About the Author: Ryan Rockwood is an Electrical Engineer who works in the design lab at Yorkville Sound (parent company of Traynor and VTC). He has been playing guitar for most of his life and has been tinkering with the electronics associated with guitars since he was a teenager. You can submit suggestions or requests for blog articles to guitar.amp.ryan at gmail dot com.
Amplifiers and Speaker Impedance
Ok, so you’re a guitar (or bass) player, and you’re thinking about getting an amplifier head. Or you already have one, but you’re thinking about getting a second speaker cab to hook up to it. That’s awesome, but you should know a little bit about impedance first.
What is Impedance?
Impedance is a measurement of how hard an amplifier has to work to drive a particular load (load is a tech term that refers to your speakers in this case). The lower the impedance, the harder your amp has to work.
You may have noticed on your speaker cabs that they say something like 4Ω or 8Ω (that symbol means Ohms). That’s the impedance of your cab. Some cabs even have selectable impedances via a switch or based on which jack you hook up to.
You may also have noticed that your amp’s speaker jacks have an impedance rating. That’s because your amp has a limit on the range of impedances it can drive without blowing up.
How to Not Blow Up Your Speaker Cabinet
First, there’s a rule that I’m going to ask you to follow: If you’re going to use more than one speaker cabinet, use cabinets with the same rated impedance.
This ‘rule’ can be broken safely, but it’s much easier to follow it than it is to do all the math you need to do to break it. Trust me.
So, if you follow my rule, here’s the punch line: divide the impedance of your cabs by the number of cabs you’re connecting to your amp, and that’s the total impedance your amp will see.
So, two 8Ω cabs means a 4Ω (8÷2) load to your amp, four 16Ω cabs means a 4Ω (16÷4) load, and so on and so forth. This can be extended to any number of cabinets at any impedance as long as they’re all the same impedance as each other.
Also, keep in mind that if your amp is a combo, the internal speakers should be counted as a speaker cabinet when you’re working out your total impedance. Unless, that is, you plan on disconnecting them…
Great! I’m Ready to Rock!
Not so fast. What about the power ratings of your cabs? If you’ve followed the rule about matched impedance cabs, then each cabinet gets an equal amount of power from your amp.
So all you’ve got to do is divide the power rating of your amp by the number of cabs you’re running. For example, with a 100W amp and two cabs, each cab gets 50W. If each cab is rated for 50W or more, you’re good to go. If either of them is rated for less than 50W, then it’s bye-bye speakers!
The Outputs on Your Amp
If the back of your amp looks something like the above picture, and you have two 8Ω cabs, how would you hook it up?
You would have a total impedance of 4Ω (8÷2), so you should hook up to the jacks in the 4Ω group. If you connected two 4Ω cabs to the 4Ω jacks, you would have a total load of 2Ω, and you could damage your amp, your speakers, or worst of all, both.
You should also know that you should NEVER connect to multiple groups of output jacks on a tube amp. Use ONLY one set at a time, or even more bad stuff could happen.
Tubes vs. Solid State
If you have a tube amp, you should ONLY connect the rated load to the amplifier. With some amps, it may be safe to connect a higher impedance to a lower impedance output, but I can’t guarantee this. If you want to do this for, you should consult with the manufacturer of your amp.
On the other hand, if you’ve got a solid state amp, you can hook up a higher impedance cab to a lower impedance output, (i.e. 8Ω cab hooked up to a 4Ω output). Solid state amps should be perfectly OK with this. The catch when you do this is that your amp won’t put out as much power as it would into the rated impedance. You’ll get the best performance from your amp if you use a matched impedance.
One Last Thing
I think I’ve covered everything a beginner needs to know, but I’m not perfect and I may have left something out. If you still have any questions, please post them in the comments below. I’ll try to check back frequently and answer any questions that come up. If you decide you’d like to learn more, or if you feel that you need to break one of the “rules” I’ve outlined above, I would suggest that you start by learning a bit about Ohm’s Law and a few other basic electronics principles. A great place to get started with all that is here.