to get a nice clear vocal sound when the mics are used up close. Of course, if the singer moves further away they'll sound a bit thin. There are mics that can counteract that, but they're not cheap.
Chris
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