bi-amping with an 8 ohm cab and a 4 ohm cab

Discussion in 'Amps and Cabs [BG]' started by goodwillsidis, Sep 21, 2005.

  1. Phew-- I been looking at the interent for a couple hours now trying to figure out the answer to this question, and I've learned a lot from this site... but I don't think I've quite learned exactly what I want to know. So I'ma ask you all point-blank:

    I have a 4-ohm Ampeg 4x10 cab and an 8-ohm custom cab with a 18" Eminence speaker in it. I also have an Tech 21 RBI pre-amp. I want to buy a power amp and a crossover so I can run these two cabinets together using the 18 for the lows and the 4x10 for the highs. I'm looking at Crest and Crown power amps, but how do I figure out what amp will supply each cab with enough power? The 4x10 runs at 600 watts RMS and the 18 runs 800 watts RMS-- so, do I want a power amp that will give me at least 800 watts to 8 ohms in stereo? I used to run just the 4x10 off a 350-watt Ampeg SVT head but I felt like it just wasn't loud enough to fill up the big barn I practice in, so I'm trying to get into a situation where I've got more than enough volume for each of these speakers... does that make sense? Reading about ohms and stereo and parallel and etc all day has made my brain a little mushy.
  2. Bi-amping requires an electronic crossover and two amp channels. The impedance difference makes no difference to the crossover, or the final sound.

    I suggest using a Rane MX22 for the crossover, because they are high quality, cheap, and plentiful on ebay. I paid less than $50 each for my pair. One is in my bass rig, the other in my PA rack.

    I run QSC power amps because I like them. I find them reliable and pretty much uncolored.
  3. How loud do I need the power amp to be? Which spec am I looking at to make sure that I won't have less than 800 watts going to my 18?
  4. Using one amp channel for the Highs, and the other channel for the Lows, you will need one big-ass amp to hit 800w into 8 ohms per channel.

    Almost none of the Eminence drivers will accept full rated power without exceeding the Xmax (linear motion) of the cone. This means the driver will be increasingly distorted as you approach maximum power. Life in the fast lane.

    Add more cabinets if you need more noise.

    An alternate solution is using two power amps, and bridging one of them. Anything above a QSC RMX-850 will provide more than 800w into 8-ohms when bridged. But... you will still need another amp channel for the Highs. I run a PLX-1202 for the Highs and PLX-3002 for the Lows when doing street gigs. This gives me 4x15 on the subs, and (2) 2x10 on the Highs.

    If you really need a boat load of noise, build a few Tuba horn loaded cabs instead.
  5. Thanks a lot, bgavin. It's all coming together in my head now... when I finally get this system up and running all that volume is gonna feel all the more sweet, given the mental gymnastics it takes to wrangle the right setup.
  6. Just be aware if you get a stereo amp that puts out 800W at 8 ohms in one channel, it will be able to put out around 1200 into 4 ohms, so you need to make sure you don't have distortion on the 4x10, cause the amp has enough power to cause a little havoc. Its not unreasonable though, conventional wisdom allows up to twice the amp RMS rating compared to the RMS rating of the speaker. Just make sure you don't get so loud its distorting.

  7. A9X


    Dec 27, 2003
    Sinny, Oztraya
    Turn down the level control on that channel of the xover. It's going to need to be done anyway as the cabs are different (so different sensitivities etc) and are working in different frequency ranges.

    Otherwise bgavin has given good advice above.
  8. The Rane MX22 offers individual gain controls for each channel, and both will do better than unity gain.

    This means the crossover will provide some boost for preamps with weak signals (mine) and power amps that use standard input voltages (mine).
  9. Turning the level down on the crossover (or on the one channel the power amp) doesn't mean the amp still can't put out more watts than the cab can handle. You still have to be careful. Turning it down will help balance the two cabs though.

  10. invisiblebass


    Sep 19, 2005
    I love seeing new bass-ers hook up 8 Ohm cabs to 4 Ohm Cabs thinking everythings cool, after ignoring advise given them watching everything go up in smoke or occasionally flames!!

    I figured i'd share that with the bass world! Its really funny to watch after they don't believe you!
  11. So, uh... are you watching all kinds of "new bass-ers" fry their amps when you're hanging out at some kind of crossroads near the on-ramp to the bass-playing highway?
  12. tadawson


    Aug 24, 2005
    Lewisville, TX
    If the amp will handle the load (2.6 ohms in this case), there is nothing intrinsically wrong with mixing an 8 ohm and a 4 ohm cab! You just need to realize that the 4 will get 2/3 of the power, and make sure that it can handle it. That, and the 4 is going to be 3db more prominent in your sound (assuming similar efficiencies).

    - Tim
  13. A9X


    Dec 27, 2003
    Sinny, Oztraya
    Based on Post # 1 where the rig described was a 1x18 below a 4x10, then at levels below earbleed (assume the SPL levels are matched properly) the 4x10, which will likely have much higher sensitivity than a 1x18, has to move significantly less air (volume displacement) to produce the same SPL as the 1x18 as it will be crossed at least an octave higher than the fundamentals. Crossing over actively means that the 4x10 will have a 4x reduction in displacement and power requirements per octave. It also means that the porting will be rendered moot as the box will be used above tuning frequency and so the unloading of the drivers below Fb will be reduced significantly which is more likely where the drivers will be damaged in a 4x10. The 4x10 is an Ampeg so it will be of decent design, sensitivity, driver selection and power handling. All told, probably less likely to be damaged in this scenario.
  14. thedoctor


    Jun 20, 2005
    I really love bi-amping bass but I never saw an amp that didn't need to be bridged to get the full output out of it so I thought you had to use two amps to get the best out of bi-amping. I guess with a 4 ohm cab and a 8 ohm cab it should be OK but that leaves the amp un-bridged and running at lower wattage. Like I said, I get confused a lot.