Also, you can sum many channels at unity gain without overloading the stereo mix bus (more on this later).
Some mixer inputs have a switchable pad, which is a resistive attenuator before the mic preamp.
A 20 dB pad drops the signal level by 20 dB (1/10 the voltage) before it reaches the mic preamp input.
But it also reduces the S/N by 20 dB. If you turn down the gain trim all the way but the channel is still clipping, switch in the pad. Otherwise leave it out so you don’t degrade the S/N.
This advice applies to standalone mic preamps as well as console mic preamps.
Equalizer
In most consoles the signal enters a mic preamp, then goes to an equalizer, then a fader. If the signal is near clipping coming out of the preamp, and you boost that signal over some frequency band in the equalizer, the signal is likely to clip in the equalizer.
So as we said, you need to create some peakroom – some level difference between the peak signal level in the preamp and the clipping level. To do that, set the gain trim about 15 dB below the clip point.
If an EQ boost results in channel clipping, turn down the trim until the clip LED goes out. That’s assuming the clip LED is post EQ, which is not always the case.
Faders
The fader settings are important too. For starters, set them close to unity gain (design center). Do the same for the master faders.
The signals from all the channel faders sum into various sub-mixes that you set up. Every time you double the number of faders set to unity gain, feeding the same mix bus, their summed signal level increases by 3 dB. That’s when each fader is receiving a different, non-correlated signal. In other words, summed level in dB = 10 log N, where N is the number of faders set to unity gain (design center).
(click to enlarge)
Suppose you set 16 faders to unity gain and route them all to the stereo output bus. The summed signal will be 12 dB higher than the signal of one channel alone. So for example if you have 16 faders in use, it’s a good idea to set them 12 dB below unity gain. That will prevent overload in the mix bus.
If you set the master very low, say 20 dB below design center, you will tend to push up the channel faders to get enough volume and a meter reading near 0. That will overload the mix bus, giving a harsh, fatiguing sound.
If you can’t set the master at design center because the house sound is too loud, simply turn down the power amplifiers. More about this later…
If the signal going into several faders is the same (for example, hum), the signal increases 6 dB for every doubling of faders. If you have equal hum pickup in every channel (not likely), a 16-channel mix will boost the hum by 24 dB! That’s one reason to mute or turn down unused faders.