Impedance - what you need to know

If you loiter about any music forum for a while (as I tend to do), sooner or later someone is going to ask about impedance. No other subject seems to generate such confusion, but with a few facts under your belt it's all pretty straightforward. Here I'll attempt to explain why you may need to know about impedence and the essentials of making sense of it all.

Impedance - what is it?

Simply put (and that's how I'll try to keep it), it's a measure of the resistance of some electrical component to the flow of an electric current. In this article I'll limit the discussion so we're mostly talking about speakers and speaker cabs as our component. The electric current comes from the output of an amplifier.

Why is it important?

If you ever find yourself putting together any kind of amplification system that combines an amplifier with separate speakers, it is vitally important to get your impedances 'right'. It could be a bass rig with separate cabs like this one pictured right, adding an extension speaker to a combo or even speccing a full PA system. The consequences of getting it wrong will result in either a) your system not being as loud as it could be, or b) damage to your amplifier.

Ok, so tell me how it works

Let's start with a simple analogy - think of your amplifier being a water pump and it is the flow of water through your pipes (or speakers) that results in the production of sound. A speaker can be like a narrow pipe (high impedance, ie. it impedes water flow through it) or a wide pipe (low impedance, the water flows easily). The loudest system is when the size of the pipes exactly matches the output of the pump, ie. all the water output by the pump is flowing through the system. If the total impedance is too high, the pump can't output enough water through the narrow pipes. If the total impedance is too low, the pump has to work too hard to keep the pipes full of water and is in danger of overheating and blowing up! Maybe a few air bubbles (distortion) might start creeping into the pipes (ok, let's not take this too far!). Are you getting the gist though?

Ok, let's take it back into the real world. All amplifiers have a specified minimum impedance which will produce maximum output or 'volume'. In the world of the musician, this is more often than not 4 ohms (Ohm being the measure of impedance), but can sometimes be 2 ohms (eg. some heavy duty PA amps) or something else. Assuming 4 ohms in this case, this means that if all your speakers combined result in an impedance which is more than 4 ohms, you won't get maximum power from the amp. Less than 4 ohms and you're risking damage to the amplifier.

So how do I know what my speaker impedances are?

Every speaker cab will have its impedance specified somewhere, be it marked on the cab itself or hidden away on a spec sheet somewhere. The most common values are 8 ohms and 4 ohms, but as usual there can be exceptions. So, if you have an amplifier with a minimum impedance of 4 ohms and you connect up a 4 ohm cab, Bob's your uncle - a nicely matched amp and speaker cab. Instead, you could connect an 8 ohm cab and the amp would be perfectly happy, but remember the 'pipe' will be narrower so you won't get as much output ('volume').

So what happens when you connect multiple speaker cabs? Ah, well the important thing here is *how* they are connected. There are two ways of doing it, known as 'in parallel' (side by side) and 'in series' (one after the other). Again, in the world of instrument amps and cabs there is a common way of doing things and that is in parallel...

Parallel

Take a look at this diagram.

Think of the water again, flowing out of the amp and into the speakers. The minimum amplifier impedance is 4 ohms (the lower the impedance, the wider the pipe remember), but in this case there are two 8 ohm speakers attached in parallel. Well whaddya know, these 2 narrower pipes actually share the water coming from the amp which maintains the overall flow through the system. Voila - another nicely matched system. So back in electricity language, two 8 ohm cabs in parallel results in an overall impedance of 4 ohms!

Working it out

Of course there is a way to work out what the impedance of a combination of cabs will be without drawing diagrams of pipes. If (and only if) all your cabs are of the same impedance, then simply divide the impedance of one of them by the number of cabs. In the above case, 8 ohms divided by 2 = 4 ohms. Easy huh? If there were four cabs it would be 2 ohms and so on.

If you are mixing cabs of different impedance then you have to do it the proper way which works in all cases. What you have to do is put one over the value of each cab, add them up and then invert the result. Huh?

It's easy really. For our example:
1/8 + 1/8 = 2/8 = 1/4, so flipping the result we get 4/1 = 4 ohms.
For four cabs, 1/8 + 1/8 + 1/8 + 1/8 = 4/8 = 1/2, the answer is 2 ohms.

So, mixing in a few different value cabs, let's say two 8's and a 4:
1/8 + 1/8 + 1/4 = 2/4, the answer is 4/2 = 2.

What about a 4 and an 8?
1/4 + 1/8 = 2/8 + 1/8 = 3/8. The answer is 8/3 or 2.66.

These last two examples bring up an important point about mixing cabs of different impedances. Take a look at this diagram which represents the last example and think of the water again.

Firstly, note that we've had to change the amplifier to a model that will go down to 2 ohms, as the combined impedance is going to be 2.66. Now notice that the 8 ohm cab only gets half the water that the 4 ohm cab gets due to its narrower 'pipe'. What this translates to in the real world is the higher impedance cab will put out less power than its 4 ohm partner as it is receiving less than half the power from the amp. Therefore, if you want a matched pair of cabs, always use identical impedances. Only mix impedances if you know what you're doing!

Series

You're unlikely to come across connecting cabs in series unless you're doing something rather out of the ordinary. If you decide to build your own cab or replace the drivers, however, it'll be handy to know how things work when they're 'in series'. Ok, it's really quite simple, all you need to know is that when speakers are in series, their impedances are added together. Easy eh? So, two 4 ohm speakers in series will produce a combined impedance of 8 ohms. Three of them will be 12 ohms and so on. Stretching the water analogy a little further than is elegant, you can imagine it becoming much harder work for the pump to push the water through a string of cabs, one after another.

The most common occurence of speakers in series for the average muso is inside cabs which contain several speakers. For example a 2x10 cab might have two 4 ohm speakers in series to give an overall cab impedance of 8 ohms, or a 4x12 might have two pairs of 8 ohm speakers in series (=16 ohms), with the pairs then wired in parallel to result in again, an 8 ohm cab. Got that? Have a look back at the parallel calculation and check it works!

Parallel or Series - how do I know?

It's all very well this talk of speakers being in parallel or in series, but how do you know how your cabs should be connected? Quite simply, if it doesn't specifically indicate a 'series' connection on the cab anywhere, you can fairly safely assume that any connections made through that cab will be in parallel.

Many people quite understandably get confused at this point, as they think if they run a cable from an amp to a cab, and then out from that cab to another cab then they must be in series, right? Nope! The secret lies in that connection panel in the back of the cab. You can imagine it as the T-junction between the two cabs, with one branch heading off to the innards of one cab, while the other branch heads off to the second cab. The fact the cable physically goes into one cab first and then into the other really doesn't matter. The diagram below might help you visualize it.

Alternatively, your amplifier may offer a pair of outputs, in which case they are almost certainly wired in parallel, ie. if you plug an 8 ohm cab into each, the amp will 'see' 4 ohms. If you have the choice there is a *very* marginal advantage to using both these outputs instead of daisy-chaining through the first cab, but it is only to do with keeping speaker cable length to a minimum. I'd be amazed if you could hear the difference (unless your speaker cables are really bad!).

So what's the actual difference between series and parallel?

The electrical difference between series and parallel is actually very simple, it's all down to how you connect the speakers together. As speakers only have two connections - a positive and a negative terminal (+/-), there are only two ways you can possibly wire them together (and have them both work I mean!). This gives us the parallel or series methods. The best way to show this is in a diagram:

Ah I see. I understand it all completely now!

See, told you it was easy.

© Jez Stephenson 2005. All rights reserved.