I know that the lower the ohm rating the higher the watts it puts out... right?...

Im trying to buy a new amp rig (really just a cab and a head) and i know that for my needs i need at least 300 watts. Simple enough, or so i thought. Im just lost with what im supposed to be looking at with ohms. For instance, im looking at an ampeg cab, its "Nominal Impedance" is 8 Ohms and i think its a 300 watt cab (i say think because it says its "RMS handling" is 300 watts, while its "Program Handling" is 600 watts.), so when im looking for a head to go with it, should i look for an 8 ohm cab? What if its put with a 4 Ohm cab?

ah... thanks for any help fellas :/
First of all, look at the RMS, it's the most reliable source for wattage handling capabilities. Secondly, ohms. Your head puts out a different amount of watts depending on the ohmage. An example would be 300 watts @ 8 ohms and 500 watts @ 4 ohms. So, if you have an 8 ohm cabinet you get 300 watts. If you have two 8 ohm cabinets or one 4 ohm cabinet you get 500 watts. (note: cabinets are wired in parallel and so when you add two cabinets of the same ohmage together you get half the ohmage.) So really, all you need to know is what your head puts out at different ohmages, what your cabinet can handle and what the ohmage of your cabinet setup is.
Oh! thanks. So if i had a 300 watt @ 8 ohms cab, then what would i get with a head that puts out 300 watts @ 4 ohms? 150 watts?
Cabs don't have watts @ ohms. A head does. So if the head is 300 watts @ 4 ohms and you're putting it through an 8 ohm cab you'll probably get about 180 watts. A cab with handling anywhere above that would be good (preferably not too much higher though).
if you had 300 8ohms watts into a 300w 8ohms cab,
or 300 4ohms watts into a 300w 4ohms version of the same cab, would the 4ohms be louder and by how much?
there would be no difference because you'd be getting 300 watts either way.

Also, remember that added speakers(speaker cone area really) add more volume also. so if you plan on running at 4 ohms you may want to look into two 8 ohm speaker cabinets.

It was mentioned earlier that speaker outputs are wired in parallel(this isn't technically always true, but99.9% of the time it is true, so we'll run with the idea that this is a definite fact) This means that when you put two speaker cabs of the same ohmage through one head it halves the ohms.

here's a little table for reference: (note, 16 ohm cabs are extremely rare)
in parallel:
16ohms and 16 ohms = 8 ohms
8 & 8 = 4
4 & 4 = 2
16 , 16, 16, &16 = 4
8, 8, 8, & 8 = 2
8 & 4 = 2.66(only heads that can be run at 2 ohms will be able to do this)
16 & 8 = 4.66(only heads that can be run at 4 ohms or lower will be able to do this...which is pretty much every bass amp head out there now)
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Last edited by Revelation at Nov 29, 2009,
I'd like to point out that because of the way tube amps work, they always put out the same wattage regardless of what impedance(ohms) they're running at. Very few amps are full tube though.

Also, here's a good rule of thumb to go by when picking cabs. If you go from 8-ohms to 4-ohms, you nearly double the wattage. This does not double the volume. To get double the volume, you need 10 times the wattage. Adding a cab will raise the volume by about as much as doubling the wattage will.

What that means when picking cabs is, two 8-ohm cabs are much louder than one 4-ohm cab, because you've added speakers and raised the wattage.
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Last edited by Mutant Corn at Nov 29, 2009,
Ok. this is very helpful information im getting. So, when im shopping, if i want to get the amount of watts thats stated on the head, i should try to match the recommended impedance? I.E. if the head says 300 @ 8, if i want to get 300 i need an 8 ohm cab
You got it dude.

But most head goes to 4 ohm stable.

So if its 300 @ 8, that means like 400 @ 4.
So if you want the most, get a 4 ohm cab thats 400+ rated.
^^ basicly look at your head and it will say for example RMS= 300w@8ohm 450w@4ohm

then if you plug it into a 8ohm cab you get 300w or if you plug it into a 4ohm cab you get 450w.

it is then your responsabillity to first check the impedience of the cab your looking at (ie Ohms) and then see if its RMS power handleing is equal to the power output of the head at that givin impedence
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there is not exact formula to what amps give at certain ohms you have to check with each amp differently.

thare are amps that do 300W at 8 ohms and 400w at 4 ohms there are amps that do 300W at 8ohms and 550W at 4 ohms
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