Often I see leftover mixes (that would be the positions the faders were left after the last use) that portrayed a rather chaotic mix. Some of the faders would be all of the way to the top, other faders somewhere in the middle and, always, a few near the bottom but not off.
When asked about it the sound operator would usually state defensively "Well, I can barely get enough volume out of some inputs while others are blasting!".
What we clearly have here is a lack of understanding of what I call proper signal management - usually beginning at the mixer's first setup (or, by subsequent Monkeying around with, after being set by a professional, by those who know just enough to be dangerous).
Proper signal management is accomplished when the sound operator adjusts the incoming (input) signals to optimum levels. Optimum levels are ones that have sufficient gain to provide sufficient signal-to-noise ratios while also providing sufficient headroom for transient signals.
Let me describe that in less engineery terms.
Most all input signals are analog - that is...regular electrical voltage. Various types of input devices (microphones, guitars, keyboards, etc.) output different levels of voltages. Our task as sound operators is to blend all of these various inputs into a mix where all inputs can be heard without any being overbearing. This requires us to match, as close as possible, the input voltages of those input sources.
Enter the Head Amp...or Gain, .....or Trim, .....or anything else manufacturers want to call it.
One amplifier sits at the entry point of the signal path to each input channel. With them we can manage the setup of the gain structure of the input signals across the board.
To help you visualize this process imagine a box. This box represents the input channel into which we've plugged a microphone. When someone speaks into the mic a small voltage is generated. That voltage travels down the mic cable and into our input channel on the mixer (our box). The signal fills the box to the level proportional to the amount of volume being put into the mic - a small amount for whispering or a large amount for screaming.
Now one thing we must realize is that there was already something in the box. Don't blame the last person who used the mixer - it's only the inherent noise that's generated in an energized analog circuit.
I hear some of you saying now... Wait! But I have a digital console so I shouldn't have any noise in my circuits... I paid big money! Keep this in mind - while your mixer is digital our input devices and output devices (speakers) are analog. That means that inside of your nice digital mixer are two types of analog circuits - Analog-to-Digital converters for inputs and Digital-to-Analog converters for outputs. So you got some noise.
Back to the box
So we've put some input signal into the box and have discovered that noise was already in the box. Now, for us to hear the input signal above the noise we have to increase the input signal over the noise (which we'll call the noise floor). This is where we use the head amplifier to increase the input signal. So if we had marked our box with level markings at, say, every tenth and had found our noise to be at the ten level and our input signal also at the ten level what will we hear. Yep, equal parts of noise and signal. What to do? We must increase in the input signal using the Head Amp to a sufficient ratio above the noise as to make the inherent noise negligible.
We call that signal-to-noise ratio. The nifty part is that the mixers manufacturer has already figured out the optimum S/N ratio for you. If you look at the fader around the zero mark you will usually find a band that is colored differently (that's not at the bottom of the fader - the bottom mark is infinity, then, as you go up from the bottom, the mark numbers decrease in negative decibel levels (db) until you reach zero near the top. Above zero you see increasing db level number markings). This differently colored band near zero is where the fader should be operated to achieve optimum Signal-to-Noise ratios.
Don't overfill the box.
Oh, yeah. Our box has a top on it. Bad things happen when you try to overfill the box. Your life may never be the same... Well, at least your input signal will never be the same!
When more input voltage is sent to the input circuit than the circuit can handle (overfilling the box) clipping happens. At the output end (the speakers) you hear that clipping as distortion. Clipping is simply taking that lovely sound you are inputting - which has a beautiful rounded sine wave pattern - and clipping off the round tops turning them into square waves (distortion). It doesn't sound very good unless you happen to like the sound of a buzz saw. By the way, distortion is the number one killer of speakers. I saw it on a poster in the Post Office.
So you gotta fill the box with enough signal to overcome the noise floor but leave enough room for our transient signals (someone shouts or drops the mic un-expectantly). We call that Headroom.
So what was going on with those left over mixes I first mentioned?
Someone was trying to mix with improperly set up Head Amps. Some boxes were overfilled - others barely had any signal (other than noise) in them.
How to properly fill the box
Read the instructions. They will tell you something like this. With all other inputs off begin to input signal into the channel you desire to setup. Have someone talk or sing - if a mic (at the volume of the actual performance), or play the instrument if a line level input. Select Solo or PFL if the mixer does not have individual input meters on the input channels. Turn the Head Amp all of the way down (could also be labeled Gain or Trim). Raise the Fader to the Zero mark (optimum S/R ratio). Then, slowly, increase the Head Amp watching the master or solo/PFL meter until you reach close to the zero mark. On some mixers that will be just before the meter light colors turn to red. Leave the signal a little below that zero mark to allow for those big transients.
Rinse and Repeat
Now, one by one, go through all of the remaining channels and repeat the process. Once complete, and if done properly, there should be very little need (if any) to change the Head Amp during a performance. The need would only arise if something changed on the input side like taking a mic from a singer and throwing it into the kick drum (bad idea). Stranger things have happened though - in such cases you gotta go through the Head Amp setup process again (just without turning off all the other inputs!)
Don't Mix from the Head Amps
Better yet don't touch them once set unless absolutely necessary. Why? As you can now visualize the Head Amp controls all of the signal going into the input channel. You, likely, are using your aux mixes for things like monitors, effects sends, and other outputs. When you adjust the Head Amp, turning the level up or down, you are turning the level up or down to all outputs associated with that channel - like monitors. You're gonna get some dirty looks.
After you have set up all inputs to their optimum Signal-to-Noise ratio settings you will be able to mix all faders at or around the zero marks. Obviously, to get a good mix, they won't all be in an exact line across the board but they should be at varying levels near zero. Faders at the very top or near bottom is a big indicator that someone has not properly setup the mixer.
Don't leave an ugly leftover mix.
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