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Power Amp in Bridged Mono

Discussion in 'Amps and Cabs [BG]' started by huskies90, May 16, 2005.


  1. huskies90

    huskies90

    May 17, 2004
    I am thinking of going to a power amp set up. I currently have two 8 ohm cabs. Two questions: 1) I assume that bridging into 4 ohms, the wattage would be split equally to each cab (as it is from a mono amp) is this true? and 2) How hot does an amp like a QSC run bridged into 4ohm? And is it safe for constant use or only once in awhile? I know my mono amp really "cooks" when I run it at 2 ohms.
     
  2. lo-freq

    lo-freq aka UFO

    Jan 19, 2003
    The Republic of Texas
    If the cabs have the same impedence, they should each get half the output of the amp.

    How warm the amp gets depends on at least a couple of factors:
    ambient temperature (outside in 100 degree heat?)
    how close to clipping you drive the amp and how often it's that close

    Some people claim that bridging into 4ohm is very stressful on an amp.

    My take is if you give your amp plenty of fresh air (if I play outside in the summer, I set a stand fan behind my amp) and are not pushing it hard (I just occasionally blimp my -10dB lights) then it will be fine (another good reason for having plenty of headroom available).
     
  3. Transverz

    Transverz believer of the Low End Theory

    May 3, 2004
    Los Angeles, CA
    This is something I don't see mentioned that much here on TB regarding bridging amps and whether or not they cause more or less strain on the amp. This is from a thread I started a long time ago. I quoted this same comment on another thread to which Bob Lee was apart of and did not correct nor dispute, so I'm guessing its at least got some merit. And I personally agree with it :D Check it out:

    FWIW...

    -T
     
  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
    Running bridged mono into, say, 4 ohms, is electrically equivalent to running 2 ohms per channel. An 8- ohm load in bridged is equivalent to 4 ohms per channel.

    How hard it is on the amp depends on hard hard you push it, but pushing to near clipping is a lot more stressful on the amp at low impedances than with higher ones.
     
  5. huskies90

    huskies90

    May 17, 2004
    That makes sense. So, in theory, what would be worse for the amp, pushing a bridged 8 ohm load to full power or a bridged 4 ohm load at half power or would they have the same effect? If I got the plx 1602 and bridged it at 4 ohms I could (should) only push 800 watts out of the max 1600 because my 8 ohm cabs are only 400 watts each and if I push it too hard I would probably blow my speakers anyway...Is that logic correct?
     
  6. Producing 1500W is harder on the amp than 1000W, make sense? I don't think running 1000 W @ 4 ohms is any harder on the amp than running 1000W @ 8 ohms.

    Running 4 ohms bridged can be harder on the amp than 8 ohm loads because it allows it to produce more power, therefore more waste heat, therefore higher temps compared to the same amp running at an 8 ohm load. The excess heat can cause problems, stress, shorten the lifespan of stuff.

    End of the day, watts is watts, at 4 ohms there's less voltage, but more current across/through the output transistors, compared to more voltage/less current at 8 ohms. The changes cancel each other out, its a wash. IME

    Randy
     
  7. 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
    That's making it a lot more complicated than necessary.

    Stop worrying about how much power you should "push" out of the amp, since you can't control it that much anyway without external other gear, like a limiter.

    Is 400 watts the continuous power rating of the cabinets?
     
  8. huskies90

    huskies90

    May 17, 2004
    Yes, 400 watts RMS. I don't want to get too technical. I'm worried about three things, 1) having enough power to do a few outdoor gigs this summer with no PA support 2) not blowing my speakers 3) not pushing my amp too hard. Right now I have two 8 ohm cabs rated at 400 watts RMS each and a mono amp - 350w at 4 ohms which is definitely running out of head room for these type of gigs. I wanted to add a QSC power amp to power the cabs. Please help with the easiest least expensive way to do this...
     
  9. 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
    Okay, they should work fine with a PLX 1602. The two cabs connected in parallel would amount to a 4-ohm load with a continuous power handling capacity of 800 watts.

