Audio mixer channel gain can be set in three ways. Someone reading that last sentence just went ballistic. Is there only one right method for setting the channel gain? Let’s explore.
Follow the signal
Audio comes into a mono mixer channel via a balanced connection. There are three wires in the input cable; one is ground and the other two carry the audio. Of these two, one carries a mirrored signal image of the other. This allows the console to compare the two audio signal images and remove differences caused by noise picked up after the signal left the source.
The signal comes in at microphone level (a few millivolts), gain is applied, and the result is boosted to line level (a couple of volts) via the channel preamp. Though we talk of these levels generically, there is some fluctuation of the signal strength such as 5 to 50 mV for a microphone level signal. For example, if a vocalist sings softly, they will send a weaker signal compared to if they were singing loudly.
The gain (a.k.a. trim) control allows the FOH tech to allow for more or less of the signal to come into the console. For example, a hot signal from an instrument would need less of that signal to come into the console. It would be like turning a faucet valve so less water comes out although the water pressure behind the valve stays the same.
Some signals come into the console so strong they can still be heard with the gain at zero. When this is the case, the Pad option should be used. It cuts 20 dB from the signal and places it into a manageable range.
The signal sent to the console needs to be a strong clear signal. This is why vocalists need to put their mic to their mouth. It’s also why a guitarist needs to turn up the volume on their guitar. Otherwise noise can be heard.
Noise can be picked up within the signal path either via interferences on instrument cables or any time the signal goes through a connection. Noise also comes from microphones as even air particles hitting the diaphragm will produce a sound. Turn on the audio system and open a mic channel. Hear the small noise hum.
The mixer needs a strong CLEAR signal for optimal mixing. This happens with a high signal-to-noise (S/N) ratio which means the true source (guitar, vocal, etc.) is so strong in the audio signal that natural noise is overpowered.
One more point on the S/N. Equipment can be set up so the signal from the stage first goes into a rack component or other off-board processor and that equipment has gain control. If there’s an initial low S/N, by the time the signal reaches the mixer, that noise can sound substantial in relationship to the desired sound. This is another reason you should get a high S/N as soon as possible.
Ideally, you want strong signals with a high signal-to-noise ratio where the signals are all in close relativity to each other. This enables precise volume mixing on the faders.
One aspect of gain setting is gain-before-feedback. The microphone properties, the speaker properties, and the room properties all contribute to the level in which the audio system can produce sound without producing feedback caused when the reverberation field crosses over into the microphone field.
This is why floor monitor placement, microphone type, and microphone placement are so critical. The best sound is the strongest one produced at the source and contains the greatest amount of the source as possible. A keyboard routed directly into the system is 99.99% pure keyboard. A vocal microphone, not so much, but the microphone’s proximity to the singer’s mouth along with the strength of the singer’s voice plays into that percentage.
How to Set Gain
There are three primary ways to set the gain and much has to do with the audio environment because of how the faders come into play.
In a studio environment, it’s all about having the clearest cleanest sound possible. In the live environment, the highest level of fidelity can’t be appreciated because of the nature of the environment itself.
In studio work, gain is set and then hours are spent on EQ and effects manipulation with a little fader manipulation tossed in. In the live environment, the gain is set and then fader adjustments are frequently made. This will be an important consideration in two of the methods of setting gain.
Role of the fader
When a channel fader is set at unity, the console is neither boosting nor cutting the signal. Using studio gain theory, the fader location (fader is post-gain) isn’t critical as long as it allows for good granular volume control. That was an overly-simplified statement but it gets the point across. It’s also where opinions start to fly.
A quick note on clipping, the process of clipping off the signal when it gets too high. Occasional clipping is ok – that’s why the console has the clipping process. However, excessive clipping is a sign of an incorrect gain setting.
The first method of gain setting follows the studio mentality and says the fader should start at the infinity position (at the bottom) and the gain is increased until the input meter reads almost to the red while allowing for signal increases without distorting, such as when a vocalist sing a lot louder for a passage or a chorus or a single line. Then, the fader is raised to the point where the volume is right in the room.
Some people use this method but aim for the 0 dB level on the channel’s metering.
Be aware, every three channels at the same output level will create a louder combined sound by around 3 dB. Therefore, while the main fader would initially be at unity, after setting all gains, it’s possible to have a hotter output than you’d expect. If I correctly recall, on my typical weekend setup, I see about an extra 6 dB overall.
The result of this method is the strongest signal in each channel and with faders all over the place. They may or may not end up at unity. This can depend somewhat with what’s happening on stage and how the system is configured regarding the amplifiers.
The second method of gain setting, the one I use, follows a live environment mentality which says the volume balance (channel volumes in relation to each other) for one song are often different in the next song and therefore, fader control is very important and it helps to have a BASELINE balance.
To use this method, set the fader to unity and increase the gain until the volume level sounds right for the room and for a general mix. The result is faders all set at unity with a general volume balance between channels. For example, lead vocal always on top. Therefore, when moving from one song mix to the next, subtle volume changes can easily be made with the most granular control (remember fader controls are logarithmic). Is it the strongest signal? Maybe, maybe not. Will it be a noticeable strength difference and produce a sub-par mix? I’m saying no, with one exception.
There are times when we have little-to-no control over the source. For example, someone hands you a CD they burned from a file they made after they did who-knows-what to it. In such cases, set the gain for the strongest signal and then set the fader. You don’t have to ride a fader for a backing track or for the audio from a DVD. Well, you shouldn’t.
The key to making method #2 work is using proper microphone selection and usage for the cleanest strongest signal from the stage.
The third method of gain setting goes a completely different route and uses the location of the faders to indicate where the channel should sit in the mix. For example, the lead vocal channel fader is higher in relation to all other channels. A lead guitar fader would be lower than the lead vocal but higher than the keyboard. By looking at the faders, you are seeing where everything sits in the mix. You’re seeing the volume balance. Only then do you increase the gain.
Of these three methods, I’ve seen professional live FOH engineers use all three. They use a method they were taught or what they found works best, such as with the third method.
It comes down to getting the best signal from the source and that’s the area where I see churches doing it wrong because of incorrect microphone selection or usage.
The Next Step
Mic selection and usage is critical for setting good gain so check out this post on microphone properties. It’s also got links to even more posts on mic usage and vocal mixing.