Hooking up a head to a single or group of cabinets is often a confusing topic. Most of us just want to play music and don’t have time to learn about watts and ohms. But this is an important topic to understand so you don’t damage you or your amp.
First, it’s important to use speaker cables when hooking up a head to your cabinet. Instrument cables used between your guitar and amp might look similar, but they are drastically different. Speaker cables are thicker and not shielded. Even gauge numbers can be confusing – just remember, the thicker the cable, the lower the number. Use at least an 18 or 16 gauge speaker cable so you don’t damage your rig.
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Impedance is a measure of resistance. When electricity flows to a load, some of the electricity is resisted or impeded. Impedance is measured in ohms – denoted with the Greek letter omega ( ). Most cabinets have an impedance of 4, 8 or 16 ohms. More ohms mean more resistance and with more resistance there is less power flowing to the speaker.
Amp heads specify how much power is sent to a load (a cabinet or group of cabinets) of a certain impedance (the amount of resistance measured in ohms). For example, you may see an amp rated “400W @ 4 ohms.” Likewise, a particular amp head might send 200 watts into an 8-ohm load, but the same amp will send 350 watts into 4 ohms. More power will flow from an amp as the ohm rating (resistance) decreases. If you went down to 2 ohms, that same amp might send 600 watts to the load.
All of this is important because amps are designed to work within a certain resistance. If there is not enough resistance, the amp will put out more juice than it can handle, leading to overheating, and eventually burning up!
You probably don’t want your amp catching fire, no matter how cool it looks onstage.
If you are running your head to one cabinet, things are pretty straightforward. If your head says 300W @ 4 ohms, you need to plug it into a 4-ohm cabinet that can handle 300 watts. If you plug it into an 8-ohm cabinet, you might only get 200 watts of power and you’ll probably get less volume. If you plug it into a 2-ohm cab, there’s not enough resistance and the amp will overwork itself, which results in FIRE!
When you hook up more than one speaker to an amp, there are two ways they can be arranged: series or parallel. Series means chaining the cabinets together, one to the next. Parallel means sending one output of the bass amp head to one cabinet and a second output from the head to another cabinet. Parallel is two or more side-byside connections.
When you add a second cabinet, realize there are now two places for the amp’s power to go. Adding a second cabinet causes the total impedance (amount of resistance) of the load to change.
It’s easiest having each cabinet with the same impedance (e.g. each cab is 4 ohms or 8 ohms). In order to determine the total impedance of the cabinets (all with the same impedance), take the impedance of one cabinet and divide that amount by the total number of cabinets. For instance, two 8-ohm cabinets wired in parallel will have a total impedance of 4 ohms; two 4-ohm cabinets would have an impedance of 2 ohms. Remember, if your amp can’t handle a 2-ohm load it could go up in flames – that’s why this is so important. If your amp says it can put out a certain number of watts at 4 ohms, you can only hookup a total load of 4 ohms, 8 ohms or greater – not 2 ohms!
If you are hooking up two cabinets of different impedances, there’s a little more math involved. For instance, if you have a 4-ohm cab and an 8-ohm cab, the equation is as follows: 4 x 8 = 32, then 4 + 8 = 12. Divide 32 by 12 and you get 2.667 ohms. If your amp is rated only for 4 ohms, you can’t use this configuration of cabinets with 2.667 ohms.
If you are still confused, get someone with experience to verify that your amp hookup is okay before you turn it on. Make sure you use the correct cables and have extras to avoid the temptation of using an instrument cable. Most importantly, know the specs of your head and cabs. If you’re heads up, you’ll be able to get down! Go forth and hook up!
Rick Wheeler
Rick Wheeler currently works as Larry Carlton’s guitar tech and front of house engineer. He is also an accomplished jazz guitarist, vocalist, and educator. You can contact Rick at rickwheeler@hughes.net