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JBL powered Sub into a bass cab?

Discussion in 'Amps and Cabs [BG]' started by Low end Eddie, Aug 13, 2019.

  1. Low end Eddie

    Low end Eddie

    Aug 9, 2019
    I am new to this site and this is my 2nd (and hopefully last) question:

    I understand that subs have no mids or highs, and sound ‘muddy’ if used as is.
    My question is what determines the ‘just low end sound’? Can I change out the speaker, or by-pass electronics (or both) to utilize the power amp, and turn it into a bass cab? It is a JBL JRX 118sp model. I got it off a friend real cheap and was hoping to convert it.
    Any input would be appreciated.
    JC Nelson and zon6c-f like this.
  2. Stumbo

    Stumbo Wherever you go, there you are. Supporting Member Commercial User

    Feb 11, 2008
    Song Surgeon slow downer. https://tinyurl.com/y5dcuqjg
    Welcome to TB! Good questions.:thumbsup:
    Have you tested it? Do you have any other cabs or amps?

    Just so we know what we're talking about: https://www.jblpro.com/ProductAttachments/JBL_JRX118S.PDF

    Key Features:
    1) 350 watt (continuous), 1400 watt (peak) power capacity (note c)
    2) 18" compact direct-radiation subwoofer designed to supplement and extend low frequency performance of full-range systems in live sound and music playback applications.
    3) The rugged enclosure is constructed from 19 mm (¾ in) MDF (Medium Density Fiberboard) using advanced adhesives and mechanical fastener technology for durability and improved low-frequency performance. The enclosure is covered in black carpet.

    4) A top mounted, cast aluminum 36 mm pole mount receptacle can receive an optional SS3BK pole to facilitate use of the JRX118SP as a base for a mid/high speaker cabinet.
    5) Non-resonant, all steel handles are used.
    6) Attractive 18-gauge perforated steel grille protects components from damage.
    7) JRX118S is driven by a massive JBL 18" woofer with a cast frame and 3" voice-coil. It’s rated at 350 watts (continuous) and 1400 watts (peak) – based on JBL Professional’s 100 hour extended life test.

    8) Specifications: System Type: 18" compact subwoofer Frequency Range (Note a)(-10 dB):1 38 Hz - 300 Hz Frequency Response (±3 dB):1 47 Hz - 300 Hz Sensitivity (1w/1m): 96 dB SPL(note b) Nominal Impedance: 4 ohms
    9) Rated Maximum SPL2: 133 dB peak, @1 m (3.3 ft)

    10) Recommended Amplifier 350 - 700 watts Power: @ 4 ohms
    11) Dimensions (H x W x D): 605 mm x 508 mm x 551 mm (23.8 in x 20.0 in x 21.7 in) Net Weight: 32.2 kg (71 lb) Shipping Weight: 36.8 kg (81 lb)
    12) LF Driver: 2043-G Input Connectors: Neutrik® Speakon® NL-4 (x2); ¼" TS phone jack (x1) parallel
    13) Optional Accessory: SS3-BK 1

    a) Frequency Range and Frequency Response are based on half-space conditions.
    b) Calculated half-space performance based upon peak power capacity.
    c) “Power Capacity” and “Peak Power Capacity” ratings are based on the average and peak power capacity of product samples subjected to a 100 hour power test using random noise with a 6 dB crest factor, in accordance with IEC standards.

    JBL continually engages in research related to product improvement. Some materials, production methods and design refinements are introduced into existing products without notice as a routine expression of that philosophy. For this reason, any current JBL product may differ in some respect from its published description, but will always equal or exceed the original design specifications unless otherwise stated.
    Last edited: Aug 13, 2019
    zon6c-f likes this.
  3. Wasnex


    Dec 25, 2011
    The built in amp has a 100hz low pass filter (LPF). I don't believe the LPF can be bypassed, so I don't think it is possible to do what you want. I would suggest two courses of action:1. Sell or trade the JRX118SP for what you want, 2. Buy a powered speaker to use with it in biamp mode. I believe the JRX118SP is designed to work with a powered JBL Eon 15.
    JC Nelson likes this.
  4. klokker


    Jan 7, 2009
    Steele City, NE
    Well, that depends on the player. There are lots of different "low end sounds". But in general subs don't work well for a bass rig. Some people use them a little, but in general that's not the case, at least in my experience.

