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Bridged Mono

Discussion in 'Amps and Cabs [BG]' started by mrpackerguy, Apr 11, 2005.


  1. mrpackerguy

    mrpackerguy Supporting Member

    Jul 3, 2004
    Madison, Wisconsin
    Bridged mono seems to always supply more watts than the individual channel wattage added together. For instance, my QSC RMX runs about 850W bridged mono, but 400W + 400W per channel stereo.

    In a set up with one amp powering 2 separate speaker cabs, would you always run stereo? ( 400W + 400W)

    Or is there a way to safely run the bridged mono signal into both cabs?
     
  2. Well yeah, it would. If you're using two 8ohm cabs in stereo the RMX 850 puts out 200 watts per channel. Put the two cabs together and you have 4ohms and the 850 puts out 830 watts bridged into 4ohms.
     
  3. mrpackerguy

    mrpackerguy Supporting Member

    Jul 3, 2004
    Madison, Wisconsin
    Sorry if the questions are dumb, but I only recently went to a amp/preamp after 25 years using combo amps/amp heads.

    How would you output a bridged mono signal into two speaker cabs? In my set-up it would be two 4 amp cabs. Would each cab really still only be getting about 400W per cab just like if they were run in stereo? If so, is there an advantage to stereo over mono in a stacked set-up?
     
  4. You mean two 4ohm cabs? Oooo sorry, it can't be done. Two 4ohm cabs together makes a 2ohm load and you can't bridge an RMX (or any other power amp i know of) into a 2ohm load.
     
  5. mrpackerguy

    mrpackerguy Supporting Member

    Jul 3, 2004
    Madison, Wisconsin
    Yea, thanks. I think I got it now. I can run a parallel mode with one input but using both channels feeding each cab about 400W.
     
  6. Each channel individually will handle a 2ohm load, but you can't bridge both channels into a 2ohm load. So you could run both cabs off of one side and use the other side to run a whole nother stack of cabs...muhahaha! :cool:
     
  7. Funkengrooven

    Funkengrooven Turn it down? You gotta be nuts!!

    Don't let anyone fool you. Your amp only delivers as much power as it delivers. Period. If you have 200 per side in stereo you have 400 in bridge. However... What you have in bridge that you don't have in stereo is HEADROOM, and not Max Headroom either.
    What that means is your peaks can go way up, alot higher because you have more headroom. The other condition is that the bridge uses the power supply differently which effects headroom but you can still only get 400 watts.
    What you might get is 215 watts in stereo and 430 in bridge, but you better pay attention to the lowest recommended impedance otherwise you are in for at best a meltdown and at worst a nice fire.
    AND you CAN hook two 4 ohm cabinets up they just have to be in SERIES making a very safe 8 ohm load, then you can add another 8 ohm cabinet and have lots of speakers and still not go below the 4 ohm limit (if that is the case)
    HOWEVER...there are lots of those pesky howevers...the amp is designed to absorb only so much reverse voltage from your speakers..(known as damping factor) wanna see reverse voltage? hook up a DC voltmeter to the speaker and press down and release...see the voltage?? Get 20 speakers doing that and even if the impedance is right the amp will burn. which is why the most efficient setup is the most power thru the least number of speakers.
     
  8. mrpackerguy

    mrpackerguy Supporting Member

    Jul 3, 2004
    Madison, Wisconsin
    I'm considering buying another 2x10 cab. Both cabs are rated at 4 Ohms. To power them, 'm running a QSC RMX 850. Currently, I run bridged mono into the 4 ohm stand alone 2x10. If I do get the other 4 ohm 2x10, what's the recommendation on how to run it?
     
  9. Stereo with each cab having it's own power amp channel. More power into each cab that way.
     
  10. Funkengrooven

    Funkengrooven Turn it down? You gotta be nuts!!

    Absolutely Right, Go back to stereo mode and run one cabinet from each side.

