Audio Mixer

Audio Mixers: A Simple Guide

An audio mixer is a device with the primary function to accept, combine, process and monitor audio. Mixers are primarily used in four types of environments: live (at a concert), in a recording studio, for broadcast audio, and for film/television. An audio mixer can come in either analog or digital form.

There are two builds of mixers: in-line and split monitor. An in-line mixer means there are two paths per channel. For example, in a recording environment, an in-line mixer can use the same channel to send and receive sound to and from a digital audio workstation (DAW). A split monitor console has one path per channel. Each channel can be used to either send or receive sound to or from a DAW. Both of these builds have two areas for us to explore: the channels and the master section.

Before we begin, let’s discuss some terminology and vocabulary. An audio mixer may also be referred to as a “console,” “desk,” or “board.” All three of these terms are synonyms with “mixer.” Additionally, we must define “signal.” Signal is the generalized term for any audio passing through a mixer. This could be a vocal, drum, bass, synth, guitar or another instrument. All of it is referred to as signal.

Regardless of the application, build or form of the mixer, most are structured in a similar fashion. Let’s examine the parts, features, and prices of audio mixers.

Parts of a Channel

Input Section
The input area of the channel strip may accept any or all of the following levels of signal: mic, instrument, -10 line level or +4 line level. Depending on which level of signal is desired to be accepted, specific knobs and/or switches must be engaged and used.

For example, if a microphone is wired into a channel, the channel must be configured to accept mic level signal. Then, the mic pre knob must be raised in order to add gain to the signal. Wiring a guitar, bass or another instrument may use a separate input jack (probably for an “unbalanced” ¼” cable). In this case, there may be a switch or button labeled “DI” in the input section. If so, pressing this button will tell the mixer to accept the instrument level signal (rather than mic level or line level).

The mixer will also be able to accept +4 line level, -10 line level or both. This normally takes the form of deselecting all switches and buttons but may vary between consoles. Some mixers have a knob referred to as the “line trim.” This serves the same purpose for line level signal as the mic pre does for mic level signal: it adds or subtracts gain at line level.

EQ
Most mixers have an equalization (EQ) area on the channel strip. The amount of flexibility and precision of frequencies that can be adjusted in the EQ area is typically related to the overall sophistication of the board. For example, most budget and moderate-level mixers have fewer frequencies that can be targeted and adjusted with broad strokes, while high-end desks have EQ areas that can be used as a scalpel.

Dynamics
High-end mixers commonly have a “Dynamics” area, either on each channel or in the master section. Common dynamic effects included in this area are compressors and gates. When a switch is engaged to activate the dynamics area, the compressor will make the voice or instrument sound more even (loud parts become softer, soft parts become louder). The gate will open to allow any signal to pass through the channel, then close to keep out background noise and bleed.

Fader
The fader is the device that raises or lowers the amount of audible signal from the channel. A fader is actually a resistor: as it is lowered, it increases resistance. This is why the numbers start high at the bottom of the fader bank on each channel and decrease to the zero point (usually located about two-thirds of the way to the top). These numbers indicate a general idea of loudness (more advanced users will recognize these numbers as the logarithmic decibel scale). The most important number on the fader bank is zero. The zero point on the fader is referred to as “unity,” meaning there is no resistance applied to the channel’s signal. Any fader position above zero is amplifying the signal as opposed to varying the amount of resistance as it is moved up or down below zero.

Group Faders
Group faders on an audio mixer are used to control multiple channel faders at once. A common use for a group fader is to raise or lower all of the faders that contain drum signals together at once — the collective mix of the drum faders can be raised or lowered using a group fader (also referred to as a subgroup). If included in a console’s construction, there may be anywhere from two to eight group faders.

In many cases, these group faders are actually voltage-controlled amplifiers, meaning there is no audio passing through them. These are commonly referred to as VCA Group Faders.

Auxes
Mixers have a dedicated area for auxiliaries (auxes) used to send a copy of the channel’s signal to another destination. In the studio environment, auxes are used for creating a headphone mix or adding a time-based effect (such as reverb or delay). On a live console, auxes are used for time-based effects, but also may be used to send sounds to in-ear monitors or a monitor wedge on stage.

