Audio mixers are a big topic, spanning everything from passive combiners to huge recording and live music consoles. Not to mention virtualized mixers in DSP. If you’re combining two audio signals together, you’re mixing. The topic is so large that this introductory article will take two parts.
If I were entering the pro audio or AV world today, and especially if I spent time in physical music stores or on vendor websites, a “mixer” might suggest those small or medium-size digital boards made by companies like Allen & Heath, Soundcraft, Behringer, and the like, that are everywhere in studios, schools, churches, performance venues, and corporate events.
But long before digital mixing of any kind, there was (and still is) analog, which is where the basic principles were established. Or perhaps “dreamed up” is a better term, since the earliest mixers were often hand-made by audio engineers who needed to perform a specific task, such as recording music in the 1950s, when magnetic tape became a practical reality.
In the earliest days, recording was monophonic, and the mixer was used to combine some quantity of microphones into the final signal going to tape. Analog mixers increased in complexity as music recording moved from documenting live performances to productions with mics, electronic instruments, and effects. Likewise for sound reinforcement and other uses. But the underlying principles have remained mostly the same.
Simple Analog Mixers
In its simplest form, an audio mixer might consist of a few pots (potentiometers; variable resistors) that adjust the level of several input signals, perhaps followed by a combining amplifier. Sometimes that’s all you need (Fig. 1). Add a second output and some more pots for panning, and you have a stereo mixer! Some commercial analog mixers for utility purposes are not much more than that (Fig. 2).
On a small mixer used for music or speech, each input channel might also have a microphone preamp, and maybe a simple equalizer to adjust bass and treble content. Channel levels can be controlled by rotary or linear pots (faders), and there’s probably a master level control for the final mix. It’s also helpful to have some meters that indicate the actual signal level in the final output stage.
A bus (or buss) in electronics is a common point where signals combine, and this term is often used when talking about mixers. The primary output from a simple mixer might be the Main, Program, or Stereo mix bus. When source channels are “potted up,” they are added to the bus. As it happens, the physical arrangement of channel strips and controls in most analog mixers provides a nice visual clue to the signal flow (Fig. 3). Or, to use a plumbing analogy, the bus is like a drainpipe, with each channel strip being a sink that empties into the drain.
Fig. 3
When the idea arose to add artificial reverberation (or other effects), it became necessary to add aux buses. An aux (auxiliary) bus is just another mixing bus, but it gathers signals from individual sends on each channel strip so that a unique mix can be sent to an effect device. Returning to plumbing, an aux send is like a “tap” that lets some audio from the channel flow into the aux bus. This allows a desired amount of any input channel signal to be sent to the reverb or other device.
A large analog mixer might have many aux buses, some of which are used to create monitor mixes for musicians in the studio or on stage. Because each aux bus has its own send knob on the channel strips, there could be several unique monitor mixes. Again, the physical layout can often imply the signal flow.
The taps for some sends may be taken before the main fader (pre-fader), while others are taken after (post-fader). The former is useful for monitor mixes, so if the engineer changes the channel level in the main mix, it does not affect what the musicians hear. Conversely, a reverb send would typically be post-fader, so that when the instrument’s level is changed in the main mix, its reverb level follows.
The underlying assumption here is that the operator is “mixing” with the channel faders on the main stereo bus, say for sound reinforcement, a recording, or the audio for a broadcast or streaming feed. Aux buses are independent of that primary mix and available for other purposes. Arguably, this arrangement is as much historical and practical (regarding control panel layout) as anything, and digital mixers have gone in a different direction.
In most cases, each mix bus, whether main or aux, has a master level control. These ensure that the combined level of all the contributing signals does not overload the final bus output stage. They also allow for useful functions, like fading the entire mix in or out.
Other Mixer Functions
Mixers designed to take signals from microphones will include a preamp on some or all input channels, with a control for adjusting the preamp gain. Mic output levels can vary quite a bit, so preamp adjustments are important. Using too much gain with a strong mic signal can overload the preamp, causing distortion. Conversely, if the mic level is very low, bringing up the gain may introduce an undesirable amount of noise (hiss). That noise is not a malfunction; it’s an inherent byproduct of amplification. (For more on audio levels, see https://www.svconline.com/needtoknow/analog-audio-interfacing)
Mixer mic preamps typically also have one or more switches to apply phantom power to the mic inputs. Phantom is a DC voltage used to power condenser microphones that uses the same mic cable wires as the audio. Note that plugging other devices into mic jacks when phantom is on can do some damage!
Because analog mixers have many stages of amplification and attenuation, designers try to optimize for both good noise performance and sufficient headroom (available signal level before overload). This concept of “gain staging” is also important operationally. For example, if the main fader on a channel is set too low, it may be necessary to bring up the level at the mic pre or the bus master, adding unnecessary noise.
A good starting point is to set channel and bus master faders for “unity” gain (usually the 0 mark), and use the preamp trim to get a good level for that input. The same is generally true for line-level inputs. Under nominal conditions, with appropriate equipment connected together appropriately, I like to see controls sitting comfortably in the middle of their range. If you have to turn a control drastically up or down, look for a problem before leaving it that way.
This is not the place to go into detail about equalization, since that’s a whole topic in itself. Suffice it to say that onboard EQ can include various types of fixed and variable filters, parametric equalizers, etc., depending on the cost and intended usage of the mixer. Analog mixers may also include analog processing such as compression, or digital effects (internally interfaced to the analog circuits).
