When setting up any type of routing in Ableton Live such as Sidechain, MIDI routing, or routing audio from one channel to another, you may have noticed you get presented with the following 3 options: Pre FX, Post FX and Post Mixer.
When Pre FX is selected Ableton will route the signal as it is before it is run through any of the effects chains, plugins, or mixers that may be applied to that channel. The signal gets routed completely dry.
So if you have a vocal sample on Channel A with a delay and a reverb on it, and set Audio From on Channel B to Channel A Pre FX, and then solo Channel B. You will not hear the delay and reverb, just the dry signal.
When Post FX is selected Ableton will route the signal only once it has passed through all effects chains and plugins but not the mixer so the fader level will have no effect on the routed signal. The signal gets routed wet.
So if you have a vocal sample on Channel A with a delay and a reverb on it, and set Audio From on Channel B to Channel A Post FX, and then solo Channel B. You will hear the delay and reverb.
Post Mixer routes the final output of a track, once it has passed through all effects chains and the mixer. However, soloing the routed channel (Channel B) in this instance will not allow you to hear the routed channel. You will have to un-solo all channels to hear this.
Post Mixer is a good option if you want to create multiple copies of a wet signal and control the volume of all copies using the original channels mixer settings. This is useful for things like creating fake vocal doubles.
In most common sidechain cases like sidechaining instruments to your kick, you would want to use Pre FX (unless you have no FX applied to your kick, in which case it doesn’t matter if you choose pre or pose fx).
If you are sidechaining reverb to your vocal to achieve a cleaner vocal you would want to sidechain it to the dry signal – so in this case, Pre FX is the best option.
This depends on how you have processed your vocal. If you’ve done all your compression, timing, and eq adjustments on the source channel but no reverb/delays. You would want to route the audio Post FX, or Post Mixer so that all the copies of the signal carry the processing. You can then apply things like reverb and delays to the vocal group/bus or on a return track.
When doing things like New York or Parallel Compression you should consider what effects have been applied. If for example, you have reverb applied to the source signal just be aware that if you choose Post FX routing the compression will bring out the reverb even more, so if you are happy with your reverb then opt for Pre FX.