Running Mono Bridged vs. Stereo (2 4x10's)

Discussion in 'Amps and Cabs [BG]' started by jnprather, May 29, 2002.

  1. Hi all,

    I currently am running an SWR SM-900 with a single SWR Goliath III 4x10 cab. HOPEFULLY, i'll be getting my other SWR Goliath III back this week from service (Thank you to all who replied to my thread 'SWR Service Problem, San Jose, Ca.')! At any event, this will be the first time i'll have two cabinets running from my SM-900. Right now I am using my amp primarily to practice with a drummer and a guitar player. They both have loud setups but we don't play particularly loud. For now i've been able to get by just fine with one Goliath III running in Stereo mode (240 watts x 8 ohms), so volume isn't really a concern. I know i'll be able to get a little bit better tone at volume with both Goliath III's than just my one.

    Anyway, my question is concerning ohm loads and running amps/cabs in bridged mono vs. stereo. I understand the math behind it completely, but i'm trying to understand from a user standpoint (with volume not really a concern), what my best option would be. I've been running my amp in Stereo 8ohm mode right now because I always figured if i'm loud enough, it would be less "wear and tear" on my gear and less likely to damage my cabinet. Is this correct logic? And if so, does this logic transfer over to a setup with two Goliath III's? I'm not going to bi-amp or anything (or have any stereo effects), so running in Stereo would only be for the sake of running less power. Running both of my cabinets in Stereo would give me 240watts per side @ 8 Ohms. Running them Bridged Mono would give me 900watts @ 4 Ohms. That would mean that each cabinet would be getting 450 watts, correct? If they are able to handle 700 watts each, I shouldn't be even remotely worried about overpowering them, even at Bridged Mono mode, right? Now from the amp's standpoint, would it be "easier" on the amp at all to run two 8 ohm stereo loads than one 4 ohm mono load? Would it run cooler?

    The bottom line is that I want to know if there's any benefit for me to run in Stereo mode rather than Bridged? If Bridged gives me more volume but Stereo is easier on my setup i'll run in Stereo because volume is not an issue right now...

    Thanks in advance for the replies (and sorry if some of my post seemed redundant)

  2. Hi John,
    I've got an SM-900 which I'm running bridged mono into Aguilar 2x10 and 2x12 cabs. I'm goin' from the amp to the 2x12, and the 2x12 into the 2x10. It sounds really great.. just huge and powerful. Getting the full 900 watts is nice, because even if you don't need that much volume, you have the headroom to be able to turn up louder and keep it clean. The 2x10 is only rated for 350 watts and it's getting 450, but I'm not running anywhere near peak power. I went through an entire band rehearsal today without clipping once.
    As far as whether it's easier on the amp to run it in stereo, someone else on the board might be able to chime in. I've never run mine in stereo, so I really don't know. My amp is running as hot as it always has. It's doing more work than it used to with one 4x10.
  3. malibu

    malibu Guest

    Dec 26, 2001
    maybe this is only a typo, but then what you say after that doesn't seem so right .

    there's no per side with the bridged mono, so if it is putting out 900 watts into a 4 ohm impedance
    - and if that load is made up of two cabs in parallel - then they are each getting up to 900 watts.
  4. With the SM-900, bridged mono is 900 watts into 4 ohms. In stereo, it's 350 per side into 4 ohms, I believe. As I understand it, in bridged mono, if it's going into two cabs, they're each getting 450. That's at peak power, which is not the norm.
  5. Wxp4759cb


    Nov 23, 2000
    Kansas City, MO
    I've got a similar question.

    1) If you run the power into one Goliath 3 8ohm, and the out of that Goliath into another Goliath III 8ohm, do you have a 4 or 16 ohm load?

    2)If this generates a 4 ohm load, then is it better to run 1500 watts bridged into one with another hooked up to it. Or to run 750 watts stereo to each?

