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Bridging question

Discussion in 'Amps and Cabs [BG]' started by Captain Awesome, Aug 10, 2001.


  1. Captain Awesome

    Captain Awesome

    Apr 2, 2001
    PDX
    Excuse my ignorance, but is it possible to run two cabs in parallel from a bridged amp? If so, if the cabs were 8 ohms, would the load on the amp be 4 ohms?
     
  2. No.

    Bridged mode uses both amp channels, and each channel is presented with half the total load impedance.

    Your combined load is 4 ohms, which means each amp channel will see a 2 ohm load. Check with your manufacturer, as this is THE most harsh operating environment for your amp.
     
  3. MikeyD

    MikeyD

    Sep 9, 2000
    Yes, you can run the two in parallel. If they are both 8 ohms, the net impedance will be 4 ohms. There are many amps on the market that can safely power 4 ohms while bridged, but check the manual to be sure, or else call the manufacturer. You have to really pay attention to the load and how it's wired. As mentioned, bridging the amp as such, or otherwise feeding each side (unbridged) into a 2-ohm load, would probably be more stressful to the amp at high output levels.
    - Mike
     
  4. Captain Awesome

    Captain Awesome

    Apr 2, 2001
    PDX
    So basically I can't count on an amp to run safely at its advertised (i.e. 4 ohms bridged) output?
     
  5. 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
    If the amp is rated for 4-ohm bridged operation, it should be okay.

    Yes, 8 ohms || 8 ohms = 4 ohms.

    -Bob
     
  6. Captain Awesome

    Captain Awesome

    Apr 2, 2001
    PDX
    One more question: Is the "through" jack on a cab wired in series or parallel?
     
  7. 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 depends on the amp and the circumstances. Even for an amp rated for 2 ohms/channel or 4 ohms bridged, it's pretty close to the hairy edge, and it's the hardest way to run the amp. Some amps can better handle this type of operation than others.

    Plus, speaker impedances vary with frequency, and some speakers dip well below their nominal Z ratings at some points. So instead of a 2-ohm load on each channel, you might sometimes have maybe a 1.5-ohm load. That sucks a lot of current from the amp if you're running high levels.

    So if you run the amp hard, you run a higher risk of overheating it or making it current limit. But if you're careful and pay attention to your levels, you can probably run it successfully with no problems.

    -Bob
     
  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
    Parallel.
     
  9. All of the above being true, it still doesn't mean you can't run 2 cabs from a bridged amp. I believe there's still a bit of confusion on this topic.

    It all depends on the total load seen by the amp. If the amp handles 4 ohms when bridged, 2 8 ohms cabs can be connected.
     
  10. Captain Awesome

    Captain Awesome

    Apr 2, 2001
    PDX
    So as long the amp can handle 4 ohms bridged and only mono operation is needed, more output can be achieved through this configuration than running in stereo, correct?
     
  11. 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
    Yes, that's right.

    -Bob