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QSC USA series. Why no 4ohm bridged rating?

Discussion in 'Amps and Cabs [BG]' started by MuzikMan, Mar 17, 2005.


  1. I've read the specs on a lot of different sites for my QSC USA900 power amp. They all specify a 550watt rating at 2 ohms per channel (both channels driven) but there is no rating available for 4 ohm bridged, only 8 and 16. What's up with that? Is QSC afraid to back up their stuff, or could this possibly be an oversight?
     
  2. Hmmm..... Just found this at QSC's FAQ.

    Can I drive a 2 ohm load with my QSC amplifier in bridge-mode?
    The minimum rated impedance for an amplifier in bridge-mono is 8 ohms. A 4 ohm load is possible as this represents a 2 ohm per channel equivalent. Two ohm load precautions still apply, as it would with any amplifier, so supplemental cooling may be found necessary. It will also be important to watch for any impedance transients that arise from the speakers in use. Four ohm bridge-mono loading is the absolute minimum across the entire audio range (20Hz - 20kHz.)
     
  3. 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
    The amp has no reverse gear or rear-view mirrors, so, yes, we're afraid to back it up, especially when parallel parking. ;)

    But seriously, you might be able to run the amp in bridged mono into a 4-ohm load, but its cooling really isn't heavy duty enough for doing that. A lot depends on the actual load (not all 4-ohm loads are alike) and how hard you push the amp. Because in bridged mono you would drive both channels equally hard if you drive the amp hard (whereas in stereo you might have some difference between channels), we felt it was best not to recommend running it bridged into 4 ohms.
     
  4. Selta

    Selta

    Feb 6, 2002
    Pacific Northwet
    Total fanboi of: Fractal Audio, AudiKinesis Cabs, Dingwall basses
    Since the question's answered I don't feel so bad hijacking... but Bob, what excatly is "pink noise"? It's on http://www.qscaudio.com/products/amps/rmx/rmx.htm that page right under power requirements. Thanks :)

    Ray
     
  5. HA!!!!!!!! I deserved that one! Ok this falls along the lines of the reason I suspected and that is fair. I must say I am very pleased with the performance of this amp especially considering it's age and level of abuse it has taken. Unfortunaltely I am not one to leave well enough alone so I am going to look into additional cooling and have at it!

    What is the worst that could happen? Will the protection circuitry save my stupidity?
     
  6. 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
    Pink noise is an electronically generated test signal made up of random noise of all frequencies, with equal intensity per octave. That means there is as much energy in, say, 50 to 100 Hz as in 200 to 400 Hz or 350 to 700 Hz or 5000 to 10000 Hz. It sounds like a waterfall. That equal energy per octave relationship makes it a reasonable analog for many forms of music and other audio programs, and it's also useful for doing a spectral analysis of an audio system.

    Read on: http://www.google.com/search?hl=en&lr=&safe=off&oi=defmore&q=define:Pink+Noise
     
  7. Selta

    Selta

    Feb 6, 2002
    Pacific Northwet
    Total fanboi of: Fractal Audio, AudiKinesis Cabs, Dingwall basses
    Hah, kick---. Thanks man!

    Ray
     
  8. 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 may seem strange until you try it, but using pink noise is also an excellent way to find anomalies in a sound system without any test equipment.

    If you run pink noise through your home stereo or your PA system, for example, and walk about the room, you can very easily and distinctly hear changes in the tonal character caused by acoustic reflections, cancellations, etc., that occur in various places around the room.

    Or for an experiment, have someone hold a small speaker emitting pink noise and have him or her hold it close to a wall and then move it gradually away, and back again. You'll hear the comb filtering effect of the varying time-delayed reflections. It's interesting, if you're at all interested in acoustics and why things sound the way they do.
     
  9. Selta

    Selta

    Feb 6, 2002
    Pacific Northwet
    Total fanboi of: Fractal Audio, AudiKinesis Cabs, Dingwall basses
    I'm going to have to do all sorts of testing now with my roommate once he gets in. I listened to it on my headphones... neat stuff! Do you have any clip longer than 15 seconds I can have?

