The EM Poll

P.A. 101

Mar 14, 2008 3:24 PM, By Emile Menasche

           

HOW TO CHOOSE THE RIGHT SOUND SYSTEM FOR YOUR NEEDS WITHOUT TEARS

Feel the Power

If you use an unpowered mixer, it's important to mate it with a power amp that has enough juice to process your music cleanly. A powered mixer or all-in-one system makes these decisions for you, of course, but some of them let you bypass the internal power amp and use an external power amp instead.

Ideally, you should shop for your power amps and your speakers at the same time. Make sure the amp and speakers match in impedance (see the sidebar "Impedance and Speaker Load") and power rating. Matching the power rating is very important: if you overpower your speakers, the result can be thermal or mechanical failure. But if you underpower your speakers, distortion-and ultimately, speaker damage-may result. When the signal clips (distorts), the amp could be called on to suddenly deliver double the usual amount of power, which can be bad news. We'll return to this point in a moment.

Sound-reinforcement speaker systems usually have three power-handling specifications: continuous power, program power, and peak power. Loosely defined, continuous power is the average long-term level of power that the speaker can handle. However, it is usually measured using a sine wave, which does not reflect the demands a music signal will place on the system. Program power is the maximum average level of power, measured over the medium term, that the speaker can handle-but it is measured using a test signal that approximates an actual music signal. Peak power tells you how much power the speaker can handle for an instant, which is less than a second.

For a professional P.A. system, which usually has to handle large amounts of power over several hours, you want a power amp that can deliver approximately the program power rating of the speaker system but-and this is important-does not exceed half of the peak power. The reason: if clipping causes the system to demand twice the normal power from the amp, you need to make sure that doesn't exceed the speaker's peak power rating, even for an instant. If your speaker has only continuous and peak ratings, you'll have to use some common sense in approximating the probable program power, remembering to keep the power at no more than half of the peak power.

Keep in mind that the considerations here are different than with a home stereo system, where sound-pressure levels are lower and power demands are much less.

Figure 2: If you have access to more than one power amp, you can use one to power the mains and two sides of the second amp to feed two separate monitor mixes.

Are Two Heads Better?

In live situations, you can use a typical 2-channel power amp in a number of ways. Note that the house mix and the monitor mix may call for different approaches.

Figure 3: Running a stereo power amp in bridged mode turns it into a more powerful mono amp.

For example, you could run a stereo house mix, which allows you to create a more realistic soundstage and utilize stereo effects. However, operating a stereo sound system can add considerably to the challenge of mixing-among other things, it invites complex phase problems resulting from sound reflecting off of walls and other surfaces-and it can create a lot more problems than it solves. Running a stereo monitor mix can really open up a can of worms, and most club bands probably shouldn't do it.

In many club situations, therefore, you probably will run the house and monitor systems in mono. One way to do this is to run in dual mono, feeding the same mono mix to both sides of a 2-channel power amp, so that each side of the house and each side of the stage has its own power amp channel (see Fig. 2). That setup gives you the ability to adjust the sound-pressure level on each side of the house or the stage without affecting the mix. If you don't need that sort of flexibility, however, you could use one amp channel to drive the house mix and the other to drive a monitor mix.

However, if your gig requires only a mono stage mix, running your power amp in bridged mode may be the most efficient method (see Fig. 3). In bridged mode, the two sides of the amp operate in tandem to form a single, monaural power amp. One channel's hot output remains positive, while the other channel's positive output actually becomes negative, so you connect the positive output of each channel of the power amp to the speaker. Because the two sides are combined, the amp delivers significantly more power when bridged to mono. Although logic would indicate that the power should double when the left and right channels are bridged (assuming the same output impedance), the exact amount of power you actually get in bridged mode depends on the amplifier's design.

You could use a bridged power amp for the house mix, too, but since you probably are driving two speaker stacks that require significant juice, you probably are best off either running a powerful amp in dual mono or using two or more amps for the mains. If you have more than one power amp, naturally, you get more flexibility.