In addition to channel faders, most mixers have mute (or on/off) buttons on channel strips and sometimes output buses. Mute is important because it removes the audio instantly, and without disturbing fader settings. Whether aux sends can be pre- or post-mute depends on the mixer design (and does not always match pre/post-fader). Pan controls, as the name suggests, allow a channel signal to be placed “panoramically” between the left and right buses in a typical stereo mix.
Most mixers also have some facility for monitoring the audio signals entering and leaving the mixer. The simplest might be a headphone jack that lets you hear the main mix. Going further, the monitor section may have outputs to feed speakers, and the ability to listen to different buses or other mixer sections. Metering is also usually part of the monitor section. As a rule, monitor sections are independent of the output mixes, but in some cases, monitor signals can be routed to other buses as well.
The last key function to mention is solo. Solo buttons may appear all over a mixer and allow the operator to monitor signals at that location without disturbing primary signal flow (sometimes called non-destructive). Solo is technically just another bus (or pair of buses), but is really key to keeping things sounding good and under control in live situations. In addition to hearing and adjusting individual mics or instruments, engineers can, for instance, listen to the monitor mixes going to musicians.
Solo may also be called PFL (pre-fader listen) and AFL (after-fader listen), depending on where the solo signal is derived. In many cases solo is mono, since it’s picking up a single point in a signal path, but mixers may also have “solo-in-place,” which, for stereo, keeps the soloed signal in its correct location in the stereo field.
Over time analog mixers have acquired other new functions, such as on-board USB interfaces for computer connection. And some manufacturers include different routing configurations, such as output matrix mixing, or alternative buses when the mute button is pressed (Mackie’s classic “Alt 3-4” arrangement). The best way to find out about a mixer’s routing and control functions is to consult the block diagram.
Specialized Mixers (Analog or Digital)
The description above covers fairly generic mixer functions. A mixer with some or all of those functions could be used for mixing a band on stage or in a studio, covering meetings and conferences, podcasts, live streams, location video recording, you name it. But as uses for audio have evolved, some specialization has occurred to address particular needs.
For starters, recording large ensembles (or small groups with many microphones) requires lots of input channels. So the large “mixing desks” (aka consoles) often seen in music studios have a lot of channel strips. Even if outboard mic preamps and processors are used, it’s likely their signals will come through the console for recording and/or monitoring.
Plus, as music recording moved from straight documentation to creating a whole from individual parts, in order to record instruments on separate tracks (of a DAW or tape machine), many outputs are needed. Rather than doing this with lots of aux sends, more mix buses were added so that channel strips can directly feed recorder tracks using the main channel fader.
Mixing multitrack sessions within a DAW is very common, but if you want to mix all those tracks, plus countless outboard effects, through the console, many channels are needed. This applies whether the desk is analog or digital, but digital mixers can have a lot of channels without needing as many physical controls (coming in Part Two).
Another place to find very large mixing consoles is in film dubbing stages. As with music, creating the final movie soundtrack is a process of mixing dozens or hundreds of individual tracks—in this case, music, dialogue, and effects—while watching the visuals. In production parlance, a “dub” is a copy. In this case, dubbing (aka re-recording) comes from the days when the source tracks were played from sprocketed magnetic tape that ran in sync with the film, and the resultant mix was “rerecorded” onto a final tape. (“Stage” is a Hollywood term for a place where film production happens.). Here’s an example: https://www.foxstudiolot.com/john-ford-mixing-stage/#gallery
Then again, one of the oldest uses for mixers is live radio, and those have their own special attributes. Often, the actual mixing for radio is minimal, with one or two voice mics and some music and commercial playback. But before computerized automation, radio “board operators” (often the on-air talent) had to cue up and control playback of vinyl records, tape and CD players, “cart” machines (cartridge tape loop players), and electronic file players. (Fig. 4).
So conventional radio desks have few buses, but they’re configured in particular ways that allow the operator to cue (hear) and start the next audio clip without disturbing the on-air mix. There may be very few controls, and they are arranged to facilitate reliable on-air results. In many cases, the familiar layout remains even if the guts of the mixer are now mostly software.
Yet another purpose-evolved mixer is for DJs. Here the aim is easy control of certain parameters, like cross-fading between music cuts, and fast access to effects and audio clips. DJ mixers may have just a couple faders, with a variety of other controls sprinkled around. Some have turntable-style playback controllers built in, or the ability to run lights and other outboard equipment during shows. Dance DJs rely heavily on cue/solo functionality to create seamless transitions between music beats and cuts.
Coming Next
One obvious trait of analog mixers is the large number of controls. They can look intimidating until you realize that every channel strip is just a repeat; the knobs and buttons do the same things. Although servicing one can be a major hassle if every one of those knobs (and underlying mounting nuts) must be removed to get inside!
The key point is that every knob and button is mounted to a circuit board and carries either actual audio or a control signal for audio. This is a control-per-function environment that does not support virtualization or delegation. Making adjustments on the fly while recording a final mix sometimes required multiple people working together with well-rehearsed moves.
Mix automation for analog boards arrived in the 1970s with the advent of either motorized moving faders or digital capture of fader positions. In both cases, the fader moves were recorded during consecutive playbacks and then recalled when the final mix was recorded. A further development captured the position of every control on the board, which allowed them to be reset to a previous state (by manually turning while watching a display that showed the target position).
Digital mixers make it possible to adjust many parameters from a small collection of controls, recall settings, and many other functions. Part Two will delve into the advent and state of digital mixers, with the pros and cons that they bring.
