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QSC - RMX-HD or PLX?

Discussion in 'Amps and Cabs [BG]' started by Metal Mitch, Sep 24, 2003.


  1. Metal Mitch

    Metal Mitch

    Jul 14, 2003
    NJ
    Well I've pretty much made up my mind to go with a QSC power amp. But I still have some questions about the different models.

    What's the deal with the RMX HD models? Being optimized to run at 2 ohms or 4 ohms bridged sounds great, but why would they need an HD model unless there were problems with the non-HD models?

    How do the PLX series amps handle running 2 ohms / 4 ohms bridged? As good as the RMX-HD, or not?

    Lastly, how does the sound quality compare between PLX and RMX-HD? Which has faster response time and which is punchier?

    I really don't care about the weight or fan noise of either one. Mostly looking for good reliabilty running 4 ohms bridged / 2 ohms a side, and sound quality.

    Thanks guys!
     
  2. I like the idea of an amp expressly optimized for 2 ohm loads. This would support (4) 8-ohm drivers, all in parallel. One would get all the advantages of an all-parallel configuration, and an amp built expressly to deal with it.

    The downside is the 45 pounds. I've owned RMX amps in the past and to my ears are not any different sounding than the 3 PLX I currently own.
     
  3. Schwinn

    Schwinn

    Dec 4, 2002
    Sarasota, FL
    The RMX 45 lbs might not seem like a lot...but I've got one and I figure my rack is almost 70 lbs. There's really no good way to carry it without straining your lower back. If I were to buy another power amp today I'd get a PLX because I'd love my rack to be 22 lbs lighter.
     
  4. 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 HD just means that the amp has a lot of cooling capability compared to its needs, which is good for high-current situations like running 2 ohms per channel or 4 ohms in bridged mono. It's not "optimized" or anything.

    I don't know what your phrase "response time" means, but we're talking about electrical signals traveling at nearly the speed of light.

    The PLX sound quality is overall better than the RMX, but it would still be hard to tell them apart in anything but a very controlled listening test.
     
  5. Metal Mitch

    Metal Mitch

    Jul 14, 2003
    NJ
    Thanks for the replies guys. Cooling is a good thing... we did a show earlier this year at a local rock club, and by sheer coincidence I wound up with one of the stage backlights shining directly into the back of my rack. :rolleyes:

    I didn't notice until my amp shut down on the last song of the set. :eek:

    So Bob, does the PLX "Advanced Thermal Management System" provide more, less, or about the same cooling as an RMX HD model?
     
  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
    It depends on the model. The PLX 1202, for example, has a big surplus of cooling capacity. Even shorting out the output for several minutes won't necessarily make it shut down due to overheating (but it will trigger the current limiting circuit). The PLX 1602 has very good thermal capacity, but not as good as the PLX 1202.

    The PLX 2402 has a 2-tier class H output section, which makes it more efficient than just a straight class AB, so it's not only more powerful but overall better thermally than the PLX 1602. The PLX 3002 is also class H and more powerful, but it's getting close to the practical limit of what the PLX cooling scheme can handle. The PLX 3402 (class H, too) is really right at the limit and can be made to thermal by pushing it fairly hard into 2 ohms per channel or 4 ohms in bridged mono.

    With any of them, cooling is not a problem into 8 or 4 ohms per channel; only when you get down to around 2 ohms per channel (or 4 ohms bridged) will you encounter these differences.

    With the RMX HD models, as I mentioned, it's a matter of cooling capacity versus need. The RMX 850 and RMX 1450 use the same chassis and circuit boards--just the RMX 1450 has more and different parts to make it more powerful. We should've called the RMX 850 an HD model if we'd thought of that distinction back when it came out. The RMX 1850HD is almost identical to an RMX 2450, except that it can't put out as much power. (Both are two-tier class H.) Imagine two cars--one's got a V8 engine and the other's got a smaller V6, but they have the same radiator, fan, etc. The one with the V6 isn't as powerful, but it's less likely to overheat when running hard than the V8 model. It's a similar concept with the RMX HD amps. The RMX 4050HD has a three-tier class H output, which is even more efficient than the two-tier, and it's 3 rack spaces high, so the fan is about double the cross-sectional size of the 2RU fan in the other models, and the heat sinks are about 50% bigger, so it's cooling capacity is huge. Its big near-twin brother, the RMX 5050, will be coming out next year, but it won't be an HD, as far as I know.
     
  7. Metal Mitch

    Metal Mitch

    Jul 14, 2003
    NJ
    Wow! That's great info, thanks very much Bob. You rule!