How much power do I need for my PA?

ttf_gregs70 · Post by **ttf_gregs70** » Thu Nov 10, 2016 8:17 pm

Been doing a lot of google researching and finding contradictory information and nothing that addresses my particular need. I ended up putting together a PA system for the big band I play in - we had small outdoor gigs coming and no system. I am currently running a pair of Peavey PR15 speakers and a QSC GX-5 power amp. The amp is 500 watts per channel into 8 ohms. The speakers are 8 ohms, rated at 400 wpc program, 800 wpc peak. However, efficiency is high, 97 dB. I can find formulas based on rated watts vs. rated watts, but none seem to take into account efficiency. Not blowing down the house, just running overhead mikes to amplify brass and woodwind sections and Shure SM58s for the vocalists. Bass and keyboardists bring their own amps, and the drummer needs no help. I have a chance to get a Class D Behringer power amp that is 300 wpc 8 ohms and weighs 7 lbs vs. the 28 lbs for the GSC and was thinking of switching amps to save my poor aching back. I already sold my original wood cabinet speakers in favor of the Peaveys as they were a lot lighter and easier to put up on the speaker stands. Is the 300 watts the Behringer puts out enough power per speaker, or should I just keep my GSC and hire a gorilla to schlep equipment around?

ttf_Dan Hine · Post by **ttf_Dan Hine** » Thu Nov 10, 2016 8:36 pm

The Behringer would probably be ok given you have pretty efficient speakers. You gain a little headroom from the QSC but not really any more output. That being said, I'd prefer to keep the QSC. You already have it and, in my experience, QSC makes better gear than Behringer. Why spend money on something you don't need which will also be of lower quality?

ttf_gregs70 · Post by **ttf_gregs70** » Fri Nov 11, 2016 12:45 pm

Quote from: Dan Hine on Nov 10, 2016, 08:36PMThe Behringer would probably be ok given you have pretty efficient speakers. You gain a little headroom from the QSC but not really any more output. That being said, I'd prefer to keep the QSC. You already have it and, in my experience, QSC makes better gear than Behringer. Why spend money on something you don't need which will also be of lower quality?

Only to save my 59-year-old body, and i can probably make about $50 after buying the Behringer and selling the QSC.

ttf_sonicsilver · Post by **ttf_sonicsilver** » Fri Nov 11, 2016 12:47 pm

1.21 gigawatts surely?

Oh no, that's for something else.

ttf_gregs70 · Post by **ttf_gregs70** » Fri Nov 11, 2016 4:17 pm

Quote from: sonicsilver on Nov 11, 2016, 12:47PM1.21 gigawatts surely?

Oh no, that's for something else.

No flux capacitor in my equipment, more's the pity...

ttf_BillO · Post by **ttf_BillO** » Sat Nov 12, 2016 12:21 pm

Quote from: gregs70 on Nov 11, 2016, 04:17PMNo flux capacitor in my equipment, more's the pity...
You're probably not running the current equipment at 500W in any case. Even if you were running things close to clipping, 300W is not that much quieter to human ears than 500W. Only about 2.5 dB.

ttf_gregs70 · Post by **ttf_gregs70** » Sat Nov 12, 2016 4:39 pm

Quote from: BillO on Nov 12, 2016, 12:21PMYou're probably not running the current equipment at 500W in any case. Even if you were running things close to clipping, 300W is not that much quieter to human ears than 500W. Only about 2.5 dB.

Yep, if I were playing heavy metal music I would not consider the change. The only heavy metal I play is my Holton Tr180...

ttf_anonymous · Post by **ttf_anonymous** » Sun Nov 13, 2016 5:36 am

Quote from: gregs70 on Nov 10, 2016, 08:17PMBeen doing a lot of google researching and finding contradictory information and nothing that addresses my particular need. I ended up putting together a PA system for the big band I play in - we had small outdoor gigs coming and no system. I am currently running a pair of Peavey PR15 speakers and a QSC GX-5 power amp. The amp is 500 watts per channel into 8 ohms. The speakers are 8 ohms, rated at 400 wpc program, 800 wpc peak. However, efficiency is high, 97 dB. I can find formulas based on rated watts vs. rated watts, but none seem to take into account efficiency. Not blowing down the house, just running overhead mikes to amplify brass and woodwind sections and Shure SM58s for the vocalists. Bass and keyboardists bring their own amps, and the drummer needs no help. I have a chance to get a Class D Behringer power amp that is 300 wpc 8 ohms and weighs 7 lbs vs. the 28 lbs for the GSC and was thinking of switching amps to save my poor aching back. I already sold my original wood cabinet speakers in favor of the Peaveys as they were a lot lighter and easier to put up on the speaker stands. Is the 300 watts the Behringer puts out enough power per speaker, or should I just keep my GSC and hire a gorilla to schlep equipment around?

The QSC is a better amp than the Behringer. The only amp Behringer makes that is worth looking at is EP line, and they are heavy, and a copy of the QSC. If you are looking to get lighter, look for a used QSC PLX amp, or perhaps if you can find a used Carver or Peavey digital conversion amp (or some term like that, I cannot remember it). Get a case for the amp (perhaps mixer as well) and protect them and make the carrying of them easier. Audiopile.net is a great place to get good, inexpensive cases.

You would be better served by selling the Peavey speakers you have and get a set of cabinets with 12" woofers. The clarity of your vocals will increase dramatically.

Good luck!

