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Ohm load when running amp bridged?

Discussion in 'Amps and Cabs [BG]' started by e-rock62, Oct 22, 2013.

  1. e-rock62


    Jun 25, 2007
    Winfield, Kansas
    I am planning on buying a bass preamp and a power amp. I want to run the power amp in bridged mode. I will be buying 2 4x10" cabs. If I buy 8 ohm cabs and run them both with the amp in bridged mode does that run at 4 ohms? Likewise if I buy 4 ohm cabs, will that run the amp at 2 ohms (which most will not do)? Thanks!

  2. If you buy two 4ohm cabs, you would not need to bridge, since you would be putting out decent power in the 'stereo mode'.

    Many PA type power amps can be bridged into a total 4ohm nominal impedance (i.e., two 8ohm cabs connected in parallel). This results in each side of the amp running at 2 ohms).

    I would suggest two 4ohm cabs. You get away from the '2ohm per side' issue, which is 'safe' with many PA power amps, but increases THD, heat and everything else, and for smaller gigs, you have a nice 4ohm 410 that will pump with a little 500 watt micro amp.

    That being said, either bridged into two 8ohm cabs or run 'stereo' into two 4 ohm cabs will work (assuming the power amp can bridge at 4ohms).
  3. Dave W

    Dave W

    Mar 1, 2007
    White Plains
    It really depends on the specific amp you're talking about, they are not all the same.

  4. Most PA type power amps will run 4ohm bridged, none of them that I know of will run 2ohm bridged.

    So, for the most part, his choice is two 8ohm cabs run bridged or two 4ohm cabs run 'dual mono', stereo.

    Advantage of the 2 x 8ohms bridged:

    -A bit more power than 2 x 4ohm stereo
    -The ability to get almost any 'all in one bass amp' at a
    later date if the component thing doesn't work out
    - Nice power bridged at 8ohms for a single cab for
    smaller gigs with the rack

    Downside of the 2 x 8ohm bridged... pushing that power amp to the max by running at 2ohms per side (which is what happens when bridged at 4ohms.

    Advantage of 2 x 4ohms running 'dual mono/stereo':

    - Each side running at 4ohms instead of 2 ohms, but still
    good power levels
    - Ability to leave the rack at home and run a single cab
    optimally with a micro for smaller gigs

    Downside.... must get a '2ohm stable head' if the OP moves to an all in one bass head later on

    Either will work fine though, again assuming the power amp can bridge to 4ohms.
  5. BurningSkies

    BurningSkies CRAZY BALDHEAD Supporting Member

    Feb 20, 2005
    Syracuse NY
    Endorsing artist: Dingwall Guitars
    If you find a power amp that tells you it will bridge into 2ohms, its best to walk away quietly. Its probably a lie.
  6. +1

    Since you live in this world, what is your take on bridging at 4ohms with two 8ohm cabs, versus running 'dual mono' with two otherwise identical 4ohm cabs (the OP's situation)?

    I've always tried to stay away from 2ohm operation (i.e., bridged into 4ohms) for a variety of reasons, but probably doesn't make much of a difference (i.e., you would get a touch more power bridging into the tow 8ohm cabs, but the increased THD probably makes the two ways of doing this pretty similar regarding total, clean output).
  7. BurningSkies

    BurningSkies CRAZY BALDHEAD Supporting Member

    Feb 20, 2005
    Syracuse NY
    Endorsing artist: Dingwall Guitars
    If you're careful of your amp choice, it shouldn't really be needed. One of the reasons I use a Pre/Power setup is that I have a setup that's flexible given my speakers and/or other amplification needs on stage.

    For instance here's what I work with:

    Rated Power (2 x 2 ohms) - 1,490 watts per channel @ 1 kHz 1% T.H.D. both channels driven.

    Rated Power (2 x 4 ohms)-840 watts per channel @ 1 kHz at <0.1% T.H.D. both channels driven.

    Rated Power (2 x 8 ohms)- 440 watts per channel @ 1 kHz at <0.1% T.H.D. both channels driven.

    What that gives me with 2 8ohm cabs is the option to:

    - Run each off of a channel for 440w per cab

    -Run both off of one channel, giving 420w per cab and an extra channel for something else (monitors, keys, extra front end, etc)

    -Bridge into both cabs for a 4ohm load, and get a *theoretical* 1490 per cab I only really do this in bigger venues and when I worry about house voltage (class D can get picky about low line voltage, and I have less problems with low headroom when I bridge). I also don't ask as much of my amp when bridged...I have my attenuators rolled back even further than when I'm running dual mono, so I'm not going to hit anything close to protection unless the house voltage is low.

    In most cases, if you're planning on bridging and you're using your amp solely as a bass rig, you maybe should consider looking at a bigger power amp. And with high power there is always the concern and extra caution that comes from more than potentially damaging voltages hitting your speakers.

  8. I don't know about "asking less from the amp" in bridge mode. As far as I know bridging involves one channel pushing a high voltage into the next channel to get amped again, massively increasing the heat in the 2nd channel, so backing off the attenuator probably only gets you back to square one at best.
  9. megafiddle


    May 25, 2011
    Bridging involves driving the two amps in opposite phase (the signal to one is inverted).
    The speaker then connects between the two amp outputs, instead of between an amp
    output and common.

    One speaker terminal is driven positive while the other terminal is driven negative. Total
    speaker voltage is double that of a single amp output driving the speaker to common.

  10. Bridging runs one amp inverted and one non-inverted (input signal), they combine at the speaker load.

    ^ I was late to the party. :)
  11. seamonkey


    Aug 6, 2004
    There's really so much watt per dollar available today in power amps that you don't need to worry about squeezing out every single last watt. You'll have plenty of head room on the cheap these days.
  12. BurningSkies

    BurningSkies CRAZY BALDHEAD Supporting Member

    Feb 20, 2005
    Syracuse NY
    Endorsing artist: Dingwall Guitars
    Um...no. Bridging is not cascading.


    Think of your amp as couple of old timey lumberjacks. With a cab on a single 'side' you've got one lumberjack doing all the work (back and forth, both sides of the wave). One cab on each side, two lumberjacks doing all the work to saw down two trees (each side of the amp laboring the same amount, but in isolation, working on that specific cab) . But now when you bridge, they have that big two man saw and they're working together! One on each side of a BIG tree (two cabs), each one is responsible for pulling in one direction only(doubling the voltage rails) and together exerting twice as much power.

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