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QSC is clipping

Discussion in 'Amps and Cabs [BG]' started by shatner, Mar 9, 2005.

  1. shatner


    Sep 22, 2004
    Isle Of Wight, UK
    I have a Sansamp RBI into a QSC rMX1450 into two Ampeg svt cabs (2x10 and 1x15). The whole speaker load is 450 watts and 4 ohms. I am running them both off one channel so the power is matched by the QSC (450 watts at 4 ohms).

    I did a gig last night (outdoor pop gig on a ship) and had to push the QSC to full volume and could still have done with more power but the red clip light kept flickering on it so I kept backing off with the output level of the Sansamp. Is the clip light to do with me pushing the input of the QSC? ie. the output of the sansamp.

    I am tempted to run the QSC bridged mono and just hold back on the volume thus allowing me to run the sansamp at a lower level. The level ended up just past 12 o clock with the treble backed off to 9 o clock, the bass and mids at just over 12 o clock and blend completely off.

    Would this solve my clipping problems? Do I need higher powered speakers?

    It is frustrating cause I have played through combos that can be run much louder at lesser output level and here I am with a big power amp, big speaker stack and nice preamp but still suffering from not enough volume (without clipping).

  2. Munjibunga

    Munjibunga Total Hyper-Elite Member Gold Supporting Member

    May 6, 2000
    San Diego (when not at Groom Lake)
    Independent Contractor to Bass San Diego
    Not much headroom when you use only 450 watts to drive speakers rated at 450 watts. Bridge it and be careful. You want your amp to be capable of 1.5 to 2 times the power rating of your speakers.
  3. natrab


    Dec 9, 2003
    Bay Area, CA
  4. No, there isn't an input level clip light. The amp will clip when it's being asked to put out more power than it can - regardless of what your input level going into the amp or gain level on the amp is. You hit the max.

    I too would recommend you try bridging it and being careful not to push anything too hard - should give you some more headroom.
  5. Run it in bridged mono. I have the same setup (with the addition of a presonus mic preamp and compressor). I run bridged mono and can easily compete with 2 LOUD guitars and drums. Just watch your volume levels so you don't over do it. You will have tons of headroom so you will not clip. This is a much better way to run that setup.
  6. Joe Beets

    Joe Beets Guest

    Nov 21, 2004
    And just be thankful you don't have a 300 watt Combo Amp. :eek:
  7. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    You do have an interesting choice, don't you?

    Your choices are 450W or 1400W, and you have 450W speakers.

    Clearly the 450W is "underpowered", since you needed more.

    Equally clearly, 1400W is plenty to overpower your speakers. You can probably get away with being careful, but don't use a 5 string and crank lows.......

    I'm not particularly worried about straight power, because you won't use a lot of "rms" power in most situations. I am more worried about lows and cone motion. You will easily be able to get too much cone movement at low frequencies.

    It would probably be a different deal if you had speakers rated around 700 or 800W. Much as I hate to say it, you may want to consider an upgrade in the future. It's a pretty significant mismatch.
  8. The RMX series offers both a clip limiter and high-pass set of dip switch settings. Engage the 50 Hz low cutoff and engage the clip limiter. This will keep the low rumble and string movement signals out of your drivers.

    If you are not making enough noise, you don't have enough speakers. The wattage rating is for the most part useless. It only tells you how much power the coils will accept before melting into goo. The drivers will over excurse long before hitting the maximum power rating.

    Just for grins, use a battery and check polarity between both cabs. Make sure the cones move all in the same direction when the battery is made in the circuit. Out of polarity cabs will kill your output levels and bottom response.

