Every so often someone will ask a question on the forums regarding wiring guitar speaker cabinets, and rather than repeating myself in multiple forum posts, I thought an article that I could link to would make more sense. So in this article, we're going to go over the basics (with as little theory and as few mathematical formulas as possible) of the main ways of wiring up a guitar speaker cabinet. We'll start with the simplest configuration - a single speaker - and move up to two-speaker and then four-speaker cabinets. We'll also look at how the various wiring methods can change the impedance load presented to the amplifier.
Cabinets with just one speaker in them are relatively easy to wire.
Some single speaker combo amps have two non-detachable wires that come out of a grommeted hole in the chassis that are designed to connect to the speaker. Other than any challenges posed by the cabinet itself, mounting a new speaker and wiring it in is easy. Make sure the amp is turned off and unplugged and just attach the positive wire to the positive terminal on the speaker, and the negative lead to the other terminal, and you're done. The positive speaker terminal is usually marked in some way; usually with a + symbol or some red paint. Many speakers have two lugs for each to facilitate the more complex wiring configurations that we'll describe momentarily, and if yours does, you can feel free to use either lug.
When wiring up a cabinet, it's important to use heavy gauge "speaker wire" and not the thin-gauge wire you'll find inside guitar cables! In most commercially-built speaker cables, the white wire is usually "hot" and is connected to the "tip" of the speaker cable plug, and the black wire is neutral and is connected to the plug's sleeve. Connect the white wire to the + terminal on the speaker, and the black wire to the - terminal lug.
When connecting wires to the terminals (or lugs) on the speakers, you can either solder the wires directly to the lugs, or attach female crimp terminals to the ends of the wires and connect those to the lugs on the speaker. Never just wrap the wire through the hole or around the lugs, since doing so may result in the wire, and thus the speaker, becoming disconnected. This can result in a "no load" condition that can fry your amp!
Many amps have a 1/4" jack that is labeled "speaker out" or something similar. There is usually also some information about the minimum impedance load the amplifier can safely handle marked on the amp somewhere near this jack. It is important to always heed the manufacturer's recommendations since an incorrect speaker load can damage the amplifier. If it says "8 ohms minimum", you should never connect a 4 ohm load.
Sometimes an amp will have a switch that allows you to match the amp's speaker output to speaker cabinets of different impedances, or possibly multiple speaker output jacks for different loads. If your amp has one of these features, it's important to make sure you're using the right setting or correct jack for whatever speaker load you have connected.
With single speaker cabinets, the speaker cabinet is rated the same as whatever the impedance and power handling ratings are for the single speaker. When one 8 ohm speaker is in the cabinet, you have an 8 ohm load, and likewise, the power handling of the cabinet is the same as the power handling rating of that single speaker. In general, you want the power handling to at least equal the output power of the amp, but excess power handling capacity won't hurt anything - I have a speaker with a 200W rating in one of my 20W amps, and it works just fine.
Things start to get more complex when you're dealing with a cabinet that is loaded with more than one speaker, in terms of impedance, power handling capacity, and wiring. To simplify things, you should generally stick to using speakers with the same nominal impedance, sensitivity, and power handling capacity.
A two-speaker cabinet can be wired in one of two ways - either in parallel, or in series. With both parallel and series wiring of multiple speakers, the total impedance load changes compared to running a single speaker. Let's take a look at each method, along with what happens in terms of the total load and power handling capability with each wiring approach.
Parallel wiring connects each of the speakers directly to the amplifier's output, with wires from the positive terminal of the amp's output routed to the positive terminal of each speaker, and wires from the negative terminal going to the negative terminal on each speaker.
When you connect two 8 ohm speakers with parallel wiring, the impedance is cut in half, so you have a total load of 4 ohms whenever you use two 8 ohm speakers in a cabinet wired in parallel. As far as wattage and power handling capacity, let's assume that each of the speakers is rated for 50 watts of power. With parallel wiring, you'd add the wattage of the two together, giving you a total power handling capacity of 100W.
If the impedance is the same for all of the speakers in a multi-speaker cabinet, with parallel wiring, you divide the impedance by the number of speakers to arrive at the total load. For example, with two 8 ohm speakers wired in parallel, you have a total load of 4 ohms.
The other option is series wiring. For series wiring, the positive output terminal of the amp is connected to the positive terminal on only one of the speakers. The negative terminal of that speaker is then connected to the positive terminal of the second speaker, and the second speaker's negative terminal is connected to the negative terminal on the amplifier.
Series wiring has the opposite effect on total load impedance as parallel wiring does. When speakers are wired together in this way, you add the impedance of all the speakers together to arrive at the total load value, so those same two 8 ohm speakers present a 16 ohm load to the amplifier when they are wired in series. As with parallel wiring, two 50W speakers wired in series can handle up to 100W.
When we move on to even larger cabinets with more drivers, such as the classic 4x10 and 4x12 speaker enclosures, series and parallel wiring configurations are still possible, but a third option also opens up. This is a combination of the two previous methods, and is called series-parallel wiring. First, here's a couple of illustrations to show parallel and series wiring with four speakers.
Four speakers, wired in series:
Assuming we're using the same 50W 8 ohm speakers, the impedance of the cabinet is 32 ohms (8+8+8+8=32) and the power handling is 50W x 4, or 200W.
And here's the wiring diagram for four speakers wired in parallel:
Again, assuming we're using the same 8 ohm 50W speakers, the impedance of the cabinet is 2 ohms (8/4=2) and the power handling is 50W x 4, or 200W.
Both of these wiring configurations are sometimes used, but many cabinets with four speakers use series-parallel wiring instead. To connect the speakers in a 4x12 cabinet with series-parallel wiring, the four speakers are wired together using a combination of the series and parallel wiring techniques.
The advantage here is that the total load for the cabinet remains the same as the value for a single speaker. If you wire four 16 ohm speakers together with series-parallel wiring, the nominal speaker load for the cabinet will remain 16 ohms. Four 8 ohm speakers in a series-parallel configuration will give you an 8 ohm cabinet.
Multiple cabinets can be connected to a single amplifier, just as long as you match impedances and don't go below the amp's minimum load. If an amp can run at 8 ohms, connecting two 16 ohm cabinets to it with parallel wiring will result in a safe, 8 ohm nominal load, while connecting two 8 ohm cabinets with parallel wiring will give you an unsafe 4 ohm load. It's much less common, but if special cables are used and the cabinets are wired to each other and to the amp with series wiring, two 16 ohm cabinets will give you a 32 ohm load, while two 8 ohm cabinets will yield a 16 ohm load.
Well, there you have it - the basics on speaker cabinet wiring methods. Of course, this isn't all there is on the subject. It is possible to use speakers with different power handling capacities and even different impedances in the same cabinet, but this requires more math and much more careful consideration, and all of that is a subject for another day, another article…