The Focusrite Compounder was an inexpensive two channel compressor/limiter from their popular Platinum range. A VCA compressor with a separate optical limiter this was quite an unusual piece but it gained notoriety in some quarters courtesy of its Bass Expand feature and the temptingly titled “Huge” button.

As Focusrite explained:

That mysterious Bass Expansion circuit. 

Many people think the ComPounder is generating artificial sub-bass frequencies, but this is not what actually happens. The enhancement circuit actually comprises a wire-wound inductor (low frequency filter) that allows some of the bass energy to bypass the compressor circuit, and hence avoid gain reduction. This may seem to defeat the object of a compressor, (since the bass sounds are the ones where much of the energy resides,) but you don't always want to use a compressor just to control gain - sometimes you want it to make the material sound compressed in a creative way. Because the bass sounds contain so much of the energy in a typical mix, the compressor works hardest during bass notes or kick drums, and this often robs the mix of some of its punch. By sneaking some of this energy past the compressor, the low end energy can be made proportionally higher than the rest of the mix, and the nature of the inductor-based filter causes hard-driven bass sounds to pick up even order harmonic distortion, much as they would in a heavily driven valve circuit. This increases the level of subjective bass. Depending on the settings chosen by the user, this unique feature can be used either simply to replace lost LF content, or to actively boost it for a bass- heavy effect.

Inductors also store energy, so bass sounds may be 'stretched', which increases their average energy and hence their perceived level. A slight dip in the frequency response an octave or so above the point at which the filter is adding bass energy helps keep the bass sounding tight, even when massively boosted. Furthermore, the effect of the filter combined with the compressor gives the bass end a degree of dynamic control as though it's passing through an expander. 

Result? Huge tight bass. 

I wanted to see if I could replicate this process in Pro Tools and it’s pretty straightforward and a fun effect (as long as you watch the headroom!). It’s not quite the same because the inductor based LPF in the hardware as that introduced quite a lot of colour but if you want to add some saturation, or anything else then that is of course easy to achieve.

Watch the free video tutorial to see some of the things you might try.