Mixing Desks come in a wide variety of sizes, with a range of facilities. There are many different configurations such as; a common split console or multi-track in-line desks, knob-per-function or assignable control surfaces and digitally controlled analogue or true all digital board.
All mixers share broad outlines of a common signal path design. A mixer combines signals from a number of sound sources, processes them to produce an acceptable balance and quality sound, and passes the resulting mix on to a recorder, broadcast chain or PA system. Most mixers are multiple mixers because they provide more than once combined signal. A mixer requires a number of input channels, each acceptable of handling signal at either microphone or line levels with facilities to adjust their levels and equalisation. It may generate extra, separately controlled, mixes for effects units, fold-back or cue feeds and multi-track recorders. The desk must provide a means of listening to and metering individual channels so that the complete master mix or alternative output mixes can be adjusted correctly and any problems identified and fixed.
Most desks have a similar signal path: The Input signal from Mic or Line source passes through the Mic Amp or Line Buffer stage, where the signal level is optimised for headroom and noise performance, then passes through 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 and then returned to continue through the desk.
Signals are routed to available outputs or groups. A group is where a 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 affect a collection of channel signals simultaneously.
The first element in a signal path is a microphone amplifier and general input stage. the design of a mic preamp defines the sound character of the entire desk, as any quality loss at this stage can never be regained. The Mic amp must provide a lot of signal gain with absolute minimum background noise, it must have high headroom so that unexpected peaks do not cause overloads, it must preserve every subtle nuance of the waveform captured by the microphone with a wide dynamic range.
High quality microphones are generally of the electrostatic variety and these need a power source to polarise their capsules and power their internal pre-amps. The mixer provides this in the form of “Phantom-Power”, independently switchable from each channel. Mic pre-amp designs usually have a wide gain range so that a sensible signal level can be obtained. Provided in the form of a switched course-gain control (5dB – 10dB) and a separate, continuously variable fire trim. Cheaper desks economise with a single variable control which covers the entire gain range. For maximum flexibility, an input stage with up to 70dB of fain is desirable. To avoid overloading the microphone input stage of the desk there is a switch to insert a “Pad” or attenuator ahead of the pre-amp reducing signal strength by 30dB, this is used for when placing mic near loud noise, i.e. a kickdrum.
The input stage often includes a switch to invert polarity of the input signal “Phase Reverse”. This is useful when combining outputs of several microphones all of which are capturing a common signal source. The “Phase Reverse” switch is provided , allowing the operator to control how the outputs of different microphones add to or cancel out each other.
Most desks include a means of selecting microphone or line-level inputs to desk channels. Expensive desks provide separate gain controls for the microphone and line inputs. Cheap desks merely have a selection switch. When setting input gain on a channel, it is important to deselect any equalisation and have the channel fader set to 0dB, before adjusting the gain to bring the second source to an appropriate level. If this is not done the input stage won’t be operating under ideal conditions and will suffer from reduced headroom or an increased noise floor.
A number of auxiliary outputs from a channel will depend on the intended use of the desk, but normally ranges between one and eight. Sends may be switched to derive their signals from “Pre” or “Post” the channel fader, so that the output signal level will be either independent or dependant on the position of the fader. Usually “Pre-Fader” auxiliary sends are taken from a point after the channel equaliser. Some desks have an option to take it from a point before the equaliser. “Pre-EQ” feeds might be better for foldback purposes so that adjusting EQ doesn’t rick creating feedback, where as “Post-EQ” feeds would be better for effects or headfone cue signals.
Pre-Fader seeds 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 audience only hears sources when fades are up,and also for most types of signal processing, particularly “artificial Reverb”. Post-Fader auxiliary sends are 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 pre-fader is used, then the channel will still be contributing to the effects send so when the channel fader is closed the effects will continue to be heard through the effects return.
In short, a bus is a fader with its own dedicated output, or said differently a bus is a major pathway from all channels to a single fader connected to an output. Everything going to that fader can be taken out of the mixer and can be sent to another piece or rack of gear. The signal can also be brought back in to the mixer on spare channels. On mixers with busses, there are routing buttons on each channel that lets you route the whole signal to one of the busses. The Main bus is often called the L/R bus. Other busses are often grouped in pairs. There may also be another switch that lets you route these bus faders to the Master fader.
Typical uses of busses are to send a track or groups of tracks to a digital multitrack, or to a soundcard or audio interface. Yet one can also be very creative with them, such as sending them to samplers, grooveboxs with analog inputs, surround encoders, separate compressors and more. Some busses may have inserts. These are nice as they let one to return the external signal back to the mixer without eating up more channels.
The output from one channel has to be combined with that from other channels. In simple desks 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 a role of a desk, there may be anything from 2-48 groups with varying levels of sophistication in terms of additional equalisers and auxiliary sends. Groups are usually allocated in pairs with channel pan-pot providing the means of restricting signal to a single group and image positioning within a pair of restricting signal to a single group, and image positioning within a pair of groups for stereo working.
It is always better to use dedicated stereo channels for a stereo source rather than a pair of mono channels panned left and right because channel gains, fader positions and equaliser setting must be matched between to sides of a stereo signal. Unused channels should not be left routed to groups or main outputs as this often degrades the noise performance of the mixing stages.
Most general purpose mixers have a very simple structure where input channels are routed to a small number of groups and from there to main outputs. This simple structure becomes complicated if the desk is intended to work in conjunction with a multi-track recorder, particularly if many tracks are involved. In the case of multi-track mixers, the normal convention is to feed each tape track from its own group, allowing multiple channels to feed a single track. 24, 32 or even 48 groups may be needed.
A monitor section is actually another complete mixer so the structure of the desk becomes: Input channels-groups-Tape Monitor Channels- Stereo Output. This kind of structure goes under the name “Split-Console”, because the recording input and monitoring functions of the desks are entirely separate.
In-Line arrangements, more complex in concept but more flexible and requires less physical space. Channel sections become input/output modules because each strip incorporates all functions for the channel inputs, group outputs and monitor return corresponding to the relevant strip number. During a mix down from tape, unused channel paths can be used to provide inputs for sequenced keyboard or returns for effects. Extending the idea of re-using redundant hits of the desk during mix-down. Group routing facilities can also be re-used as extra post fade auxiliary sends. The down side to an In-Line concept is it is easy to become confused about the signal path of a particular send source unless meticulous attention to labelling and logical thought process is used.
Each operational control or a mixer has its own control knob. The 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 so a vital role is to act as memory.
One or more control knobs have been allocated to every channel allowing a specific parameter to be allocated to these knobs. Ideally, a well- designed system has the ultimate in control ergonomics, the benefits of total Automation, single-operator control of ridiculously large number of channels and the high-quality performance of analogue electronics.