Extension Speakers: Quick Tips for Musicians Part 2

In the first part of this article we learned the basics of configuring extension speakers- as long as you know the total impedance load of your cabinet(s).

So How Do You Calculate the Total Impedance Load?

I'm so glad you asked! Because when it comes to amplifiers they have a limit as to how low the impedance can be without effectively shorting the output, and there is a practical limit to how high it can get and still utilize your speakers efficiently. When you plug that extra cabinet straight into your amp via the extension speaker output you are "parallel-wired" because the jack connects the positive side of both speakers to the positive terminal of the amp output, and the negative side of both to the negative terminal in parallel (remember to double-check your extension speaker jack is wired in parallel- it usually is). We know the total impedance will go down, but how do we calculate the actual load? The formula for the total load of multiple cabinets in parallel is:

Total impedance (Zt) = (Za x Zb) / (Za + Zb)
(Where Za and Zb equal the impedance of each cabinet wired in parallel).

It almost sounds like you might have to fire up that calculator app on your smart phone, and indeed you might at some point. But fortunately most commercially made extension speakers are standard impedances of 16 ohm, 8 ohm or 4 ohm, so a lot of the time you are connecting two cabinets of the same impedance together. When the loads are the same the formula simplifies itself and you only need to divide the impedance of the cabinets by the number you are connecting in parallel:

Zt = Zc / Tc
(Where Zc is the impedance of the cabs, and Tc is the number of cabs wired in parallel).

If you want to add a Carvin Audio 212E cabinet to your Carvin Audio Belair combo, you have an internal load (of the amp's built in speaker pair) which matches the impedance of the extension cabinet (both are 8 ohm). Your total load is 4 ohms (8 ohm / 2 cabinets). Using a pair of 4 ohm cabinets like the BR410 with your BX700 bass amp head yields 2 ohm (4 ohm / 2 cabinets = 2 ohm). Sometimes you will encounter a 4x12 cabinet for guitar wired to 16 ohm, and two of those in parallel will have a total load of 8 ohm.

Where does the series-wired circuit come into play?

In all of our former examples, we used self-contained speaker cabinets and did our calculations based on the impedance rating of the entire cabinet. It would be impractical to daisy-chain (parallel) multiple individual speakers. While a very small number of amps offer series-wired extension jacks, series-wiring is almost always used within an individual cabinet (see note below). It is pretty common for the musician to want cabinets with two to four speakers in them, so the designer might wire a pair of 8 ohm speakers in series inside the cabinet to get a convenient total load of 16 ohm. By using parallel wiring to lower the total load, and series wiring to increase it, you can achieve some control over the total output impedance. This is especially useful with multiple speakers where the total load in parallel is less than the minimum load-rating of the amplifier. Some 4x12 cabinets for guitar (like the Carvin Audio VX412T), utilize two pairs of 16 ohm speakers wired in parallel to offer stereo 8 ohm inputs, but also have a switch that puts both pairs in series so you can use the cabinet in mono at 16 ohm.

The VX412T offers stereo 8 ohm inputs or a mono 16 ohm input

The formula for speakers wired in series is simple:

 Zt = Za + Zb
Where Za and Zb are any number of speakers. You just add up the total impedance of all the speakers in series.

Note: To series-wire your cabinet, the positive terminal of the first speaker attaches to the positive side of the output amp, but the negative terminal is connected to the positive terminal of the next speaker in line, which in turn has its negative terminal attached to the positive terminal of the speaker after that, etc., until the last speaker has the negative terminal attached to the negative side of the amp.

Being familiar with how to calculate your total impedance can help you get the most out of your equipment, and can also help you avoid causing damage to your gear! Every amplifier has a minimum rating that you cannot drop below. Guitar amps usually have 4 ohm minimum output-ratings and bass amps can often support a 2 ohm total load. It is important to check the label on the back of the amp or look up the specs, so you know for certain. Less than the minimum load can essentially short your amplifier's output and can damage your power amp. And knowing how different cabinets will work together can be useful in getting the most out of your speaker system, so you always sound your best.

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