The EM Poll

Tech Basics Without Tears

Mar 14, 2008 5:29 PM, By Jon Chappell With Steve Oppenheimer

           

A PAIN-FREE EXPLANATION OF FIVE CONCEPTS ALL MUSICIANS SHOULD UNDERSTAND

Figure 1: Wiring diagrams for connecting speakers in series, parallel, and series-parallel configurations

Watts That?

Electrical power measures how much work a given voltage and current can do when presented with a specific impedance load. This is why we said earlier that the concepts of electrical power and impedance are related. Electrical power is measured in watts, the symbol for which is the letter W.

When you check the power-output rating of an amplifier, therefore, you'll 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 @ 4ž.” This means that each channel outputs 250 watts, assuming the amp channel is connected to a speaker cabinet providing a 4 ž load.

Matching the impedance of the load (the speakers) to the output of the amplifier is crucial for achieving maximum efficiency in a system. Maximum efficiency means that all of the power is being used to drive the speaker, and none is being wasted as heat.

If an amp expecting a resistance of 4ž encounters a lower impedance (a larger load), such as 2ž, 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 (a smaller load) — say, 8ž — 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're 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 designed to work with 8ž speakers might be fine with a 16ž speaker system but not with a 4ž or 2ž 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, they'll simply blow up. Having put out power-amp fires onstage in midshow, we can tell you that 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 output 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 measured in watts RMS.

When One Is Not Enough

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 8ž speakers can have a total rating of 4, 8, or 16ž, 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 you want to install into a cabinet, or you want to change the existing wiring, you can hook them up in series, parallel, or series-parallel. (The same rules apply when driving more than one speaker cabinet with one power-amp channel.) Fig. 1 shows how you connect the terminals of the speakers (or entire speaker cabinets) to produce series, parallel, or series-parallel configurations. Each scheme yields a different total impedance.