Power amp speaker matching

Discussion in 'Location Recording' started by ste, Aug 7, 2006.

  1. ste

    ste Guest

    Im thinking of gettin a pair of AA151and a pair of AA15HL both 400watts at 8 ohms for FOH and a pair of JBLPro JRX112m 250watts at 8ohms for monitors. what power amps do i need to be looking at?
  2. sheet

    sheet Well-Known Member

    The power ratings on those speakers mean nothing unless you provide sensitivity at 1W/1M. Who knows how they got the power rating? Is it AES or what? Either way, you will not blow your monitors if you power them correctly, provide up to 6dB of headroom, and watch the feedback. With JBL, they use AES I think.

    How much headroom do you need? That will be the big cost factor, as it could double or quadruple the price.

    Not all amps are built alike. It is not about where the amp is made, but the engineering behind it, and the manufacturing quality/tolerances after that. I can show you many US made Crowns that have had major recall and reliability issues, Peavey's that have never had an issue, and foriegn made DJ market amps that will crap on you very quickly.

    Headroom: A monitor placed 1.5M away from the listener, at 110dB, with a sensitivity of 98dB, with 3dB of headroom, requires 71W. But this does not factor in heat and cable length, which eat up about 90% of your power in worst cases, so you should look at an amp around 150W per channel. Double that to 300W if you want over 6dB of headroom.

    Mains 10M away, playing at 110dB, with a sensitivity of 98dB, with 3dB of headroom require 3162W. So obviously you won't be able to use those mains 10M away at 110dB. But you can do 100dB at 30', which would require 316W, then double that for heat and cable, so say 650WPC to be safe.
  3. ste

    ste Guest

    The sensitivity for the JBLs is 99db 1w/1m

    The AA 151 is 98db 1w/1m

    and the AA 15HL is 100db 1w/1m

    So would 3 700watt amps at 4ohms be enough do the ohms have to match up
  4. sheet

    sheet Well-Known Member

    I do not know what the AA speakers are. Are those Architechural Acoustics by Peavey?

    The only time that the impedence is an issues is if you drop it to 2 ohms. There is never a good reason to run an amp that low, as it will get very distorted, and give up the control of the speakers (damping). That said, you do want to keep the impedence the same on both sides of the amp, unless you have an amp that is designed to be a true dual mono, with seperate power supplies, etc. Otherwise the channels share one, and it is not usually good practice.

    Can you mix a 8 ohm speaker with a 4 ohm? Sure.

    Anytime you parrallel speakers you double the wattage and divide the resistance. So 8 ohm 250W + 8 ohm 250W = 4 ohm 500W.

    If you buy 700W amps, or somthing close to it, you will be good to go. You will have plenty of headroom and the ability to expand in the future.
  5. ste

    ste Guest

    AA are adlib audio

    This has been very helpful thanks

    also if i link the monitors then go into one channel off a amp can i do that?
    and then would it be best to link the subs and go into one channel and link the tops or link a sub and a top together for each side?
  6. sheet

    sheet Well-Known Member

    If you can afford it, a pair of amps for your full range cabs amd subs, and then one amp for your two wedges. Since your wedges are 8 ohms, you could run four from one amp at 4 ohms per side and not cause problems.

    You don't want to run subs on one side and mains on another Though you may have power for it, unless your amp is totally discrete, with seperate power supplies and cables, your amp will not function well for very long.

    Good luck.

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