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Rig Help! (Acme, QSC content)

Discussion in 'Amps and Cabs [BG]' started by Stereo Joe, Aug 15, 2007.

  1. So I finally was able to get all the components together for my rig (all thanks to members of TB!):

    QSC RMX 1850HD
    Acme Low B4 (8ohms)

    I absolutely LOVE the sound of the Acme, but I was finding it too quiet with my Little Mark II. So, I switched to the pre/power amp setup to get a nice whallop of power.

    However, when I set this up for the first time to run at rehearsal, I found myself running the QSC in bridged mono mode (1100 watts) at full volume and I was only barely loud enough to keep up with a drummer and guitarist (who uses a 20 watt tube amp). The preamp was cranked up to the point of almost clipping the input of the QSC. I tried turning on and off the filters and clip limiters on the QSC and even running it in stereo mode off one channel, but I still found myself maxing everything out. My band isn't THAT loud. I used to have the Little Mark II running at 300 watts into an Avatar 1X12 speaker and it was about as loud as I'm getting now.

    Is something wrong? I know Acmes are notoriously innefficient, but could my Low B4 be so innefficient that 1100 watts is not loud enough for a 3 piece?

    Maybe the QSC isn't bridging properly? I didn't notice a discernable volume difference between running the amp in stereo or in bridged mono, so I'm wondering if it might not be putting out the full power I need.

    Oh, one other thing, the speakers in the Acme were barely moving at the volume I was playing. If I understand correctly, these speakers are meant to be able to really pump and move a LOT. Could I still be underpowering this cab with 1100watts?

    I appeal to all Acme users to help me. I absolutely LOVE the sound of the cab, but I need to hear it at useable volume levels. Is this impossible?

    QSC (or other power amp) users, do you notice a large volume increase when running your amp in bridged mono? Could my lack of volume difference mean a problem with the amp (it was banged up a bit in shipping...)

    Anyway, any light that could be shed on this issue would be MOST appreciated! I have a big show on Thursday and I'm pretty bummed that I can't have a crushing wall of bass. :crying:
  2. bongomania

    bongomania Supporting Member Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    Are you sure about this? The IOD is notoriously low-output, and the QSC is notoriously not very sensitive at the input, so it would seem like there's your problem- unless you're right, and you were actually clipping the QSC's input.

    The other thing is, what EQ settings are you using? If you scoop the mids much, you're doomed to inaudibility in a live mix, even with a ton of watts.
  3. Sounds like there was an issue with your bridged mono mode... was it all connected properly?

    Using speakon cables?

    Is there a switch to engage for the mono mode?

    Just throwing out a suggestion althgouh I have no idea how to remedy the situation.
  4. Eric Moesle

    Eric Moesle Supporting Member

    Sep 21, 2001
    Columbus OH
    When running in bridge mode, you must alter the amp-end of the speakon cable to +1 and +2 instead of +1 and -1.

    Is that your problem?
  5. I had the input gain and the master volume almost dimed on the IOD and when I played a note hard, I was getting the red clip light lighting up on the QSC.

    As for EQ, I started flat and gradually increased the mids and highs until I could cut through a little better, but I still wasn't getting any more real volume. (oh, and I had the Aural Enhancer turned all the way off - no scoop for me!)

    The other thing I tried (which I forgot to mention) was to switch the QSC's filter to the 50hz setting which gave me a bump in the lows. It helped to cut through a bit more, but again, no real volume gain.

    I'm stumped!

  6. ****. That is probably it exactly. I wondered, but I couldn't find anything in the manual specifically stating I needed a different cable for bridge mono to work. Grrr....

    I've just been using my stock speakon cable. Any literature online how I can go about changing it? Has the amp just been running at the volume from one side then?

    THANK YOU!!!

    Edit: I only have the 2 connector speakon cable. Can I make this work, or do I need a 4 connector cable?
  7. For bridged mono you are going to need a 4 pole speakon connector. Supposedly speakons are easy to wire with just a screw driver. Here's a thread on the QSC board about how to do it.

    BTW, I got similar results when trying to drive my RMX850 with just my low output Fishman Pro Bass EQ (low volume, clipping when I turned up the power amp). Adding a cheap ART tube pre for more gain in my signal chain solved that problem. If you really like the sound of your IOD, you might consider having the input sensitivity of the amp matched to the output of the IOD (.70v?). From what I understand it can be done at any QSC service center and it's not an expensive mod.

    good luck,

    Attached Files:

  8. CrackBass


    Aug 10, 2004
    yea, try wiring up a new speakon for your amp. i did the same thing years ago with a crown amp. btw the connections should be labled on the back of your amp. speakons are very easy to wire. you were probably only pushing about 250 watts with the configuration you were using. same as running one side at 8 ohms.
  9. Bob Lee (QSC)

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

    Jul 3, 2001
    Santa Ana, Calif.
    Former Technical Communications Developer, QSC Audio
    Joe, I don't think you were clipping the input of the amp. It takes about 10 volts (14 volts peak) to do that. The amp's clip LEDs indicate clipping on the output.

    For bridged mono, you have to wire the output the way the others in this thread have mentioned, which is what is described in the owner's manual and on the back of the amp. Otherwise, you're just running the rig on one channel, the same as you had before.
  10. Sorry if I was not clear on that. With the VERY low input (in relation to the sensitivity), I was clipping the output without getting very loud.

  11. Dan Molina

    Dan Molina TalkBass Secular Progressive

    Jul 17, 2002
    Murr Town, California
    Use banana plugs instead. It's far easier. You don't even have to worry about the wiring at all.
  12. Bob Lee (QSC)

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

    Jul 3, 2001
    Santa Ana, Calif.
    Former Technical Communications Developer, QSC Audio
    Now I'm getting confused. ;)

    Sorry. No, it was Stereo Joe who said he was clipping the input of his amp.
  13. Johnny Crab

    Johnny Crab HELIX user & BOSE Abuser

    Feb 11, 2004
    Fix the cable and you'll be PLENTY loud.
    Two ACME B2's(4 ohm) here are very adequate in a Texas/blues/rock/cover setting with a LOUD drummer. They're each getting one side of a QSC 2402.
    Do leave the filter "ON" as it helps preserve the life of the 10's a little longer without giving up too much.

    Be SURE your cable has a decent size/gauge wire. I've found an audible difference in power/fullness/bottom when comparing "budget" speaker cables to decent ones.
  14. Kindness


    Oct 1, 2003
    When you get that cable wired up properly be prepared for a very noticeable difference. :)

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