Hum or Buzz
• Maybe nothing is plugged into the channel input. This creates an open circuit, and that high impedance input picks up lots of hum and buzz. Make sure the instrument or mic is plugged into the snake.
• The mic cable’s shield might be broken. Replace the cable.
• Maybe the mic is picking up a humming guitar amp. Turn up the guitar and turn down the amp.
• A guitar with a pickup is plugged in and turned up, but nobody is playing it. Touch the strings or turn down the pickup volume to see if the hum stops.
• A direct box can create hum because of a ground loop. Try flipping its ground-lift switch.
• A compressor stomp box is raising the gain because there’s no input signal. Temporarily shut it off and see if the hum goes away.
Distortion
• If the mixer channel is clipping, turn down the input trim until it stops.
• Check the gain staging of the musician’s stomp boxes.
• Replace bad cables.
• Switch on pads in condenser mics (you’ll hear this only when the musicians play).
Crackles and Noise
• Replace bad cables. There might be a cold solder joint. Clean cable connectors with a cleaner such as DeoxIT by Caig Labs.
• Replace weak batteries.
• In some microphones, make sure the mic capsule is screwed fully onto the mic handle.
At this point, you might dial in some levels and EQ based on past experience. You could insert compressors in vocal channels, and so on.
THE SOUND CHECK
Overview
After verifying that all the lines have the right signal and sound clean, you’re ready to begin the sound check itself.
It helps to use a talkback mic. At the mixer, plug a mic into a spare channel and turn up its monitor send. Talk to the band through the monitor loudspeakers – it beats yelling instructions across the room or using sign language.
Before the gig, make sure the talkback mic and its cable sound clean.
I recently did a sound check in which I heard crackles and noise, and wasted time tracking it down – until I found it was the talkback mic’s cable!