A FEW NOTES ABOUT WIRING YOUR OWN SPEAKER CABINETS
By Captain Ogre

So, you're planning to try and use those speakers you have laying around and drop them into a cabinet so you can try to improve your sound, eh? Well, let's see if Ye Cap'n can lend you a hand here.

Let's talk a bit about your cabinet. There are a myriad of different box calculators available on the Internet. Not all of these calculators are great or accurate, and if this is your first build, it will be helpful to run your calculations on more than one of them. I'd recommend running your speaker parameters through at least four calculators, because almost every one that I've tried has had at least one parameter difference in the measurements between all of them. For the most part, however, the ones to pay attention to are the ones that will give relatively the same measurements between the calculators.

There are also a series of given factors in every speaker that need to be calculated in any given box design. These can be completely confusing to those that can't or won't take the time to find out what these parameters are and how they affect both the efficiency of the speaker and the resonance of the cabinet. I very much recommend that you take the time to learn a bit about Thiele-Small Speaker Theories. Most writeups on speaker theories developed by Thiele and Small are usually dead-on, and they cover both sealed and ported cabinet designs, the hows and whys of crossover networks, as well as what factor does what in regards to a speaker.

But, for this discussion, I will show the basics of speaker wiring. And really, it's not as difficult as some may think.

HOW WORKS A SPEAKER
First, let's talk about what a speaker really is. Basically, a speaker can be described as a magnetically-controlled moving coil melded onto a conical compression plate. This "plate" can be thought of as a resistive spring. When the coil moves with the help of a magnet and a touch of electricity, the plate will move forward and backward. The coil in a speaker is exactly that- a coil of wire that's wrapped around a cylindrical core (usually paper). This coil is, in effect, a resistor- although it doesn't resist very much- and is expressed with the suffix "Ohms." This is why speakers are given terms like "4-Ohm Load" or "8-Ohm Load." Speakers are also rated by the amount of wattage that the coil can handle. In a nutshell, a higher wattage rating means the speaker coil is made with a larger-gauge wire.

Next, the wattage rating. Most speakers are sold to you with their selling point showing their peak wattage rating. For example, you might find a 12" subwoofer that's rated at 400 watts peak. READ THE FINE PRINT! What this actually means is that the speaker can handle a 400-watt power surge (the "peak") for a very short period of time. The actual number to pay attention to the most is it's nominal rating (Root-Mean-Square, or RMS)- this is the speaker's actual operating wattage, or what the speaker will handle under continuous use (such as a 4-hour gig at the Room Sans Acoustique, at almost full volume coming from your amp). In this example, the nominal wattage rating might be closer to 150 watts RMS. It's noteworthy to point out that it is rare to see any speaker sold nowadays that is sold based on it's nominal rating. So please, be sure to read all the documentation that comes with the speaker.

Another thing to consider is that speakers have a polarity. That is, they have a positive (+) and a negative (-) terminal. OBSERVE POLARITY! As I showed with the Speaker-Mic, a speaker that is reverse-wired (positive lead from the source attached to the negative terminal of the speaker, and vice-versa) causes the speaker to pull IN instead of push OUT. This has a great use as a kick-drum mic or a reflexive subwoofer, but it ain't worth a damn as a guitar amp!

HOW ARE THEY WIRED?
OK, having said all this, it's time to explain a few wiring techniques (and why they are what they are). First, there are actually three types of wiring configurations- Series, Parallel, and Series-Parallel. Let's take a look at the Series wiring scheme:

When comparing a speaker to a resistor, the function (and the math) is the same. Since we're really talking resistors here, the speakers are referred to as R1 and R2. In any resistor network that involves configuring them in series, the math is simple. The total resistance simply add up. So, if R1 is 8 Ohms, and R2 is 8 Ohms, the total is (you guessed it) 16 Ohms. Does this mean that you can mix impedances? Not usually, but that also depends on your amp. For consistency in both sound and functionality, it's best to use identical speakers.

