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Under Powering PA Speakers

Discussion in 'Live Sound [BG]' started by Bart277, Jul 26, 2012.


  1. I am thinking of getting a pair of JBL JRX215 Dual 15's for our front of house. At the moment our front of house amp is only 300 watts per side @ 4 ohms. The JBL's are rated at 500watts - 1200 watts @ 4 ohms.
    My question is will this setup be ok or will I have to get a bigger amp?
     
  2. You won't damage the speakers by under-powering them, but you may or may not be happy with the results of a less powerful amp. Dunno how big a crowd you're playing for.
     
  3. Start there and only concern about "broken" factors
     
  4. uhdinator

    uhdinator

    Apr 20, 2010
    Maine
    100 lbs each and not great sound.
    A pair of prx415m's 100 bucks more but 35lbs. Sound 10 times better.

    And yes you will be underpowered.
    Those things need 750-1000w and have known tweeter failure issues (jrx series)
     
  5. lowfreq33

    lowfreq33

    Jan 27, 2010
    Nashville
    Endorsing Artist: Genz Benz Amplification
    Don't do it. The JRX series JBL's are the worst speakers ever made. You'd be better off getting the equivalent price range from Peavey, or even Behringer.
     
  6. seamonkey

    seamonkey

    Aug 6, 2004
  7. 1) Don't buy a 215 PA cab.

    2) Don't buy a JBL JRX.

    3) DON'T buy a 215 PA cab.

    You're better than that.

    -jb
     
  8. Munjibunga

    Munjibunga Total Hyper-Elite Member Gold Supporting Member

    May 6, 2000
    San Diego (when not at Groom Lake)
    Independent Contractor to Bass San Diego
    I'm lost.
     
  9. Munjibunga

    Munjibunga Total Hyper-Elite Member Gold Supporting Member

    May 6, 2000
    San Diego (when not at Groom Lake)
    Independent Contractor to Bass San Diego
    Agreed. I like 12s for vocals. I do have to say I really like my PRX612Ms.
     
  10. Hactar

    Hactar

    Sep 25, 2011
    Boulder, CO
    This.

    You will be much better off with 2 112's or 115's on sticks with a sub or two in the center. 215's are hard to get up in the air, and simply make things muddy.
     
  11. walterw

    walterw Supportive Fender Gold Supporting Member Commercial User

    Feb 20, 2009
    alpha-music.com
    Too heavy to put on poles, horns are too low when you don't, stack 'em on subs and the bottom 15 loses any reason to even be there.
     
  12. I don't know how this got to be conventional wisdom around here, or what everyone else is doing wrong, but my 2x15 tops sound great.

    The problem with the JRX double 15 isn't that it's a double 15. The problem is that it sucks.

    A triamped stack of two 18" subs, two 15" mid bass drivers and a 2" exit compression driver is the SVT of rock club sound reinforcement. There are roughly a billion venues using that configuration with great sounding shows happening all the time.
     
  13. joelb79

    joelb79

    Mar 22, 2006
    Lansing, Michigan
    The JRX series are notorious for having blown tweeters. The tweeter also is very very short on frequency response. The quasi-3-way 215 also will not be any much louder than the 125's in any place but the low end (marginally louder, but as said not when stacked above a subwoofer or anywhere but direct floor placement). If you are going to go into a subwoofer system, save your money and buy the 125's if you MUST shop JBL's cheapest speaker.

    Owners of JRX series highly recommend replacing the horn with a selenium driver. This will make the speaker sound much more worth the money they already charged you (without a crossover change).



    Notices how his sound went from muffled to much better? That is because the horn driver in the JBL JRX series is cheap. Total crap. Starts to fall off around 12khz like a rock. Fine for speech, but not for sound reinforcement over loud drums. (unless you like to sound like your singing through a blanket?) This will give your listeners fatigue and not something you should do to an audience.

    My advise, a Yamaha Club Series C115V (with a 2" horn driver) will make you a much happier person, for around $390 a speaker. As far as 300w of power vs the power ratings: you will not blow the 15's but you will most certainly blow the horn when clipping starts. Then you can do what I talked about and have less money for buying a bigger power amp. the difference between 300-500w is only 2db (rounded up). Not really a major improvement on headroom, especially when you factor in the JBL is near its limits already and likely will not deliver 2db more output between 300-500w because of simple physics of the 15" driver.
     
  14. And if the OP were looking to install a ****load of big, heavy speakers in a club, and never had to move them (let alone fit them into a non-commercial vehicle), there *might* be a justification for using 215s.

    As it is, a crossed-over 215 means you're using a 15" driver for midrange, which is a *terrible* choice for midrange. If I were going to lug a 95+ pound, 4.5'-tall cab around, it would definitely be a three-way with a dedicated midrange, like the QSC KW153. I would never get a 215 when there are much more portable, and better-sounding, alternatives out there.

    As for conventional wisdom, there are thousands of bassists out there using 410+115 rigs out there, including the likes of Victor Wooten, but there are still plenty of reasons why that's a sub-optimal set-up.

    Hey, if you've already invested in 215s and like the sound, more power to you. But the OP hasn't, and there are better-sounding, smaller and more portable alternatives out there.

    -jb
     
  15. lowfreq33

    lowfreq33

    Jan 27, 2010
    Nashville
    Endorsing Artist: Genz Benz Amplification
    I've heard dual 15's that sounded good, but the JBL JRX125's are not among them. Even with a dbx driverack it was impossible to get a clear sound out of them. Not very reliable either. In the two years I toured with a band that used them we had to replace a tweeter twice, woofer 5 times, and a crossover once. Could've taken that money and bought something better, but it wasn't my call.
     
  16. DuraMorte

    DuraMorte

    Mar 3, 2011
    True... but a 2x15 cab will never sound as good over a large area the way a 15/6/horn will. The relationship between cone area and dispersion is an important factor to remember.
    It may not be rocket science... but it is science. ;)
     
  17. modulusman

    modulusman Banned

    Jan 18, 2004
    montana
    The JRX125 is the double 15 cab. There is no JRX215 model.:confused:
     
  18. Oy...

    Okay, I haven't heard those EAWs, but I bet they sound awesome. I also notice they have two dedicated midrange drivers, and aren't using the second 15" as a midrange, so they're -- wait for it -- true three-ways that happen to have two 15s. I also also notice they weight over 160 pounds. Again, if you're flying a permanent installation in a club, those look like an awesome choice. The OP isn't.

    The JBL is a two-way, not a three-way. And I would never bet my life on manufacturers' published specs anyway, but that frequency response chart looks *very* suspect for a direct-radiating cab with a pair of 15s crossed over to a horn.

    Those "little 3 way QSCs" are almost exactly the size and weight of most 215s. Just a more intelligent design, and with a dedicated midrange driver, built-in amps, DSP and crossovers. A totally different animal from the JRXs the OP was asking about.

    And all things being equal, the QSCs and the JBLs have the same number of speakers. That isn't rocket science either.

    Oh, I understand you're trying to make a point that I shouldn't make blanket statements about how "good" 215s are. But for the vast majority of people here, who have limited budgets, aren't playing arenas, need to move rigs to gigs and don't have roadies, I think 215s are a dumb choice. And if you own an arena, or even a large club, line arrays of 12s are a better choice anyway.

    -jb
     

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