Effects of Bridged Mono?? Attn: Power Amp guys

Discussion in 'Amps and Cabs [BG]' started by jonasp, Aug 19, 2004.

  1. Hey TB. Transverz and myself were talking and we got some conflicting answers about the effects running amps in bridged mono. Transverz posted some info he got from Ampeg_Insider.
    Here's the full thread: http://www.talkbass.com/forum/showthread.php?t=135927

    I emailed Mark Hellinger a question about bridged mono and got this reply:

    Now, we figured both of these men are pretty knowlegable and know what they are talking about. Is there a right answer? Does running an amp in bridged mono have any longer term effects?

    Pete and Transverz
  2. Transverz

    Transverz believer of the Low End Theory

    May 3, 2004
    Los Angeles, CA
    Yeah...what he said... :ninja:

    Thanks in advance :D

  3. brianrost

    brianrost Gold Supporting Member

    Apr 26, 2000
    Boston, Taxachusetts
    The simple answer is that if you run two 8 ohms cabs in stereo, each side of the amp sees an 8 ohm load. Run them together in bridge mono and each side of the amp sees a 2 ohm load. That's why it puts out so much more power, you've effectively cut the impedance by a factor of 4.

    If you check specs on power amps, running each side at 2 ohms instead of 8 ohms yields higher much distortion figures, makes the amp run hotter and draws more current. The heat is the biggest thing, even if the amp is well cooled you will stress the components more by running them hotter.

    A second issue is that speakers are not fixed impedance. An 8 ohm speaker may have a much lower (or higher) impedance at certain frequencies. Go to any speaker maker's web site to see specs that show this. If the speakers dip to say 4 ohms at 100 Hz, then in parallel this will result in a 2 ohm load at this frequency. Now each side of the amp sees only 1 ohm...oops, smoke starts pouring out :cool: On the other hand, in stereo mode each side of the amp sees a 4 ohm load at that frequency which it can easily handle.

    So why bridge at all? $$$$$$$$$$ :meh:

    If I need to drive 500 watts into each 8 ohm speaker I can buy a stereo amp that puts out 500 watts at 8 ohms or I could buy a bridgeable one that outs out 500 watts at 2 ohms, bridging to 1000 watts at 4 ohms. The bridging amp will be much cheaper (in general). For applications that use lots of power, the bandwidth (thus impedance swings) is limited and higher distortion is OK (i.e. SUBWOOFERS) bridging is a real boon but you do have to understand the pros and cons of doing it.
  4. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    All interesting points of view...

    AI's comments were interesting because I never considered that bridging would place less strain on any part of the amp.

    I would have to believe that bridging puts a greater strain on the output devices at a given load impedance (since the effective load impedance that each channel sees is half of the load impedance).

    From personal experience, I remember the first/only time that I ran my QSC PLX2402 in the bridge mode...my power amp was at neck level and I was stuffed in the back of the stage...I remember the PLX' hot breath on the back of my neck (bridged into 4 ohms...each channel effectively seeing 2 ohms (for which it was rated)). In the stereo/parallel mode, I don't recall it getting that hot with 8 ohms per channel or even one channel at 8 and one channel cranked into a 4 ohm Acme B2.

    I recently bought a nice used Hartke HA5000. While I've only used it at home, the volume that I get (judging by how much I'm shaking the house apart and how quickly my wife and kids object to the noisyest kid in the house :D ) with 170 watts per cabinet (one 8 ohm cabinet on each channel) is right up there with my PLX2402 running at 425 watts per cabinet. Theory would dictate that when using the QSC it should be a little louder, and maybe it was, but this exercise told me that I really don't need a huge power amp. My point...you probably really need a lot less power than you think (assuming that you're not using inefficient cabinets)...so why bridge and place extra stress on the output devices of your power amp?
  5. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Apples to apples, please.

    If your amp is loaded in bridge or mono to put out the same power, bridge mode may be considered slightly less stressful.

    If you simply hook the same exact loads first in stereo and then parallel them in bridge, well, surprise.....you are putting out lots more power in bridge, and it would be very surprising if the stress level were not different in those two different conditions.

    It is load dependent, first, THEN it is slightly mode dependent. In that order.

    Now, If you take two 2ohm loads, and put one on each channel in "dual mono", you can put out power "X". Peak current from each half of the power supply will be "Y", drawn from each alternately (assuming signals in phase...i.e. dual mono).

    If you hook them in series and set up that load in bridge (net 4 ohms), each channel sees exactly the same 2 ohm loading as before. You still get at least power "X", maybe slightly more.

    However, now each supply must put out a peak current of only "Y/2", half as much as before. The total power remains the same, because each "half supply" sources the same current (but opposite polarity).

    Because the peak currents are halved, the peak power supply losses go down to 1/4 of those in dual mono. (resistive losses go as current squared).

    The most hard-worked, and most often under-rated parts in the power supply are the filter capacitors. The capacitors and rectifiers are far less stressed in bridge, due to lower peak currents.

    Does it matter in the larger scheme of things? Dunno.

    It does NOT affect the amplifier sections, if the total power is the same in each mode. The amps operate the same either way, their load and losses are essentially the same either way.

    Only if you change the loading does the amp's output device stress change. Loads of 2 ohms per channel or 4 ohms bridge are the exact same amplifier stress.

    It is the power supply that sees the difference in bridge. For it, 2 ohms per channel dual mono and 4 ohms bridge are different, and 4 ohms bridge is an easier load.

    The issue is most applicable to subwoofer amps, they are typically run the closest to full power of any portion of the sound equipment. Bass amps are rarely run as hard as 1/3 power continuously.

    I wouldn't lose sleep over it, but you usually get slightly more power, and better transient peak performance running bridge.
  6. 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
    Contrary to common myth, running an amp in bridged mono is not destructive to the amp, nor does it cause more distortion, nor any other bad things. As Jerrold and Mark Hellinger said, bridged mono is just two amp channels working together--one pushing while the other pulls, and vice-versa--to produce twice the potential voltage swing and quadruple the potential power of a single channel.

    That can be a good (even vital) thing if you want a lot of headroom and only one monaural source of audio power.

    Operating bridged into a 4-ohm load is electrically equivalent to operating at 2 ohms per channel, and bridged into 8 ohms is like 4 ohms on each channel.

    It's not worth making more complicated than it actually is.
  7. RAM


    May 10, 2000
    Chicago, IL
    ...so, bringing this down to my level, correct me if I'm wrong...I have a Stewart World 2.1, which is 2100 watts if I bridge both sides into a 4 ohm load. Here's my question...with an Epifani 4x10 UL, which is a 4 ohm cabinet (in my case), bridged-mono would stress the power amp to a 2 ohm load???
  8. 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 would be the equivalent of a 2-ohm load on each channel.

    If you push the amp very hard, yes, that will be stressful on the amp, but if you were to push the same power out of one channel alone, it would be stressful on that channel plus you would have about 6 dB less headroom.
  9. TheChariot


    Jul 6, 2004
    Boston, MA
    Ah... the "H" Word. ;)

    Let's not forget people.... with the almighty power we draw from our amps in bridged mode... we ususally dont have to crank the gain up nearly as much. That's gonna save a little bit of stress.

    Let's hypothetically say we're trying to power an 8x10 fridge. Is it more secure to have a super-powerful amp that could give you a Kilowatt or more in parallel? Technically... yes. But I cant see a major problem with running a power amp bridged on a regular basis. You likely wont have to really crank out all those watts anyways, in all likelyhood... unless your playing a giant outdoor show with no PA. and honestly, I dont think that's very common for most of us at all.