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who bridges @ 4 ohms successfully?

Discussion in 'Amps and Cabs [BG]' started by jbass67, May 12, 2004.

  1. jbass67


    Nov 17, 2003
    talked to crest today & one of the techs said the manual for the LT1000 does state do not bridge below 8 ohms. he also said they do advertise on the website as bridgeable at 4 ohms. he personally does not recommend bridging any amp at 4 ohms
    because the extra wattage has negligable volume as opposed to the stress on the amp. does anyone else bridge at 4 ohms without any problems?
  2. thetaurus


    May 28, 2002
    Muncy, PA
    i bridge my rmx 1450 into @ 4 ohms with no problems so far.
  3. bassic1959


    Jan 16, 2003
    I bridge my Behringer 1500 into 4 ohms. No Problems at all.
  4. joeybcdt


    May 6, 2004
    SE Texas
    I bridge my RMX 1850HP @ 4 ohms on my PA. No problems. The manuals I've read only warn about 2 ohm loads.
  5. I'm about to start bridging my QSC PLX-3402 to 4 ohms for my brand-new Acmes that haven't arrived yet. If there are problems, I'll jump in a vat of acid. Then yell at Bob Lee.
  6. NJL


    Apr 12, 2002
    San Antonio
    same with me. my QSC is the only thing in my arsenal that has yet to give me any problems. sometimes i'll feed it the wrong cable, but it still works like a horse.

    QSC is the bomb
  7. jivetkr


    May 15, 2002
    I bridge my QSC PLX 2402 into a 4 ohm load & it runs like a champ.
  8. QSC PLX1602 - bridge at 4 ohms - beautiful...
  9. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    Stewart 2.1 here, no problems.
  10. Same here. :ninja:
  11. Seymour Duncan 300x300 bridged into 4 ohms, no problems,

    SWR SM-900 bridged into 4 ohms, some problems (overheating), even though it's within it's limits. An external fan seems to have done the trick though.
  12. when you're bridging, both channels not only produce power, but they also share the total load. so while you're presenting the amp in total with a 4 Ohm load, each channel only sees half of that, so each channel is running into the equivalent of a 2 Ohm load. if your manual warns against using 2 Ohm loads, you should be aware that applies to bridging into 4 Ohm loads as well.

  13. jbass67


    Nov 17, 2003
    website says 2 ohms ok in stereo but manual has no mention of 2 ohm loads, pro or con. i guess the general concensus would lead me to bridge 4 ohms when i need to & run parallel with 2 cabinets.
  14. 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
    Operating at bridged mono into 4 ohms is electrically equivalent to driving 2 ohms per channel. For most amps, that's operating close to the hairy edge, and how you run it can make the difference between success (clean, uninterrupted operation) and failure (distortion, shutdown, or intermittancy).

    For any given output voltage below the clipping point, an amp is putting out four times as much power into a 2-ohm load as it would into an 8-ohm load. So operating at 4 ohms in bridged mono tends to make the amp work hard, and it'll run hotter.

    Most fan-cooled amps can tolerate running into clipping into 8 ohms per channel almost indefinitely, and into 4 ohms reasonably without incident. But any significant clipping into 2 ohms per channel (or 4 ohms, bridged) can make the amp overheat and/or current-limit (most solid state amps have current-limiting protection circuitry to help avoid melting the output transistors). But with the extra power available pushing 4 ohms in bridged, you would be smart to choose an amp with enough headroom to make that unnecessary.

    Also, make sure that air can flow freely through the amp to take away the heat. If you obstruct the cooling vents or have a lot of dust clogging them or the heat sinks, you may have trouble.

    And make sure you have enough AC to power the amp; it can draw considerably more current than when it's driving higher load impedances or operating in stereo or parallel modes. If you use extension cables, use heavy-duty ones that are only as long as necessary.
  15. jbass67


    Nov 17, 2003
    thanks rcz & bob lee for jumping in & elaborating. i was getting conflicting messages from dealer & manufacturer
    that i wanted to clear up so i don't run into any trouble
    setting up my rig.
  16. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Bob is correct.

    I would go further..running 4 ohms bridged is EASIER on the amp than 2 ohms per channel "dual mono".

    This is because in bridge, the power supply is symmetrically loaded, the "+" and "-" sides of it (typical) are both drawn from at once, but at a lower peak current.

    In "dual mono", BOTH amplifiers draw from the "+" or the "-" supply at one time, alternating between them. So even though the power draw is approximately equal, a much higher (double) current is drawn as a peak. Power supply components must be sized to handle the peak draw without too much "sag" or voltage reduction.

    Generally, an amp will actually put out slightly more power in bridge than the sum of the channel powers. And ability to handle peaks (dynamic headroom) is likely better also.

    Units with a regulated switching power supply are less affected in power, but still may have slightly lower losses in bridge.

    Oh, yeah. Ampeg amps that are bridgeable will run 4 ohms bridge if they do 2 ohms per channel. Example: SVT-4PRO, which many folks run bridged at 4.
  17. smarvelous


    Jun 2, 2002
    Albany, CA
    My Stewart World 600 overheats and fails when bridged at 4ohms (600 watts). I've heard that a fan will help prevent this (the stewart is normally convection cooled) But mine is a perfect example of pushing an amp close to clipping bridged at 4ohms, I should get a fatter amp or one with a built in fan. I have no problem running one side at 4 ohms (190 Watts) so that's what I do.
  18. secretdonkey


    Oct 9, 2002
    Austin, TX
    SWR SM-900 bridged 4 ohms, never ever any problems. Ran her close to clipping at a loud (ie, no PA support for me) outdoor gig last weekend for 3 sets with no troubles.

  19. I ran just the left side into my 8ohm 1X15 and 8ohm 2X10 running on half power (12 noon) because I thought I'd need to run the monitors out of the right side of the amp. The sound was good, but it would have been nice to turn up the low end a little.

    This was at sound check and the monitors were fine with another amp so I switched the config. and ran the amp bridged mono and turning the volume to about 9 o'clock. Wow, the amp had much more low end, extremely solid. None of the tone controls were touched and neither my bandmates nor the sound guy perceived more volume.

    Was this just my imagination or was this just more power and and headroom which made me believe more low end? Did this have anything to do with bridging the amp versus running only one channel?
  20. To any of you SWR SM 900 users, have you had the amp fail AND how do you rack/carry them?

    I've never had any problems with mine, I also have run the amp with 2 - 3 other bands playing at extreme volumes for 3 - 4 hour shows BUT I just sit the amp right on top of the cab with nothing on or around it.

    I beleive I'll never run it too hot, but I can't see what would happen if I put it in a two space rack.