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Putting together medium PA, a few questions...

Discussion in 'Live Sound [BG]' started by bigblondeafro87, Mar 12, 2009.


  1. Hey all,

    So me and my band have been getting the pieces for a PA together over the past two semesters at college.

    Currently our mains are B-52 LX1515s (4 ohms each, 800 watt power handling), I know these aren't very popular, but its what we have. Down the road we will add subs and possibly replace the mains, but for the present I feel these should be sufficient. Do any of you feel otherwise?

    My friend is buying a Mackie 12 channel mixer and speaker cables from a buddy of his, so we're set there.


    Now for the current issue:

    I just got in touch with a guy who has a Crate SPA 2000 power amp that is rated at 750 watts per channel at 8 Ohms and 1150 watts per channel at 4 Ohms. Is this a decent power amp? I couldn't find any reviews on it, so I am a little wary. Also, if I'm running two 4 ohm cabs from it in stereo, it is running at 4 ohms, correct?

    If this isn't a good idea, I know where I can get a QSC 2450 for rather cheap, but it has less power than the crate.


    I know some of you may think that this is a large PA, but believe me I have looked at the alternatives (renting) and really thought about it. I like the idea of having our own equipment to set up where we please.

    Thanks
    -Paul
     
  2. First I would stear clear of the Mackie. Alot of their mixers have quality issues. Next 800 watts power handling, I would not push that much power into those speakers. The QSC amps are solid.
     
  3. Correct me if I'm wrong, but as long as I don't crank the amp gain to full blast, I shouldn't have a problem, right? And I've heard running your system like this gives you plenty of headroom, so you don't have to worry about the amp clipping. Also, I believe that the 800 watts is continuous power handling, not peak.
     
  4. The QSC amp will not only give you better sound but will be more consistent. The control on the amp is not a volume switch. It is recommended to run your speakers at 1.5 times the RMS rating. If your speakers are 800 continuous then they are only 400 rms. I would not put more than 600 watts into those cabs. You will be surprised at how loud 600 watts is. Read this:

    Clipping, in itself, does not cause loudspeaker drivers or crossover components to heat up any more than unclipped signals do. Dynamic loudspeaker drivers are <<10% efficient, so they heat up when power is applied regardless of whether it's clean or distorted. Over 90% of the power in the signal turns into heat and not acoustical energy.

    When an amp clips, it puts out power in excess of its rating. With severe clipping, it may put out much, much more power than its rating would indicate. Therefore, an amp rated at, say, 400 watts into 8&#937; could be a safe pairing with 8&#937; loudspeakers rated at, say, 500 watts continuous &#8230; but only if you're careful to avoid significant clipping. If you drive the amp into heavy clipping, you could easily cause it to put out greater than 500 watts for periods of time, possibly causing the loudspeaker to fail. Keep in mind, though, that you could also destroy the loudspeakers with the same amount of power from a more powerful amp that doesn't clip.

    In short: It's not the clipping, it's the power.

    Clipping distortion causes additional harmonics in the signal, and that may endanger tweeters and compression drivers. But clipping also tends to partially obliterate HF signals that ride on the usually larger LF signals, so the net power delivered to the HF driver by a passive crossover may be something of a wash, depending on the program material.
     
  5. Ok thanks a bunch, I'll skip out on the crate amp for now and just get the QSC.

    So our set up will be the two speakers, a QSC 2450, and a Yamaha (not Mackie) 12 channel mixer. Is there any other items you'd see as essential for a decent sound in this set up?
     
  6. You will need and eq. A good eq is the DBX 231. They can be found used for about $100-125. Will you be running any monitors? If so,you can run the mians on one channel and the monitors on the other. When you do upgrade the mains,you will already have a good start with the amp and eq. Good luck!
     
  7. Yeah I was planning on buying one, and thankfully a used EQ is very easy on the wallet. On that note I'll probably be picking up a dbx166 for some subtle compression on the whole mix. I am picking up a QSC 2450 tomorrow or early this week, I'll post how it goes.
     
  8. NEWS:

    The PA system (named EVAN 9) has made its first noise. The setup as of this moment is a QSC 2450 running two B52 LX1515 through a Yamaha 12 channel mixer. Next step is to buy an eq and a compressor, and after that a set of 12 or 15 inch speakers to be used as monitors with the whole PA, or standalone speakers for small gigs.

    Tomorrow we'll run the whole band through the PA and turn it up to see how it all sounds. One possible problem I ran into is that the tweeters on both speakers were not making any noise, although we were turned down considerably. Could both tweeters be blown?
     

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