Here is an early attempt at building a rudimentary 3-band compressor in Drambo.

It’s nothing fancy so far, but the results are still quite pleasing (see video below).

Explanation:
– the input signal is split into 3 bands, controlled by the lowpass and highpass filters in each band.
– A layer mixer is used to keep the layout tidy and puts each band of the effect on its own layer (1,2,3)
– for each band, an envelope follower then tracks the transients in each filtered band and creates an envelope with which to modulate the gain downwards. Note – attack and decay parameters on the envelope follower work like attack/decay on a normal compressor
– a graphic shaper for each band helps adjust the curve of the envelope
– there is a makeup gain per band
– the layer mixer allows you to adjust the levels of each band relative to one another
– finally there is an A/B mixer to control the Dry/Wet mix.

In future versions, I hope to incorporate some advice received from RS2000 about improving the quality of my amp envelopes. And, ideally, I’d like to develop this into an OTT clone so that it’s doing compression and expansion of all three bands at once. Watch this space!

You can read more about this project here:
http://forum.beepstreet.com/discussion/223/building-a-multiband-compressor-all-help-appreciated