Now that you’ve received your new unloaded Mojo cabinet, it’s time to install a set of speakers to finish it up. The following is a guide for wiring a speaker cabinet correctly to match the output impedance of your amp.
Impedance is an equivalent resistance that an inductor or capacitor produces in a circuit. Therefore, without delving into Ohm’s Law, from which our process for calculating total impedance is derived, we can start with the basic equations for calculating series and parallel resistance. The difference between impedance and resistance can be disregarded for this application.
Loads that are wired together in series result in a total resistance (impedance) that is equal to the sum of all loads or resistances in the circuit.
Wiring loads together in parallel results in a total resistance that is equal to the reciprocal of the sum of all reciprocals of each load.
This will come in handy when a circuit has more than two loads in parallel, but by rearranging variables, we get a more accessible equation for calculating the total resistance of two loads in parallel.
This form can certainly be used for cabinets with more than two speakers. Calculate the resistance of two of the speakers in parallel, calculate the resistance of the next two speakers in parallel (assuming the cabinet has four speakers), then calculate the total resistance of the two results in parallel.
If all of your speakers have the same resistance value, the calculation can be further simplified. Notice above that four 16Ω speakers in parallel have a total resistance of 4Ω. It is safe to assume that dividing the resistance value of the speakers by the number of speakers will yield the total resistance. Remember, this is true only for loads in parallel with the same resistance value.
Calculating for Power
The big thing to consider regarding speaker installation is the power your amplifier outputs. If the amp is rated for 100W, the cabinet should be able to accept more than 100W for safe operation. So, if you have a 4×12 speaker cabinet and a 100W amplifier, four speakers rated at 30W would be safe. The cabinet is rated at 120W. Failure to plan for a safe cabinet power rating could result in blown speakers if the amplifier output is higher than what the cabinet is designed for. If a voice coil fails, the resistance load of the cabinet can alter depending on the wiring method. If the value falls below what the amplifier’s output impedance is set for, the output transformer is likely to fail. So, avoid the expense of replacing your equipment and plan ahead.
The following is a set of wiring diagrams for the most popular speaker cabinet configurations.
Note: The series-parallel and parallel-series configurations function the same electrically. The names for each are interchangeable which is apparent when searching for these configurations on the web. We’ve labeled them in this fashion simply because they show up in other web resources more often than the other way around. Furthermore, we know of no real proof that supports theories of tonal differences between the two. However, if there is good scientific data to back it up, let us know about it; we’re eager to learn. Please, no AB listening tests.
Steve Gerard sent in this photo of his rig, featuring a few great looking custom Mojo cabs. Check them out! Interested in your own custom cab? Try our Custom Cabinet Configurator.