The Channel Strip Part III: Auxiliaries

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  • July 12, 2017

Until now, we’ve followed the signal path from the microphone preamp, which boosts the level of the signal to make it useable further down the line. Then we went on to the EQ section, so the frequency spectrum could be shaped to taste. The path so far is linear and pretty simple to follow. Enter the auxiliary – an extra path for routing signal.

So if things are sounding good already with a straightforward signal path, why complicate things with an entirely different one? Utilised judiciously, auxiliaries are incredibly powerful sonic tools and in some cases, downright essential. As we’ll discover, the reasons why auxiliaries are employed have practical and creative benefits for recording, mixing and live performance.

Here is part three of our series of articles across the channel strip; this time we take a look at the auxiliary section.

How It Works

On a console channel strip, the auxiliary sends signal from any given channel to an auxiliary master, which controls a physical output on the back of the desk. The auxiliary section on a channel strip is usually comprised of a series of numbered knobs.

As an example, let’s assume that there are four auxiliary knobs on each channel strip. Turn the knob for auxiliary 1, and you are sending a double of the signal from that channel strip to the auxiliary 1 master. Turn the knob for auxiliary 2 and the signal will go the auxiliary 2 master, and so on.

In and of themselves, auxiliaries do not have any tone shaping characteristics. So, again, why do it? Well, it gets more interesting after the signal gets sent from the auxiliary master, out of the console, to a different device.

How Auxiliaries Work With Effects

Using auxiliaries to access effects is perhaps the most creative way to use this alternative signal path. Imagine you want to add reverb to a snare drum.

Firstly, you’d need to connect a cable from an auxiliary master output on the console, (for example auxiliary master 1) to the input of the reverb unit. When you crank up the auxiliary 1 knob on the snare channel (assuming the auxiliary master is turned up to a sensible level) the signal will be feeding the reverb unit.

To hear the reverb, you then need to use another cable to connect the output of the reverb unit to a spare channel on the console. Push the fader up on that channel and a juicy, reverberant snare should appear.

But if you want to use reverb as an effect, why can’t it just be on every channel, like EQ, rather than accessing it via the comparatively convoluted auxiliary signal path? Aside from the sheer impossibility of adding a reverb unit to every channel of a multichannel console (expense, size, heat generation etc), it’s not the best way to employ an ambient effect such as reverb.

The real beauty of auxiliaries is that they can be blended with the raw tone. Having a mostly dry snare, with a little wetness from the reverb might be appropriate, or a huge snare, swimming in reverb, that wouldn’t be out of place in the 80s might be a better choice.

Having the option to blend this tone into the mix on a separate fader with dynamism and artistic flair is part of the fun of mixing. The auxiliary makes this possible.

Also, imagine you want the toms, or the overheards, or even the vocals to kissed by a little bit of this same reverb. All that is required is to dial in the corresponding auxiliary on those channels for them to be affected by the same sound. This too would be impossible without the help of the auxiliary path.

In fact, it can be helpful to think of the auxiliary path as a way to create an entirely different mix
from the main one. In this way, auxiliaries can be used to serve more practical ends.

A Separate Mix For Performers

When playing live on stage, or recording in a studio, it’s of great benefit to the performers to hear their own mix. They may need to hear specific sounds to hit their cues or click tracks to keep in time. In any case, the ideal control room mix, or front of house mix is apt to be different from mix for the musicians.

Auxiliaries can facilitate this need. Yet to make full use of this function, the difference between pre-fade and post-fade auxiliary paths needs to be understood.

By default, auxiliaries function in post-fade mode – this means that the auxiliary level will be affected by the level of the output fader. If the fader is all the way down, or if the channel is muted, the signal won’t be sent to the auxiliary master, no matter how much you’ve cranked the knob.

If the pre-fade button is engaged, the auxiliary knob acts independently from the fader – meaning a different mix of levels can be achieved even if the fader is down or the channel is muted.

So if you’re in a recording session with a vocalist, and they want to hear a lot of the bass drum to get the energy right for their vocal take, it need not affect the mix in the control room. This gives the engineer maximum flexibility in honing the mix to taste while giving the vocalist the ideal ingredients for a great performance.

Understanding auxiliaries can perhaps commandeer a little more brain power than other aspects of the audio signal. The extra layers of inputs and outputs and mixing options can be overwhelming on first inspection. Yet the principle of the auxiliary path and its sheer usefulness and sound-sculpting power is worthy of understanding, and surely an essential stepping-stone on the pathway to engineering expertise.

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Dan Shaw is recording and mixing engineer at Enmore Audio. He also plays bass in Wells and Circle.

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