    The QSC clipping indicator shows when too much input signal (Sansamp) is coming into the amp.
  9. I second the use of the filter options on the RMX. I run mine in parallel mode with my 210 8ohm on CH1 with the 50 Hz cutoff engaged. CH2 powers my 410 4ohm with the 30Hz filter on. Clip limiting on for both channels. I run my preamp with output fairly high and the RMX attenuators at 12 o'clock. No clipping and lots of volume. It certainly makes my 210 speak better. Speaking of clipping, it's my understanding from the manual that the clip lights operate when the output of the RMX is limiting. (or did I misunderstand bgavins post?) :)

  10. Nope, it shows output clipping or limiting depending on whether the limiters are engaged. The clip lights on every power amp I've ever encountered are referenced to output clipping.

    Of course, that's a result of too much signal being applied at the inputs.....:D
  11. TheChariot


    Jul 6, 2004
    Boston, MA
    I highly recommend changing your cabs. You probably need more power... but I dont know if I'd be too comfortable putting 1400Watts into a pair of cabs rated 200Watts RMS each..... no matter how high or low you have it cranked, it still seems like an unecessary risk.

    Option #1:
    Try hooking up your cabs into separate channels. 8ohms per side. That would give you a little more power. 280Watts per side, I believe. It might be just enough to get you over that hump your stuck on.

    Option #2:
    Get an 8ohm cab that handles 500-900Watts RMS, and running that cab bridged for a 900Watt output from the amp. If you get a cab that happens to be diverse enough, it will be plenty for most gigs.

    Option #3:
    Another option is actually adding a 3rd cab in 4 ohms. I know it sounds unorthodox, but all you would need is something small... and you could either chain it into the same channel as your SVTs (for 700Watts) or you can put it on its own channel, and get 450Watts into that cab @ 4 ohms. It actually seems like a practical alternative to getting a whole new larger cab. An avatar SB112 is only about $220 now, and its rated at 500Watts RMS. It'd also be handy for taking to practices or smaller gigs.

    Explore the possibilities :)
  12. Bob Lee (QSC)

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

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Try putting a cabinet on each channel instead both on one, which doesn't make sense to do. Set he low frequency filters as was suggested, so you don't waste amp power on frequencies that are too low for the loudspeakers to handle well.

    A little quick bit of clipping now and then isn't anything to worry about as long as you don't get audible distortion.
  13. You are absolutely correct, the LED indicates output clipping.

    As a consequence of overdriving the input...

  14. You're probably joking, but output clipping wouldn't neccessarily have anything to do with the input level being too high.
  15. Not joking at all.

    The QSC is a voltage amplifier. The model 1450 provides 32dB of gain, which multiplies the input voltage by a factor of 40x. The input sensitivity is 1.15v and this produces a maximum output of 46 volts.

    If the Sansamp or other preamp can provide more than 1.15v input voltage to the 1450 amp, this will drive the output stage over its 46v maximum, and into clipping.

    Demeter preamps are capable of up to 20 ~ 26 volts output, which is far more than the QSC requires, or can handle. On the other hand, my SWR IOD only produces 0.75v maximum. To clip my QSC, I have to have additional gain stages (active crossover, EQ, subsynth) between the IOD and the QSC.
  16. Bob Lee (QSC)

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

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Everything you say is correct, Bruce, but I want to clarify that voltages above 1.15 volt will drive the amp into clipping at full gain. The gain knob is there so that you can turn down the gain and use the amp with devices that put out higher voltages, too. :)
  17. Bob Lee (QSC)

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

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    It would, along with gain. Too much input signal with too much gain leads to clipping. You can decrease either or both to eliminate it, for the most part.
  18. davepack

    davepack Supporting Member

    Jun 16, 2004
    Denver, CO
    Can too SMALL of an input signal and cranking the gain on the power amp to compensate produce clipping as well??
  19. Bob Lee (QSC)

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

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Too small a signal won't. It would be too quiet, that's all.
  20. Aram


    Feb 2, 2003
    New York, NY
    Bob, do you have any tips on how to successfully match preamp outputs to input sensitivity? When I called Demeter because my 201s was clipping my RMX 2450, they just said 'use your ears'. With so many volumes (two on the Demeter, and one per channel on the QSC), it's hard to know where each should be set...