Basics Of Analog Audio Mixers

From a 1994 article for the University of New Mexico’s Intro to Electronic Music class:

Signal Flow:

  1. Mixing boards contain multiple “channels”, each of which has separate input, and is controlled separately. These channels are then assigned to either a “buss” or a master output. Every channel assigned to a given buss or master is mixed together and output in it’s blended fashion through the output jacks corresponding to each buss or master output.
  2. Channels on all mixers (with rare or no exception) have either an On switch or a Mute switch, or both. On switches simply turn that channel on. If no sound is being output of a channel you think should be outputting, the first thing to check is the On or Mute switch. Mute switches do the opposite. They turn the channel off. If an On switch is not engaged, no sound will be outputted. If a Mute switch is not engaged, sound will be outputted.
  3. Signal fed to a given channel’s input procedes first to it’s trim section, which allows the input level (volume) to be adjusted; then to it’s EQ (equalization) section, then to the Pan-Pot, which simply adjusts to which side of the stereo spectrum (see stereo concepts) the channel is placed, then to that channel’s “fader” (or gain knob, depending on the board). A fader (or gain knob) is simply a volume control. It controls how much overall level is being output from that channel. This signal flow is represented somewhat by the physical layout of each channel. Input jacks are at the back, Trim knobs are next, then EQ section, then Pan-Pot, then Fader.
  4. After flowing through Trim, EQ, Pan-Pot and Fader, signal is fed to all busses or master outputs that a given channel is assigned to. For example, if a channel is assigned to buss 1 (usually by pressing a button above or near the fader), then signal will be fed to buss 1, buss 1 will output that signal. Busses are simply outputs. All busses have a volume control of their own. That volume controls the overall level of all the sound being fed to that buss (meaning this sound is usually a mix of many channels).
  5. Their are two basic configurations for output on mixing boards:
    $nbsp$nbsp$nbsp 1) All busses are automatically fed to the mixer’s outputs (which usually consist of at least two sets of outputs: one pair for monitoring, and one pair to go directly to tape. “Monitor Out” and “Tape Out”).
    $nbsp$nbsp$nbsp 2) The mixer has a “master left-right”, (which is simply the main master control buss) and individual busses can be assigned to that output. In this case busses are not automatically outputted to the master output jacks. In either situation, most boards have another set of output jacks, which correspond to each individual buss. (For example, if their are four busses, there are four “buss output” jacks, in addition to the master output pairs.)

Common Channel Characteristics:

  1. INPUT – Channels on most boards consist of at least two input jacks (usually at the back): 1) Line input. This is usually an RCA (or “phono”) jack or Quarter-Inch (large “headphone”) jack (although really high-end professional equipment rarely uses anything except XLR connectors, even for line input…most good boards have a choice). This input accepts line-level input, that is, input that does not need to be boosted before proceding to the channel controls. Examples include cassette decks, CD players, Synths, etc. This input is usually labeled “line in” or “tape in”, or similar (sometimes there are both “line in” and “tape in” jacks. They are essentially the same).
    2) Mic input. This is usually an XLR (three-pronged, or “cannon” input) connector, but it can be other types. This input accepts mic level input, that is level that DOES require boosting before it will be at an acceptable level before proceding. Examples include, of course, microphones, and assumably some other devices that ouput at mic level. This boosting is done by the Pre-Amp section of the channel. This input is usually labeled “mic” or “mic input” or simaler.
  2. DIRECT OUT – Channels usually consist of a “Direct Out”. This is (as far as I know) always a line-level ouput. Direct out jacks consist simply of the signal that is being output of the given channel. Direct out jacks are not effected by other channels, busses, or master output levels. Plugging into a direct out does not effect the channel’s output into busses or master outputs, and does not interfere with the flow of that channel into a mix.
  3. INSERT or ACCESS – Channels usually consist of an “insert”, or “access”. This can be accomplished in one of two ways: Either with one jack, that outputs the channel’s output, then returns it, within the same jack, or with one jack that consists of the channel’s output and another that returns it. Inserts interupt the signal, and thus allow you to “insert” another device into that path. (An example is a person may use the insert to take the output of the channel to a compressor or effects processor, then return the modified sound to the channel…the modified sound is then what is fed to any busses or master outputs.) Plugging into an insert DOES effect the channel’s output to any busses or master outputs. It does NOT effect other individual channels.
  4. INPUT SELECTOR – Channels must have an input selector switch if they have more than one input jack. Typically this switch chooses either the mic or line input. IE: If the switch is switched to “mic”, then input for the channel will be accepted from the mic jack, and not from the line jack, and vise-versa.

Thanks for reading! This was quite some time ago but still applies. If you want some current flavor, I’ve got free downloads available!

— Aaron

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