There are many mixed opinions on this subject, but I gotta say, I'm a fan of having a compressor on my mix bus. Actually, I have two! Let's dive into some of the pros and cons of mix bus compression, and a few different ways you can use it.
The main benefit for using a mix bus compressor is that it tends to "glue" the mix together. As you're mixing, you may find that the instruments just feel too separated and don't work together to support the overall sound. By gently decreasing the dynamic range on the mix, you start to feel a sense of unity.
Secondly, you can use a mix bus compressor while mixing to get a sense of how the mix will shift during mastering. For instance, a common problem between mix and master is the snare drum disappearing. This happens because the mix will hit a limiter in the mastering stage, and the snare drum transient that was poking out gets squashed, while the rest of the mix stays where it was. Therefore the snare gets quieter while the rest stays at the same volume. Now, if you mix into a compressor, you can find ways to work around this issue with volume automation, EQ, multi-band compression, etc.
A lot of mastering engineers prefer to work on an uncompressed mix down. Why is that? Because they have less room to work with when the mix has already by compressed. Fair enough! So in one sense, you can pigeon-hole your mastering engineer but doing a part of their job for them.
Second, your clients can get used to a compressed sound, and it can be pretty easy to take it too far. As with most things in working with audio, you have to be careful not to overdo it.
Having said all that, I still use mix bus compression and I send to the mastering engineer that way. My reasoning is that I want it to sound how I want it to sound, and the mastering engineer has to work off of that. Now, I'm not applying all that much compression, so it should still be manageable for the mastering engineer. As with all my techniques, this is just what I'm currently using, but it will always be evolving:
1) UAD Fairchild 670 - I have two ways of using this one. The first way is to set the time constant to 5, put a full 100% mix, set the sidechain filter to about 12 o'clock, adjust the threshold until I'm knocking off at most 2 dB, then adjust the output volume to match the volume before the plugin. In this method, I'm just trying to slightly round off peaks and get a little glue. My second method is to set the time constant to 1, sidechain filter at about 9 o'clock, adjust the threshold until I'm knocking off 5-7 dB, set the mix to about 20%, then adjust the volume again. Yes, this is parallel compression! In this approach, I'm trying to give the whole mix a bit more energy, without it destroying my balance. Try either!
2) UAD SSL G Compressor - I tend to always use this one on the same settings, and here they are. Attack at 3 ms, Release on Auto, Ratio at 2:1, adjust the threshold until I'm at most knocking off dB, match the gain using the makeup. I find this compressor adds a lot of punch to the mix, but you have to be subtle about it.
3) Fabfilter Pro-C 2 - I am effectively using this last one as a last resort limiter. I set it up for fast attack and release, and find the threshold that knocks off less than a dB at most. It should only engage on strong hits. I've usually gain staged my mix so that I can set this threshold to -4.0 dB. My rationale there is that I want to give the mastering engineer ATLEAST 3.0 dB headroom. Again, this one is hardly being used, it's simply hitting the highest of peaks so that we don't see any odd clipping.
And that's my mix bus compression! I am doing some EQ moves within all this, however this post was focussed on compression, so I left that stuff out. I'm sure my approach will evolve very soon, but that's where I'm at now!
Thanks for reading, and let me know if you have any further ideas! Follow us and stuff :)