The top-most channel strip module controls aspects of the channel input sources from external devices. 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 and pre-amp, or charge the baffles for a ribbon mic, or provides power to an active direct box.
WARNING: When connecting gear to a board, do so with all phantom power OFF. Turn ON phantom only after everything is hooked up. Otherwise, hot-wiring an expensive mic may at best produce a loud pop if the speakers are powered on (they shouldn't be), but can also burn out a super-thin mic ribbon or blow a mic's pre-amp.
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 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—means you need to trim the channel down or lower the strength of your input signal.
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, the channel may still overload the preamp and distort. For this reason, channels are often supplied with a PAD switch that attenuates the input signal by up to 25 dB or more. 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. The HPF may alternatively be found in the EQ section on some boards.
[On this Mackie board, the MIC/LINE-TAPE switch chooses whether the mic and line input OR the tape input is sent to the channel fader. The other input is then sent to a separate MIX-B output].