Anatomy Of A Mixer

Hugh Robjohns shrinks to microscopic size in order to show you around the conglomeration of knobs, wires and circuits that make up a mixer.

Mixing desks come in an extraordinarily wide variety of sizes, with an equally wide range of facilities, from the most basic 6:2 design through to the all‑singing and dancing 100+ input multitrack monsters. They are available in many different configurations too, such as the common split console or multitrack in‑line desks, knob‑per‑function or assignable control surfaces, and now, of course, digitally controlled analogue, or even true all‑digital boards.

Despite this enormous diversity, many aspects of operation and anatomy are common to all mixers — you'll find that with an understanding of the fundamental principles, together with a bit of common sense, even the most daunting state‑of‑the‑art console will become (almost) child's play.

Basics

All mixers share the broad outlines of a common signal‑path design, simply because they are all intended to perform the same basic functions. The exact detail will inevitably vary to suit the intended use and price of the desk, but the principles remain the same: a mixer combines signals from a number of sound sources, processes them to produce an acceptable balance and quality, and passes the resulting mix on to a recorder, broadcast chain or PA system. (Most mixers are actually multiple mixers, because they provide more than just one combined output signal.) The mixer will therefore require a number of input channels, each capable of handling signals at either microphone or line levels, with facilities to adjust their levels and equalisation; in addition it may generate extra, separately controlled, mixes for effects units, foldback or cue feeds, and multitrack recorders. On top of all that, the desk must provide a means of listening to and metering individual channels, the complete master mix, or the alternative output mixes, so that the controls can be adjusted correctly and problems identified.

Most desks have a similar signal path: the input signal from a microphone or line source passes through the microphone amplifier or line buffer stage, where the signal level is optimised for headroom and noise performance, then passes on to the equaliser, before reaching the channel fader. Auxiliary outputs will usually be immediately before or after the fader, and there may also be insert points where the signal can be extracted from the desk, processed externally (perhaps with a compressor or noise gate), and then returned to continue through the desk.

Next, the signal is routed to the available outputs or groups as appropriate. In the case of the groups, the signal may pass through an additional equaliser stage in the groups before reaching the fader, and further routing to the main desk outputs. Groups are provided to make it easier to control a large number of signals, or to allow a single signal processor to affect a collection of channel signals simultaneously.

In the following paragraphs, I'll look at some of the aspects of each section in the signal path, and some of the alternative design and operational concepts.

Input Stage

The first element in the signal path is the microphone amplifier and general input stage. The design of the mic preamp really defines the sound character of the entire desk, since any quality loss at this stage can never be regained. For this reason, it's common practice in multitrack studios to use a few extremely high‑quality (and therefore expensive) external microphone preamps in place of the often indifferent ones built into an otherwise good mixer.

The mic amp has a very difficult job to do: it must provide a lot of signal gain with the absolute minimum of background noise; it must have very high headroom so that unexpected peaks do not cause overloads; and it must preserve every subtle nuance of the waveform captured by the microphone, from the lowest frequency to the highest, with a wide dynamic range.

Assuming that the basic design is capable of achieving all these things, there are some practical demands too. The highest‑quality microphones are generally of the electrostatic variety, and these usually need a power source to polarise their capsules and power their internal preamps. The mixer normally provides this in the form of 'phantom power', independently switchable from each channel.

All directional mics are susceptible to low‑frequency mechanical vibrations such as handling noise, and these unwanted subsonic signals can very quickly use up any amount of available headroom. To counteract this problem, the microphone stage will normally include a switchable high‑pass filter which will remove subsonic rubbish, preferably without affecting the wanted sound in a detrimental way.

The better mic preamp designs usually have a very wide gain range so that a sensible signal level can be obtained no matter how loud or quiet the original sound source, or how close or distant the microphone (within reason, of course). This is often provided in the form of a switched coarse‑gain control (with maybe 5dB or 10dB steps), and a separate, continuously variable, fine trim. However, cheaper desks usually economise with a single variable control which covers the entire gain range. In pure engineering terms, the former approach is technically superior, but there are a number of perfectly respectable designs using the latter technique these days. For maximum flexibility, an input stage with up to 70dB of gain is desirable, but this places great demands on the circuit design. Most home‑studio applications can manage with as little as 50dB of mic gain, which relaxes the design constraints considerably.

