MIXING IT: Mixer Anatomy

Published in SOS August 1994
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PAUL WHITE explains the key features of a typical recording mixer, and weighs up the pros and cons of Split and In-line monitoring.

Once you move up from an integrated device like a Portastudio to a separate multitrack recorder and mixer, the most onerous task (other than wiring it all up) is figuring out how best to use your mixer. Thankfully, although mixers come in all shapes and sizes these days, the underlying principles are very similar, and much of what you learned from using your Portastudio can be carried through to your new system.

In essence, mixers combine two or more audio signals, provide control over level and EQ, and enable signals to be routed from one place to another. The latter is easy enough to comprehend in the case of a 'something into two' (eg. 16:2) PA or keyboard mixer, but when you're confronted by a large multitrack mixing console, the routing system might not be so obvious. Let's take a look...

In order to mix several inputs into stereo, we require two signal paths: one to carry the left channel signal and one to carry the right channel signal. These signal paths are known as buses, and in the case of our simple 'something into two' mixer, these would be called the Left/Right buses or, more commonly, the Stereo Mix Bus. Further buses are required to handle any Auxiliaries (ie. effects sends, foldback) the mixer may have, while a recording console also needs Group buses. An 8-bus console, for example, would have eight Group faders to control the level of the signals being sent to the multitrack tape machine inputs.

Once the tracks of your multitrack tape have been filled up and it's time to mix, the channel routing buttons and Group faders may be used to create Sub-Groups of logical groups or sets of instruments or voices, but we need some way of feeding these Sub-Groups into the stereo mix.

The most common method is to use a button called 'Groups to L/R', or something similar, which routes the signal from the Group fader into the main stereo mix. The usual protocol is to route all odd-numbered Groups to the left of the stereo mix and all even-numbered Groups to the right. That way, if you want to create a stereo sub-group, the channels concerned may be routed to Groups 1/2, 3/4, 5/6 or 7/8 and then the channel Pan pot used to move the channel signal to the desired left/right position. The Sub-Group level will then be controlled by an adjacent pair of Group faders (1/2, 5/6 or whatever).

When assigning channels to Sub-Groups when mixing, it is important to realise that any effects used on those channels (via the channel Aux sends), and fed directly back into the stereo mix, will not change in level if the Sub-Group fader is moved. The way around this, where possible, is to route your effects returns back to the same Sub-Group, so that when you now turn down the Sub-Group fader, both the dry, uneffected Sub-mix and its effects change together.

Mixer circuitry is designed to work within a particular range of signal levels, usually up to 12 Volts or thereabouts, but not all input signals come in at the right level. For example, the signal produced by a microphone may measure only a few thousandths of a Volt, so any mixer designed for use with microphones must incorporate a microphone preamplifier which boosts the microphone signal up to the mixer's internal working level. This microphone preamplifier may well also be fitted with phantom power circuitry, enabling it to be used with capacitor microphones.

Signals from electronic keyboards, CD players, tape machines and so on, are said to be at 'line level', because they require very little amplification to bring them up to the mixer's internal operating level. Because both mic(rophone) and line signals vary in level depending on the source, gain controls are fitted; on most home recording and semi-pro mixers, a common control is used for both mic and line gain.

All recording mixers have some facility for connecting external effects units and for producing a cue mix for the performers, both of which require Auxiliary sends. An Aux send is simply another level control feeding a mono mix bus, which runs across the mixer to an Aux Master level control and then to an output socket.

To set up a cue mix, we need to use a pre-fade Aux send -- 'pre' signifies that the signal is tapped off before it reaches the channel fader. This means that, because the signal is taken pre-fader, the cue level doesn't change if the channel fader setting is varied. On a large mixing console there may be several pre-fade sends per channel, which makes it possible to set up a number of different cue mixes to meet the needs of different performers. For example, the drummer might want more bass guitar in his cue mix, while the singer may need to hear more keyboards in order to pitch properly.

Aux sends are also used to feed external effects units, but this time we need to take the signal post-fader so that the effect level will move in proportion to any adjustments you make to the channel level fader. For example, if a post-fade send on a vocal channel is being used to feed a reverb unit and we fade out the vocal, we usually want the reverb to fade out too. If we were to use a pre-fade send to our reverb unit, the vocal reverb would remain in the mix, even if the channel fader was pulled right down.

It is important to note that any effects unit used in conjunction with a channel Aux send should be arranged so that it produces only the effected sound and none of the original. This is usually set up using the unit's mix/balance control or parameter.