Notice in the Series Speaker Wiring configuration how they are wired. You have a red wire coming from the positive source that connects to the positive lead of the first speaker (and, of course, the black lead connecting to the negative terminal of the other speaker). Now, you also have a "Jumper Line" that connects the two speakers together to complete the circuit. It's good practice, when using a Jumper Line to connect, to use a wire that is colored differently than your main lines (red and black) to help distinguish it from your main lines. You should also use red for positive and black for negative as your mains whenever possible. And, use a wire gauge that is suitable for the amount of wattage your cab will be rated for (more on that below.)

Now, let's take a look at a Parallel speaker scheme:

The math on this gets a little strange here- you might want to bone up on converting fractions. Notice that they are tied from the positive lead to both positive terminals of the speakers, and the same with the negatives. So, what does this do to the total impedance of the speakers? It gets reduced. This is why you have to choose your speakers wisely- your amp will deliver it's best at it's given rated load. Usually, that load is either 4 or 8 ohms. Sometimes, an amp will be rated to run on 16 ohms, and there's a few out there that run on 2 ohms.

So, how does it calculate? Well, a good-working formula for calculating parallel-resistance networks is to use this one:

Let's say you're using two identical speakers, each rated at 8 ohms. In the parallel formula, you'd figure the bottom numbers first:
1/8 + 1/8 = 2/8 or 1/4
Now, when 1/4 is divided by 1, this turns out to be (yes, indeed) 4 ohms. Your amp will like that.

OK, so what happens when you're NOT using identical speakers, and they have different impedances- for example, one is an 8-ohm, and one is a 4-ohm? Well, you'll have 1/8 + 1/4 = 1/8 + 2/8 = 3/8. Divide this by 1, and you have a resistance of 3.75 ohms. Sure, it's pretty close to your 4 ohms nominal impedance that your amp requires, but consider this (and this needs mentioning now)- that load rating is actually a "rounded" number- if you put an ohmmeter to a 4-ohm speaker, you will find that it's exact load rating is more like 3.2 ohms for a speaker rated at 4 ohms! And, it's about 6.8 for an 8-ohm speaker! So, for an exact electrical measurement, the original 3.75-ohm figure is more like 2.176 ohms- trust me, this is a pretty drastic change that your 4-ohm rated amp will not tolerate for very long.

So, is it possible to "mix-and-match" your speakers? Sure it is! But, you still have to be careful when doing this, as it involves some extra math and probably a crossover network (which I won't go into here). For guitar and bass cabs, it's just wise to use identical speakers (or, speakers with the same load rating) to get a consistent sound and best functionality from the cabinet.

And how are those 4x10 and 4x12 cabinets wired? Enter the Series-Parallel Network...

This wiring configuration could be typical of most of your 4x10 and 4x12 speaker cabinets. Essentially, what is going on here is you're creating two "arrays" of series-wired speakers and changing the final result to parallel. So, let's break it down:

Series Circuit 1 is denoted by speakers R1A and R1B, and Series Circuit 2 is denoted by R2A and R2B. Since resistors (and speakers) in series sum their total resistance, you now have ONE array of speakers made from summing the resistance of two speakers, R1A + R1B. Series Circuit 2 is wired exactly the same as Series Circuit 1, and is also ONE array of speakers (R2A + R2B). You now have, in essence, TWO speakers that can be wired in parallel to give the desired load impedance that your amp needs. So, if all four of your speakers are 4 ohms each, you would have:

SERIES CIRCUIT 1: 4 ohms + 4 ohms = 8 ohms
SERIES CIRCUIT 2: 4 ohms + 4 ohms = 8 ohms
PARALLEL CIRCUIT: 1/((1/8)+(1/8)) = 1/(2/8) = 1/(1/4) = 4 Ohms

PUTTING IN ADDITIONAL SPEAKERS
Are we stuck using ONLY speaker arrays that are either 2 speakers or 4 speakers per cabinet? Well, not necessarily. A good example of this is in horn-loaded bass cabs. Technically, these cabs have either three speakers or five speakers- the third and fifth speaker being a tweeter horn. One very important thing to consider with these types of cabs is the load impedance of the speakers you intend to use (see below).

