Beginners and pros alike have debated the relative merits of having a compressor on the master output. The main argument 'for' is that it generates mix cohesion; the counter-argument is that it can relegate the mastering engineer to damage control for the most part.
Nowadays, it's less of a contentious issue as there are a large range of quality compressor plug-ins designed expressly to handle master bus compression. In particular, there's a proliferation of virtual emulations of SSL's famous bus compressor, but while Propellerhead, Waves, Universal Audio and even SSL have released exacting virtual replicas of this classic circuitry, PSP has taken a slightly different approach with BussPressor, which offers its own take on bus compression without being a clone of a specific device.
All the compressor controls you would expect to find are onboard: Attack, Release (with a range-definable Auto option), Ratio (up to 10:1 - the start of limiter territory), Threshold and Make-up gain. The Attack, Release and Ratio controls are 'stepped' in true analogue hardware-emulating style, albeit in smaller increments than the SSL Duende Bus Compressor.
It's the little touches that make BussPressor interesting, though. First and foremost, the dry and wet signals can be balanced for parallel (aka New York-style) compression. This can be more sonically effective than just using a fully compressed signal, as it blends the punch of the dry sound with the 'crush' of the compressed one. Back in the day, this was achieved by running the compressed signal on one channel or bus and the dry signal on another, each with its own level fader.
BussPressor also offers sidechaining, with a high-pass filter adjustable before the detection stage. This is particularly useful for drum busses and full mixes, as it enables reduction of the 'pumping' reaction to kick drums and basslines. The high-pass filter in the sidechain lets you compress signals and pull the sounds together without the kick making the level of the whole mix drop dramatically every time it hits.
On the right bus?
Having tried BussPressor on a variety of signals, we'd summarise its sound as having a loose and rough character. It's not as clinical or sharp as a lot of the compressors favoured by modern dance and pop producers; we'd say its feature-set and sound make it more suited to the kind of natural, organic response required for acoustic and rock styles.
That said, it is designed to work like a VCA unit rather than warm/colourful vintage tube gear, so it is relatively transparent in the mix - vaguely reminiscent of a 90s Neve sound, actually.
BussPressor isn't the best-sounding bus compressor we've ever heard, nor is it the most feature-packed. Ultimately, we think it works a little better on busses than the master output. However, it does have a very natural sound and a pleasingly distinct response, and the parallel compression is a creative delight. Thus, we believe that it certainly deserves a place in your plug-in folder.