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QSC PLX 2402 internal noise issue

Discussion in 'Amps and Cabs [BG]' started by vicenzajay, Aug 21, 2005.

  1. Yes, I did a search....

    O.K. - after four gigs with my PLX 2402 over the past couple of weeks (no problems - great performance all around), I brought everything home, set it all up again, and turned everything back on today to retune the basses and reset all the levels on the PA.

    And...there's an internal "flapping" noise inside the power amp when I play the low G (also there slightly if I play one octave up). The note is reproduced faithfully, and all other notes don't cause this "rattle" of sorts. It's as if I have found some kind of resonant frequency with something inside the amplifier. Obviously, once everyone is playing, the rattle is completely unnoticeable - you have to put your ear by the fan outlets to hear it when others are playing.

    Although the amp seems to be working fine (apart from the rattle) - something's not right.

    Any help?

  2. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Is the amp on top of the speaker? If so isolate it.
  3. inazone


    Apr 20, 2003
    do a search, there was a few of us that had our plx amps rattle. If I remember right, most of us (myself included) put a grama pad under the amp, or a piece of foam. Someone else opened theirs up and wedged in a shim or something. It sounded easy to do but I dont remember what he used.
  4. So I went down to ACE hardware - got some adhesive foam weatherstripping, and then returned home and took apart the amp. I applied the weatherstripping to wedge the fish paper heat sink "tunnel" and then put the amp back together.


    Rattle's not completely gone - It is obviously the fish paper (cardboardy heavy paper), but what I did didn't completely secure it down. I then put two towels and some cardboard supports underneath the rack case, and that helped more (paper rattle is pretty hard to hear now).

    Lastly, I ordered a set of MoPads to put underneath the rack case. Hopefully that will put an end to this.

    Anyway, I think I'm now in the market for a Gramma pad for putting underneath the 2x12. All these gigs on raised stages (and the endless EQ issues) are making me want to isolate the cabinet a bit.