Watts an Ohm?

a guide to buying amps and cabs

Probably the single hardest part of buying a new amp or cab, other than paying for it, has to be figuring out how many watts of an amp for how many ohms of speakers. There is a good reason for that, to fully understand exactly what it all means, one has to have a degree in engineering, it’s that complicated.

Taken from wikipedia.org

The watt (symbol: W) is the SI derived unit for power. It is equivalent to one joule per second (1 J/s), or in electrical units, one volt ampere (1 V•A).
It is the rate in joules per second at which energy is being converted, used, or dissipated.

Lol don’t worry everyone’s head hurts now, okay so now that you see what sort of complications we are working with,I won’t use anymore equations or forumlas, I’m going to attempt to put it all into simple terms that everyone can understand, so please don’t send me emails, giving detailed explantations on how I was wrong in oversimplifying and then attach a 4 page equation that will explain it better.

Ok lets get started with the amp or head. They are rated by watts like everyone knows, watts are basically how loud the amp is, a 100w amp is louder than a 50w, a 200w amp is louder than 100w and so on and so forth. This is just a generalaztion, so don’t send me stories of how much your 50w amp smokes.

Now keep in mine this is the peak amount of power they amp can produce when running wide open/turned up all the way, and only for millaseconds, a 100w amp doesn’t pump out a continous 100w of power, this is the part where amp makers get tricky. The actual numbers that mean something, which are usually hidden on the back or buried in the spec sheet, is RMS rating. Continuous or RMS power simply tells us how much power a given amp can deliver day-in/day-out on a steady basis. (RMS stands for root means square, an equation that specifies average power). So a 300w amp with a RMS of 150w, would run 150w, all the time and peak out to 300w for small amounts of time. (just an example each amp’s rating is different).

That leaves ohms, and keeping with my promise of no more math stuff, I’m going to skip explaining it, as long as you know how to use the number of ohms, you really don’t need to know what they are or what they do, a google search will provide more detailed info, but it is basically the resistance the power has to go thru, using a plumbing example I saw somewhere, if your amp was a water system, water is sound, the water pressure is the watts, and the ohms would be the size of the pipe; which restricts how much water can actually get thru at one time.

Confused yet? I’m going to explain how this all correlates with the cabinet and then give you some easy to understand examples with real products, and a cheat sheet to match up cabs and and amps. Now that you kind of have an understanding of all those numbers and words, you have I’m sure started to realize that the cabs have the same nonsense rating on them as well.
1200w 8 ohm cab 1000rms, I’m sure you have seen something like before, and using our knowledge gained can deduce that the cab takes an 8ohm single and can handle up to 1200watts of power for a small amount of time, and can handle taking 1000watts all the time. Do I see lightbulbs popping up yet?
All right lets break it down in a real world solution using a Behringer bx3000T amp and avatar speakers.

Behringer:300w RMS into 4 ohms.
Okay first we know our cabinet has to be able to handle 300w all the time.
By looking at the cheat sheet below we can run 2 - 8 ohm cabs, or one 4ohm cab.
I like using a 4x10 over 15’s, so I want to buy a 4x10 cab and a 1-15 cab.
So I have to get both cabs in an 8 ohm version, also both cabs must be able to handle 300watts of power at all times. Now off to http://avatarspeakers.com/ , to get what I need.
B410 Neodymium - 1200 watts RMS 4 or 8 ohm
B115H NEODYMIUM - 300 RMS Watts 4 or 8 ohm
Sweet, the 8 ohm version of both speakers match my amp’s guidelines so I’m set.

You should have the basics down enough to go out and buy an amp or cab without fear.
You always have the cheat sheet below to refer to, in case you get stuck.

Cheat sheet
for cabs wired in parallel
2ohm amp
2 - 4 ohm cabs
1 - 2 ohm cab

4 ohm amp
2 - 8 ohm cabs
1 - 4 ohm cab

8ohm head
2 - 16 ohm cabs
1 - 8 ohm cab