When you’re doing sound mixing, you want to make sure that the level for each sub-system within your console—mic preamp, equalizer, compressor/limiter, channel fader, etc. is set correctly—high enough to be well above circuit noise, but low enough so as not to be constantly clipping and distorting the signal.
Take a look at how your faders are set. If you find you’re mixing with the channel faders and sub or master faders at far different settings (like faders low, masters high), try to figure out why you feel you need to mix that way. This isn’t an optimum setting as you’re adding unnecessary noise to the mix.
The solution may just be simply readjusting the levels of the channel fader and masters. But perhaps the problem lies further upstream. Are you trying to bring levels down to try to reduce some distortion? Then look to the mic preamp to see if you’re overloading the input. Adjust the input sensitivity control or if that doesn’t do the trick, insert a pad between the source and the console. If you get distortion at this stage, it will carry through the entire mix.
Also make sure you’re not connecting a line level source to a mic level input. Check your EQ settings. Are you boosting some frequencies that are overloading the next stage? On dynamics processors, is there an overall gain setting that’s too high?
Are you using any outboard gear through insert sends and returns? There could easily be a level and impedance mismatch between the gear and the console.
Adjusting levels on the outboard gear may get you through the mix. However if you have a console with unbalanced send/receive I/O at -10 dBV, and outboard gear with balanced I/O and expecting and producing +4 dBu, seriously consider adding interface devices between the outboard gear and the console to match levels and impedance.
Continue to work your way from the input, through the processing stages, to the channel fader, the subs, and then masters, checking levels at each stage until the problem is solved.