Notice in this configuration, we've added a single tweeter horn in parallel with the rest of a Series-Parallel Network. This is where your speaker selection becomes critical. Mathematically, to attain a 4-ohm load, you would use four 8-ohm speakers and an 8-ohm horn. So, how does this one figure?

SERIES CIRCUIT 1: 8 ohms + 8 ohms = 16 ohms
SERIES CIRCUIT 2: 8 ohms + 8 ohms = 16 ohms
TWEETER HORN: 8 ohms
PARALLEL CIRCUIT: 1/((1/16) + (1/16) + (1/8)) = 1/((2/16) + (1/8)) = 1/((1/8) + (1/8)) = 1/(2/8) = 1/(1/4) = 4 ohms.

One thing to pay attention to, when using a horn, is that most tweeter horns will come with a capacitor attached (or sometimes just included in the box) to the positive terminal. DO NOT BYPASS THIS CAPACITOR! This capacitor is acting as a crossover. This is what's filtering out the lows and putting the highs into the horn. Tweeter horns don't like low-end frequencies pushed through them at all, too much bass will make them distort like crazy. (I found this out the hard way once. I bypassed the cap on one that I was testing, hooked it up to my stereo, heard some really funny sounds coming from it, and almost got shot in the head by the tip of a tweeter cone coming out at relatively high velocity.)

IS THERE A DIFFERENT APPROACH?
So, is this the only way to wire your speakers to match an amp? No, of course not. All I showed here is just some basic speaker cabinet wiring. You can use these formulae to "plug-in" your numbers and develop your own wiring schemes. Of course, be sure to double- and triple-check your wiring on paper before you commit to your design.

WATTS- SO VERSATILE, YET SO MISUNDERSTOOD
Wattage is something else that can be considered versatile. This is one of those unique factors in speaker design- wattage adds up, regardless of what wiring scheme is used (Series, Parallel, or Series-Parallel). What this means is that any set of speakers you use, the total power handling of the cabinet is derived from the sum of the wattage rating of all speakers. Let's say you have two speakers, each bearing a 150W RMS load. When placed into your cab and wired properly, the cabinet will be rated at 300 Watts RMS. But, be careful about this- if your amp puts out a nice, beefy 450 Watts, you should use an array of speakers that will give a total wattage rating that is higher than your amp's rated output (and this is generally a wise idea, it avoids speaker blowouts).

A common misconception among a lot of people is that amps should be powerful enough to drive the speaker, otherwise you won't hear anything (and might toast the amp). The fact is, any amp is going to deliver as much sound as it can, at any volume, through the speakers it's attached to. 50 Watts will still push an 800-watt rated speaker (I have tried and proven this fact!). The problem is, what you're hearing is 50 watts of amplified sound coming from an 800-watt speaker. However, it really doesn't sound too bad, it just sounds lower in volume than you'd expect it to. As long as the rated impedances match both speaker and amp, you'll get sound out of it without any major problems.

Where people get into trouble with their designs the most is when they "overdrive" their speakers- putting too much wattage from the amp into a speaker that's not rated high enough for the amp. When a speaker moves with sound, the motion from it's dead-stop point to it's full outer extension is called excursion. Also, when a speaker moves, the coil creates friction. So, when you put too much wattage into a speaker that's not rated to handle it, you get "over-excursion." This creates heavy speaker distortion because you're making the speaker coil reach farther than it should, and it's thumping against a wall while generating exponentially more coil friction. So, while the speaker coil is essentially trying to knock a hole through that wall, it burns itself out very quickly, causing you to run in panic for the nearest fire extinguisher. Trust me, this is NOT a pretty picture. This is why it's highly recommended to use speakers that will give a total sum of wattage that is greater than the output of your amp.

