It’s the knob at the top. The importance of getting the gain right can’t be overstated. It’s the fundamental first setting on each channel that affects every knob, fader and external processor all the way down the chain.
The simplest way to define gain is as a volume control on the way into the console.
Gain is a volume control on the way into the console.
Microphones output a very quiet signal which we call “mic level”. The exact level varies based on the kind of microphone, the source volume and the distance from the source to the microphone, but it’s in the millivolt range. Mixers and other powered devices like iPods, computers and wireless microphone systems, work at a much higher level called “line level”. [If you’re wondering about DI boxes, they convert line level sources to mic level in order to transmit them over a mic cable. More on that some other time!] Line level can vary too, but it should be a little more than one volt on professional devices. This means that a mixer operates at a level a thousand times louder than a microphone!
In order for a mixer to work with a microphone signal at all, it needs to be made louder. A lot louder! And the tool for this job is the microphone preamp.
You’ll find a mic preamp on every microphone input on your audio console. The gain knob controls the preamp to add gain to a signal and this is measured in dBu. If you have a loud line level sources you can also take gain away, often by employing a “PAD” switch. Negative gain can be called “trim” although the terms “gain” and “trim” are usually just used interchangeably.
The goal in either case is to match the signal level to the console’s ideal internal operating level – its “nominal level”. If a source is too quiet you’ll have to make up the gain somewhere else – like at the fader – which adds unwanted hiss and noise. If an input is too loud the signal will distort and you’ll probably end up with the fader quite low to compensate. Either way, your mixer won’t be living up to its potential.
On analog mixers this “nominal level” is 0 dBu and can be read at the console’s input meters. To set the input gain it’s helpful to PFL or solo the individual channel so its input signal shows up on the console’s biggest meter. Then adjust the gain until the signal averages 0 dBu and peaks no higher than +6 dBu. This puts the average signal level at the highest green segment on the meter with peaks a little higher but clearly in the yellow zone.
Digital mixers are a little different. Because they deal in zeroes instead of volts, signals can only have negative values with 0 dBu being the absolute loudest possible signal. So, while the concept is the same, the metering is different. I can’t vouch for every desk out there, but since we’re demoing a Yamaha CL5 this week I asked the rep for his advice. He suggested averaging -18 dBu with peaks no higher than -12 dBu. This once again puts the average signal level at the highest green segment on the meter (the virtual one on the touchscreen) which seems to be a trend. Whatever your equipment, it’s a good idea to find out where the nominal level is for your console and aim for that.
Another way to set gains is to position the channel faders exactly at “0” then adjust the gain until you hear the desired level in the mix. This approach keeps the fader levels as close to “0” as possible which is where they’re designed to operate best. A case can be made that this produces a cleaner main mix but, I must admit, I’ve never been comfortable setting gain this way. It’s much more subjective than the meter method so I don’t recommend it for beginner or intermediate audio techs. I also find it more challenging when mixing monitors from FOH because it prioritizes the needs of main mix over the monitor mixes. Of course, I’m always up for sparking a global internet controversy so feel free to disagree with me in the comments!
Even though I don’t recommend the “Flat Fader” method, it teaches us a good lesson. If you find your faders are consistently outside the shaded -5 to +5 region it could be a symptom incorrectly set gain. It’s worth taking three seconds to throw that channel up on the big meter and see what’s going on. Of course, the problem could just as easily be on the output side – power amplifiers and system processing – but we’ll have to leave that topic for a future post!