    Here are some guidelines for helping make sure you don't blow out the loudspeakers or make the amp overheat and shut down:

    • Avoid clipping the amp.
    • If you see the clip LED light up, back off on your levels, by turning down the amp's gain, turning down the preamp, whatever.
    • Make sure air can flow freely into the back of the amp and out the front.

    BTW, are you a UConn or Northeastern or UW fan? ;)
     
  10. huskies90

    huskies90

    May 17, 2004
    Thanks Bob!! I borrowed a QSC rmx1405 from a friend to try it out. When A/B'ing the power from my swr 350 and the QSC I thought the swr had a little more depth and low end so I think I will power my 15" speaker with the swr and then power a pair of 210s with the QSC. With the set up I should be be able to run the cabs in stereo even at 8ohms from the QSC. Three cabs at about 250-300 watts at 8ohms each should be sufficient, no? For extreme power needs I could always bridge the power amp.

    Now, I tried hooking it up in bridge mode. I barely turned it up and started playing at a very low volume. My speaker made the most ungodly popping/crackling sound I had ever heard. I almost had a heart attack thinking I blew a speaker or my tweeter. I unplugged the QSC and plugged the swr back in everything sounded fine. What did I do wrong??

    BTW, UCONN HUSKIES ALL THE WAY. I am assuming you are from the hartford area. I went to Uconn in the late 80s and had a very close friend who was the Whaler ticket manager plus, my brother was a season ticket holder. So, I attendend many Whaler games before they left. Dat dat dat da-da da-da dat...duh dat da-da da-da dat!!!!
     
  11. 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
    How did you have it hooked up and set up?

    Re Hartford and UConn: Yup, I lived in Vernon for many years.
     
  12. huskies90

    huskies90

    May 17, 2004
    Here is how I set it up:

    Effects send from the swr to ch 1 input.

    For Stereo:

    Ch 1 output to one speaker and ch 2 to the other. Then I set up as follows: 30hz filter on parallel input on strereo on everything else off.

    For Bridged:

    I just switched it from strereo to bridge mono. Connected one speaker to channel 1 output and then daisy chained to speaker two. Everything else I left the same.
     
  13. 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
    For bridged mono, you un-parallel the inputs (switches 4 and 5), turn channel 2's LF filter off, turn channel 2's gain control all the way down, and connect the speaker cable to the bridged mono outputs, which are the two red binding posts and/or the upper Speakon's 1+ and 2+ terminals.
     
  14. Jeb

    Jeb

    Jul 22, 2001
    USA
    Hey Bob,

    How "hardy" are the RMX or PLX series amps running bridged at 4ohms? Honestly.

    I'm sure its a pretty common practice, but is the design such for long term durability in that application? Any thoughts?

    Thanks.
     
  15. 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 all depends on how hard you run the amp, how much heat (waste power) it has to dissipate, and how effective its cooling system is. Bridged mono into a 4-ohm load is electrically equivalent to stereo or parallel into 2 ohms per channel.

    The PLX amps all have the same heat sinks and fan, so the ones that don't have to dissipate as much waste power have an advantage over the ones that do. The PLX 1202 driving bridged into 4 or stereo into 2 ohms, I'd rate a 9 on a scale of 1 to 10 because it's almost impossible to make it shut down due to overheating. The PLX 2402 is the lowest-power model with a class H output section, so it has some extra efficiency; I'd rate it about an 8. Next, the PLX 1602 and PLX 3002 would both be a 7. The PLX 3402 I'd give maybe a 5.5 to 6.

    The RMX HD models have a higher ratio of cooling capacity to power dissipation, so the RMX 1850HD and RMX 4050HD would both be 9s. The RMX 850 I'd rate as an 8, RMX 1450 as a 7, and the RMX 2450 and RMX 5050 as maybe 6.5 to 7.

    A couple rules of thumb for driving 4 ohms in bridged mono: don't drive the amp into clipping, and make sure cooling air can flow freely into and out of the amp. If you have an amp that draws a lot of current, either plug directly into the AC outlet or power distro, or if you have to use an extension cord, use the shortest and heaviest gauge one possible.
     
  16. Jeb

    Jeb

    Jul 22, 2001
    USA
    Thanks for that! You're the best!

    jeb.