Matching Sound System Components
By hcadmin |
Don't Impede Me!
By Jon Chappell
P.A. systems and guitar heads and cabs are often modular affairs, with the speaker cabinet being separate from the power amplifier (unless your P.A. have powered speakers). Having separate speakers provides more versatility, as you can often mix and match speaker systems to suit the job.
But before you start using your Crown power amp with your guitar player’s Celestion cabinets, or attaching a 4x12 cab to your Marshall head where you’d been using 2x10, you have to know that you can safely match up those particular components. If you get it wrong, you could permanently damage your amplifier, speakers, or both. (Of course, a speaker system designed for guitar amps might not be the best choice for a P.A., but that’s a separate question.) So we need to deal with, understand, and read the labels regarding power and impedance. These specs are related, so we’ll discuss them together, and give you enough to assemble a properly matched system.
Keep in mind that a given power amplifier is designed to deliver a specific amount of AC power, and a given speaker system is designed to handle a specified maximum amount of AC power. If your amp delivers too much or too little juice to the speaker, the amplifier, the speaker, or both can be damaged. The amplifier and speaker are a team, and they need to be properly matched. So let’s see how to make a good match.
Let’s start with the obvious: a power amplifier increases the level of an electrical signal to the point that it can cause an electromagnet-powered speaker coil to move in and out in the same pattern as the electrical waveform, which in turn causes compression waves in the air that we perceive as sound. The whole thing is predicated on the electrical signal, the speaker cone, and the resulting sound wave moving back and forth identically (to the extent possible) between positive and negative values. Alternating current (AC) is so named because it alternates between positive and negative values, which are what we need here, so a power amp always sends AC power to the speakers. DC power is deadly for speakers because it continually drives them in one direction (positive or negative).
THE HIGHS AND LOWS OF IMPEDANCE
You undoubtedly have heard other musicians talk about high- and low impedance-inputs on mixers, power amps, speakers, and guitar amps. Indeed, impedance is an important factor in matching parts of the signal chain, and it pays to understand at least a little bit about it.
For practical purposes, an electrical current always meets some opposition when flowing through a circuit, even if that circuit is merely a straight copper wire. Therefore, a speaker’s electronics always present a certain amount of opposition to the AC electrical flow coming from the power amplifier. The opposition to alternating current is called impedance because it hinders—that is, impedes—the flow of electrons. Impedance is represented by the letter Z in equations, and it’s measured in ohms, which is symbolized by the Greek letter omega (Ω). (Impedance is closely related to resistance, but the two are not identical.)
Every speaker system has an impedance rating that indicates how much opposition its circuitry presents to the signal coming from the power amplifier. Common impedance ratings for speakers are 2, 4, 8, and 16Ω.
If the speaker impedance is high, it won’t let much current flow, so it doesn’t demand much from the voltage source (the power amp). In such a case, the speaker is said to present a small load to the power amplifier. On the other hand, if the impedance is low, a lot of current can flow, and that puts a high demand on the power amp—that is, it presents a large load.
Remember: high impedance, small load; low impedance, large load. Got it? Good, let’s talk about power.
Electrical power measures how much work a given voltage and current can perform when presented with a specific impedance load. This is why we said earlier that the concepts of electrical power and impedance are related. The unit of measure for electrical power is watts, the symbol for which is the letter W.
When you check the power-output rating of an amplifier, therefore, you will see that power rating expressed as a number of watts of power into a number of ohms of impedance. On the back of the power amp, you’ll often see a notation that reads 250W/channel @ 4Ω. This means that each channel outputs 250 watts, assuming the amp channel is connected to a speaker cabinet providing a 4Ω load.
Matching the impedance of the load (the speakers) to the output of the amplifier is crucial for achieving maximum efficiency in a system. Efficiency means that all of the power is being used to drive the speaker, and as little as possible is being wasted as heat.
If an amp expecting a resistance of 4 ohms encounters a lower impedance (a larger load), such as 2 ohms, it will work harder and harder to deliver current to keep up with the current-sucking load. Eventually, it will heat up and burn out. On the other hand, if the amp encounters a higher impedance (smaller load)—say, 8 ohms—it simply will deliver half as much power (in theory), which is wasteful but generally not dangerous in and of itself.
However, you can still have problems if the amp’s power drops so low that it can’t properly drive the speakers. In that event, the amp can start distorting (clipping) the signal, and distortion can rip speakers apart. In fact, assuming we aren’t talking about extremes, you are more likely to blow out a speaker by using an amp that is not powerful enough than by using one that is too powerful.
