QSC PLX 2402 and Pre-Amp Compatibility

Discussion in 'Amps and Cabs [BG]' started by Tobias Wilcox, Sep 21, 2016.


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  1. Tobias Wilcox

    Tobias Wilcox Supporting Member

    Aug 18, 2006
    London, ENGLAND
    Just got tis QSC power amp to go with my SWR Grand Prix Pre. I intend running it in stereo so that I'm putting 700w into my 610.

    The SWR manual states-

    "For a power amp to be compatible with the Grand Prix while using the unbalanced out jacks, its sensitivity must be 7V RMS or LESS and have an input impedance of greater than 600 ohms."

    Will the 2402 be driven hard enough by the Grand Prix?

    Is it best to run the 2402 with the attenuator fully up and control from the pre?
     
  2. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    Yes, 7 volts is about 9dB more than is necessary to drive the 2402 to full rated output.

    Start with the attenuators fully up, and if you find that you have more than enough drive signal you can reduce them, but I would recommend leaving the limiters of the PLX on and reduce the attenuators by 3dB which will get you 6dB of limiter compliance before the output driver of the preamp distorts. I would also recommend the HPF's on and set for 30Hz.

    If you plan on bridging, you will need to look up the rated sensitivity, sometimes it's the same, sometimes more sensitive and sometimes less sensitive depending on the way the internal bridging circuitry is designed. Usually it's the same, resulting in 6dB additional power amp voltage gain (will sound louder, but just more sensitive for the given output power).
     
    BasturdBlaster and Bob Lee (QSC) like this.
  3. Tobias Wilcox

    Tobias Wilcox Supporting Member

    Aug 18, 2006
    London, ENGLAND
    Hey, thanks for that! Sounds as if there shouldn't be a problem there. I don't plan on bridging as the PLX 2400w @ 4 Ohms and my cab being 900w may not enjoy that?
     
  4. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    Nice to see a good application of common sense here. I think you will be happy with the results.

    (I toured with a few dozen PLX amps up until a few years ago, one of the most reliable SMPS based power amps of its day. I still have maybe 10 or 12 that I use for one-offs).
     
    deepbluesuk likes this.
  5. murphy

    murphy Supporting Member

    May 5, 2004
    Canada
    It was not a good amp for my Alembic F2b
    The SWR stereo 800 power amp was heaven
     
  6. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    The output impedance is very high on the F2b which results in a lot of loss (and some loss of low frequency) when driving a 22k ohm input impedance at high (line) level like the QSC PLX (IIRC).
     
    murphy likes this.
  7. Passinwind

    Passinwind I know nothing. Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    Yep. Here's a spitball LTspice model of the loading effect on the F2B of changing the power amp input impedance downward from 50K ohms to 5K in 5K increments:



    F2B_loading5K_50K.jpg
     
  8. Tobias Wilcox

    Tobias Wilcox Supporting Member

    Aug 18, 2006
    London, ENGLAND
    I was after a SWR Power 750 and I'm beginning to wish I'd held out for one. Not feeling the "whump" that I was hoping for from the QSC after running a couple of pre's through it.......
     
  9. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    Not due to the power amp, but perhaps due to the driving capabilities of the preamp, or the input module setting on the power amp. Where are the HPF's filters set on the power amp? Are you bridging?
     
  10. Tobias Wilcox

    Tobias Wilcox Supporting Member

    Aug 18, 2006
    London, ENGLAND
    Not bridged, HPF filters on 50, clip limiter on
     
  11. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    Set the HPF to 30Hz as I mentioned in post 2.

    50Hz is too high for all but the most compact cabinets.
     
    staccatogrowl and wcriley like this.
  12. Tobias Wilcox

    Tobias Wilcox Supporting Member

    Aug 18, 2006
    London, ENGLAND
    No difference there. Will setting the dip switches to Parallel instead of Stereo make any difference?
     
  13. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    Shouldn't make any difference between stereo and parallel.

    What cabinets are you using? When you say you miss the whump, did you have it with these specific cabinets or others?
     
  14. Tobias Wilcox

    Tobias Wilcox Supporting Member

    Aug 18, 2006
    London, ENGLAND
    Ashdown 610 - yes, definitely there before and ran the SWR pre through the FX return of a couple of different 500w amps, absolutely loads of clout.
     
  15. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    I don't know why you aren't getting what you want. I am out of ideas, sorry.
     
  16. Tobias Wilcox

    Tobias Wilcox Supporting Member

    Aug 18, 2006
    London, ENGLAND
    Thanks for all your suggestions. i guess that pre just isn't compatible.
     
  17. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    Maybe the amp you used before is not flat response, but has a small rising low frequency response. Some MI power amps do voice their power amps in a way that suits their marketability.

    It's really hard to say, but I'm confident that it's not just the amp itself (I have just too much experience with them). There's something else going on, I just don't know what.
     
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    Here are some related products that TB members are talking about. Clicking on a product will take you to TB’s partner, Primary, where you can find links to TB discussions about these products.

     
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