OK, SO HOW DO I HOOK IT UP TO THE AMP?
Another thing to consider is how you want to interface your cabinet to your amp. This involves simply using the appropriate plug-and-jack combination. Now, since technology moves forward in general, you have a variety of choices. You could use the tried-and-true 1/4" tip-sleeve phone jack (just like the ones on your guitar cord), or you can use XLR jacks (although this is NOT a common procedure, and therefore not recommended), or you can use Speakon (tm) jacks. Each has advantages and disadvantages-

• 1/4" T-S jacks are cheap, and relatively easy to acquire. And, if one breaks, they're easy to replace. The disadvantages are that they're susceptible to contamination by dirt and wear, and the female jacks have a tendency to get loose from the constant vibration factor inherent in every running cabinet, and the ever-present setup and teardown of equipment by pulling the jacks out at the end of one gig and putting them back in at the next, thus creating a bad contact point.
• Speakon jacks lock in place. You plug them in, twist it a bit, and it's locked down. This pretty much guarantees a solid connection that doesn't go away. The disadvantages are that they are kinda spendy, and, if you break one, they're not always available as replacement parts at your local electronic parts store, nor are they as easy to reinstall as the T-S jacks are.

DO NOT USE A GUITAR CORD!
Also, remember that you are attaching a speaker cabinet to an amplifier. This means you need speaker wire to "patch" this with, not (and I repeat, NOT) a guitar cord. Guitar cords can be thought of as a condensed version of TV coaxial cable. It's suited for reducing noise to your amp's input because it's shielded. However, these are made with a resistance factor built into them at around 30-50 ohms per foot. This resistance factor is an undesirable trait when you're dealing with the back-side of an amp. Think about it- if you've just designed the perfect speaker cab to match your amp at 4 ohms, why would you add the extra 30-50 ohms to the total impedance by using a guitar cable? Your amp won't like that added resistance at all. So, use the proper gauge speaker cable and not a guitar cord.

WIRING THEM UP PROPERLY
Be sure to consider matching the proper gauge wire to the amp's output rating, including your amp-to-cab patch cord. A homemade zip-cord speaker patch cable might work for an amp that's pushing less than 100 watts or so, but if you're using some pretty serious equipment, you'll want to go larger with the wire gauge. Bear in mind that electricity is abrasive when not contained properly. This means that a small-gauge wire with a high-output amp signal shooting through it is going to get very hot. So, play it safe and find the proper gauge wire.

How do I find the proper gauge? Well, it's time to go back to the basics of Ohm's Law. You know that you have two electrical factors from either your amp or your speaker cab- POWER (expressed in WATTS, suffixed by the letter P, and we're going to use the PEAK wattage here), and RESISTANCE (expressed in OHMS, suffixed by the letter R, and meaning the total resistance of the cabinet, or the impedance of the amp). What we would be looking for is the ENERGY (expressed as VOLTAGE, suffixed by the letter E) needed to help determine the proper gauge wire. Now, let's say that your speaker cab is rated at 400 watts PEAK, and the total resistance is 4 ohms. The formula would read:

So, to calculate this, you would say the square root of 400 watts times 4 ohms equals the voltage. This means that the square root of 400 x 4 = square root of 1600 = 40 volts. BUT, are we done yet? No, unfortunately. Now we know that we need a wire that's going to handle 40 volts, but at what amperage? Back to Ohm's Law- INDUCTANCE is a measure of AMPERAGE (expressed in AMPS, suffixed by the letter I). The formula for determining the amperage that's going to be shot out the amp is:

So, we're looking at Amps = 400 Watts / 40 Volts = 10 Amps (or Amps = 40 Volts / 4 Ohms, still equals 10 Amps). That looks to be a fairly sizable wire, about 14 gauge or better. (It's amazing to think that your amp is actually pushing an electrical current damn near equal or higher than that of your house wiring, isn't it?) This is why it's important to use the proper gauge wire, there's a lot of electrical factors going through this stuff that I bet you didn't know was actually there, did you? And, you can also use this to determine the "hidden factors" of what your amp can actually deliver to your speakers.

ANOTHER ADDITIONAL TIP
Here's another neat-o tip that should be engineered into your design- put an inline fuse in the positive end of your jack. This is dirt-cheap speaker protection in the event of a catastrophic amplifier meltdown. The fuse should be rated at about the same amperage as your amp's output (use the above example to figure it out), with a top-end voltage of about 250 volts. Of course, there's a lot of different styles of inline fuse holders to choose from. But, it will be wise to choose one that you can access easily, unless you really want to tear your cab apart for a minor fix.