You often hear that people like to use a compressor on their stereo master bus. Historically large format mixing consoles often had a high quality compressor built in that you could assign to the stereo master bus. A classic example would be the SSL compressor as found in the master section of the mixing console. This is a well known compressor and it has a fairly clean yet aggressive character when set to it’s fastest time constants. This is a stereo bus compressor that is often used on rock mixes to give them a little extra drive and punch. It can however be set just to tickle the music and gently apply gain reduction on peaks in the programme material.
Why use a bus compressor?
Using a bus compressor can often add a little sonic enhancement to the sound, often applying a stereo output bus compressor can glue the mix together adding a little dynamic control which helps round out the sound and homogenize the sound a little. It is important not to over compress the mix because over compressing a mix stifles the sound, reduces it’s width, reduces the space and stops the mix from breathing. It can also cause problems in mastering . Typically 2dB-3dB compression on peaks (such as drums) would provide a valuable enhancement without stifling the transients and losing high frequency detail. You can try some of the following settings and vary them to suit your track.
Ratio 1.5:1 / 2:1
2-3 dB gain reduction suggested
Set your side chain filter to 100Hz to avoid the low end making the bass ‘pump’
Which type of stereo bus compressor?
Not all compressors work well as a stereo master bus processor, 2 classic and common examples of good stereo bus compressors would be a Vari mu and an SSL type, the SSL type is for adding a little punch to the drums and a Vari mu adds a smooth subtle sheen across the mix and maybe a touch of warmth. Both are useful on the right type of mix. You can also try your software plugins as they could provide a beneficial contribution to the sound of your mix. Remember subtlety is the way to go.
Using a bus compressor is somewhat different than using a compressor across a stereo track during mastering. When you mix the compressor will influence your fader positions and mix balance to a small degree whereas in mastering the goals are slightly different. In mastering compression can be used to add a tonal character as much as anything, maybe a dark sounding compressor to smooth harsh upper mids or a fast and punchy compressor to enhance the rhythmical movement in a track.
Is there always an improvement using a stereo bus compressor ?
The main difference is that a stereo bus compressor can affect your mixing decisions so you should use one lightly. I always recommend frequent A/B ing of your mix down with and without a bus compressor on to ensure it is actually making an improvement. Try and set the make up gain so that when you switch the compressor out you are hearing the same perceived volume as the ear can easily be fooled into thinking the loudest sounding of the two is the best.
When the compressor is applied to the master output listen carefully to the bass, mid range and high frequencies and check them with the compressor in bypass. It is not always an improvement. If it is not you may need to try another compressor type or remove it completely. There is no guarantee that it will work for any given mix and you need to determine if it is an improvement yourself.
Stereo bus compression is not a panacea for a badly balanced mix.
One last note, a bad mix is a bad mix whether it has a bus compressor on or not. You should not attempt to compress you way out of a bad mix. Instead you should use eq, fader automation and freshly rested ears to improve it first. Heavily squeezing a mix may give the sense of keeping everything in place but the mix will sound flat, lifeless, stifled and suffer compression side effects.
A good mix takes many man hours to perfect and you should expect to be doing a lot of fader automation to get your mix sources to gel in an effective manner before resorting to stereo compression to solve problems. Adding a stereo compressor to a mix that is not well balanced will usually cause as many problems as it may appear to solve. So expect to work hard at crafting your mix down. To contextualize a professional mix engineer may take a whole day automating a single vocal to fit the mix.