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Microphone Month 3

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Powering Jbl SRX722

Trumpetguy7

Howdy!
I have a powering question.
I know the 722's are 4 ohm speakers rating 1200/2400/4800w.
I can't imagine our church ever needing to push them(two 722's) past the 1200w.
Here are the Crown XTi ratings:
XTi 1002= Bridged Mono 1400w @ 4 ohm ($500) X2= $1000
XTi 2002= Bridged Mono 2000w @ 4 ohm ($700) X2= $1400
XTi 4002= Stereo 1200w @ 4 ohm ($900)
XTi 6002= Stereo 2100w @ 4 ohm ($2000)
-Using the XTi 4002 in stereo is the cheapest choice. Would it be enough though? We don't run things very loud. I'm picky with protecting people's hearing including my own.
-Using two XTi 1002 in bridge(one per speaker) would be the next cheapest. But is having 1400w enough and is it worth the trouble of adding ANOTHER power amp to the rig?
Are the XTi 2002 and 6002 setups even worth paying the extra money for?
Plus I haven't mentioned the subs yet.
I like the XTi 4002 in stereo the best even though it barely meets requirements because it saves ALOT of money. But will I regret it?
Thanks y'all.
May 18, 2012 @07:41pm
eRoland

Your manual says 1200 continuous and 2400 program. I would go with the 6002 becuase the max output of the 722 is 97db at 1 meter at 1 watt input. The lower power amp would have a hard time reaching higher levels which means the system would not deliver on peaks. Even reaching 90db on loudest peaks like kick or snare punch would be difficult with out pushing amp. The JBL site recomends continuos x 2 so 1200 would need 2400. the weaker amp will put stress on speakers. http://www.jblpro.com/catalog/support/faqgeneralaudio.aspx
You will get sound out of the system with weaker amp but for the best sound and matching of componants the 2000 watt would be best.
Same with sub amp. 2x continuos power rating.
Your 722 also is rated to output 80hz so make sure and cross over high pass around 100 on mains and sub 120 to 130hz low pass this will make amps work less hard and add clairity. You may need to move numbers up and down to find sweet spot for your room. good start point.
May 19, 2012 @04:18am
michaelhoddy

Even reaching 90db on loudest peaks like kick or snare punch would be difficult with out pushing amp.

I'm really not following you here- this isn't true. Explain. Plus, "90 dB" is a meaningless number unless quantified by a frequency, distance, and whether this is in free space or something else.
May 21, 2012 @01:52am
TimmyP1955

A friend is using an iTech 4000. I'd say it's just emough to keep from clipping any of the peaks on most shows - though I'd feel better using a 6000. He uses the iTech 8000 on his 728s, and it's just right.
May 21, 2012 @02:22am
eRoland

Speaker specs say 2400 watt to drive the drivers at their best. The lesser amp can punch it up to the 97 db rated level at one meter I am sure but it would stain the drivers and amp. the manufacturer did the math and suggest a amp that can get to 2400 watts. You can get peaks higher than that from the speaker but if you run at max output continously you need more speaker. In a church you would not need it most likely at this level.
If you use a good pro db meter in most churches (even quieter ones) during song service the people singing, instruments and air conditioners running you could need up to 90 db peaks at ear level in the seats. Depending on distance (20 to 60 feet) and nosie floor you could need all the speakers have at times to get this level. Our church is much loader than most and we hit 100 db at the mix postion at peak most likely average 85 to 90 db. My point is you should get the suggested amp to operate speakers at their best operating conditions and you may be running them closer to max than you realize at times. You would have all the distortion free head room you need most likely.
May 22, 2012 @02:46am
michaelhoddy

2400 watts is simply a spec which achieves 3 dB of headroom over full RMS rated power for transients. There's nothing special about it that makes the drivers "perform" better.
Every doubling of amplifier power is equivalent (in a perfect world) to an increase of 3 dB in output. The 722 is rated at 135 dB (peak) output, and we can deduce a roughly 129 dB RMS output at what I would assume is 1 meter. If you take your 97 dB 1w/1m figure and multiply it upward to the rated power RMS, it's pretty close to the same figure of around 129 dB average.
Using this figure, the sensitivity figure, and the inverse square law, you can then ascertain a worst case scenario of how much amplifier power it will take for a given RMS performance at any point in free space, with the knowledge that the same speaker will be "louder" in a room with walls, floor, and ceiling because of reflections.
The thing that fools people is the last few "doublings" of power to squeeze out those last few decibels of performance. Driving the 722 with only 600 instead of 1200 RMS watts will only result in a 3 dB decrease of output. If you do some quick inverse square law calculations, you realize that this is still adequate to produce roughly 111 dB continuous average of output 52 feet away from the speakers in free space , and all while maintaining 6 dB of headroom for transients and peaks. The calculations are rough and there are always unpredictable factors, but they're in the ballpark. Granted, and this is very important, there's no bandwidth factor, and this doesn't take into account things like musicality, but it does demonstrate that the thing gets plenty loud, if that's the deciding factor.
I'm all for the bigger amp thing, but it doesn't require the biggest amp you can find to produce quite a lot of noise in the average church. If this is a portable rig that's going to see a different venue and different program material every night, you obviously want all the amp you can get.
But if the space is consistent (i.e. this is not a portable setup) and the SPL targets are predictable, you can design a system with plenty of headroom without going ballistic on the amp (and thus the power, wiring, racking, and cooling) specs.
May 22, 2012 @05:07am
Trumpetguy7

This is not for a portable system. It's pretty stationary. If I understood correctly, then I think 2 Crown Xti 4002 bridged is plenty. They offer 3000watts plus the useable DSP. But then again the Xti 2002 offers 2000 watts bridged and after reading what HODDY had to say I think even that would be plenty. Even though I don't have a measuring tool, I'm confident we never go over 100db. I like keeping things pretty quiet.
I think it's always better to have room just incase without over doing it.
We'll probably go with the 4002 since that's a safer choice incase we ever rent our place for events.
Thanks a lot for your help guys. I'm confident that this system is going to sound very professional and if it doesn't, then the problem is probably us and not the gear! haha!
WAIT! so if I get the 4002 and run it stereo, which is 1200 watts per channel, then what would the output db be? And would it be enough for a good sound? Not a loud sound.....just a good quality sound?
May 23, 2012 @01:34am
jfinnigan

WAIT! so if I get the 4002 and run it stereo, which is 1200 watts per channel, then what would the output db be? And would it be enough for a good sound? Not a loud sound.....just a good quality sound?

i think it'll give you good quality, my friend used it before and told me that
Sound quality doesn't improve with higher wattage output on the amp, higher SPL before peak is possible. 3db increase when you double your wattage - the reason you usually get amps rated higher than your speakers is to prevent an amp signal spike or distortion from damaging your speaker, if you don't have any distortion at the volumes you are running you're unlikely to damage them but you always want to give yourself some headroom before peak.
December 9, 2013 @09:38pm