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unbalanced power amp loads

Discussion in 'Amps and Cabs [BG]' started by Stinsok, Sep 2, 2004.


  1. Stinsok

    Stinsok Supporting Member

    Dec 16, 2002
    Central Alabama
    I have been running a mono signal to my power amp(bridged.) Would it hurt the amp to run a splitter with 2 mono signals into the amp-running a 4 ohm cab on one side and an 8 ohm cab on the other?
     
  2. Passinwind

    Passinwind I Know Nothing Supporting Member

    No worries. Many PA setups end up doing this, and it's not a problem at all.
     
  3. Trevorus

    Trevorus

    Oct 18, 2002
    Urbana, IL
    that's actually a good way to do it so you have control over each load. That way the 4 ohm isn't getting too much more power than the 8 ohm load.
     
  4. Stinsok

    Stinsok Supporting Member

    Dec 16, 2002
    Central Alabama
    When I run the two cabinets together it comes out to 2.67(?) ohms. I am not completely comfortable doing it, so I think I will try the splitter. BTW- are BBE's and The Sansamp rackmount preamps mono or stereo?
     
  5. Both mono, but I think the Sansamp has two mono outputs.
     
  6. CrackBass

    CrackBass

    Aug 10, 2004
    huntsville,AL
    most amps have a mono imput that will feed both stereo outs. just check your manual or get one on line. there should be no reason to run two imputs. most amps.

    but if you had to it would work fine
    just trying to save you a cable
     
  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
    You can't run a cabinet on each side in bridged mono. What you describe is parallel mono, not bridged.
     
  8. Fuzzbass

    Fuzzbass P5 with overdrive Supporting Member

    I often use a 4-ohm 8-ohm stack. Sometimes I run them from 2 channels of my amp, sometimes from just one channel. 2.67 ohms isn't a problem if the amp can handle 2 ohms.

    I can hear both of my cabinets just fine when I run them both from one channel. But this shouldn't be too surprising: the 4 ohm cabinet is the larger cabinet. A typical 4-ohm 8-ohm rig would be a 4x10 over a 2x10. The 4x10 gets twice as much power, but it also has twice as many speakers so the cabs are balanced (same wattage per speaker). My 4+8 (ohm) rig is typically 3x10 over 1x15, but still seems to be balanced. That probably wouldn't be true for similar cabinets that have different impedances: say, a 4-ohm 1x15 over an 8-ohm 1x15.
     
  9. Stinsok

    Stinsok Supporting Member

    Dec 16, 2002
    Central Alabama
    I had been running out of one cabinet into the other using the amp in bridged mode (not a cab off each side.)
     
  10. 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
    Okay, I see. The amp's minimum load impedance in bridged mono is probably 4 ohms or higher; a 4-ohm minimum would allow you to use two 8-ohm cabs connected in parallel or one 4-ohm cab, but not an 8 and a 4.

    In bridged you can put one mono signal into the amp.
     
  11. Stinsok

    Stinsok Supporting Member

    Dec 16, 2002
    Central Alabama
    After reading the QSC manual you sent me I realize that running the 4 and 8 ohm cabinet together while the amp was bridged probably smoked my Carvin FET 900. Not to worry, I sold it in "non working order" and"as is" for 103.00 on ebay.
     
  12. 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
    I don't know the specifics of the FET 900, but IMHO an important aspect of any power amp design is to put in adequate protection against excessively stressful loads.
     
  13. Passinwind

    Passinwind I Know Nothing Supporting Member

    Stinsok:

    I didn't catch the possibility that you would try to run the two cabs off two channels in bridged mode. By your question, I assumed you knew to go dual mono (if available), or just "stereo" position with a splitter feeding the inputs. Sorry for any confusion. Good thing Bob caught this, sounds like you've already smoked one amp, eh?
     
  14. Stinsok

    Stinsok Supporting Member

    Dec 16, 2002
    Central Alabama
    You would think that it would shutdown before any real problems occured. I was actually running the rig at a low volume. It may not be that expensive for the buyer to fix, I just didn't want to mess with it.
     
  15. I have a similar question...I want to get another 15 cab, so I can run 2 single 15 cabs and 1 2x10 cab. I would want to run stereo, which would mean one channel would have a 4 ohm load (2 1x15 cabs in parallel) and the other channel would have an 8 ohm load (single 2x10 cab). Is it OK to run like that?
     
  16. 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.