Mixer Tip: Using the Gain
What shall it profit a man if he whirls the gain? But he ain't got no soul.
One of the first steps to successful audio mixing is properly setting channel trim controls. Also known as gain, trim should not be confused with the channel fader or level control. Most consoles have a trim knob near the top and a fader at the bottom of each column of channel controls. As a rule, the trim is set for a given sound source and left. The fader is then used to make level adjustments during the program.
To minimize self-noise and distortion, audio consoles mix signals at line level. This is well above microphone output levels. And mic levels vary greatly due to differences in sensitivity and in the levels of the sounds they are picking up. Trim adjustment provides the amount of gain needed to bring the incoming signal to the optimum mixing level. Gain simply means "more of the same." (While the Apostle Paul never taught church sound technicians to mix, he did teach sound doctrine and understood the meaning of gain: "For to me to live is Christ, and to die is gain." Phil 1:21).
Setting the trim correctly is important yet simple. First, mute, or otherwise turn off, all channels but the one to be adjusted. You should also bring the mixer's Master level controls all the way down, or simply turn the power amplifier down or off to avoid feedback. Turn the gain control all the way to the minimum position, usually fully counterclockwise. If the channel has a Pad or Attenuator switch it should be in the off position unless the gain range proves insufficient for a high-level source. Set the channel fader (or rotary level control) to the nominal operating mark adjacent to the control. This may be a heavier line, or a "U" for unity gain. Engage the channel's solo button, making sure it is in the
PFL (Pre-Fader Listening) mode. (This is to route the channel's signal to the mixer's PFL solo metering. Your mixer's means for doing this may vary.) Next have someone speak, sing or play into the microphone at the distance and level at which it will be used. Now turn up the channel's input trim. You should see the channel's input level on the mixer's meters. Adjust the trim to achieve a level of around zero dB.
That's it! Now just repeat the procedure for each channel and you're on your way to better sounding mixes.
[Ed. note: Greg is Commercial Sound Specialist with Mackie Designs.]