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Bridged 4 ohms vs 8 ohms, difference in sound quality?

Discussion in 'Amps and Cabs [BG]' started by Mo' bass, Mar 31, 2006.

  1. Mo' bass

    Mo' bass

    May 4, 2000
    Have you guys ever noticed any difference in sound quality when bridging an amp into 4 or 8 ohms?

    Does bridging an amp sound different then running both sides stereo?
  2. 'fraid not in my experience.
  3. Running a 'mono block' amp at 4ohms is, for the most part, a very good thing. Most mono bass amps are designed to run 'optimally' (i.e., running cool and putting out maximum power within the amps spec) at 4ohms.

    As I understand it, bridging a 'stereo' power amp is somewhat different. Bridging a stereo power amp into 8ohms is the virtual equivalent of running each side at 4ohms in stereo.

    However, bridging a stereo power amp into 4ohms results in each amp seeing the equivalent of a 2ohm load, again from my layman's understanding of how this all works.

    So, the 4ohm/8ohm question is quite different regarding mono amps and bridged stereo amps.

    That being said, if a stereo amp is rated for 4ohm bridged operation, which I think means by definition, each side is rated for 2ohm operation, there should be no problem, and I don't think you'd hear a degraded sound at all.
  4. Mo' bass

    Mo' bass

    May 4, 2000
    Thanks guys! KJung I think your layman's understanding is 100% correct. I am talking specificaly about bridging a stereo amp by the way.

    If you take a look at the QSC specs you will see that they spec the 8ohm bridge at 0.1% THD and the 4 ohm bridge at 1% THD. It seems a big difference, I have no idea how (or if) this could be heard in a real world scenario... Anyone??
  5. That increase in THD doesn't surprise me, because IMO running amps at 2ohms (4ohms bridge) really is starting to 'push the envelope.

    I'll look forward to the more technically-oriented TBers comments on if there is any 'practicle' difference in sound quality between lower wattage at .1THD vs. higher wattage at 1%THD.
  6. greenboy


    Dec 18, 2000
    remote mountain cabin Montana
    greenboy designs: fEARful, bassic, dually, crazy88 etc
    I think that's more to do with AES ratings specs ... and the amp will drive a load at 4-ohms bridged with the lower distortion figure, it just will have a slightly lower wattage rating attached. If you look in the PLX manual several ratings are given at different distortion levels.

    For live use 1% THD is nothing - a lot of heads go way higher than that even before making much wattage.
  7. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    If you're talking about THD figures in power specs, do not confuse them with THD performance specs. They are just arbitrary definitions of the threshold of clipping.

    The answers to your questions in the original post are:

    Yes, because the loudspeakers are different, not because of the amp.

    No, unless the amp is defective.
  8. McHack


    Jul 29, 2003
    Central Ohio!
    Ok... so running a stereo power amp, in mono bridged mode into one 8ohm speaker generates a result. But, if you take a 2nd 8ohm cab of the same type & daisy chain it to the first, the net result SHOULD be, the amp is now loaded to 4ohms.

    Correct? And, under this implementation, would it cause degradation in sound quality?
  9. greenboy


    Dec 18, 2000
    remote mountain cabin Montana
    greenboy designs: fEARful, bassic, dually, crazy88 etc
    No. Unless the amp is crap. And if it's crap it's probably not doing so well at 8 ohms either.
  10. joelb79


    Mar 22, 2006
    Lansing, Michigan

    dont forget that speaker wire eats up power as well, especialy when you drop the ohms down to 4 and 2 ohms. ;)
  11. greenboy


    Dec 18, 2000
    remote mountain cabin Montana
    greenboy designs: fEARful, bassic, dually, crazy88 etc
    And some people use lots of sugar in their tea and coffee. But what does that have to do with this? ; }
  12. Mo' bass

    Mo' bass

    May 4, 2000
    Thanx Bob and greenboy! Good to know this. I'll keep on bridging happily than:bassist:
  13. greenboy


    Dec 18, 2000
    remote mountain cabin Montana
    greenboy designs: fEARful, bassic, dually, crazy88 etc
    My theory on stuff like this is that unless YOU can hear the difference (and you are not doing something electrically/electronically wrong that could bust equipment) that until someone you acknowledge as having smarts - and a very developed ear - tells you you could improve your sound by doing such-and-such, then just keep on folllowing your own ears.

    Experiment, build experiences and keep perceptive, and your ear will begin to hear subtle things it missed before.
  14. Mystic Michael

    Mystic Michael Hip No Ties

    Apr 1, 2004
    New York, NY
    Interesting that Beller would hold those views...

    I wouldn't hesitate to run any amp at 4 ohms, as long as it's rated accordingly - especially if I had lots of headroom. Running at 2 ohms is a different story. It's not something I would do, even if the amp is rated @ 2 ohms - other than just temporarily, in a pinch...

  15. Mo' bass

    Mo' bass

    May 4, 2000
    Right of course, but I like some theorizing now and then... :)

    When an amp is bridged into a 4 ohm mono load the same current flows as when driving a 2 ohm stereo load... Maybe that's why Bryan Beller has a problem with this?

    That would also mean that you'd get the same power from the amp in 4 ohms mono / 2ohms stereo. However, my Mo' Bass is rated bridged 900W @ 4 ohms and stereo 2*600W @ 2 ohms. Why is is not 1200W @ 4 ohms????
  16. Jack


    Sep 6, 2003
    Newcastle, UK
    Different THD figures?
  17. KPJ


    Oct 2, 2001
    Methuen, MA USA
    Bridging the amp is essentially summing two amps into one.Bridging at 4 ohms is the same as running both channels at 8 ohms. The Mo'Bass is most likely 450 watts a side at 8 ohms, hence the 900 watts bridged at 4. The amp would be 1200 watts at 1 ohm bridged, not recommended.
  18. 4ohm bridge is equivalent to each channel at 2ohms, not 8. The SWR900, which I think has the same power rating as the Mo Bass is listed as 400 watts per channel at 4 ohms, and 900 watts bridged. It lists a minimum of 2ohm per side capability (which is by definition if you can bridge to 4ohms), but does not list the wattage on the website at 2ohms per side. My guess is, that output is 450 watts per side into a 2ohm load, resulting in 900 watts bridged to mono. There are a number of heads out there (Walkabout, EBS350, etc.) that are safe to operate with a 2ohm load, but show minimum or no power output increase from 4 to 2 ohms... I have a feeling this is the case with your amp.

    Not sure about the Mo's 600 per side into 2ohms.. that would not jive with the 4ohm rating... I think that might be incorrect.
  19. pickles

    pickles Gold Supporting Member

    Mar 23, 2000
    Ventura, CA
    I've noticed that some amps are rated for 2 ohm operation, but list basically the same maximum output as 4 ohms. So the SWR might not be able to make any more than 400 watts even into a 2 ohm load.

    I can't remember if this is true or voodoo, but I've heard that running higher loads results in a higher system damping factor, so running an 8 ohm cab would sound "tighter". :confused:
  20. Mo' bass

    Mo' bass

    May 4, 2000
    Thanx guys! The 2*600W@2Ohms spec is in the Mo' Bass owners manual, so it should be correct...:eyebrow:

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