Mixing consoles are invariably fitted with both pre‑fade and post‑fade Aux sends, ostensibly with the intention of allowing the user to set up foldback mixes or add effects from external effects units. These are the typical applications described in most mixer manuals and textbooks, but by applying just a little lateral thinking, you can find numerous novel uses for the Aux busses that are rarely documented. This article outlines the more conventional uses of the Aux busses, followed by some useful and less obvious tricks.
Pre‑Fade Conventions
When overdubbing, a cue or foldback mix is generally needed so that the performers can hear any tracks already recorded. This is set up using a pre‑fade send control, a knob found in the mixer's channel strip which feeds some of the channel signal onto a mono mix buss running the length of the mixer and out via an Aux master level control. The output from Aux 1, for example, feeds onto the Aux 1 buss and then to the Aux 1 output socket, via the Aux 1 master output level control. Foldback or cue signals are referred to as 'pre‑fade' because they are picked up before they reach the channel fader. The significance of this is that, once set, the level of the Aux 1 signal doesn't change if the channel fader setting is varied.
It follows that an independent mono mix of all your channels can be set up using the Aux 1 controls, and this will appear at the Aux 1 output where it may be fed to a headphone amplifier or other monitoring system. If your mixer has more than one pre‑fade send, you can set up a number of different monitor mixes to satisfy the requirements of each musician, providing you have access to a multi‑channel headphone amplifier system. A typical situation is where the backing vocalists want a lot of lead vocal in the cans, whereas the drummer and bass player want to hear primarily each other.
Many of you will already be intimately familiar with this conventional use of pre‑fade sends, but later in this article I will be exploring a few alternative uses for this versatile control.
Post‑Fade Conventions
Post‑fade Aux send controls pick up their signal feed after the channel fader, so any change to the channel fader position will also affect the Aux send level. This is exactly what we need if the Aux send is being used to feed an effects device, such as a reverb or echo — as the channel fader setting is modified during the course of a mix, the amount of effect needs to change by the same amount to maintain the correct proportion of effect to dry signal. By using different settings of the post‑fade send control on each mixer channel, it is possible to send different amounts of each channel's signal to the same effects unit.
Utilising the Aux send system has the advantage that different amounts of the same effect can be added to different instruments in a mix. A typical example might be where one reverb unit is used to provide a rich reverberation treatment for the vocals, less reverb for the drums, and little or none for the guitars and bass.
It is important to remember that any effects unit used in conjunction with a channel Aux send should be set up so that it produces only the effected sound and none of the original (dry) source. This is usually achieved using the unit's mix control, which is either in the form of a physical knob or a mix parameter accessed via the effects unit's editing software. In either case, the mix should be set to 100% effect, 0% dry. This ensures that the amount of effect added to any channel signal is determined solely by the setting of the pre‑fade Aux send control.
The output of the effects unit may be fed back into the mixing desk via spare input channels or via dedicated effects return inputs, often known as Aux returns. These Aux returns are electrically similar to the input channels but usually offer far fewer facilities (limited EQ, for instance). If a spare input channel (or channels) is used for an effects return, ensure that the Aux send control on those channels is turned fully down to prevent feedback. To keep the effect in stereo, the two Aux returns or channels must be panned hard left and right.
Also note that although most stereo reverb units have two input jacks, the reverb itself is generated from a sum of the left and right input signals, so you only need to tie up one Aux send. Usually, one of the effect input sockets will be labelled 'Mono', indicating that you should send the input to that jack for mono in/stereo out operation.
Effects & Insert Points
An Insert point is simply a connector that allows the normal signal path to be interrupted and re‑routed through an external device. Insert points usually take the form of stereo jack sockets wired to carry both the send and return signal, and if you don't have a patchbay, you'll need a 'Y' splitter lead with a stereo jack plug on one end and two mono jacks on the other. Inserts are normally fitted to all mixer channels that have mic inputs, and often to the group outputs and main stereo outputs too. If soldering up Y‑leads is not your forte (though you'll save a lot of money if you learn), you can buy stereo to dual mono jack adaptors or ready‑made Y‑leads.
You can connect either an effects unit (eg. a delay or reverb) or a signal processor (eg. compressor or gate) to a mixer via an insert point, and in situations where an effect is required only on a single mixer channel, then connecting it via the insert point will provide a cleaner signal path than going via the Aux sends. That's because you will avoid picking up mix buss noise from all the other channels that are also connected to the Aux send buss. It will also avoid tying up Aux sends unnecessarily. Note that when an effects device is connected in this way, via an insert, then the dry/effect balance must be set on the effects unit itself.
Inserts As Aux Sends
Stereo effects can be fed from an insert send and the effect outputs brought back into a pair of spare channels or a stereo effect return as normal. The effects mix should be set at 100% wet (effect) in this case and the effects return channel controls used to set the effect level. Note that since the insert point comes before the channel fader, the effect level will remain constant even if the channel fader is moved. If there is ever a need to adjust the channel fader during the mix, it helps to return the effects to two adjacent channels so that all three faders can be moved together. If you don't have your mixer insert points wired to a patchbay, you may have to make up a special lead to allow you to take a feed from the insert point without interrupting the signal flow through the channel. Some mixers, such as Mackie models, allow you to take a direct feed from the insert point by using a mono jack and only pushing it half‑way into the socket.
You can also use this method for providing a direct channel feed to a multitrack recorder input. In this case, turn the channel fader right down, and/or ensure that all routing buttons are in their 'off' position, then use the input gain Trim control on the channel strip to set the recording level going to tape.
