bass amp faq : how to hook up heads and cabinets
Hooking up a bass head to a bass cabinet, or group of cabinets, is often a confusing topic. Most of us musicians just want to play music. We don’t want to have to learn about watts and ohms. But, this is the price we pay for playing electric instruments. All of this is important to understand so you don’t damage your bass amp and you can play your gig with less trouble. I’ll try to break it down and explain some important things about hooking up your bass amp…
Use Speaker Cable!
It’s important to use speaker cables to hook up your bass head to your cabinet. Though instrument cables used between your bass and amp input look similar, they are not. Speaker cables are thicker. Gauge numbers can be confusing. The thicker the cable, the lower the number. Use at least an 18- or 16-gauge speaker cable so you don’t damage your bass rig.
You will run across the term “load” a lot with bass amps. A load is something that uses power to do something. A load could be a motor, a light bulb, a speaker, a group of speakers in a bass cabinet, or several bass cabinets together. A bass amp head is used to power the load (bass cabinet or cabinets).
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 Ω). Most bass cabinets have an impedance of 4 ohms or 8 ohms.
More ohms (higher number) means more resistance. The more resistance there is, the less power will flow to the speaker.
Bass 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 “400 Watts @ 4 ohms”.
Let’s look at an example so you better understand all of this. A particular amp head might send 200 Watts into an 8-ohm load. That same amp might 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 with a certain resistance in mind. If there is not enough resistance, the amp will put out more juice than the amp can handle, and it will overheat and burn up! You don’t want your precious bass amp catching fire no matter how cool it looks on stage.
Hooking Up a Single Bass Cabinet
If you are running your bass head to one cabinet, it’s pretty straightforward. If your bass head says 300 Watts @ 4 ohms, you need to plug it into a 4-ohm bass cabinet which can handle 300 Watts. If you plugged it into an 8-ohm cabinet, you might only get 200 Watts of power and probably less volume. If you plugged it into a 2-ohm cab, you’d get…FIRE! Don’t do that because there’s not enough resistance and the amp will overwork itself!
Hooking Up Multiple Bass Guitar Cabinets
Where all this gets more complicated is when you hook up multiple speaker cabinets to the same head. Let’s look at that…
Series and Parallel
When you hook up more than one speaker to an amp, there are two ways they can be arranged: in series, or in 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-by-side connections.
Most of the time you will wire bass cabinets in parallel. That’s how we will wire the upcoming examples.
Calculating Total Impedence of Equal Impedance Cabinets
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 each is 8-ohms). To determine the total impedance of the cabinets (all with the same impedance), take the impedance of one cabinet and divide it by the total number of cabinets.
impedance of single cab / number of cabs = total impedance
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, we said 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!
Calculating Total Impedence of Different Impedance Cabinets
If you are hooking up two cabinets of different impedances, there’s a little more math.
(impedance of cab 1 X impedance of cab 2) / (impedance of cab 1 + impedance of cab 2)
For instance, if you have a 4-ohm cab and an 8-ohm cab:
multiply 4 x 8 = 32
then 4 + 8 = 12
32 / 12 = 2.667 ohms
If your amp is rated only for 4-ohms, you can’t use this configuration of cabinets with 2.667 ohms.
An alternate method is to theoretically treat the single 4-ohm cab as two 8-ohm cabs. When they’re all the same impedance use the prior calculation method: divide the ohms of one cab by the number of cabs. Our same example would be 8 ohms/3 cabs = 2.667 total ohms.
If you are still confused (and I don’t blame you), get someone experienced to verify your bass amp hookup is okay before you turn it on!! And, don't forget to take your calculator to the gig. Make sure you use the correct cables and have extras to avoid the temptation of using an instrument cable. Know the specs of your bass head and cabs. Maybe copy the page from your manual and keep it in your gig bag/case. Maybe have a flashlight with you in case you need to read the specs on a borrowed cabinet. Never blindly hook up cabinets to amps or you may have to buy a new bass amp. Good luck!