The main advantage of connecting effects via the Aux sends (instead of via your mixer's channel insert points, if it has them) is that the same effect can be used on several mixer channels at the same time but with independent control over the effects level; in other words, the channel Aux send controls each act as independent effects level controls for that channel.

The output of the effects unit needs to be fed back into the mixer somewhere, and when I first got into recording I thought that you always had to use a dedicated Aux return input for this purpose. It was only after thinking about it for a while that I realised that an effects return is just another input channel stripped of most of its frills. So you could return your effects via spare input channels or, conversely, you could bring any line level signal (such as a drum machine or synth) into the mix via an Aux return -- there's no difference. On smaller mixers, the Aux returns feed straight into the main stereo mix, though on larger recording consoles they may have the same routing as the input channels. Effects units with stereo outputs should be connected to either two spare input channels or to a stereo Aux return input.

Before moving on, one word of warning: If a spare input channel is used as an Aux return, ensure that the corresponding Aux send control is turned right down on that channel or the effect signal will be fed back on itself, resulting in howling feedback.

It is important to note that while it's fine to connect any type of effect or signal processor via an insert point, there are restrictions on what can be used via the Aux send/return system. As a rule of thumb, only delay-based effects units such as reverb, echo, chorus, phasing, flanging and pitch shifting should be connected via the Aux system. Equalisers, compressors, gates and so on can't be used in the Aux send system -- they are connected via your mixer's insert points instead.

Another way to connect an effects unit or signal processor to a mixer is via an insert point. All properly equipped recording consoles have insert points on the input channels, the groups, and the master stereo outputs. Unlike Aux sends, insert points are simply connectors that allow the normal signal path to be interrupted and re-routed through an external device, the most usual type of insert point being a stereo jack socket wired 'Tip Send/Ring Return'.

In a recording context, the term monitoring simply means 'what you hear out of the control room speakers'. When recording, you always need to hear the new signal being recorded at the same time as the material already on tape, and unless you have a very old multitrack tape machine, the chances are that your multitrack recorder will do most of the thinking for you. That's because modern multitracks invariably have an 'auto monitoring' mode, which simply means that if you monitor the multitrack's output sockets, you'll always be able to hear the signals you need to hear without having to repatch or press switches on your mixer.

The monitor mix is traditionally set up in a part of the mixer known as the Monitor section. On a split console, this will typically comprise a separate set of Monitor channels located above the Group faders on the right-hand side of the desk, though the more usual approach is to adopt so-called in-line monitoring, where the monitor controls are located within the input channel strip along with the main channel controls.

Because in-line consoles have monitor controls in every input channel strip, a 24-channel desk can handle a monitor mix from a 24-track tape machine, whereas split desks tend to offer either 8-track or 16-track monitoring regardless of the number of input channels. Furthermore, because the monitor inputs aren't required when you come to mix, they're free for use as additional line inputs; on an in-line console, this effectively doubles the number of usable line inputs. This is obviously very attractive for people with large MIDI systems, where sequenced music is run 'live' into the mix. In-line consoles have switching on the channel strip to reconfigure the mixer for mixdown, often taking the form of an Input Flip switch which swaps the inputs to the main and monitor signal paths, so there's no need to repatch when you come to mix.

In practical terms, an in-line console may be physically smaller than an equivalent split model, simply because the master section doesn't have to accommodate the monitor channels, but there are compromises that you need to be aware of. A split console will have Aux sends on the monitors, and maybe even EQ, but on the majority of affordable in-line consoles, one lot of EQ and one set of Aux sends have to be shared between the main channel and the monitor. One way to take the edge off this compromise is to make the EQ splittable, so that the high and low equalisers can be used in one signal path (eg. monitor) and the two mids in the other (eg. channel). In-line console EQs designed to work this way usually offer very wide range mid controls, enabling them to cover the whole audio spectrum. If you come across the word 'assignable' in the context of an in-line console, it simply means that part of the EQ, or some of the Aux sends, can be switched between the main channel and monitor signal paths -- rather like timeshare.

There is another approach to setting up a monitor mix which doesn't rely on either in-line or split console design, and that is to use input channels for everything. For example, if you have a 24-channel mixer and a 16-track tape machine, you could use channels 1-16 (routed to the Stereo Mix bus) for monitoring the tape outputs and channels 17-24 (routed via the Groups) to handle any input signals being recorded on tape. In practice, most people who have enough input channels tend to work this way, and even if their console does have split or in-line monitoring, this isn't used except to provide either additional effects returns or further line inputs at the final mixing stage.