     
  11. IvanMike

    IvanMike Player Characters fear me... Staff Member Supporting Member

    Nov 10, 2002
    Middletown CT, USA
    ok, here's the deal and why bridge mono works for more power.

    with a poweramp you have ratings for 8 ohms per side, 4 ohms per side, and (if the amp will do it), 2 ohms per side.

    let's pretend we have an amp that puts out 200 watts into 8 ohms per side, 350 into 4 ohms per side, and 425 into 2 ohms per side.

    say you have two 8 ohm cabs. run them in stereo and you end up with 200+200, or 400 watts.

    when you run an amp in bridge mono, each side gets 1/2 the load, so it "sees" 1/2 the impedance.

    so you take those same two cabs, and daisy chain them together for a 4 ohm load (unless you have special cables or jacks daisy chaining always ends up with the cabs wired in parallel, ditto for stacking bannana plugs). then you run the amp in bridge mono mode into them.

    each side "sees" a 2 ohm load and delivers 425 watts. 425+425=850 watts, compared to the 400 watts you put into them before when running in "stereo".

    what does that do for you? well, it gives you 3 dB of headroom. plus, many speakers tend to sound a bit different when hit with a lot of watts. IME, cabs sound a bit more articulate and "open" with fewer watts, and "thicker" and "fuller" with a lot of watts. what you like is up to you.

    unfortunately for those of us with two 4 ohm cabs, daisy chaining them together and running an amp in bridge mono mode ends up with each side "seeing" a 1 ohm load, which means smoke, fire, a voided warranty, and one bummed out player.

    bridge mono is very cool when using one 8 ohm or one 4 ohm cab. with our hypothetical amp it makes the difference between running an 8 ohm cab at 200 or 700 watts, or with a 4 ohm cab from 350 to 850 watts.
     
  12. huskies90

    huskies90

    May 17, 2004
    I am thinking of going to a power amp set up too. 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 2ohms.
     
  13. arjune

    arjune

    Oct 8, 2006
    SF, CA
    I had to bump this one because I had a question that fit right in.

    So, say I've got a 400W/8Ohm Cab and a 600W/8Ohm Cab, and I've got a Power Amp that's rated at 1200W/4Ohms Bridged Mono and 200W/8Ohms Stereo.

    So, if I want to hook up my speakers Bridged Mono, do I just use one cable to my first speaker, then a second cable daisy-chaining the first and second speaker together?

    I'm not exactly sure how the whole Bridged Mono thing works.
     
  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
    The reason bridged mono offers more power than with a single channel is because the two channels' outputs are put in series, which results in twice the output voltage capability of a single channel. (Even though they're called "power" amps, they're really voltage amps with a good amount of output current capacity for driving relatively low-impedance loads.)

    That's also why there's no point in series-connecting loudspeaker loads for use with a bridged amp. It would be like putting two 6V batteries in series and then connecting them to two 6V lamps in series; it would make more sense to connect a lamp to each battery separately. But if you had a 12V lamp, then two 6V batteries in series would a good way to power it; you could even power two 12V lamps that way, as long as the batteries have the oomph to put out twice as much current.

    [​IMG]
     
  15. amos

    amos

    Oct 23, 2003
    SE Portland Oregon
    Word. What he said. ^^^
     
  16. BassmanPaul

    BassmanPaul Gold Supporting Member

    Aug 25, 2007
    Toronto Ontario Canada
    Sorry but I'm confused. If you are running in bridge the separate channels sort of ceace to exist. If you have switched the amp to bridge you connect the hot of channel one to the + of your speakers and the hot of ch2 to the (-) of your cabinets.

    If you are connecting one cabinet to each channel you are running papallel with one cabinet out of phase to the other!

    Paul
     
  17. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Technically true......

    But there is a lot of perceived loudness and power due to the following (also described somewhat by Bob Lee):

    At least continuous average ("rms") watts.

    The "uses the power supply differently" is actually quite important.... with two channels in parallel (stereo) each "polarity" of the power supply is loaded by both channels at once, each demanding max current. Both load the "+" supply, then both load the "-" supply, alternating. This puts a big demand on the transformer and filter capacitors.

    In mono, each channel gets one polarity to itself, halving the current draw per "polarity", and allowing the filter capacitors to hold up the voltage twice as well as in stereo. it is quite a bit easier on teh power supply than "stereo" mode, and you can usually hear the difference.

    Not at all clear to me as a long-time amp designer why that would be true..... If it were, then two 810 cabs would be a horrible, amp-damaging load...... but they aren't.
     
  18. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Um...SVT? That's 16 drivers the way it was originally designed. Not quite 20 but close. I've heard of the heads catching on fire on rare occasions, but never because of the speakers. Usually it's a loose wire or a bad tube or something. Or pyro ;)

    This sounds like something worked out on paper but not actually put to the test before the author published it.