user error or problem w/ amp?

Discussion in 'Amps and Cabs [BG]' started by ColForbs, Sep 1, 2001.

  1. ColForbs


    Jul 21, 2001
    I just bought a used qsc mx1500a over ebay. No I go to hook it up and plug my Eden Navigator into channel 1 of them amp, and send the powered signal, out through channel 1 via a banana plugto 1/4" cable to my cabinet.No sound whatsoever. I then realize that the amp is not detecting any input signal. I am using a "guitar' patch cable to run from my navigator into the RTS this the problem? I want to make sure this amp works, and I think it probably does, I am just not hooking something up right...Yes I already RTFM, and I just can't figure out what I might be doing wrong...

    ps..I figure qsc tech support is not open till monday and that is why I am posting this here.

    Also I have tried a multitude of patch cables on the input stage, and the banana to 1/4 " speaker cable is brand new.
  2. Munjibunga

    Munjibunga Retired Member

    May 6, 2000
    San Diego (when not at Groom Lake)
    Independent Contractor to Bass San Diego
    Have you tried Channel Two? Have you tried plugging something else into Channel One? Did you turn on the Navigator? Are you getting anything out of the Navigator? Help me out here. I use an Mx 1500a for my PA, and it works bitchen.
  3. ColForbs


    Jul 21, 2001
    Yes the Navigator is on and I am getting signal out of it. I have tried both channels of the amp, and the same thing happens...that is why I think there is something wrong w/the way I have it hooked up.
  4. From Da Manual
    An MXa amplifier features balanced inputs,connected
    via barrier strip terminals or 1/4 ” (6.3 mm)tip/ring/sleeve
    phone jacks.The tip of the input jack is wired for “nega-
    tive ’ or inverted polarity,while the ring is the non-inverting
    input.Thus,if the voltage at the tip is positive with respect
    to that at the ring,the amplifier channel will produce a
    negative voltage at its output.
    For use with unbalanced signals,connect the unused
    terminal to ground to avoid a reduction in gain caused by
    a floating input;use a two-conductor (tip/sleeve)plug,or
    connect a jumper between the inverting input terminals
    on the barrier strip and their adjacent ground terminal.

    Do you have a TRS (Tip, Ring, Sleeve) 1/4 phono plug patch cable possibly? Sounds like you've been using TS (Tip, Sleeve) 1/4 plugs I'm guessin. Use one of the stereo TRS patch cables and ya should be fine possibly. Also, check ALL fuses. I bought a Carvin amp offa Ebay, and the thing got signal, but no output. Changed out the Output fuses. Unit works perfectly. Got a GREAT deal on it cause it was not guarenteed, and supposed to be DOA.

  5. ColForbs


    Jul 21, 2001
    I forgot to mention this earlier, and this might possible be the cause of the problem. When i got the amp both negative inputs on the barrier strip are terminated to the ground...
  6. Bob Lee (QSC)

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

    Jul 3, 2001
    Chester, Connecticut
    Former Technical Communications Developer, QSC Audio
    Yup, that would do it. The 1/4" tips are negative, so the barrier strip connections were shorting them to ground. Remove those jumpers and use a regular instrument-type cable to connect to the amp input.

    You can download an owner's manual at