  6. If you run into an 8 ohm cabinet, and out of that into another 8 ohm, you are running 4 ohms...

    The problem with your second question is... running the same two cabinets in stereo does not give you half the power of running then in mono... for instance, me running two Goliath III's in mono would give me 900 watts, and stereo would give me 240 watts per side... not 450...
  7. malibu

    malibu Guest

    Dec 26, 2001
    oops, my bad.
    the voltages would be the same across all the parallel components - amp -cab1 -cab2 -- not the watts .

    cab outlet are parallel in Goliath III, so yeah parallel 450 watts to each
  8. No malibu, you were right the first time... it was a typo on my part...
  9. malibu

    malibu Guest

    Dec 26, 2001
    except I guess I'm still not so sure about the 450 watts part -
    and at least anyway they wouldn't both have to get an exact equal 450 watts -
    -any slight ohms difference could mean one cab gets slightly more amps of current
    and does this really have to add up to 900W total always - could the bridged mono get 500 watts delivered to one of the cabs ?

    stupid maybe example - one of those lights with like 4 multiple bulbs on a strip above a bathroom sink
    you can unscrew one and the others stay lit so I guess parallel , anyway. . .
    it seems I could put in various watts of bulb 75w or 100w - then throw in a 60 watt (or do they just all end up at some same power ?)

    it just seems like it doesn't need to be divided up so equally -
    say your at half volume still with headroom to go but one cab has a very different efficiency - I don't think the amp drives the two parallel voice coils with the same power.
  10. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    The amp should run cooler in the stereo mode with an 8 ohm load on each channel.

    In the bridged mono mode with a 4 ohm load (i.e. two 8 ohm cabinets in parallel (the jacks are in parallel on the goliaths (and probably every other cabinet unless it's marked to indicate a series connection)), each side of the power amp will "see" a 2 ohm load...that's why there is such a dramatic power increase. Each amp section will work much harder to drive a 2 ohm load and will consequently run hotter. I don't know about SWR's take on bridged mono operation at the minimum impedance, but I seem to remember reading that QSC says that running their amps that way significantly reduces the amp's life span.

    The SPL difference between going stereo and bridged mono would be minimal...if it's adequately loud in stereo, I'd recommend that you run it that way.

    Have fun!!!!
  11. I checked with SWR and they said it was fine to daisychain cabinets in bridged mode, as long you don't go below 4 ohms.
  12. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    While it may be true that you can run the amp bridged into 4 ohms, I took a look at the manual for the amp and saw something interesting...

    In the stereo mode, SWR rates it at 240 per ch @8 ohms, 400 per ch@4 and no rating at 2 ohms.

    Typically in the bridge mode, each side of the power amp sees half of the load impedance. If you're using two 8 ohm cabs (total impedance 4 ohms) each power amp sees 2 ohms. This suggests that the 2 ohm rating in the stereo mode is 450 per channel...not much more than it's 4 ohm rating. The power supply must be running out of steam...I mean current.

    If you double the 4 ohm stereo output rating, you should get the 8 ohm bridged rating...which should be 800 watts (2x400) but SWR claims only 650.

    Unless I'm missing something, it seems like the amp is really being pushed in the bridge mode.

    So, unless you really need the extra watts I'd still recommend that you stay in the stereo mode with an 8 ohm load on each channel.

    Bear in mind that the specs are just have to honor the minimum impedance (so you don't blow your amp up) but beyond that, go with what sounds best!

    Have fun!!!!
  13. ihixulu

    ihixulu Supporting Member

    Mar 31, 2000
    getting warmer
    I recall reading in one of the threads here that SWR employs a "current limiting" circuit to allow the amp to operate at low impedances without frying itself. Whatever that means, I don't know.

    FWIW, I've been running an SM400 bridged into a 4 ohm cab for hours on end for a while now and it hasn't crapped out on me once.

    Also, in the manual for the SM900 it does say that 2 ohm per side in stereo is the minimum load, without specifying output power. I'd imagine that you'd get about 450W per side at most.
  14. Interesting stuff... Thanks for your replies. That pretty much goes along with my logic but I just needed to hear someone confirm that.