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PA Basics

There are essentially four different sections to a PA System? These are input sources (eg. microphones, CD player), signal processors (eg. mixing desks, effects processors), amplifiers and speakers.

The input source is what generates the sound signal and sends it down the cable. This can be either a live conversion, in the way that a microphone picks up a sound and turns it into an electrical signal, or it can be a recording, in the way a CD player converts the recorded track into an electrical signal. The most important difference between the signal from a microphone and that from a CD player is the amplitude of it (signal level). Microphones generally give quite a low signal level and this needs to be amplified to the same sort of level the CD player gives out (referred to as line level). This is done by means of a pre-amp which is usually found just after the microphone input on a mixing console, but can also be an external unit prior to the mixing desk.

Signal processors are any item in the chain that is used to alter or mix the sound signals. Most systems will have some form of mixing desk, where various input signals can be mixed into one or more output signals to go to the next item in the chain. Most mixing desks also contain equalisers on each channel so that the signals coming in can be adjusted - for example, increasing the low or high frequency. Other signal processors commonly found are graphic equalisers which are used to increase or decrease certain frequencies in the signal being passed through and effects processors which are used to add echoes, reverberation and other effects to the signal that is passed through them.

Amplifiers take the line level signal, and increase it so that it can drive the speakers. They are often standalone units which you put the line level signal into and take the speaker level signal out of, but they can also be integrated into mixing desks (referred to as powered mixing desks) and also into speakers (generally referred to as powered speakers or active speakers).

The last piece of equipment in the chain is the speaker. The speaker level signal in the amplifier causes the voice coil on the speakers driver to generate a variable magnetic field which in turn causes the diaphragm to move and generate the sound. Depending on the speaker, it may have a crossover in it, which splits the signal coming into the speaker into different frequency bands and passes each split signal to the appropriate driver in the speaker.

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Damon Oldacre trading as RKDO Sound & Light