phase needs to be inverted ( polarity reversed ) on the bottom mic .
12 . Preamp . The preamplifier is used to set the level for the microphone or device connected to the Mic / Line input .
13 . Gain . The gain potentiometer controls the level of the preamplifier and is used to either boost or pad the input signal .
14 . High-pass filter . Most mixers include a high-pass filter on the channel strip that lets the high frequencies pass through unaffected while cutting the lows below a set point . The high-pass filter in this channel strip cuts the lows below 100 Hz . Other mixers might have high-pass filters that cut below 60 , 70 , or 80 Hz . The high-pass filter is an important factor in creating mixes with a controlled low frequency band . Trimming the lows on all channels that don ’ t need those frequencies helps clean up the low end and makes the entire mix cleaner , easier , and more enjoyable .
15 . Path to the equalizer . The signal , after leaving the preamp , routes to the insert jack and then back down the channel , into the equalizer circuit . The ring and tip of the insert jack connects , so the signal always goes through the insert jack , even if there ’ s nothing plugged into it ; however , once a 1 / 4-inch TRS insert cable is plugged into the jack , the signal is routed up the insert cable to the connected device , and the affected signal returns back through the cable to continue its path to the equalizer . Notice that the signal entered the mixer as either a balanced mic or line-level signal , but after leaving the isolation transformer , the signal going to the insert and onward is unbalanced — it will be balanced again when it leaves the group , aux , and main outputs .