    Ray
     
  10. 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
    I don't, but you might be able to find a WAV file online that you can splice onto itself to make longer, or you might be able to find a CD with test signals. I think Alan Parsons put out a pretty good test CD once, IIRC.
     
  11. Passinwind

    Passinwind I Know Nothing Supporting Member

    If you have a sound editing program on your computer, you can either record a longer clip directly if it has a pink noise function (IIRC,Wave Lab, Sound Forge and Audition/Cool Edit all do), or loop the 15 second clip. You can learn tons about your preamps, effects, and whatnot with a wave editor and pink noise as well.

    I could make you a longer one, but you can probably find what you need online pretty easily. Standalone generators are fairly easy to build too.
     
  12. Selta

    Selta

    Feb 6, 2002
    Pacific Northwet
    Total fanboi of: Fractal Audio, AudiKinesis Cabs, Dingwall basses
    I do have sound forge and just looked for this, but didn't see it... guess I just gotta look harder. Thanks for the info. Is it safe to put the signal through a bass amp? :eyebrow:

    Ray
     
  13. ihixulu

    ihixulu Supporting Member

    Mar 31, 2000
    getting warmer
    So what is white noise? (and please don't say country, metal or punk)
     
  14. msquared

    msquared

    Sep 19, 2004
    Kansas City
    I was going to say "Kid Rock"...
     
  15. Passinwind

    Passinwind I Know Nothing Supporting Member

    Which version of SF? I could be wrong, I can ask someone who has lots of versions if you want. I use Wave Lab myself, and it's not in an obvious place. I'd think there must be freeware that'll do it though?

    Yeah, it's safe if the level isn't stupid-hot. It'll work things fairly hard though, so be a little careful. It's commonly used to come up with speaker power ratings, among other things. It makes heat well, so you can expect you amp to get warm if you push it.

    White noise is more like static or hiss, pink noise is filtered white noise and sounds more like waves.
     
  16. Selta

    Selta

    Feb 6, 2002
    Pacific Northwet
    Total fanboi of: Fractal Audio, AudiKinesis Cabs, Dingwall basses
    I've version 5.0... but it doesn't matter, I found a freeware that does it... if anyone's interested, I can host my .wav that it generated. I think I'll put it through my amp just for ****s n giggles.

    Ray
     
  17. Passinwind

    Passinwind I Know Nothing Supporting Member

    The part of my post you quoted about white noise was incorrect or at least unclear. I'll try to scare up a better definition, but I edited it out of my original post.

    Here, from Bob's link: http://www.yamaha.co.jp/product/proaudio/homeenglish/faq/glossaries/glossarie/promix01_glossary.html

    Pink Noise: "A type of random noise that contains an equal amount of energy per octave. The bands 100-200, 800-1600, and 3000-6000 all contain the same amount of energy: White noise, on the other hand, has an equal amount of energy per frequency band. That is, l 00-200, 800-900, and 3000-3100."
     
  18. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Yes please. I can then use my recording software to splice it into a longer wave file.
     
  19. Tim__x

    Tim__x

    Aug 13, 2002
    Alberta, Canada
    Those of you who are really curious might like too know that there are yet more colours.

    Red/Brown (after Brownian motion): Noise density decreases 6db an octave.

    Pink: Noise density decreases 3db an octave

    White: Flat, noise density equal at all frequency.

    Blue: Noise density increases 3db an octave.

    Purple: Noise density increases 6db an octave.
     
  20. Selta

    Selta

    Feb 6, 2002
    Pacific Northwet
    Total fanboi of: Fractal Audio, AudiKinesis Cabs, Dingwall basses
    OK, well, the porgram I used generated two files.
    One is a 4m40 some odd sec. file of just the noise... then another one that's 6m30some odd sec. file of the noise, then each frequency. Both are up, here's the link:
    http://www.bleedingsoul13.titaniumhosting.com/pink/

    Ray