Doug

ttf_gregs70 · Post by **ttf_gregs70** » Sun Nov 13, 2016 5:19 pm

Quote from: dougm on Nov 13, 2016, 05:36AMThe QSC is a better amp than the Behringer. The only amp Behringer makes that is worth looking at is EP line, and they are heavy, and a copy of the QSC. If you are looking to get lighter, look for a used QSC PLX amp, or perhaps if you can find a used Carver or Peavey digital conversion amp (or some term like that, I cannot remember it). Get a case for the amp (perhaps mixer as well) and protect them and make the carrying of them easier. Audiopile.net is a great place to get good, inexpensive cases.

You would be better served by selling the Peavey speakers you have and get a set of cabinets with 12" woofers. The clarity of your vocals will increase dramatically.

Good luck!

Doug

Thanks for the input. Yep, have it in a good SKB hard case. I decided against the Behringer even though the price and weight are right - read that they are not reliable, a fact proven by our vocalist who has had the keyboard amp he uses for his vocals rebuilt twice already.

ttf_cmillar · Post by **ttf_cmillar** » Thu Dec 08, 2016 6:01 pm

Go for the power...the OSC is putting out more watts per side, so I'd go with that.

Rule of thumb is to double the sustained wattage intake of your speaker if you can. (ie: if speaker is rated for 500 watts continuous, then you could give it 1000 watts of power per speaker.)

You have to shop around for the best amp deal, as more power is more money.

But worth it in the long run! Get quality power amps for sure.

ttf_Piano man · Post by **ttf_Piano man** » Fri Dec 09, 2016 12:34 am

The key is that you're playing outdoors. Outdoor sound reinforcement is very challenging. You don't want to give up power, period. A small amp will disappear pretty quickly outside and you'll end up clipping trying to get it loud enough. Two-and-a-half db is a lot when people are shouting, "We can't hear the singer!" It helps a lot if there's a bandshell or other backing. Moving the speakers back in the space can help if you can do it without feedback or unwanted resonance.

Likewise, speakers. As was pointed out, vocals generally sound nicer through cabs with 12" speakers and horns, but they'll sound tinny outside. Ideally you'd use maybe 12s with horns along with subwoofers, but that gets into more lugging.

There isn't a small, lightweight solution that I know of. You have to have some power, and you have to move some air. My personal sound reinforcement for keys is a thousand-watt 10" two-way QSC speaker, but outdoors or in a medium to large room I'd always run through the mains. It wouldn't even come close by itself.

ttf_Piano man · Post by **ttf_Piano man** » Fri Dec 09, 2016 12:36 am

For your application, powered speakers and a small mixer might be worth considering. Make sure you get a mixer with effects for your singer. EV makes some relatively lightweight and affordable powered speakers.

ttf_BillO · Post by **ttf_BillO** » Fri Dec 09, 2016 5:56 am

I think people are forgetting the OP is trying to make a quality of life decision here.

If he's not already moved on, perhaps he could rent a smaller (lighter) amplifier of better quality to see if it gives what he's after before making the final decision. Or another solution might be to use two smaller amplifiers, each one being easier to carry/set-up. With some of the better units, the outputs of the L & R channels can be bridged for more than double the power. As Piano man said, horns direct vocals and mid-range much better than cone speakers do, so moving to a speaker system that utilizes horn mid-range drivers might be more 'efficient' too because they will better direct the sound to your audience. This is important outdoors. BTW, there is an easy and effective way to make LF horn type speaker systems that are more directional than regular speaker units. I'm not sure of commercial availability, but if you're handy, they are not too complicated to build and can be made quite small using 4 x 6" drivers.

If he has back issues to deal with, further aggravation may lead to considerable disability later in life. Why not try to advise him on solution that 'lightens' his load so he can live better with in the long run. Just sayin'.

Quote from: cmillar on Dec 08, 2016, 06:01PM (ie: if speaker is rated for 500 watts continuous, then you could give it 1000 watts of power per speaker.) This is mixing units here, I think (peak vs RMS/continuous). Ideally, using RMS rating on both the speakers and the amplifiers, the speakers should be rated at or higher than the amplifiers for long term reliability. ie; If you have a 500W RMS/ch. amplifier, you should use speakers rated for 500W RMS or higher.

ttf_gregs70 · Post by **ttf_gregs70** » Fri Dec 09, 2016 3:29 pm

Thanks for the input, group! I passed on the other amp and am sticking with my Plastic Fantastic PR-15s and the QSC amp I have already. If I had more than one outdoor big band gig and one car show DJ gig a year I might re-think that, but in the mean time I'll stick with what I got. Yes, I would love a cutting edge system, but I pieced together something that does the job on the cheap. The band isn't complaining - if it weren't for me, they would have nada! The leader kicks me an extra $10 or $20 when I supply the sound; at that rate I'll die well before I break even

ttf_gregs70 · Post by **ttf_gregs70** » Fri Dec 09, 2016 3:29 pm

Thanks for the input, group! I passed on the other amp and am sticking with my Plastic Fantastic PR-15s and the QSC amp I have already. If I had more than one outdoor big band gig and one car show DJ gig a year I might re-think that, but in the mean time I'll stick with what I got. Yes, I would love a cutting edge system, but I pieced together something that does the job on the cheap. The band isn't complaining - if it weren't for me, they would have nada! The leader kicks me an extra $10 or $20 when I supply the sound; at that rate I'll die well before I break even