Sends As Tape Outs
A pre‑fade send is simply an independent mono mix based on the channel pre‑fade send control settings, so even if the main channel faders are turned down you can still create a pre‑fade mix. This is potentially useful in a mixer that has fewer output groups than you might like, because you can use any pre‑fade send output to feed a multitrack recorder input. In other words, you're using the pre‑fade send as an additional output group. You have to control levels using knobs rather than faders, but in all other respects the result is exactly the same. For example, if you have a couple of microphones on mixer channels 1 and 2 that you want to mix together and record on tape track 6, all you have to do is turn the faders on channels 1 and 2 right down, turn the pre‑fade sends up, and route the corresponding pre‑fade send output to tape input 6. The send master level control may then be used to set the correct signal level to tape.
The labels associated with mixer inputs and outputs relate to their most common use, but that's not to say you have to use them solely for that purpose.
Post‑fade sends may also be used as tape outputs, providing your mixer has routing buttons. This is vital, because you don't want the channel signal going anywhere else other than to the Aux sends, so it's important to ensure that any group and L/R routing buttons are turned off. If you have the facility to do this, then using the post‑fade sends instead of the pre‑fade sends means you can use the channel faders to adjust the signal level going to tape, which may be far more convenient.
Sends & Processors
Conventional wisdom tells us not to connect signal processors via the Aux sends, but what if you have a mixer or cassette multitracker with no insert points? In this case the pre‑fade Aux sends can provide a solution. Here, the channel to be compressed is channel 1 and the channel fader must be set fully down. The pre‑fade send, on that channel only, is then turned up and the compressor connected to the pre‑fade send output as shown. The output from the compressor can then be fed into a spare mixer channel and the signal routed normally. If you study this patch, you'll see that because the fader is down on channel 1, the dry signal isn't routed anywhere; the only signal you use is the output from the compressor. In essence, channel 1 has been used as a mic preamp with the signal being tapped off via the pre‑fade send for convenience.
Summary
When I was first introduced to mixers, I found pre‑fade and post‑fade sends quite confusing, and it was only after studying the block diagram of the mixer that I realised how simple they really were. Once you grasp that the Aux send busses are much like the main group busses, you can use them in any way you wish. Both pre‑ and post‑fade sends may be used to create new tape feeds, and with a 4‑buss mixer this can make all the difference between 4‑track and 8‑track recording. Cassette multitracker owners can use pre‑fade sends to patch in compressors or equalisers where there are no insert points, and on more than one occasion I have used the pre‑fade foldback sends on a cassette multitracker to create additional tape outs for remixing. It's often useful to remix a 4‑track cassette via a more serious mixer, but on cheaper machines you don't have separate tape outputs. In such instances, I've used the left and right outputs to carry a mix of three of the tracks and a pre‑fade send to carry the remaining track, and though three sources may not seem like a lot, it's often enough to make a big difference to the final mix. On a machine with two pre‑fade sends, you can separate out all four tracks — which is the ideal situation.
I haven't even mentioned the PFL (Pre‑Fade Listen) buss in the course of this article, but that can provide yet another way out of the mix — if you PFL solo one channel while its fader is down, its signal will appear at the headphone and monitor outputs while the remaining channels will be routed to the main and output groups as normal. With just a little imagination, a 12:2 mixer can be used to handle 4‑track recording, a 16:4 PA mixer can function well for serious 8‑track recording, and the limitations you thought your cassette multitracker had can be turned on their head.
What I have written has by no means exhausted the less obvious uses of pre‑ and post‑fade sends, but I hope that by pointing out some of the possibilities, it'll help you think of your own ways to use and abuse your equipment to get the most out of it. The labels associated with mixer inputs and outputs relate to their most common use, but that's not to say you have to use them solely for that purpose. Quite often, the only way around a sticky problem is to use your mixer in an unorthodox way, and the more you think about it, the more ways there are.
Effects Patching
Before you can work out the best place to patch in your outboard equipment, you need to know what counts as an 'effect' and what does not. I tend to define the various bits of outboard equipment as either effects or processors, according to what they do and how they do it. The main reason for this is that there are certain restrictions on how processors can be connected, while effects enjoy a little more flexibility. (I've covered this before in SOS, so if you're already happy with the distinction, feel free to skip back to the main text. )
Processors are boxes that modify a signal, whereas an effect is a device that leaves the original signal intact and adds something else to it. Processors include EQ, compressor/limiters, expander/gates, panners and single‑ended noise reduction units. If you have a box that modifies either the gain of a sound or filters it in some way, it's a reasonably safe bet that it is a signal processor. Echo units, delay lines, reverbs, chorus/flangers, pitch‑shifters, phasers and suchlike are effects, and all effects should have some means of balancing the original (dry) and effected (wet) signal. Effects may be connected either via mixing desk Insert points or via the Aux sends, but processors can normally only be used at Insert points or connected between two pieces of equipment. However, there are exceptions, hence the need for this article.
Post‑Fade Sends & Subgroups
When you come to mix down, you might create subgroups for things like drums, backing vocals or keyboards, so as to reduce the number of mixer faders you have to worry about during the mix. However, if your effects returns come back straight into the main stereo mix, they'll stubbornly refuse to change in level, no matter how far you move the group faders.
The answer is to route the effects used within the subgroup back to the same numbered groups, so that when the group fader is turned down the effect level goes down with it. The only time a problem arises is when the same effect is used in two different subgroups or in both a subgroup and in channels routed directly to the stereo mix. There's no easy solution to this, and in extreme cases you might have to patch in a second effects unit to get around the problem.