Ohm-ulators and Watt-ologists: Please check my math

[Lowbrow]

My Acoustic B600H delivers 600 watts at 2 ohms.

If I run this head into:
* Two 8-ohm cabs, and
* One 4-ohm cab

The amp will operate at 2 ohms, with the 4 ohm cab receiving ~300 watts and each 8-ohm cab receiving ~150 watts - correct?

[bacid]

Yes... It will be two ohms, as far as who gets what, I have no Idea, and am still trying to figure this out myself. I don't really know how the amp distributes the watts to each cab, just that it will be 600 @ 2 ohms total.

[Sneakypete]

In theory this is correct and I myself run a 16 ohm 2x10 and an 8 ohm 1x15 (making five point something) and it sounds fine to me although you will hear plenty of reasons on TB as to why this is bad practice. Although obviously subjective I would say that the 16 really is delivering something like half of the perceived loudness of the 8 ohm. This is fine because it also has half the power rating. On paper both cabs combined have a 575W rating but as I only use an old school TE head (250W) I can turn it up way louder than I ever need too and it still has the nice clean, slightly growling sound I like. I always use the rule of thumb that a decent middle of the road cab can safely handle something like half the power it claims on the plate. Personally though I would never run an amp at 2 ohms - even if the manufacturer says it's OK it still seems too much like a short-circuit to me!

[RickenBoogie]

The op's math is correct, but it would help to know which cabs, exactly, we're talking about.

[Lowbrow]

Originally Posted by RickenBoogie

The op's math is correct, but it would help to know which cabs, exactly, we're talking about.

Well, what I'm really up to is just a bit of tomfoolery not unlike the hi-jinks I used to get up to when my high school band rehearsed in my parent's basement - piling up and connecting all the band's gear to see what kind of sound it would make.

I normally run a Walkabout (300w at 4 ohms) into 2 Bag End S15Ds (very efficient; 300w @ 8 ohms each), and it sounds great. The Acoustic 2-ohm amp and a Schroeder 115 cab (efficient 400w @ 4 ohms) are collecting dust, and I wanted to hook all cabs together to the Acoustic just for s**ts and giggles - without frying anything. Not planning to really go with this setup unless the outcome is somehow miraculous.

[IvanMike]

try it - you won't break anything.

chances are one type of cab will be louder (hard to say which one). Likewise, the two different types of cabinet may not sound good together, or they may not.

BTW, your math is correct.

[hyp.spec]

Would not the one with least resistance carry the higher wattage?

I'm assuming it would depend on exactly how the cabs are wired up. If the 2-8ohm cabs are daisy chained and connected into your head with one input then it would see a 4 ohm load.

If this is the case, each 8 ohm cab would receive exactly half the wattage of the 4 ohm cab. So a 500w amp would put 125w into each 8-ohm cab and 250w into the 4ohm.

I am by no means an expert... but I sure hope I learned SOMETHING in Physics II...

[Bass_Pounder]

Originally Posted by hyp.spec

Would not the one with least resistance carry the higher wattage?

Yes, you are correct that the 4 ohm cab would get the higher wattage.

However, that alone does not dictate which cab(s) would be louder.

If one of the 8 ohm cabs has a sensitivity that is 3db higher than the 4 ohm cab, they will be equal. Anything above +3db, and it will be louder than the 4 ohm cab.

[dsanders]

Great title .

[bacid]

Originally Posted by hyp.spec

but I sure hope I learned SOMETHING in Physics II...

I'm glad someone did, I sure didn't. Hence, why I'm an Economist and not a Physicist. What about when using a tube amp that "will produce its full power at all of these impedances?" I understand path of least resistance and all that good stuff, but does a tube signal not discriminate? The tube amp is a Mesa 400 and will be running through a Bag End D10XD (4 ohm) another D10XD (8 ohm) and an S18D (8 ohm), in case you were wondering.

[Lowbrow]

Originally Posted by IvanMike

... chances are one type of cab will be louder (hard to say which one).

I'll go out on a limb and predict the Schroeder will be louder as they are some of the most outspoken cabs I've ever played through.

[Implosion]

Originally Posted by bacid

I'm glad someone did, I sure didn't. Hence, why I'm an Economist and not a Physicist. What about when using a tube amp that "will produce its full power at all of these impedances?" I understand path of least resistance and all that good stuff, but does a tube signal not discriminate? The tube amp is a Mesa 400 and will be running through a Bag End D10XD (4 ohm) another D10XD (8 ohm) and an S18D (8 ohm), in case you were wondering.

It will be the same as it would be with a solid state amp. The output of a tube amp doesn't depend on the load, but with solid state amps the output depends on the total load presented. How the watts distribute to cabinets are the same on both cases.

[bacid]

Thanks! Now I just need to wait for this monster to arrive.

[Lowbrow]

Originally Posted by Lowbrow

Well, what I'm really up to is just a bit of tomfoolery not unlike the hi-jinks I used to get up to when my high school band rehearsed in my parent's basement - piling up and connecting all the band's gear to see what kind of sound it would make.

I normally run a Walkabout (300w at 4 ohms) into 2 Bag End S15Ds (very efficient; 300w @ 8 ohms each), and it sounds great. The Acoustic 2-ohm amp and a Schroeder 115 cab (efficient 400w @ 4 ohms) are collecting dust, and I wanted to hook all cabs together to the Acoustic just for s**ts and giggles - without frying anything. Not planning to really go with this setup unless the outcome is somehow miraculous.

Update: tried out the 3 1x15 cabs setup powered with my Carvin B800 2-ohm head.
Actually sounds GREAT! And it looks cool, too. Will post a photo soon. The varying impedances and speaker efficiencies worked out to a very even output between all 3 cabs. Thanks for the guidance