So if you’re faced with a mismatch, remember, assuming you have sufficient power, “four into two won’t do; four into eight is great.” The same applies to other impedance ratings: an amp that is designed to work with 8Ω speakers might be fine with a 16Ω speaker system but not with a 4Ω or 2Ω speaker system.
It is important to note that in the real world, cutting the speaker impedance in half does not necessarily cause the amplifier to deliver exactly twice the power. There are many places in a circuit where power is lost, including the speaker wire. Higher current can cause greater losses in transistors and the power supply, as well as in the wire. Heat, that mortal enemy of electronic equipment, also adversely affects an amp’s performance. Eventually, if you abuse the amp, its protective circuits should kick in—but some amps don’t use a lot of protective circuitry and expect you to behave yourself, so if you abuse them, and they’ll simply blow up. Having put out power-amp fires onstage in mid-show, I can tell you this is not a fun addition to your light show.
Finally, keep in mind that there are several ways to rate power in an amplifier, including continuous power (the long-term average heating power with typical program material), program power (maximum average levels over the medium term, typically up to a minute), and peak power (calculated for short-term peaks, usually about a tenth of a second). For P.A.s, you generally want the program-power rating, preferably rated in watts RMS.
WHEN ONE IS NOT ENOUGH
Fig. 1. Series wiring, where the impedances are added together, makes the total impedance larger than any one of the speakers.
When you put two speakers in a cabinet or wire two cabinets to the same power-amp channel, how they’re wired affects the total impedance presented to the amplifier. For example, a cabinet housing two 8Ω speakers can have a total rating of 4Ω, 8Ω, or 16Ω, and wiring multiple cabinets or speakers can get a mite complicated. Let’s take the fear out of dealing with a mishmash of speaker setups.
If you have two speakers that you want to install into a cabinet, or you want to change the existing wiring, you can hook them up in parallel, series, or series-parallel. (The same rules apply when driving more than one speaker cabinet with one power-amp channel.) Figure 1 shows how you connect up the terminals of the speakers to produce series, parallel, or series-parallel configurations. Each scheme yields a different total impedance. Let’s look at each.
Series. In a series setup, you merely add the impedances together. I know we said we’d keep the math to a minimum; this is easier than it looks. Let’s assume we have an 8Ω speaker and two 4Ω speakers.
For a series setup, the equation is simple:
Z1 + Z2 + Z3 = Z
So in our example:
8Ω + 4Ω + 4Ω = 16Ω
Thus, if we wire the speakers in series (see Fig. 1a), our power amp will be dealing with a 16Ω system.
Parallel. If we wire the speakers in parallel, and all three speakers have the same impedance, the formula is easy: the impedance of one speaker divided by the number of speakers. So if we had three 8Ω speakers wired in parallel, the equation is 8 ÷ 3 = 2.667Ω. If our power amp can handle 2Ω loads, that should work fine, but if the amp is looking for a 4Ω load, this is going to make our amp work awfully hard. With an amp designed for an 8Ω load, this system is going to be bad news.
If the speakers are of varying impedances, things get more complicated. This looks scary but it is easier than it looks:
Z=1/ 1/Z1 + 1/Z2 + 1/Z3 …
In our three-speaker example, that gives us this:
Z= 1/ 1/8 + 1/4 + 1/4 = 1/ 5/8 = 8/5 = 1.6Ω
Fig. 3. Series-parallel wiring, where the impedance will vary, according to which speakers are in series and which are in parallel.
Series-Parallel. Calculating the impedance for a combination of series and parallel wiring is just a matter of applying each equation as needed. Since we have three speakers in our example, we can do this two different ways.
Let’s see what happens if we wire our two 4Ω speakers in parallel, and that combination is then wired in series with the 8Ω speaker. First, we calculate the combined value of the parallel speakers. Since we have two identical values (4Ω) in parallel, we can take the easy way out:
4 ÷ 2 = 2Ω
Now, let’s put that 2Ω system in series with our 8Ω speaker, which calls for simple addition:
2Ω + 8Ω = 10Ω total
If our power amp is designed to work with 8Ω or lower impedances, 10Ω is a low load, though not out of the question. Our system won’t be terribly efficient, but it probably will work. If the amp is designed for 4Ω or less, we’re going to waste a lot of power with this system.
Instead, let’s wire one 4Ω and one 8Ω speaker in parallel, then wire that combination in series with the other 4Ω speaker:
Zparallel = 1/ 1/4 + 1/8 = 1/ 3/8 = 8/3 = 2.67Ω
Ztotal = 2.67 + 4 = 6.67Ω
That will probably be close enough for an amp that can handle 8Ω loads, and it’s fine for amps that are designed for 4Ω loads.
One final tip: If you’re in doubt, wiring in series always results in a greater impedance, and while it might be less efficient, it’s the safest way to go.