PLX1602 Bridged Clipping

Moderators: Cameron Shoffner, Christian Cook, Kirk Fyvie, Martin Barbour, Gary Evans_QSC, QSC-Dale Sandberg

Wed Mar 30, 2011 11:06 pm

  • Greetings,

    I am currently running an older JBL Sound Power rig off of a plx1202 (highs) and 1602 (mids). I have done some extensive work within the DSP (Driverack PA) in configuring limiters, gain structure etc. so that I can have a high average sound level (the room demands it, although there is no money for more rig - so it must be pushed) all without clipping the amplifiers. When the speakers are disconnected, I can run everything from pink noise, music playback, and live music without seeing a clip light. However, when the speakers are connected, I have noticed some transients catching the clip lights, and although not sustained and definitely just peaks, they are there. Would this be due to an actual load being presented to the amp (ie. the speaker is actually connected as it wasn't before)? Or, does it have to do with heat? They are definitely getting very hot, and have gone into thermal shut down before.

    One more fairly-related question is the one of the 1602's will clip much sooner than the other(about 6 or so dB), and when it is clipping sooner, it is on the right channel (CH 2) only, and never on the left channel (it is run bridged). Does this also have to do with heat? Since they first went into thermal shut down, I have made sure to leave space in the rack between each amplifier, and ensured that the back of the rack sees cool air. I have tried placing a fan in front of the amp but it still gets very hot and the clipping remains.

    Thank you for all the input!!

    Joe
    joe spitzer
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Thu Mar 31, 2011 11:26 am

  • Hi Joe,
    The problem seems to be related to the load applied to the amplifiers. First I would want to make sure that your power requirements do not exceed the amplifiers' capabilities. Here are the specs for both the PLX1202 and PLX1602 for Stereo/Parallel and Bridged operation:

    PLX1202
    Stereo / Parallel 8 ohms = 200W 4 ohms= 325W
    Bridged Mono 16 ohms = 400W 8 ohms = 700W 4 ohms = 1200W DO NOT USE a 2 ohm LOAD

    PLX1602
    Stereo / Parallel 8 ohms = 300W 4 ohms= 500W
    Bridged Mono 16 ohms = 600W 8 ohms = 1000W 4 ohms = 1600W DO NOT USE a 2 ohm LOAD

    If the above specs check out, then the next item to look at would be the pin configuration on your bridged connections at the amplifier. If you are using the binding post outputs, then you want to make sure that the “+” lead of the speaker is terminated to the “+” binding post output of channel 1 and the “-“ lead of the speaker is terminated to the “+” binding post output of channel 2. If you are using the Speakon connection, you want to make sure that the “+” lead of the speaker is terminated to the “1+” Speakon pin and that the “-“ lead of the speaker is terminated to the “2+” Speakon pin. Then make sure that you are connecting that Speakon connector to the Speakon output of amp channel 1. You would also want to verify that the connections on the speaker side line up with the manufacture spec. As long as all that checks out then it’s time to troubleshoot.
    Kirk Fyvie
    Systems Support Engineer
    QSC Audio Products, LLC
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    Kirk Fyvie
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Thu Mar 31, 2011 12:32 pm

  • Occasional brief clipping is normally nothing to worry about, particularly if you have the clip limiter engaged. Frequent or prolonged clipping would be cause for concern, though, and it would mean that you don't have enough amplifier power and/or loudspeakers for the situation. Do you know what load impedances the amps are driving?

    What is the purpose of the limiting? Is it just to prevent amp clipping?

    PLX amps' cooling air flow goes back to front. If the amps are running hot, a fan in the back pushing air into them could help. A fan in front of them will not be of any benefit. You don't need space between the amps.

    The odd clipping behavior of that one PLX1602 warrants some attention. That could be caused by a loudspeaker wiring issue (a single channel clipping prematurely in bridged mono often tends to happen if there are 1/4" connectors in the loudspeaker lines). It could also be a problem with the amp. If the loudspeaker wiring all checks out with neither side shorting to ground, perhaps intermittently, try swapping amps around to see if the problem stays with that amp or with the the position. If the amp is at fault, get it serviced.
    Bob Lee
    Technical Communications Developer
    QSC, LLC
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    "If it sounds good, it is good." —Duke Ellington
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    Bob Lee
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Thu Mar 31, 2011 4:16 pm

  • Thanks to everyone for the reply's!

    The main boxes present a 4 ohm load for the low/mid range, and are matched fairly well to the PLX's as far as power requirements, etc. I will make sure the wiring is consistent and correct from amp to speaker, and per Bob's suggestion switch the troubled amp out with the "good" one if the problem persists.
    I feel there is enough power for the room, just want to keep the amps from clipping (since a flash or two of clipping isn't terrible, I wont be so critical about it) :lol:

    When the PLX series is run Bridged, should the clip limiters and 30/50hz filter be engaged for channel two? What would be the "ideal" settings for a bridged mono situation?

    Thanks again, I really appreciate all the responses and support!

    Joe
    joe spitzer
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Thu Mar 31, 2011 5:24 pm

  • joe spitzer wrote:matched fairly well to the PLX's as far as power requirements, etc.


    One thing to keep in mind is that loudspeakers have power handling ratings but do not have power "requirements." It's common for users to employ limiters in their systems to cut down peaks, but I think those concerns are misguided. Short peaks seldom cause problems, but high average power causes high temperatures in both amplifiers and loudspeaker voice coils, and that's what burns things out.

    When the PLX series is run bridged, should the clip limiters and 30/50hz filter be engaged for channel two? What would be the "ideal" settings for a bridged mono situation?


    In the original PLX series, channel 2's input circuit is completely bypassed in bridged mono, so it doesn't matter whether its clip limiter and filter are on. The ideal settings would be to have channel 1's clip limiter on, as well as its low-frequency filter with frequency set according to what your loudspeakers can handle (if you're unsure, 30 Hz may give you a little more low end but 50 Hz will be safer for the loudspeakers). Set channel 1's gain only as high as you need. Channel 2's doesn't matter.
    Bob Lee
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    QSC, LLC
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    Bob Lee
    QSC Audio Products
     
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