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Gain Structure

The term 'Gain Structure' relates to the setting of levels at each stage of the audio chain and the idea of it is to minimise the amount of noise that is added to the signal chain whilst having a signal that will not clip and distort. The way to achieve this is by setting the level at each part of the signal chain so that at its loudest, it peaks a little bit below the point where it would clip, and allows sufficient headroom to cope with any unexpected peaks in signal.

As an example, take a PA consisting of a microphone, mixer, graphic equaliser, amplifier and speakers. The microphone picks up the source sound and converts it into an electrical signal. This is then passed on to the input stage of the mixer (the mic preamp). This is the first opportunity you have to set the gain. To do this, activate the PFL (sometimes referred to as CUE) button for the channel and ensure that the microphone is picking up the loudest signal that will be picked up during use. Whilst looking at the VU meters or peak meters, adjust the gain control for the channel until it is peaking at around +6dB. Unless you can be sure that the signal will never be greater than it is during this set up, it is always best to err on the side of caution and decrease the gain slightly to ensure that you will have enough headroom. Once this is complete, you can deselect the PFL button. Now, with the channel fader at 0dB and the master faders at 0dB, the level should be peaking at the same level set during PFL.

The next step in the chain where we can adjust the gain is at the graphic equaliser. This should have a knob marked 'Gain' or 'Input'. With the input channel and master faders on the mixer at 0dB, and the same signal coming in through the microphone, adjust the gain until the input meter on the graphic equaliser is peaking at the same level as the meter on the mixing desk.

The final stage for this system would be the amplifier. It is a common misconception that the level controls on an amplifier are gain controls and that the lower they are set, the lower the output of the amp will be. In actual fact, the controls on an amplifier are sensitivity controls and only serve to reduce the sensitivity of the amplifiers circuitry to the signal coming in. Because of this, it is still possible to get the maximum output of an amplifier without the amps levels at their highest, it just means the input level needs to be higher. For this stage of the gain structure, we need to have a signal at peak level passing through the mixing desk and the graphic. Starting with the controls at 0, slowly turn them clockwise. Once the sound level in front of the speakers is as loud as will be required, then you do not need to turn the controls any further, however, if you want the system as loud as it can go, then continue turning until the clip light on the amplifier starts to light. Immediately turn it back slightly so that the amplifier will not clip at all during use (amplifier clipping is one of the best ways to destroy a high frequency driver in a speaker).Once this is complete, you have set a gain structure for the system.

Remember that you need to do this on each channel that is used on the mixing desk and with each piece of equipment in the chain, be it effects processors, compressors etc. Some equipment may also have output gain controls which also need to be adjusted in the same way. It is also important to note that should you adjust the EQ of a channel, the gain may also need to be adjusted to ensure that it is still correctly set.

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Damon Oldacre trading as RKDO Sound & Light