The design of the mic preamp really defines the sound character of the entire desk, since any quality loss at this stage can never be regained.

Having a particularly sensitive mic preamp can be very useful, but what happens when you place a microphone somewhere very noisy — inside a kick drum or down the bell‑end of a trumpet? It's not unusual to find mics generating line‑level outputs in this kind of situation, so, to avoid overloading the microphone input stage of the desk, there's usually a switch to insert a 'pad' or attenuator ahead of the preamp, reducing the signal level, typically, by 30dB.

The input stage also often includes a switch to invert the polarity of the input signal — phase reverse. This can be very useful when you're combining the outputs of several microphones, all of which are capturing a common signal source. Although there's a recognised world standard (XLR pin 2 positive, pin 3 negative), some manufacturers don't adhere to it, and so not all microphones generate the same polarity of output signal under the same circumstances — and if mics of opposite polarity are mixed together, their outputs tend to cancel out rather than adding together. The phase‑reverse switch is provided to take advantage of this, allowing the operator to control how the outputs of different microphones add to or cancel out each other.

Finally, most desks include a means of selecting microphone or line‑level inputs to the desk channels, these normally being connected on different sockets. The more expensive desks will provide separate gain controls for the microphone and line inputs, the cheaper desks merely a selection switch.

When you're setting input gain on a channel, it's important to de‑select any equalisation, and put the channel fader at its normal operating position (0dB on the scale), before you adjust the gain, to bring the sound source to the appropriate level. If you don't do this, the input stage won't be operating under ideal conditions, and will suffer from reduced headroom or an increased noise floor.

The channel faders are provided as a convenient means of adjusting levels during a recording or performance. If the fader spends its whole time flat out or down towards the bottom of its travel, the input gains have been wrongly set and your desk is not working as well as it could.

Auxiliary Sends

The number of auxiliary outputs from a channel will depend on the intended use of the desk, but normally ranges between one and eight. The sends may be switched to derive their signals from before (pre) or after (post) the channel fader, so that the output signal level will be either independent of or dependent upon the position of the fader. Usually the pre‑fader auxiliary send is taken from a point after the channel equaliser, but some desks provide an option to take it from a point before the equaliser. There are advantages and disadvantages to both, depending on what you're using the pre‑fade sends for. For example, a pre‑EQ feed might be better for foldback purposes so that adjusting the EQ doesn't risk creating feedback, whereas post‑EQ feeds would be better for effects or headphone cue signals. In general, pre‑fader sends are used for foldback or cue signals, so that opening and closing the channel faders won't affect the performers' monitoring. Post‑fader sends are normally used for house PA in theatrical and broadcast situations (so that the audience only hear sources when they are faded up), and also for most types of signal processing, particularly artificial reverb.

The use of post‑fader auxiliary sends is crucial if a single effects processor is handling the contributions from a number of channels, because when a channel fader is closed, its direct contribution to the output is removed, as is its send to the effects unit. If a pre‑fader send is used, the channel will still be contributing to the effects send even when the channel fader is closed, and so will continue to be heard through the effects return — probably not a desirable state of affairs...

To cut down on the number of (relatively) expensive buttons, many desks select pre‑ or post‑fader status for pairs of auxiliary sends, and so a little planning may be required to optimise the use of the auxiliaries for a particular situation. Bigger desks may also provide one or more stereo auxiliary sends, normally using a pair of mono auxiliary busses, where one send control becomes the stereo send level knob and the other becomes a pan‑pot.

Routing

The output from one channel has to be combined with that from other channels. In the simplest desk, all channels may be permanently routed to a master stereo output, but more typically channels are routed through groups and from there to the main outputs.

