Chapter Two: Studio Gear
3. Mixing Console Basics | page 2
Input Channel Strips
A mixing board will consist of a number of input channels controlled by identical channel strips (vertical arrangement of controls). If you learn how one strip works, you've learned how they all work. The layout and minor differences in functions vary between manufacturers and models, but they all have the same basic elements. Inputs to a board, patched into the back or top, consist of line level inputs, from devices such as synthesizers and computer audio interfaces; microphones, for which there is often a separate preamp section; and tape inputs, for devices such as CD players. (Not all boards have separate tape inputs.)
Below, we will view sections of an input channel and their function one or two at a time.
The top module controls aspects of the input lines. In this diagram, there are actually three separate input sources per channel, a microphone input, a line input, and a tape input, 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 must be depressed for the microphone to function. This sends a 48-volt current through the mic cable to charge the plate. 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. ("Pot" stands for "potentiometer," the technical name for this type of control.) 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 preamps, 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 MIC/LINE switch, which chooses the mic or line input jack (surprise!), is located in the channel jack section. When depressed, the flip button switches the normal arrangement of the channel, sending the tape source to the main channel fader and the mic or line source to the MIX-B pot (see below).
A mixer has a separate set of outputs usually connected to effects devices. Signals can be routed to these devices from the effects sends (sometimes labeled "aux sends" or just "aux") of the input channels. Each channel can have its own relative strength of signal sent to one or more devices by adjusting the effects send pot(s). Look elsewhere on the board for an effects send master, which must be turned up for any signal to be sent. 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 effects 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 others, such as the CECM studios, route them back through other board input channels. In this way, it is possible to EQ 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. (WARNING: Do not send a returning effect signal back into the same effects bus if it has a short or no delay — this will produce an undesired feedback loop.)
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.
On this particular console (a Mackie 8-bus), the user can select the source signal for the sends as either the normal channel input signal, or a second set of line inputs called Mix-B.