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QSC PLX 2402 and SWR Grand Prix Compatability

Discussion in 'Amps and Cabs [BG]' started by bluesrx, Jun 7, 2007.


  1. bluesrx

    bluesrx Supporting Member

    Mar 31, 2006
    Lakeland, Fl.
    Just picked up a used SWR Grand Prix and am running it thru my 2402. i have to dime both the gain and the master and turn the qsc up all the way to get a decent sound.

    grand prix data says for a power amp to be compatable it must have sensitivity of 7V RMS or less and an input impedence of greater than 600 ohms

    QSC says for the 2402 .... input sensitivity is 1.5 V RMS @ full rated power at 8 ohms and 1.3 V RMS @ full rated power at 4 ohms. input impedence is 6k ohms unbalanced input.

    by the above figures they should match well. I`ve swapped out tubes and get basically the same results i`ve also measured the voltage at the unbalanced output and it`s right around 2.4 volts everything dimed with a good pluck of the bass g string....

    the level led`s of the grand prix are all green with a flash of the first red led...the qsc led`s barely get to the 2nd green led

    my ampeg svp pro has no problems driving the qsc to clip

    anybody have this problem...or have the knowledge on how to change the gain structure on the swr

    thanks for any info
     
  2. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio!

    Jul 3, 2001
    Santa Ana, Calif.
    Former Technical Communications Developer, QSC Audio
    Have you tried your rig with more than one bass? If the instrument has a weak output signal, even fairly high gain in the preamp and power amp might not be enough.

    Even so, it's surprising, because IIRC the Grand Prix has a decent amount of gain available. I'll have to look it up.
     
  3. Hmmm, interesting. I have the same pre and the 3402 amp and have no issues driving it to full power. Was the GP perhaps previously modified? Which output are you using? Have you tried the XLR out? It's typically hotter.
     
  4. Steve

    Steve

    Aug 10, 2001
    I've used that combination a bunch without any problems at all.

    There's a bunch of out's on that thing and they're pretty close together. Have you checked to make absolutly sure you're using the balanced or unbalanced "preamp out"?
     
  5. bluesrx

    bluesrx Supporting Member

    Mar 31, 2006
    Lakeland, Fl.
    hey bob... was hoping you`d chime in.

    i`ve tried it with all my basses which are all active and put out a good signal with the same result of having to dime everything.

    this is what swr has regarding the GP output
    Output Impedance
    Effects Send Jack = 10 ohms
    Line Out Jack = 100 ohms
    Tuner Out Jack = 100 ohms
    XLR Balanced Record Out Jack =1.5K ohms
    XLR Balanced Line Out Jack = 10 ohms
    Unbalanced Preamp Outs = 10 ohms
    Maximum Output @ 10K and 600 ohms
    Effects Send Jack: 9V and 7V RMS
    Line Out Jack: 9V and 7V RMS
    Tuner Out Jack: 9V and 7V RMS
    XLR Balanced Record Out Jack: 10V and 5.5V RMS
    XLR Balanced Line Out Jack: 18V and 14V RMS
    Unbalanced Preamp Outs: 9V and 7V


    however, i only see about 1.5 to 2 V at the unbalanced output with every thing up full on the pre.

    i`ve also tried the xlr out with no luck
     
  6. bluesrx

    bluesrx Supporting Member

    Mar 31, 2006
    Lakeland, Fl.
    hey steve..

    yes i`m using the unbalanced 1/4" and have also tried the balanced xlr which are side by side on the back of the pre with the same results from each.

    i reallly dig the tone from this pre so i`m hoping it can be figured out

    GP does not look to be modified
     
  7. Passinwind

    Passinwind I know nothing. Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    Any chance someone replaced the 12AX7 with a 12AU7? That'd be consistent with the lowered output voltage.
     
  8. bluesrx

    bluesrx Supporting Member

    Mar 31, 2006
    Lakeland, Fl.
    swapped out about 12 known good 12ax7 tubes...i have a great stash...no real difference in gain
     
  9. bluesrx

    bluesrx Supporting Member

    Mar 31, 2006
    Lakeland, Fl.
    i do know that if a coupling cap is leaking DC onto the grid of the tube from the previous stage it could effect the gain adversly.
     
  10. Lobaw

    Lobaw

    May 14, 2007
    Seattle, WA
    I may be stating the obvious, but could it be an issue with any of the cords you are using? I've found that ostensibly good cords fail to work now and again.
     
  11. greenboy

    greenboy

    Dec 18, 2000
    remote mountain cabin Montana
    greenboy designs: fEARful, bassic, dually, crazy88 etc
    alexclaber uses that pre and power amp, never mentioned any problems.
     
  12. bluesrx

    bluesrx Supporting Member

    Mar 31, 2006
    Lakeland, Fl.
    same cords used with my svp pro work without issues
     
  13. There's gotta be something wrong with the GP. Perhaps it suffered ill effects from being connected to a board with phantom power? There is a design flaw in some earlier units that allow phantom power to toast part of the circuit if the DI is connected to it. Don't know if this would be the effect but a possibility?
     
  14. It could just be me but it seems like the 2402 is the variable in this mix. Have it checked out and check out its settings to make sure everything's as it shoul with the amplifier.
     
  15. bluesrx

    bluesrx Supporting Member

    Mar 31, 2006
    Lakeland, Fl.
    the amp performs flawlessly with my svp pro pre
     
  16. I ran the GP w/ a QSC for a couple of years and always had plenty of gain. I think there's something wrong with your pre. Get it in and get it checked out. What year is yours?
     
  17. bluesrx

    bluesrx Supporting Member

    Mar 31, 2006
    Lakeland, Fl.
    not sure..it`s the red graphic one
     
  18. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    That's weird, mine has no trouble at all driving a PLX 3002 to full power and that's with a 5751 in it (lower gain than 12AX7).

    Balanced connections should give you another 6dB vs unbalanced.

    Puzzling...

    Alex
     
  19. bluesrx

    bluesrx Supporting Member

    Mar 31, 2006
    Lakeland, Fl.
    alex...

    just curious, what are your typical settings (settings that would drive the qsc to full power) on the GP gain and master volume?

    thanks
     
  20. Primary

    Primary TB Assistant

    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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