Extension Speakers: Quick Tips for Musicians Part 1

Carvin Audio VX112 1x 12-inch Open or Closed Back Birch Guitar Cab works great as an extension.

No matter which guitar or bass speaker configuration you select, if you're playing in different venues chances are you'll eventually want to use different cabinets or an extension speaker. Perhaps you chose a combo amp for the convenience and portability, but your next performance will be on a wide stage where you might not be able to hear your amp from the other side. An additional cabinet on the other side of the stage could be a very nice solution (and your bandmates who play over there might appreciate being able to hear you without having to add you to their crucial monitor mix). At first glance, it seems simple enough, after all your amp does have an extension speaker jack, so you might wonder, "Can't I just plug in there and crank it up?" In general you usually can do just that, but there are some things you need to know to get a good result and to protect your amp from damage. Before we look at how different speaker combinations affect the impedance load your amp has to carry, we need to know what's going on.

Know Your Amplifier

The first step to getting the most out of your extension cabinet configuration is to evaluate the features and specifications of your amp.
 - Does it provide an extension jack?
 - Is there an internal speaker or is it a head?
 - If there is an internal speaker, what impedance is it?
 - What is the minimum impedance load that it can handle?
 - How will an extension cabinet affect the output power?
 - Did the manufacturer design it with an impedance selector or multiple output jacks for different impedance loads?

Most manufacturers are pretty good about labeling their output jacks and you can usually also find someplace they've put a label that tells you some of the other specifications.

Carvin Audio V3M Guitar Amp Head with Speaker Outputs and Impedance Select Switch

If you have an amp head, all you need to know is the minimum impedance load and maximum output power (most amps produce more output power into a lower impedance) so you can match your cabinet(s) for impedance and power handling. If you have a combo, check the back panel or operator's manual to see if there is an extension jack. It should also tell you the impedance of the internal speaker and the minimum impedance the amp can handle. Never connect a load with less impedance than your amp's minimum impedance specification- you can do real damage to your equipment this way. You'll want to verify that the extension jack puts your extension cabinet in parallel with the internal speaker (most of them do, but a few designs put the extension in series instead). We'll talk about how to calculate the total impedance load of your speaker system a little later. While you're at it you should look for the total output power the amp will generate when you plug in your extension. This is pretty basic- just make sure the total amp power (at the impedance you're connecting) doesn't exceed the total power the speakers can handle. (Note: Be careful you don't confuse the AC power consumption spec with the output power specification. Output power is usually labeled as such, while the AC power requirement should specify the supply voltage and current demands as well. The AC power requirement is also most commonly found right by the AC main cable).

Understanding Series/Parallel Wiring

Once you know your amp's capacity, you need to determine the total impedance load of your speaker system. Again, check the label on the cabinet or the manual for the impedance of the extension cabinet. Your speakers will be most efficient and you'll get the most even coverage if your extension is the same impedance as your amp's internal speaker (plus it is much easier to calculate the total impedance as we'll see later). If the input jack of your cabinet isn't labeled you might need to check the individual speaker impedance and calculate the total load yourself.

Part 2 of this article will demonstrate how to calculate your total impedance with extra cabinets, and discuss how series wiring is used to adapt cabinet impedance.

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