Connecting your cab and amp: a mystery fit for Sherlock Ohms and Dr Watts-on

Ordinarily, the world of school physics lessons is a million miles from guitar heroics (unless, of course, you’re Angus Young and wear the same outfit for both). However, if there’s a place where they overlap, it’s on the back of your amp and speaker cabinet — and knowing what those scientific symbols and warnings written beside the jack sockets actually mean could be the difference between getting the best from your gear and shelling out for a repair.

The two symbols you’ll see most commonly are W, which represents power, measured in Watts, and Ω, which represents impedance, measured in Ohms, and the idea of this blog post is to explain what you need to know about them without blinding you with science. So here goes.

WATT ON EARTH?

The first thing you’ve probably noticed is that you don’t need to plug your cab into the mains. But those speaker cones have got to be powered from something, and it turns out that that’s your amp’s job. However, if your amp gives your cab too much power, you’re in trouble, and this is where the W number comes in. It’s a fairly simple but golden rule: make sure that the number of Watts (W) written on the back of your amp is less than or equal to the number of Watts written on the back of your cab. For example:

• If you’ve got an Orange Rockerverb 100 amp (100 W) going into an Orange PPC212 cab (120 W), then happy days, get shredding (don’t fret about the spare 20 W).
• If, however, you put the same amp into an Orange PPC112 (60 W), the amount of power that the amp will serve will be more than the cab can handle, which is bad news.

OHM-Y GOD!

So far, so straightforward. When it comes to Ohms, though, it’s a little more complicated, and there are endless web forums full of stuff you really don’t need to know on this topic, but it really boils down to another golden rule: make sure that the total Ohms (Ω) of your cab matches the total Ohms written above the relevant jack socket on the back of your amp. For example:

• If you’ve got an Orange Rockerverb 50 amp, you’ll see two sockets marked “8 Ω” and a single one marked “16 Ω”. If you’re plugging into an Orange PPC212 cab (16 Ω), then you want to use the “16 Ω” socket on the amp. 

If you’re running two cabs out of the same amp, though, here’s where you need to take real care, because two 8 Ω cabs plugged into the same head don’t together make 16 Ω — they actually make 4 Ω, which is obviously a bit counterintuitive. Sorry about that. Exactly why this happens is not that important here, and it doesn’t matter whether you’re daisy-chaining your cabs together or plugging them both directly into the amp. But you do need to know how to work out the combined Ω number of your two cabs, and the step-by-step trick to doing this has two routes, depending on if you’re more comfortable with fractions or decimals:

THE FRACTION WAY:

1. First, put a “1” over the Ω number of each individual cab (so 16 Ω becomes 1/16, 8 Ω becomes 1/8 etc).
2. Then, add up the two respective fractions (so, say, 1/16 + 1/16 = 2/16)
3. Then, reduce the fraction until there’s a 1 on the top (so, say, 2/16 becomes 1/8)
4. Finally, the number on the bottom of the fraction (in our example, 8) will be the total Ω of both cabs combined. Two 16 Ω cabs plugged into the same head make 8 Ω.

THE DECIMAL WAY:

1. On a calculator, type in “1 ÷ [the Ω number of each individual cab]” (so 16 Ω becomes 0.0625, 8 Ω becomes 0.125 etc).
2. Then, add up those two decimal numbers (so, say, 0.0625 + 0.0625 = 0.125)
3. On your calculator, type in that total, and then hit the “1/x” key. The answer to that (in our example, 8) will be the total Ω of both cabs combined. Two 16 Ω cabs plugged into the same head make 8 Ω.

So, if you have two Orange PPC212 cabs (16 Ω) and an Orange Rockerverb 50 amp (with two sockets marked “8 Ω”), you want to plug the two 16 Ω cabs into the two sockets marked “8 Ω” on the back of the amp, as the combined Ω of the two 16 Ω cabs are 8 Ω.

What if you don’t match the Ω numbers, you may be thinking? That’s a question for another time, but the short version is that it messes with your tone, and you don’t get the best out of either your amp or your speaker.

So, match your Ohms, and don’t exceed your Watts — case closed.