This 'all-input' approach lends itself to the use of more basic PA-type mixers for recording, providing some means can be found to route the desired input channels to tape. Back in my open-reel 4-track days, I coped for a long time using just an 8:2 PA mixer, but for any serious recording work, a 4-bus console should be considered the minimum. This way you'll be able to route your 'monitor' channels to the Stereo bus and the 'to-tape' tracks via the remaining Group buses. If you want to record more tape tracks at the same time than you have spare buses, then you can use your mixer's direct channel outputs (if available) or channel insert send points. It's also perfectly acceptable to use pre-fade Aux sends to feed signals to tape, so long as the channel fader is turned right down. If you need to connect a compressor to one of the signals that's already being fed out of an insert point, you can still do so by simply connecting the compressor between the insert send and the multitrack input.

Another way of getting signals on to tape is to bypass the mixer altogether and go direct to the multitrack inputs. For example, a guitar preamp, mic preamp or DI box could be plugged straight into the tape machine and the record levels set using the level controls on these devices. Similarly, keyboards can be recorded in this way, which is particularly useful if your mixer has no insert points.

When you're working in all-input mode, mixing is easy because you'll already have been working on your monitor mix during the session -- and your monitor mix is the same thing as your real mix; there's no repatching or resetting of controls needed. The mixer's left and right outputs are plugged into the inputs of your stereo master recorder (eg. DAT), which in turn feeds the monitoring system, and you're ready to mix. Any unused input channels are now free to accept sequenced MIDI instruments, effect returns, sound effects tapes, or whatever else you want to add to your mix.

EFFECTS & PROCESSORS: WHICH IS WHICH?

EFFECTS:
• Delay
• Echo
• Pitch Shift
• Chorus
• Flange
• Phase
• Reverb

PROCESSORS:
• Equalisers
• Compressors
• Limiters
• Gates
• Expanders
• Enhancers (though some are designed to be usable in the Aux send system as well as the insert points)
• Single-ended noise reduction units

USEFUL LEADS AND ADAPTORS

Tandy reference numbers:

• Stereo Phono-to-Phono lead (42-2351)

• Stereo Jack-to-Phonos adaptor (274-308)

• Phono-to-Mono Jack adaptor (274-320)

• XLR 3-pin Plug (274-010)

• XLR 3-pin In-Line Socket (274-011)

The above are also available from Maplin Electronics (see their catalogue for their part numbers). Tel: 0702 554161.

GLOSSARY OF MIXER TERMS

BUS
Electrical circuitry or wiring connecting several parts of a system. In the context of a mixer, buses are used to 'collect' the outputs from the various channels and mix them together. A mixer may have several buses carrying the main stereo mix, the Aux sends and -- on more sophisticated recording mixers -- sub-mixes of different groups of channels.

DRY SIGNAL
A sound source which has no added effect or processing. Conversely, a sound treated with an effect such as reverberation or echo is often referred to as 'wet'.

EQUALISATION (EQ)
A recording term for tone control. Usually comprises Treble, Mid and Bass sections.

GAIN
Put simply, a recording term for volume.

INSERT POINT
A connector that allows the normal signal path to be interrupted and re-routed through an external device, such as a compressor.

LINE LEVEL
Mixers and signal processors tend to work at a standard signal level known as 'line level'. In practice, there are several different standard line levels, but all are in the order of a few Volts. Microphones produce a much lower signal, in the order of tens of milliVolts.

PAN
Short for panoramic. This control allows you to position/move sounds left and right in the stereo mix.

PHANTOM POWER

A Capacitor microphone requires an electrical voltage to polarise the capsule, while both Capacitor and Electret microphones use built-in preamplifiers which also need electrical power. Valve microphones require special power supply units, but the vast majority of Capacitor and Back-Electret mics are designed to work using a system know as phantom powering.

This is a standardised system which passes the power supply current along the conductors of a balanced microphone cable. The standard phantom power supply voltage is 48V and is generated within the mixing console; provided a proper balanced microphone cable is used, it is necessary only to plug in the mic and turn on the phantom power supply.

In theory, it is quite safe (though pointless) to apply phantom power to a Dynamic microphone, so long as it is wired for balanced operation, as the same voltage will appear at both ends of the voice coil -- which means no current flows through it. The reason I mention this is that some budget mixing consoles have a global phantom power switch, so if you intend to use both Dynamic and Capacitor mics on the same session with such a console, you will have to ensure that all cables are balanced and that the mics are internally wired for balanced operation. As a rule, if the mic body is fitted with a three-pin XLR socket, then it is balanced -- but always check the data sheet that came with the microphone to be 100% certain. Use of unbalanced cables might cause damage to Dynamic mics, as could plugging or unplugging them with the phantom power turned on.