Running two mismatched cabs through a stereo amp... safely.

[K2000]

2 ohm subwoofer rated at 200 watts program/500 watts peak

with

4 ohm 2x12 rated at 600 watts

My amp is currently a Carvin HD1800... runs way too loud @ 900 watts/2 ohms for the 200/500 watt sub. Just right for the 4 ohm 2x12 @600 watts (efficient BFM omni cab that rarely needs to go more than halfway on volume).

My main concern is not blowing the sub accidently. Should I get a less powerful amp and run it wide open? I don't know if any commonly available low power amps can run at 2 ohms.

This is a dumb question but will the volume attenuators actually limit the wattage the speakers see? Can I simply dial way back on the sub's side? How would I know where I should set the dial? This sub will not distort when it's overdriven -- it will be clean up until failure. Using my ears wont work.

I have an old Sunn SA20 amp with better specs for the sub (300 watts @ 2 ohms) but not as good for the 2x12 (200 watts at 4 ohms, maybe not enough). Also, it's 3 rack spaces (I have space available for 2 ) and the fan is loud (probably not a factor).

Is there a way to use the Carvin without blowing the sub? Or what do you recommend?

Carvin specs, if that was unclear:
http://www.carvinguitars.com/manuals/HD900-HD1800.pdf

[bongomania]

The volume knobs don't actually limit the wattage, unfortunately. Say you manage to measure exactly 199 W while playing, with the volume knob at a certain setting--now use a bass with hotter output, or just play more aggressively, and *boom* you've got much more than 200 watts at the output.

There are a lot of 1-rack-space power amps in the under 200W range, intended for studio monitors. Or you could get a GK MB200, and just tuck it in back of the rack, behind the other amp.

[K2000]

If the sub is rated at 200/500, don't I want to have at least 200 watts (?) but less than 500?

I can't control the music in a way that would limit any spikes in volume. What amp ratings would maximize my output, while minimizing risk? Both are important: I need it to be as loud as possible, but safe (and hopefully not something I have to watch during the show... because the amp is behind me).

I'm not very tech-savvy, I'm just a musician

[K2000]

Doesn't look like the GK MB200 will run at 2 ohms. But that's an interesting idea.

[bongomania]

Unfortunately the wattage ratings (200/500) don't actually mean a whole lot. They are useful only as a broad indicator that maybe 1000 W is too much, for example.

I'd say your best bet is to put a brick-wall limiter on the sub channel, use the amp you've got, and hope for the best. When the speaker eventually blows, you can replace it with one that has better power and excursion specs, at a less awkward ohm rating.

[K2000]

If things weren't already complicated enough, the manual says the sub runs at 3 ohms.

It's a Servodrive Contrabass (somewhat rare and hard to repair) so blowing the sub is not an option I want to consider. I can't just put another speaker in there. The motor is what will likely fail, if it's driven too hard.

I will need to use a crossover... maybe I can find one with a good limiter? I have a spare crossover (dbx 223) but crossover is all it does.

[bongomania]

Even though I don't often recommend the digital stuff, I think a PA manager like the dbx Driverack, or the Behringer copy of it, would be good for your application. Crossover, limiter, EQ, everything you need for this kind of situation.

[K2000]

There's a Behringer crossover with a limiter (DCX2496). I'm going to review the manual to see if it's something I can operate

If so, it's one rack space and could be the simplest/least expensive solution.