Multitrack Mixers (Part 4)
Insert Points & Aux Sends
Last minute bargains: Guided tour around typical multitrack desk, stopping at all notable features. Bed, breakfast and evening meal not provided.
Insert points and Aux sends are often a source of confusion when you first start to use mixers, but their correct usage is central to effective mixing. Paul White demystifies them.
While I can understand the confusion that some people feel when confronted by Aux sends and returns for the first time, I'd always thought insert points to be fairly self explanatory — until a reader called me with a query. He explained that he had a mixer with stereo jacks at the channel insert points so, logically enough I suppose, he assumed these were stereo insert sends. That being the case, where on earth did he connect the returns? It was a perfectly rational question and led me to write this brief article just in case there's anyone else still in doubt as to exactly what is going on.
Of course, those of us in the know will appreciate that although the sockets are indeed stereo, they are not used to carry stereo signals — they're wired so that a single socket can handle both the send and return side of a mono signal. The problem is one of terminology — stereo jack plugs are being used for a purpose that is nothing to do with stereo. A stereo jack has three sets of contacts, as can be seen in Figure 1. In the case of most mixers, the screen connection is common for both the send and return cable, the ring connector is the insert send and the tip is the insert return. Traditional though this is, it isn't particularly convenient, as a special lead must be made up or bought to connect an effects unit or processor to the insert point. The usual lead is known as a Y-lead because it has a stereo jack plug at one end and a pair of mono jack plugs at the other. For those reluctant to indulge in soldering, there are alternatives: readymade Y-leads are available from Studiospares (2.5 metre insert leads, part number 570-010, £3.95 each. (Contact Details)), and Y-adaptors are available from Maplin (Part number JM92A, 98p each plus postage. (Contact Details)) which provide a stereo jack at one end and a pair of phonos at the other. By plugging in a couple of phono-to-jack leads (or phono leads with jack adaptors at the other end), it's possible to achieve the necessary connection using entirely off-the-shelf components. Of course the most convenient way to use insert points is to wire them to a patchbay; patchbays will be covered in detail in a future issue of RM.
As may also be seen from Figure 1, the insert point itself comprises a so-called 'normalised' jack socket which automatically connects together the insert send and return points when no jack is inserted. If this were not the case, there would be no signal path through the mixer channel unless an external processor were connected.
Figure 2 shows how the entire channel signal is routed through the external processor when it is patched into the insert point. In the case of an effects unit such as a reverb or delay unit, which works with a mix of the original signal and an effected version of that signal, the balance between the effected and 'dry' sound must be set up using the mix or balance control on the unit itself.
Both processors (such as gates, equalisers and compressors) and effects units (such as reverbs, and delays) may be connected via an insert point, but it must be realised that a device plugged into a channel insert point cannot be shared with other channels. This is in direct contrast to the Aux send and return system which allows a single effects unit to be used with any number of channels and with different amounts of the effect on each channel.
Most larger mixers also have insert points on the Groups and on the main Stereo output. The Group insert points may be used when mixing to add an effect or process to a whole subgroup, while the stereo insert points may be used to add an effect or process to the whole mix. It is unusual to add effects to a whole mix, but it is common to use a process such as equalisation, enhancement or compression to fine-tune an otherwise complete mix.
Figure 3 shows an effects unit connected via the post-fade effects send system of a mixer. The post-fade Aux send controls, sometimes called effects sends, are usually located in the middle of the channel strip, but because they come after the fader in terms of signal flow, that's where I've drawn them in the diagram. The reason they are wired after the fader is so that whenever the channel level is changed, the level of the signal fed to the effects unit varies accordingly. It is possible to use a pre-fade send to feed an effects unit, but if the level fader is moved, the level of the signal on that channel will change, while the effects level will remain the same. If you're in the situation of having to use a pre-fade send to feed an effects unit, you'll have to get into the habit of adjusting the effects send knob to match.
The output of the effects unit is fed back into the mix via the Aux return input(s) which are exactly the same as any other mixer input channels except that they usually have far fewer controls. It is essential that the balance control on the effects unit is set so that only the effect is present at the output and not the dry signal. That's because the dry signal is already being added to the mix through the input channels, and this way, the Aux send controls on the different channels determine how much effect is added to that specific channel. It is, however, very important to note that signal processors (as distinct from effects) such as compressors, equalisers and gates should not normally be connected via the Aux send controls as they don't work on a 'dry plus effect' basis — they are designed to process the entire signal. For this reason, processors are only connected via insert points or connected 'in line' between one unit and another. There are exceptions, which I'll cover when I look at some of the ways of getting around the limitations of a mixer with limited facilities, but for all general applications, these rules hold fast: both processors and effects may be used at the insert points, but only effects may be connected via the Aux send/return system.
When assigning mixer channels to Subgroups when mixing, it is important to realise that any effects used on those channels (via the Aux sends), and fed directly back into the stereo mix won't change in level if the Subgroup fader is moved. The answer is, where possible, to route the effects returns back to the same Subgroup, so that now when you turn down the Subgroup fader, both the dry Submix and its effects change together. Most multitrack mixers have signal routing on the Aux returns, but if you are working with a mixer that doesn't, then simply feed the effect unit into the mixer via one or two spare input channels, then route those to the desired Subgroup (or pair of Subgroups in the case of stereo). The only proviso here is that whatever Aux send is feeding the effects unit should be turned right down on whatever channels are being used as effects returns. This isn't the first time I've mentioned this potential problem and it won't be the last — failure to observe this particular nicety is likely to result in howls of electronic feedback!
Feature by Paul White
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