    Subs work great for home stereo, and car stereo. But that sound is mixed in a studio for that. So lots of guys just pick up a bass and think that's how it works. Tons of sub is what they understand as "low end". It doesn't work that way for a live bass mix. Usually a big low end on a live bass mix is about a low mid, or mid bump. A soundguy might add some sub to your mix in the PA, but usually not a lot. A lot of good bass amps cut out the super low stuff because it muddies the mix and wastes power. Guys like me like a real "tight" low end, not a lot of bloom and boom. So a sub is basically useless.

    It depends on the kind of band, the room, other factors as well.
    Justinian likes this.
  5. Wasnex


    Dec 25, 2011

    Per the datasheet, the JRX118SP is down -10db at 38hz and -3dB at 55hz, so it really doesn't go deeper than a good bass cab.
    Gizmot likes this.
  6. Low end Eddie

    Low end Eddie

    Aug 9, 2019
    Thanks guys,
    Jumbo: It is the JBL JRX 118sp. The “p” I believe stands for powered. I wanted to use the power from it, and the cab.

    I think I will just use Wasnex’s opinion and sell or trade it in.
    Klokker: thanks for your input also.
    I do understand they sound terrible as a bass rig. I have other speakers and didn’t even care if I had to run a pre-amp though it. I was hoping I could by-pass all the sub features and tap into the amp... I will just move it down the road..
    Thanks again to all!!!
    Wasnex and klokker like this.
  7. klokker


    Jan 7, 2009
    Steele City, NE
    OK. I don't really know what the specs are. My only experience is using them live. We have JBL's in our band PA. Must use carefully, sometimes its best to not use them at all. Classic rock band with horns. Even without them we still have plenty of "low end" sound.

    The question I tried to address is "what determines low end sound". Not a lot of that is the result of a subwoofer, at least in my experience with live music.
  8. Paulabass


    Sep 18, 2017
    The low end sound is not the problem. It's getting anything over 100Hz. out. The amp and crossover are not on separate circuit boards, so it would be a job for a pro if it can be done at all.
    I use a powered sub with one of my rigs- loop out to input, crossover out to loop in. Now the sub works from 40 to 100, and crosses over to the Big E cab. Kinda rube Goldberg meets Phil Lesh, cause the Big E is a three way cab!
  9. Wasnex


    Dec 25, 2011

    IMHO, The specs mean it's really more of a woofer than a sub. It would be useless by itself due to the 100hz LPF. It might sound okay if the OP ran a powered 12+horn on top of it, although it would depend on his tone goals.

    I believe we are in agreement; selling or trading the JRX118SP and getting a bass cab is most likely a better solution.
  10. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Wait a minute guys, that's not a powered sub, it's just an 18" unpowered cabinet. There is no crossover employed either.

    The data I have on this driver shows frequency response pretty flat on axis through ~3kHz so while it might be a little dark it's not as bad as some other options even in the 15" size. Where there may be some suffering is in the off axis response as the driver was not designed with this kind of response in mind. It's a very good driver when used within its limitations. Remember, it's a 4 ohm driver and 350 watts RMS so power appropriately.

    I would give it a try, be sure that the internal jumpers are set for 1+/1- configuration (many are used in 2+/2- in PA).

    Zos Kia Cultus likes this.
  11. Wasnex


    Dec 25, 2011
    Check again. SP is the powered version.
  12. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    That's not the same attachment as in the 2nd post which is the S version.

    OP, is this the S or the SP version??? If it's the SP (powered) version, there is no easy way to disable the lowpass filter that rolls off starting at 300Hz.
  13. Post #6 OP says,
  14. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    I missed that, so now you have your answer... no easy or practical way to use this for bass guitar.
    S-Bigbottom likes this.
  15. Welcome to TalkBass!
  16. Marhmol

    Marhmol Supporting Member

    Nov 6, 2016
    Bay Area Ca
    Give it a try. What could it hurt:bassist::hyper:
    Justinian and Skillet like this.
  17. Nice amp into a pair of cheap car subwoofer drivers! How sad! :(
    agedhorse likes this.
  18. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Except that your "subs" don't have a steep LPF rolloff like the powered JBL sub does.
    BassmanPaul likes this.
  19. It might make a cheap and entertaining biamp bottom if it can operate off the tuner out of a practice amp.
  20. Low end Eddie

    Low end Eddie

    Aug 9, 2019
    Thank you all for the help. I have decided to sell it. I took a shot at hoping I could convert it into a back-up cab, but I will leave it as is and move on.

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