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Power amp/cab impedance

Discussion in 'Amps and Cabs [BG]' started by LiquidMidnight, Apr 14, 2005.


  1. LiquidMidnight

    LiquidMidnight

    Dec 25, 2000
    Hello all;

    I have a 4 ohm cabinet, and I've been in the market for a power amp. I've been turning down a lot of nice power amps because the specifications only talk about bridging into 8 ohms. I was wondering why that is. Are the amps unstable at running bridged into a four ohm cabinet?
     
  2. 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
    There are a lot of nice power amps that can handle 4 ohms in bridged mono.
     
  3. fretlessrock

    fretlessrock Supporting Member

    Aug 8, 2002
    Corrupticut
    [Brass Bonanza plays quietly in the background]

    A lot of stereo power amps are designed to run a minimum 4-ohm load on each channel, and bridging usually doubles that minimum load requirement. So look for amps that will drive a 2-ohm load in stereo and the odds are good that they will drive a 4-ohm load in bridged mode.

    The QSC PLX and RMX amps will do this, and off the top of my head the Crest LT series, Crown Professional, and Stewart amps should be good too.
     
  4. 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
    Eight years + 1 day. :crying:
     
  5. LiquidMidnight

    LiquidMidnight

    Dec 25, 2000
    I know; it's just that if the specifications only mention that bridge output of a particular model in 8 ohms, I assume that it wouldn't be wise to bridge it into a 4 ohm cab.
     
  6. LiquidMidnight

    LiquidMidnight

    Dec 25, 2000
    Thanks. I always wondered why you get more wattage when bridged (I always thought that adding the wattage of each channel should add up, but it's always a little more. I didn't realize that you're actually running at 2 ohms). :D
     
  7. jpo259

    jpo259

    Feb 2, 2005
    Corona, CA
    I bridge my QSC RMX-850 into 4ohm cabs ALL THE TIME. No big deal at all. Now, my PLX-3402....I don't see me ever having to bridge that amp!!! Plenty of power with each channel!!!
     
  8. So Bob at QSC, why don't amps generally put out double the power at 4 ohms compared to 8 ohms?

    I've always assumed the supply voltage of the output stages must be sagging from the current requirements. Even given voltage squared /ohm load = watts (so that smaller voltage drops could cause larger power loss), seems like a lot of sagging voltage. Some amps only put out 1.5 x the 8 ohm power at 4 ohms, especially when bridged. Or have I missed another contributing factor?

    Can't someone come up with a power supply voltage bra, maybe a couple of 38 DD capacitors on the regulator? :D Keep the sagging to a minimum. Keep the power more proportional to the load.

    Randy
     
  9. 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
    Actually, they do. It's just the point at which the amp clips that droops with the heavier current demand.

    So below clipping, you double the amp's output power when you change from an 8-ohm load to 4 ohms.
     
  10. Yup, you're right, my bad, I was thinking of rated power output.... the amp rated for 1000w @ 8ohms bridged is usually only rated for 1500W or so @4 ohms bridged.... not 2000.

    Makes sense that only happens at the margins where the current demands are greatest.

    Randy