“What is the difference between a post-fader and a pre-fader auxiliary send and in what situations would I use either one?”
(See WFTD archives Pre Fade/Post Fade) A post fader Aux Send taps the incoming signal from the channel at a point after the channel fader. This means that when the channel fader is down, no signal is sent out the Aux Send(s) on that channel.
Post-fader Aux Sends are generally used as “effects sends,” to send a signal out from a particular channel to an effects processor (see WFTD Effects Loop). Since the channel fader controls the level of signal being sent to the main mix as well as the level of signal being sent out the Aux Send, when the channel fades down, the level of the “wet” signal follows the level of the “dry” signal. If the level of the wet signal did not follow the level of the dry signal, the effected signal would still be heard after the channel fades out.
A pre-fader Aux Send taps the incoming signal from the channel at a point that is before the channel fader. So, when the channel fader is down, the signal is still being sent to the aux bus.
Pre-fader aux sends are helpful for live sound reinforcement situations where the FOH console is doubling as the stage monitor mix console. When setting up stage monitor mixes, it is ideal to be able to control the level of these mixes independently from the front-of-house mix. If the position of the channel fader affected the level of that channel in each monitor mix, it would be necessary to constantly adjust the monitor mix (Aux) Sends after changing the level of the channel fader. Or more simply put, when a screaming guitar solo is boosted in the front-of-house mix, everybody on stage would get an earful from their monitor mix.
There is also another distinction known as pre or post EQ, which at this point should be fairly self explanatory. Any pre fader send could still be pre or post EQ. In a live situation, pre fader and pre EQ sends are usually best where the mixer may be feeding stage monitors.