Chapter Two: Studio Gear

4. Mixing Consoles | Page 2

Input Channel Strips

A mixing board will consist of a number of input channels controlled by identical channel strips (narrow vertical arrangements of controls). If you learn how one strip works, you've learned how they all work, at least for that board. The layout and minor differences in functions vary between manufacturers and models, but they all have the same basic elements.

Digital boards add a wrinkle. Because their controls are assignable, a single bank of physical strips can be switched between layers—channels 1–16 on one layer, 17–32 on the next. This keeps the console compact; a digital desk handling 32 inputs can be half the width of its analog equivalent. The trade-off is that you can no longer take in the whole mix at a glance, and a channel sitting on a hidden layer may be doing something you have forgotten about. Motorized faders soften this—they snap to the stored positions of whichever layer you call up—but you are still only ever looking at part of the board.

Below we take the input channel a section or two at a time, working down the strip from the inputs at the top to the fader at the bottom. The number on each heading matches the numbered section on the channel strip figure.

1Channel Strip Inputs

Inputs to a board, patched into the back or top, consist of line level inputs, from devices such as synthesizers and computer audio interfaces (usually balanced ¼"-TRS, but accepting unbalanced ¼"-TR); microphones (almost always XLR), for which there is often a separate preamp section; and possibly tape inputs, for ancient devices such as reel-to-reel tape recorders and CD players (usually RCA). Unless you buy a vintage board, chances are the channel tape inputs will be gone and replaced by one or two dedicated STEREO, TAPE or 2TR IN channels (2-track input, such as a CD player or an iPhone audio cable). In addition, one might be able to route digital input sources (such as ADAT, AES/EBU, etc.) to the channel strip on digital boards using the board's internal I/O setup. Many boards now also feature several stereo channel strips, where there are Left/Right inputs with one set of controls for both channels.

Some boards also include a single channel insert input, or a direct in and direct out. The single channel insert, which is usually a TRS jack, allows for an effects loop input and output on a single connector, tapped after the preamp but ahead of the EQ. This is handy for things like adding a graphic EQ to a mic channel to ring out feedback frequencies. Some boards split the same break into separate insert send and insert return jacks. A direct out is a different animal: it taps the channel—usually after the preamp, and on some boards switchable before or after the fader—and feeds a recorder or audio interface. Nothing comes back, so it is a one-way send rather than a loop. Where a direct in is offered, it does the reverse, dropping a line-level signal into the channel past the preamp. Since the insert is taken before the fader, the fader does not affect the level of output to the external device.

2Channel Pre-amp Trim Section

The top-most module on the strip handles the channel's input sources. A channel typically offers more than one input source—a microphone input, a line input, and often an insert point—along with various means of selecting and routing them through the channel. If a condenser microphone is being used, the phantom power switch or button (labeled +48 V above) must be depressed for the microphone to function. Some boards give every channel its own phantom switch, some switch it in banks of four or eight, and many small mixers have a single global switch that powers every mic input at once. This sends +48 volts DC through the mic cable to polarize the capsule and run its internal preamp, or to power the electronics of an active ribbon mic or an active direct box.

On this board, the line and mic inputs share a trim pot , which allows each individual channel to be balanced with the others regardless of differing input levels or microphone sensitivities. One level-setting strategy is to adjust the trims so that if the channel faders are set to the same value, all channels sound at equal strength. Since the trim pots control the channel's pre-amp, too high a setting may cause the channel to distort. Having a meter bridge or LED to warn you of a channel overload is helpful. The input overload LED will measure the input+preamp level regardless of the channel fader position, so it is really important to heed it if you want to avoid overload distortion. Often, these will have red, yellow and green LEDs—flickering green is good, it confirms you have signal, yellow OK for louder sections, red is bad for all but the more powerful transient sounds, and means you need to trim the channel down or lower the strength of your input signal.

The location of these LEDs varies widely by board. A meter bridge, or the screen on a digital board, is a more sophisticated tool for setting input levels, but be sure you know what it is showing you. Many meters can be switched between a channel's input and its output, and only the input reading tells you whether the preamp is overloading: an output meter reads after the EQ and fader, so it can look perfectly safe while the preamp is distorting.

Some boards have a single XLR input for both line level devices and mics, and these all go through the channel's preamp. This is a common spot for distortion in the signal chain. Even at the lowest levels of preamp trim, a hot source may still overload the preamp and distort. For this reason, channels are often supplied with a PAD switch that attenuates the input signal, typically by 20 dB. It is not uncommon to need this pad for +4 dBu signals coming into your board from, say, a computer's audio interface. This module may also be supplied with a HPF (high pass filter, sometimes called low cut) that rolls off low frequencies in the rumble range, for example 80 Hz and below, helpful for mics when floor noise transmits up the mic stand or you have a foot tapper. On many consoles the HPF is grouped with the EQ section instead, which we will get to on page 3.

4Channel Aux Sends

A mixer channel has an auxiliary set of outputs that can be connected to external effects devices such as a reverb (some boards have internal effects that can also be fed by these outputs). For live performance, it is common to feed on-stage monitor speakers for a monitor mix, return a mic feed to an on-stage laptop for live processing, or provide a signal to a video camera or recording booth. For DAW virtual mixers, these auxiliary channel outputs are used to send channel signals to digital signal processing plug-ins (such as reverb, delay, compressor, pitch change, EQ), even back into other input channels. Signals can be routed to these options from the effects sends, which are sometimes labeled aux sends or just aux. Boards typically have between two and six effect sends, often arranged in their own AUX section. Each channel can send differing amounts of its signal to one or more devices by adjusting the channel's effects send pots. Look elsewhere on the board for an effects send master, which must be turned up for any signal to be sent out of the board because it combines all the channel sends for that particular AUX SEND number. This is called a bus, because it is a common signal path that sums all of its individual contributors and allows an overall setting of their total strength. So while each channel send pot is designed to set the relative strength of the input signal, the overall strength can be conveniently controlled by the aux send master.

Finally, to complete an effects loop and allow the altered signal to be heard, determine how the effects devices are routed back to the board. Some studios route them back through effects returns, while most route them back through other board input channels. In this second method, it is possible to EQ and pan the returning signal differently from the original (a nice compositional idea). Additionally, it is possible to take a returning signal and send it to a second device. It is even possible to send an effect back into itself from the returning channel by carefully turning up the same aux send # pot on the return channel, creating a deliberate feedback loop. This was a key component of tape echo delay in early years, but can still be used, say, with delayed pitch change for an arpeggiated effect.

The pre/post-fader button determines whether the signal sent from a channel will be altered by the channel fader. In pre-fader position, the signal is sent out at a strength determined only by the effects send pot and not affected by the channel fader. The pre-fader position is useful for techniques where you may wish to fade out the original signal, but still hear the altered sound. In post-fader position, both the effect send pot and the channel fader affect the overall strength of signal sent to the device. The post-fader position is useful if you wish both the original and effected sound to fade out completely when the channel fader is pulled down. The pre/post fader toggle in the pictured Digital Performer virtual board is the tiny 'P' button, along with the option to mute a send ('M').