PA Systems

The Basics

One thing you will learn in your foray into the musical world is that technology is based on reiteration. That is to say that when you learn one piece of equipment, you have really learned  at least the basics of much more equipment. PA systems are no different. The abbreviation PA stands for “Public Address,” as one might hear in a high school hallway or coming from atop an ice cream truck. The same design is used in every single PA system, however large or small, simple or complicated. Every system puts into use the same four components: 1) a source, 2) a mixer, 3) a power amp, and 4) a speaker. Every sound system you find will have these same four components in differing levels of complexity and quality.

1) The Source

The source can come in many forms. It may be a guitar or a keyboard. It may be a vocal microphone (a.k.a. “mic”) or a kick drum microphone. They all can be put into two basic categories: microphones and direct injection. A simple way to put the difference between them, though very crude, is that microphones are pieces of equipment that pick up sound from an external source and send it along a low-impedance line to a mixer, while direct injection is a process by which a source (e.g. electric guitars, keyboards) sends a high-impedance signal to a small box that changes the signal to low-impedance for the mixer. Mixers only properly work with low-impedance signals, though some have an on-board “Hi-Z” (high-impedance) switch so you can plug a guitar in directly instead of using a direct injection box (a.k.a. “DI,” “DI box,” or “direct box”). Many, but not all, DIs also offer a “ground lift switch,” which removes the grounding from the signal and often eliminates buzzing. There are many exceptions, variations, and details not here, but this is a crude definition. Here is a good link if you would like to read more on direct injection and impedance. Before connecting a piece of equipment, ALWAYS MAKE SURE YOU UNDERSTAND THE SOURCE’S SIGNAL! If you connect things the wrong way, you can ruin any piece of your equipment. But never fear: you will develop a sense of what item puts out what signal as you learn more of what’s out there. Mistakes happen, but chances are there is someone around who can help you prevent them before that point.

2) The Mixer

Like the source, a mixer varies greatly in form, complexity, and quality. It may look like a mixing desk (a.k.a. “mixer,” the most common form) or a volume knob on a speaker. The point to a mixer is that it takes the signal from a source and sets it at safe and appropriate levels for the power amp and speaker to handle, as well as balancing different signals for clarity and adding effects.

INTERESTING FACT: Sometimes you will see a mixer or two on stage. A drum set will sometimes have so many microphones and pickups that a dedicated mixer will balance those pieces and send only a few signals to the house. This is not very common and may have multiple reasons—such as a lack of channels in the Front of House (a.k.a. “FOH”) mixer—but it does happen on occasion. Another situation with an extra mixer is when there are a lot of monitors but there is not a personal monitoring system, such as that offered by Aviom and others. In this situation, there will be a second sound engineer, typically next to the stage, who uses a large mixer for the monitors, with the FOH engineer mixing only the mains speakers (a.k.a. “mains”).

In the end, the basics of a mixer is that it takes a signal(s) from the source(s) and feeds a single, processed signal into the power amp.

3) The Power Amp

The job of the power amp is simple. It takes electricity and runs it through a lot of complicated circuitry in order to “make it louder,” again a crude way to say it, but a simply understood concept. The signal created by mics, pickups (as those found in guitars), or any other source is actually not sound itself. The sound created by the instrument (with the exception of digital instruments, which either use amplifiers and such or generate the signal themselves) is taken from physical movement and sound waves and transformed into an electrical current. This current is what mixers and power amps work with, being transformed at either end by the source and the speakers. You could think of a funnel like an hourglass, where it is changed when it enters one end and transformed when it leaves the other.

INTERESTING FACT: A guitar amplifier is a combination of a mixer, a power amp, and a speaker. It takes the source (a guitar) signal, processes it, amplifies it, and sends it to a speaker for the listeners to hear.

4) The Speaker

A little more varied than the power amp but nowhere near as varied as the source and mixer, a speaker takes an electrical current and translates it into physical movement. By far the most common form is a cone shape with a magnetic assembly in the back, which moves the cone forward and backward to push air and create sound waves. Other versions of this link in the chain work in specialized ways, such as transferring vibrations to the bone in one’s skull by the ear, which then interprets the vibrations as a man’s voice, an electric guitar, or whatever signal is present.

INTERESTING FACT: The tympanum in the ear are essentially a reverse of a speaker cone in that they are large surfaces that move as vibrations collide with them (via air or bone) and make those vibrations into electricity.

INTERESTING (IN) FACT: Overall, communication between the ears and the brain is like the PA system. The ears are the source, taking physical waves and transforming them into an electrical signal that moves into the brain. The brain interprets signals and sorts through what is important (e.g. “selective hearing”) and passes that into the parts of the brain that interpret the message being communicated.

These are the basics of a PA system. Once you learn these four parts, you will know the essential makeup of every sound system you will ever deal with. It helps in troubleshooting, especially, because you know that if the guitar is coming through the speakers and the mic is not, the problem is nowhere between the mixer output and the speakers. That cuts out an awful lot of places to check for the problem! Learn these basics, and you will be off to a great start as a sound engineer!