The term "PA system", describes equipment that can take many shapes and forms, from small speech  PA systems to medium-sized band PA systems up to the much larger concert PA systems. 

These are the four basic components that comprise a PA system:

MICROPHONE - MIXER - POWER AMPLIFIER and  SPEAKER

But no matter how large the PA system becomes, the 4 basic components are always there. To determine the size of the PA you need to know three things:

1. The size of the area requiring sound
2. The amount of people that need to hear the sound and most importantly,
3. What is going to be run through the system. eg: 

Speech to crowd, 1-100 people (small reception, speech only)

Band to crowd, 50-200 people. (party or small room, vocal only)

Band to crowd, 500-800 people. (pub gig, full band through system.)

DJ to crowd, 10-200 people  (party, CD and speech)

DJ to crowd, 200-800 people (rave or large venue, CD and speech).  These factors determine the size of the required power amplifier(s) and the speakers only.

Small PA systems quite often have the mixer and the power amplifier in one unit and this is obviously called a powered mixer.

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Tweeter

A loudspeaker designed to reproduce only the treble or high frequencies of the audible spectrum.

 

Driver

Speaker component not mounted in a box 

 

Baffle 

The panel upon which a driver is mounted, most commonly the front panel of an enclosure.

 

Speakers in Series

Total speaker impedance (Z) = Spkr1 + Spkr2

Two 8 ohm speakers in series Z = 8 ohms + 8 ohms = 16 Ohms

 

Speakers in Parallel

Total speaker impedance (Z) = Spkr1 x Spkr2 / Spkr1 + Spkr2

Two 8 ohm speakers in parallel Z = 8 ohms x 8 ohms / 8ohms + 8ohms = 64ohms / 16ohms = 4 Ohms

 

Bass Reflex Speaker Enclosure

Design employs a "port" or opening which greatly reinforces the bass, yet requires a relatively small cabinet.

 

Crossover

In multiple driver systems, eg. 3- way. A circuit which splits the frequency range to the particular drivers designed to handle that range. Can be active or passive.

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Amplify

To strengthen a given signal by increasing its amplitude, or raising its level. The opposite of attenuate.

 

Pre-amplifier

An amplifier which accepts low level signals from microphones, guitars etc, and boosts the signal to line level (to feed a power amp input.) Volume and tone adjustment are normally provided in the pre-amp.  

 

Power Amplifier

An amplifier which accepts a signal from a pre-amp or mixer to its input and enlarges it to be able to drive a speaker.

 

Amplifiers come in many different shapes and sizes. They can be solid state or valve, 50 watt valve amps or 500 watt per side transistor amps. They can be used for huge outdoor/arena PA systems or for guitar practice in the bedroom once a week. But they all do the same thing, amplify a small signal into a large one. This large signal is used to drive the speakers.

 

Looking after your amplifier does not take much work, but can save you a lot of heartache.

 

With all solid state amplifiers you must be very careful when fiddling with the speaker wires.

This goes for Hi-Fi amplifiers as well. If you touch the positive and negative wires together while the amplifier is on, you will short the output and blow the power amplifier. This only takes milliseconds, even if they just brush past each other - BANG!!! REPAIRS

 

Keeping the amplifier clean is important as well. If too much dust builds up inside the amplifier it will inhibit the air flow which keeps the amplifier cool. This will particularly affect solid state amplifiers, as the transistors are very temperature sensitive, but keeping anything clean and neat will always help make the time between repairs longer.

 

Making sure you have the right amplifier for the job is also very important. It is no good trying to use a practice guitar amplifier for a big stage gig, as you will overwork the small amplifier and risk blowing it up. On the other hand you really don't need a 500 watt amplifier for some ambient background music in a restaurant. The size of the amp must suit the job.

 

Here are some ideas -

Guitar player on stage in front of 200-500 people - 30-100 watts    

PA system for the whole band or DJ playing to 200-800 people - 300 watts  

Practice amplifier for home use - 5-10 watts

Bass player on stage with drums - 300-600 watts

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