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QSC PLX-2402 at 2 ohms?

Discussion in 'Amps and Cabs [BG]' started by misterk73, Mar 14, 2004.

  1. misterk73


    Apr 11, 2002
    Flagstaff, AZ
    Are there any compelling reasons NOT to run a PLX-2402 at 2 ohms to get 1200 watts out of a single channel? I'm asking because I'm considering the purchase of a couple of 4-ohm cabinets and may eventually want/need to chain them and run them off of a single channel. I don't want to be surprised by the results when/if that happens...Any thoughts? Thanks!
  2. If it says it will work at 2 ohms then it should. I've used a 2402 in PA applications working at 4 ohms bridged (same as 2 ohms single channel) with no problems what so ever.
  3. GooseYArd

    GooseYArd Guest

    May 15, 2003
    The QSC amp I use lists higher distortion ratings at lower impedances, but I'm not sure if that's just a convention or whether distortion actually increases inversely to the load. The values are still pretty small even at 2 ohms for mine, so even though it's a tradeoff, it may not matter.
  4. Yes it is supposed to work at 2 ohms. Understand the amp works hardest at the 2 ohm load.

    It might be well worth your time to run an impedance plot of your particular cabinets. This will tell you exactly how low the impedance dips. For vented cabinets, this is almost always at the tuning frequency. The impedance peak is highest at the resonant frequency in sealed cabinets.

    If the cabs are 2.0 ohms at only 30 Hz, and you don't go lower than 41 Hz, the amp will never see the 2 ohm load.
  5. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    Astute and vaild technical point there Bruce and one that most folks overlook...

    Like GreyBeard, I've used my PLX2402 bridged into 4 ohms (a pair of Goliath IIs IIRC). It worked fine although I could feel its hot breath blowing on my neck from a few feet away (and yes, I was wearing ear plugs).

    I'd say go for it but make sure that the amp gets plenty of air for cooling.
  6. Eric Moesle

    Eric Moesle Supporting Member

    Sep 21, 2001
    Columbus OH
    I've been running one channel of my QSC PLX 3002 at 2 ohms every weekend for more than a year, and while it never overheated or failed outright, I just now recently developed an intermittent problem with that channel, the sound will sometimes throttle itself down drastically for a minute or two, then kick back in. Very annoying, and its getting looked at this week.

    Of course, the problem may be due to a number of issues other than being run at 2 ohms, no way to know for sure . . .
  7. zombywoof5050


    Dec 20, 2001
    Call me conservative (or just plain wimpy if you wish) but even though it's supposed to be able to run a 2 ohm load, I think that's pushing the amp too hard. See...Eric is now having to have his looked at, and only after one year. I'd much rather go easy on it and get many more years out of it. If you need more power, get a more powerful amp.
  8. Eric Moesle

    Eric Moesle Supporting Member

    Sep 21, 2001
    Columbus OH
    Just for clarification, while I've had my 3002 for about a year, I purchased it used from eBay - its probably about four years old.
  9. Nick man

    Nick man

    Apr 7, 2002
    Tampa Bay
    For what its worth I used a QSC USA 900 which is rated for 900 watts at 8 ohms bridged, into a four ohm load without any problems.

    I asked Bob Lee for advice and he told me that the worst that could happen in the amp would overheat and go into protect mode. No damage would result. He was right, I did this without problems for a very long time in very warm humid Miami weather.

    This is when running an amp at rattings beyond what is specified as safe. I imagine that if QSC amps can handle abuse beyond their specs, theyw ill be fine within their specs.

  10. You and I are of a common mind.

    I'd be inclined to use the RMX 1850HD for constant 2-ohm loads, rather than the PLX.

    A pair of *sealed* 4-ohm cabs in parallel aren't going to hit 2 ohms down low. Maybe at a couple hundred Hz, but not at 40 Hz. It all depends on the impedance plot of your particular cabs.

    I still find 2-ohm loads a bit scary, due to the heavy load it puts on the amps. I've been driving my PLX 3002 (left channel) into 4 ohms (vented) at 900 watts, red-lined for 3 years. No worries.
  11. robareed


    Feb 12, 2004
    there is much more distortion at 2ohm loads when its cranked up. i notice this with my pa speakers however i have not attempted it with a bass cabinet. the specs say something like .05% at 4ohm and 1.0% at 2ohm
  12. Another thing to think about is that you are splitting the power from the single channel at 2 Ohms anyhow... if the cabs are identical then half the power is going to each cab.

    so... why not just run each cab on a separate channel and not have to deal with that issue at all... unless you need that other channel for running a monitor or something you aren't gaining anything by going don this road. I ran a PLX1202 for a while and I think that parallel-mono is the way to go.

    If you "have to" run 2 ohms off a single channel of the amp, it should be fine but you do run the risk of thermal pretection circuit shutting it down.

  13. Bob Lee (QSC)

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

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Yes, you can do that, but if you'll be leaving one channel unused, then it'll just be smarter to connect one cabinet to each channel instead.

    Below clipping, the distortion into 2 ohms will be about the same as the distortion into 4 or 8 ohms; either way, it's minuscule.
  14. Bob Lee (QSC)

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

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    That's not right. Are you maybe confusing the power specs with the THD specs?
  15. GooseYArd

    GooseYArd Guest

    May 15, 2003
    I was confused when I looked at the spec sheet, I'm looking at it again now and I think I was mixing apples with oranges. I see:


    8 ohms 20Hz to 20kHz, 0.1% THD 240 watts
    4 ohms, 1kHz, 1%THD 425 watts
    2 ohms, 1kHz, 1%THD 550 watts

    but theres another row that lists 8 ohms, 1kHz, 1%THD with 270 watts. So that first row is a different kind of rating, right?

    Then I saw the next line that lists the THD at < 0.1%, but at 8 ohms, so I think I must have incorrectly inferred that it would be different at lower impedances.

    Does THD increase with power?
  16. Bob Lee (QSC)

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

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    In a power spec, the THD numbers just define the onset of clipping of the sine wave test signal. It is not a THD spec.

    A higher THD number in the power spec means that the sine wave was clipping more when it was measured. A sine wave clipping with 1% THD will measure a few percent higher in power than one clipping at 0.1%. And 10% THD would be a few percent higher than at 1%.
  17. GooseYArd

    GooseYArd Guest

    May 15, 2003
    Thanks and sorry about the confusion :)

    By the way, not that it matters a great deal, but I love my old USA 850. I've had it all over the country the past 8 or 9 years, and never had even a hiccup from it. I've gone through a bunch of cabinets and instruments but only one amp. What a tank.
  18. misterk73


    Apr 11, 2002
    Flagstaff, AZ
    Thanks for the feedback, all. If I were to run the 4-ohm cabs on one channel, it would be because I need the other channel for another applications (P.A. or running audio from a keyboard or computer system, for example.)...

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