Depending on the intended role of the desk, there may be anything from two to 48 groups, with varying levels of sophistication in terms of additional equalisers and auxiliary sends. Commonly, the groups are allocated in pairs, with the channel pan‑pot providing the means of restricting a signal to a single group, and image positioning within a pair of groups for stereo working. On the subject of stereo, it's always better to use a dedicated stereo channel for a stereo source rather than a pair of mono channels panned left and right, because channel gains, fader positions and equaliser settings must be matched between the two sides of a stereo signal — awkward to do with separate channels, but very easy with a dedicated stereo channel.

A useful point to note: unused channels should not be left routed to groups or main outputs because this often degrades the noise performance of the mixing stages (although this will depend on the precise detail of the circuit topology used).

Structure

Most general‑purpose mixers have a very simple and easy‑to‑understand structure where the input channels are routed to a small number of groups, and from there to the main outputs. However, this simple structure becomes complicated if the desk is intended to work in conjunction with a multitrack recorder, particularly if a large number of tracks are involved. In the case of multitrack mixers, the normal convention is to feed each tape track from its own group (thereby allowing multiple channels to feed a single track, such as for bounce‑downs), so 24, 32 or even 48 groups may be necessary.

Although this isn't a technical problem, it would make a conventional desk rather large, especially when some means of monitoring the tape tracks is incorporated. The latter facility — a monitor section — is actually another complete mixer, so the structure of the desk becomes: Input Channels — Groups — Tape Monitor Channels — Stereo Output. Imagine a desk with 72 inputs, 48 groups and 48 monitors, all side by side: impressive it may be, but practical it ain't! This kind of structure goes under the generic name 'Split Console', because the recording input and monitoring functions of the desk are entirely separate. While it's simple to understand, this design approach quickly becomes unwieldy as the number of tracks increases, and performing simple functions such as bounce‑downs often requires external signal patching to re‑route monitor returns through input channels and then on to the group sends.

To overcome the operational impracticalities of the simple Split Console, an alternative solution was developed, which became very popular with the introduction of the original SSL 4000‑series desks. This is called the In‑Line arrangement; although it's more complex in concept, it is considerably more flexible and requires much less physical space. In an In‑line desk, the channel sections become Input‑Output (or I/O) modules because each strip incorporates all functions for the channel inputs, group outputs and monitor returns corresponding to the relevant strip number. In other words, module 6 contains the microphone and line inputs for channel 6, together with its auxiliary send controls and equaliser, channel fader (usually a short‑throw fader) and output routing. It also contains the mixing amplifier and output fader for group 6 (normally tied directly to track 6 on the tape machine). The off‑tape monitor facilities for track 6 will also be on this module, and will be provided with auxiliary sends, an equaliser (although these are usually shared with, or borrowed from, the channel paths' facilities), and a monitor fader (usually a long‑throw design).

Assignability is not necessarily the panacea of future desk design.

Building the desk in this configuration allows many economies in facilities — and therefore cost, control knobs and overall size — such as the sharing or splitting of auxiliary sends and equaliser sections between channel path (record signal) and monitor path (replay signal). Furthermore, extremely flexible signal routing for operations such as track bouncing becomes possible with the addition of a few electronic switches within the desk itself (external signal patching is rarely required); the channel and monitor fader functions can be swapped over at the press of a button, allowing the channel or monitor signals to be controlled by the most appropriate type of fader for the job in hand.

It also means that, during a mix‑down from tape, you can use the unused channel paths to provide inputs for sequenced keyboards or returns for effects units (hence the common marketing line, "48‑track desk with 96 inputs on mix‑down"). Extending the idea of re‑using redundant bits of the desk during mix‑down, the group routing facilities can also be re‑used as extra post‑fade auxiliary sends — and you won't need to patch externally, because a few internal electronic switches can re‑configure the entire desk very quickly and easily.

The down side of the in‑line concept is that it's very easy to become hopelessly confused about the signal path of a particular sound source unless you pay meticulous attention to labelling and logical thought processes. You only have to imagine a situation where a mic is plugged into channel 6, so it will be controlled by the input section and (small) channel fader in strip 6, then routed to tape track 17, so the group trim control will be on strip 17, as will the monitor return signal controlled by its own (large) fader. This signal path may not seem too bad, but the potential for confusion grows as you realise that equalisation is now available to both the record (channel) and replay (monitor) sections of the desk, as are the auxiliary sends — and it's surprisingly easy to inadvertently set up multiple effects or cue sends on the same signal but from different I/O modules.

