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Bridge vs Mono (8 ohm vs 4 ohm)

Discussion in 'Amps and Cabs [BG]' started by Ampig, Nov 29, 2004.


  1. Ampig

    Ampig Supporting Member

    I'm currently using an Ampeq SVP Pro preamp into a Crown PT 2.1 amp, biamped through an Ashly crossover into an Eden David 210 XLT and an Electovoice B115. At our gig last Ssturday the power amp shut itself down on over protect 3 times. Obviously I have a headroom problem. The question is this - would I see a significant boost in output if I were to the run the Crown in bridge mono straight to a some type of 6 x 10 cabinet? If so, any cabinet recommendations would be appreciated.

    Specs on the Crown PT 2.1 amp:
    460W @ 4 ohm stereo, 325W @ 8 ohm stereo, Bridge mono
    910 W @ 8 ohm
    howard
     
  2. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    Chicago
    Man...are you sure that your speaker cables are health and of heavy enough guage wire?

    Is the amp getting enough cooling air?

    Both cabinets (Eden and EV) are pretty efficient. If you're causing your PA to shut down you must be playing pretty darn loud!!!

    Does your crossover have any kind of subsonic filter to kill everything below a particular frequency? If so, are you using it? Perhaps excessive low end boost or subsonic noise is robbing you of usable amplifer power, creating excessive heat in the Crown and causing it to shut down.

    Unless you find a 6x10 that is a lot more efficient, I doubt that you're going to gain much volume by bridging and changing cabinets.

    In a somewhat similar thread, Brian Rost cited that from his experience it takes 4X power to make a really noticable volume difference...in your case that would be 2600+ watts (325x2x4 (assuming that your Eden/EV are both 8 ohms)) to get much louder...910 watts into a 6x10 isn't likely to do it.
     
  3. IvanMike

    IvanMike Player Characters fear me... Staff Member Supporting Member

    Nov 10, 2002
    Middletown CT, USA
    i would seriously reccomend that youstop biamping that rig and run both cabinets full range. I think you might pick up a decent amount of volume just doing that. Good chance it'll sound a lot better as well.
     
  4. Ampig

    Ampig Supporting Member

    "In a somewhat similar thread, Brian Rost cited that from his experience it takes 4X power to make a really noticable volume difference" I find that a little hard to swallow. Are you saying that there's not a noticable difference between a 200 watt and a 600 watt head?

    I'm not really looking to go much louder, but I'd like more clean headroom.
     
  5. metron

    metron Fluffy does not agree

    Sep 12, 2003
    Lakewood Colorado
    To be a little more specific, a linear volume increase requires more of an exponential increase in power dissipation. For example an 800 watt head may sound twice as loud as a 200 - 300 watt head (4x the power, 2x the volume) This is very general of course.
     
  6. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    Chicago
    Brian's comments were from his experience...of which he has much. I don't know Brian personally but have known of him for many years through both The Bottom Line and Talkbass and have always valued/respected his opinions/experiences.

    Like the rule of thumb that 2x power equals a 3dB increase in loudness which is "a little louder", I believe it takes 10X power to be "twice as loud", not 4X. So, assuming it's true, then a 600 watt amp won't drive a given loudspeaker that much louder than a 200 watt amp (given equal conditions...THD levels, etc.).

    I'm not sure how "clean headroom" works into all of this. I suspect that you just need a greatly oversized amp...or one with a very strong power supply that can supply the voltage swing that it takes for large power outputs...
     
  7. lo-freq

    lo-freq aka UFO

    Jan 19, 2003
    The Republic of Texas
    The only impedence he listed for bridged was 8ohm. [I'm not familiar with that amp.]
    If the amp can't bridge into 4ohm, he can't do two cabs unless both cabs are 16ohm apiece (not many of those around).
    If the amp is rated for bridging into 4ohm and both your cabs are 8ohm (or greater), I would 2nd IM's suggestion.
     
  8. IvanMike

    IvanMike Player Characters fear me... Staff Member Supporting Member

    Nov 10, 2002
    Middletown CT, USA
    i didnt say he should run it bridge mono, just run the full range signal to both sides of the preamp (using a y cord if needed) and power each cabinet off of its individual side. Just ditch the crossover, and run the amp in full range "stereo".

    yes it takes twice the power to increase volume by 3 dB which is generally considered to be the smallest difference a person can hear. Thus the assertion that you would need 4 times the wattage to have "an appreciable increase in volume", which translates to just two noticable degrees louder. you also do need ten times the wattage to be twice as loud.

    Right now the 210 is only getting half of the signal, and the 1x15 is getting the other half. It's a good chance that the eden 210xlt actually is capable of putting out more low end than the 15". By having both speakers put out the full signal you're going to get somewhere around 3 - 6 dB of a volume increase as you've doubled the amount of speaker area reproducing any given freqency.
     
  9. lo-freq

    lo-freq aka UFO

    Jan 19, 2003
    The Republic of Texas
    I did indeed misinterpret what you said ( :crying: ).
    You are indeed correct.

    Unless the x-over point is really low, the 2x10 is probably getting alot less than half the power (I know you said 'signal'. so I'm talking about something different).

    Ampig, do try it without the x-over; I think you'll like the tone much better and get noticably more volume.

    Note: the change may sound strange at first because you are not used to hearing it that way. If the amp has channel level controls you can tweek the balance a bit (shouldn't be bad even if the get the same amount of power).
     
  10. Ampig

    Ampig Supporting Member

    I'll try running the rig without the crossover in stereo mode and see what happens. I'm not as technically savvy as you guys, but there is a point being missed here about db's. They are indeed exponential. The difference between 90 and 100 db's is much greater than the difference between 60 and 70. Therefore if a 400 watt amp truly doubled the db output of a 200 watt amp, it would probably blow your head off.
     
  11. IvanMike

    IvanMike Player Characters fear me... Staff Member Supporting Member

    Nov 10, 2002
    Middletown CT, USA
    actually, the difference between 90 and 100 dB is the same as the differece between 60 and 70 dB. 10 dB! In either case the amount of wattage increase to effect such a change would be 10x. As dB represent points on a logarithmic scale (and have arbitrary values, they are really correctly figures of comparison, not fixed figures) they can be used to express signal strenth as well.