These copies can be taken before or after the fader, referred to as “pre-fader” or “post-fader” auxes. A pre-fader aux sends the same amount of level to the new destination regardless of the fader’s position. A post-fader aux maintains the “wet-to-dry ratio.” This means each post-fader aux has its own level control and the effected signal can be raised and lowered with the fader. With this in mind, application of these auxes becomes easy. Pre-fader auxes are used for headphone mixes so adjustments to the fader are not heard in the performer’s headphone mix. Post-fader auxes are used for time-based effects so the amount of the effect (reverb, etc.) stays consistent with respect to the fader level.

Bus Assignment
A bus is no more than a path on which signal can travel. This is relatable to any bus driving down the street — and every bus needs a destination. (Here in Los Angeles, the buses stop before they drive into the ocean…most of the time.) The bus assignment area on a mixer is the same concept: we are sending signal down a path that leads to a destination. Common destinations are external pieces of gear, audio subgroups or an audio interface. The most common bus assignment is the stereo bus. The stereo bus is a two-channel mix (left and right) of all the faders on the console. It can also be thought of as the sum of all the faders. Most consoles require a button or switch to be engaged in order to send the channel to the stereo bus (and therefore be audible in the mix).

Track Busses & The Routing Matrix
Track busses are a collection of paths for our signal to travel. If the mixer is equipped with track busses, there are commonly anywhere from four to sixteen located on the top of each channel. The busses can be routed to an audio interface for recording, other channels for summing or to a piece of outboard gear (such as a compressor). More advanced consoles contain anywhere from twenty-four to forty-eight busses and additional routing options. If so, this area is referred to as “the routing matrix.” A routing matrix possesses the same functionality as the track busses but expanded routing and destination options are available.

Pan
The purpose of the pan pot (short for potentiometer) is to pan a channel’s signal left or right across the stereo bus. An in-line console will have two pan pots, each assigned to a designated path. A split monitor desk will have one pan pot, normally located above the fader. Panning information can also be translated to track busses or the routing matrix: if more than one bus is selected on the channel and the fader has been assigned to those busses, the stereo image of the signal will be retained.

The Master Section
The master section provides areas for global adjustments to the channels or modes of the console. It is typically located in the middle of the desk. Some manufacturers refer to the master section as the “Centre Section.” Common adjustments made in the master section are toggling which level of signal the channels are accepting, master levels for auxes and busses, toggling the control room source (what is playing through the speakers), changing the control room level (volume of playback) and speaker selection. There also may be a foldback area (used for configuring headphone mixes) and stereo effect returns (a destination for the return of outboard equipment).

Patch Bay
A patch bay is a device located next to an audio mixer that has a series of jacks meant for moving signal from one place to another on the console. The jacks are organized by row and offer increased flexibility. Additionally, they serve an organizational purpose. Cables from the mixer and outboard equipment can be wired directly to the patch bay so signal can be routed easily without having to connect cables directly to a device. Patch bays are most common in the studio environment but can be used in other situations as well.

Flipping the polarity on one of the channels allows us to hear the sum frequency response of both channels and verify there are no phase issues detracting from the sound. Make sure to use whichever polarity position produces a full, thick sounding result.

Features

Phantom Power
A Phantom power button is present on each channel of most every mixer. Phantom power is required for condenser microphones: it adds 48 volts of direct current via the XLR cable used to wire the microphone to the mixer. The phantom power button may be labeled “48,” “+48,” “+48v” or “phantom power.”

Polarity
A button on most mid-level and high-end mixers is a polarity flip, commonly using the Ø symbol. Flipping the polarity on a channel changes the phase relationship. Phase is defined as a time relationship between two waveforms. An inverse phase relationship can cause frequency builds and cancellations, which will negatively affect the sound. Anytime there is more than one mic on the same source (e.g. top and bottom snare), we must flip the polarity on one of the channels in order to check the phase relationship.

Flipping the polarity on one of the channels allows us to hear the sum frequency response of both channels and verify there are no phase issues detracting from the sound. Make sure to use whichever polarity position produces a full, thick sounding result.

