Sound aficionado WILLY T talks AUXILIARY POWER, and what it has to do with live sound.

Each piece of gear in the long line from the stage to out the speakers – the signal-path – has a level at which the device operates at its best or optimum. If the gear is run too gently the signal can be swamped in the ‘hiss’ of background noise. If we run the gear too hard the signal can ‘break up’ or distort. And we are not talking about the type of creamy guitar-sound distortion that makes Strats scream and Gibsons growl, but more like the nasty sounds of analogue electrical circuits being pushed way out of their comfort-zone or digital numbers melting into non-linear chaos theory!

Most mixing desk faders have a “0” or shaded area which represents Unity Gain – where the output matches the signal input and is operating at its quietest level. Put simply: what went in is what went out – nothing more or less. On rotary faders ¾ or “7” on a scale arbitrarily marked from “1-10”, usually corresponds to unity gain.

The aim is to have an input level that sits in between Clipping – the level at which the input will distort – and the Noise Floor – the residual, low-level electronic noise present in all electrical gear. (This noise is actually due to the random movement of electrons, and until we find some way around the laws of physics and quantum mechanics we’re stuck with it – or set up and operate in a cold room at Absolute Zero temperatures.)

To get the max out of any PA rig, every time a signal is fed from one piece of gear into the next, the input and output gain levels must be set so the signal operates at its optimum level. Like the issue of consistency of wiring “polarity” that we looked at a few weeks ago, optimising the gain structure applies to all the components that combine to make a PA system, whether mic, mixer, FX unit or power amp. Failure to set even one input gain control correctly may lead either to excessive noise if the gain is too low; or ugly distortion if the gain is too high. We do not want to have mega hot output signals distorting the inputs of the next bit of gear in the chain, or too quiet a signal needing to be boosted. For example, if we are sending out of the mixing desks to an FX device and we’ve got the desk’s Master Aux Send controls turned down very low, we have to then turn up the input stage of our FX unit to compensate. Potentially boosting any noise. Not good.

FX LOOP & GAIN

When using the FX Loop of “Sends and Returns” it is important to remember that there are different points where we can alter the gain or monitor the signal’s journey into and out of the mixing desk and FX.

1: Channel Aux Send adjusts the amount of signal sent from the input channel to the Aux Master-Send.

2: Master Send (Aux) adjusts the amount of signal sent out-of-the-mixing-desk into the FX gear.

3: Input Control (FX Device) adjusts the signal level into-the-FX gear.

4: Output Control (FX Device) adjusts the level sent out-of-the-FX unit to the mixing desk’s Aux Returns.

5: Aux Return sets the amount of processed signal that is added back into the main mix.

FX INPUT & OUTPUT CONTROLS

As a lot of FX gear is relatively noisy, try to drive the FX input as hard as possible while leaving a bit of headroom for the occasional unexpected peaks in the sound. Our ears are designed so that strong sounds will mask quieter sounds so a strong signal level has the effect of masking the gear’s electronic noise, and system’s noise floor. This increased input naturally results in a stronger output level from the unit so the Aux / FX Return fader on our mixing desk can also be reduced by the same amount. This leaves the FX levels identical, but with correspondingly lower noise levels! A better “signal-to-noise ratio”, basically.

Start by setting the mixing desk’s Aux Master Send to around 7 or ¾ scale which usually corresponds to unity gain. The input gain control on the FX unit itself should be set while watching the unit’s meters. Set the input gain on the FX unit so that the signal sits between ½ and ¾ up the scale. This should get us near the correct meter reading, while leaving room for the occasional unexpected peaks in the sound. (If it has selectable +4dBu/-10dBV level switching, choose the correct setting to match your mixer. This will usually be the one that lets you work with the input gain control closest to the centre of its range. If the input gain control needs to be set much lower than this, check whether there’s a +4/-10dBu switch on the back and try the +4 setting.)

If our FX unit has an Output Level control, this should also be set at around 7 or ¾. (The Unity Gain ¾ rule of thumb!) Setting up the FX loop like this means that the actual level of effect in your mix is determined by the Effects Return control on your mixing desk.

FINALLY

Gain has structure! And there is a range in which all gear likes to communicate and operate. Not too hot … not too cold! Gain ‘tweaks’ must be carried out from the very first input, all the way from the mic to the FOH speakers – including the Auxiliary Sends and FX gear. Remember any time we boost, we boost any noises. They all add up!

WILLY T runs Giraffe Media. Providing Words, Pics and Live-Audio Recording Services to the entertainment industry. Email for more information.

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