Control Surfaces

Traditionally, each operational control on a mixer has its own control knob but, as consoles become larger, you'll find that you can no longer reach all of the controls without having to stand up or walk from one end of the desk to the other. Other practical difficulties arise too: the time needed to reset the desk between sessions, the sheer cost of fitting the control knobs, switches and potentiometers, and so on. These problems have lead to the increasing popularity of assignable consoles, whose greatly reduced number of operational controls can be assigned to alter the parameters of a selected channel.

To understand the operational implications of assignability, it's worth considering the functions of a conventional control knob. Its obvious role is to alter a particular signal parameter, such as level or turnover frequency in an equaliser, and there are two parts to this — each knob provides direct access to a specific function, but on the end of the control shaft is the actual device that changes the intended parameter. Each control knob also indicates of the current state of the parameter, so a less obvious, but vital, role is to act as a memory (the knob will not move by itself, so it effectively 'remembers' its previous setting). These are functions you take for granted, but they become crucial when you start considering assignable console designs.

Given that we only have two hands, it's been argued that an assignable mixer only requires one or two control knobs. Although a couple of desks have followed this approach, most designers accept that it's not the most practical way of operating a mixer. A better idea would be to have a single assignable channel strip with all the channel controls for gain, equaliser and auxiliaries, but also a complete set of individual channel faders. An 'Assign' button on each fader would recall the channel's parameters to the assignable strip. This is quite workable in many situations, but where faster access is required, or if one channel strip must remain continuously available, two or more assignable channel strips would be better — and, of course, this would also allow channel settings to be compared more easily.

One of the biggest problems for new users of assignable desks is that of no longer being able to gaze across a control surface to check on the relative settings of, say, the Aux 4 controls. An assignable desk usually requires the much more laborious technique of recalling individual channels, one after the other. A couple of desk designs have overcome these problems by allocating one or more control knobs to every channel, and allowing a specific parameter to be allocated to these knobs — so the Aux 4 setting across the entire desk can be seen at a glance, and crucial controls can be kept constantly to hand. In fact, being able to allocate parameters to alternative controllers is a useful spin‑off from assignability. For example, why not set up an auxiliary effects mix on the faders rather than on the traditional aux pots? A very simple idea, but stunningly effective. Assuming you can find an assignable system appropriate to your particular needs — and assignability is not necessarily the panacea of future desk design — this approach allows very easy implementation of total automation and instant desk‑wide setting recall, which are undeniably very useful.

Although assignability is widely associated with digital desks, it's also perfectly applicable to analogue desks. However, assignability requires a digital control surface, hence the term 'digitally controlled analogue' — an approach that currently represents the apex of mixer design.

Ideally, a well‑designed system has the ultimate in control ergonomics, the benefits of total automation, single‑operator control of ridiculously large numbers of channels, and the high‑quality performance of analogue electronics.

Digital Desks

To many people, the pointy bit at the top of the mixer pyramid is labelled 'digital'. As most digital mixers use assignable control surfaces, the only significant difference between a 'digitally controlled analogue' desk and a truly digital one is the audio processing path — an obvious statement, but important.

A top‑quality analogue mixer has vastly greater signal bandwidth, and significantly lower input noise floors, than any digital desk fitted with mere 16‑bit A‑Ds and D‑As. However, a digital desk can provide up to 1500dB of internal dynamic range once the signal is within the digital domain, so that it's practically impossible to overload the desk's mix‑busses, or even to hear any noise from them, no matter how many channels are mixed together.

In reality, it's still relatively early days for digital desks, mainly because the current generation of analogue/digital converters can't match the capabilities of top‑notch pure‑analogue designs. However, as the resolution of converters exceeds 20 bits, and as manufacturers increase the sampling rates — there's a growing lobby in support of 96kHz sampling — it seems certain that, in a few years, digital mixers will replace analogue ones completely.