PAD
Many mixers have a button labeled only with a number rating, such as “-10” or “-10dB.” This is the PAD for the channel. PAD is an acronym for “Passive Attenuation Device,” which softens the sensitivity of the capacitor inside of the mic, allowing louder signals to pass through the channel without distortion. The number rating (eg -20dB) is the strength of the PAD, measured in decibels. The higher the decibel rating, the stronger the PAD.

Filter
High-pass filters are available as a button on the channels of most mixers. The high pass filter will dramatically soften the low-end frequency response of the channel and is useful for decreasing or removing rumble from a signal. Low-end mixers may only have a symbol that looks like this:

Moderate and high-end consoles typically have a numerical value next to the symbol. The numerical value represents the frequency at which the filter begins.

Meters
Various styles of meters exist on mixers and for audio production in general. On a mixer, the collective area of the meters across all of the channels is referred to as the meter bridge. Three common meters that may be found on a mixer are VU, Peak, and RMS.

VU meters display the level of perceived loudness on a channel or the stereo bus, with signals far across the zero point likely to distort. Peak meters, which are most familiar in modern times, indicate the loudest part of a signal at any instant. RMS meters display the average loudness of a channel or the stereo bus, indicating the dynamic range of the signal when compared to the zero point.

The right mixer in the right hands can become powerful. Make sure to practice and purchase wisely.

Digital vs. Analog

Audio mixers are manufactured in either analog or digital form. Each of these console types has its own set of advantages and disadvantages, which are directly related to one another.

The biggest advantage digital consoles hold over an analog mixer is instant “recall.” This means a mix (or setup) can be reloaded to the exact parameters from when it was last saved. Every knob, switch, button, and fader will snap to its saved position. Analog consoles have to be recalled manually, meaning each knob, button, switch, and fader has to be documented and returned to its original position by hand. This can be a time-consuming and tedious process.

The main advantage an analog desk holds over a digital mixer is “summing.” Summing is the process of combining signals from all the channels using analog circuitry: iron, wires, faders and electrical components. Analog summing adds a desirable dimension to signal and is very familiar to listeners. Digital mixers do not sum signal in the same way: they are merely processing the signal using digital, binary code (1’s and 0’s). The sonic texture of analog summing is so popular that devices called “summing mixers” have become very popular in home and small recording studios.

A summing mixer can be thought of as an analog console minus the channels and features. They most commonly take the form of a unit that has sixteen analog inputs. In some summing mixers, each input has its own volume and pan pots. Signal passes through the analog circuitry and is summed to a physical stereo bus in the same fashion as an analog console, adding a desirable quality to the vocals and/or instruments.

Price Points

The price points for digital, analog and summing mixers vary by quality, brand, and features. Below are a few options for each, including basic, moderate and high-end mixers. We can use this information as a frame of reference for the cost associated with purchasing a mixer.

Digital Mixers

Behringer X32 Producer Digital Mixer


Street Price: $1,199.99 (at time of writing)

Midas M32R Digital Mixer


Street Price: $2,499.99 (at time of writing)

Allen & Heath GLD-112 Chrome Edition Compact Digital Mixing Surface


Street Price: $5,999.00 (at time of writing)

Analog Mixers

Mackie 1604VLZ4 Mixer


Street Price: $899.99 (at time of writing)

Yamaha MGP32X


Street Price: $1,399.99 (at time of writing)

Solid State Logic X-Desk


Street Price: $3,399.00 (at time of writing)

Summing Mixers

Dangerous Music 2-BUS LT Analog Summing Mixer


Street Price: $1,499.00 (at time of writing)

Antelope Audio Satori


Street Price: $1,995.00 (at time of writing)

Rupert Neve Designs 5059 Satellite


Street Price: $3,749.00 (at time of writing)

Final Thoughts

An audio mixer can be a great way to expand a recording studio or enhance the sound of a concert. However, diving headfirst into the purchase of a mixer without any practice or training can be a frustrating experience. Self-study or formal education will help ease the learning curve, as well as starting with a small, entry-level mixer. A person who can operate a small mixer well is more powerful than an inexperienced person behind a large mixer.

Research, education, practice, and understanding will go a long way in making sure any sort of sizable investment in a mixer will not go to waste. The right mixer in the right hands can become powerful. Make sure to practice and purchase wisely.

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