A question of compression

[icecycle66]

Does anybody have a circuit that does band compression?
How can this be done?

Build an EQ circuit as normal, such as the Six Shooter or something similar.
Then add a compression circuit to each defined EQ range so that you essentially have compression on just that frequency. 

(This sort of thing  helps a lot with extended range bass. When you use compression that sounds nice on mid to upper range bass guitar, it craps out pretty hard when you start rocking the low B)

[midwayfair]

I've discussed it with some people before. I also think there might have been a thread about running two Afterlifes in parallel.

There are two main challenges:

1) Sufficiently isolating the audio bands. This means that when you compress a particular frequency, it gets compressed while the other frequencies are left alone.

2) Sufficiently isolating the envelope bands. For instance, if you play a note with bass content, will it only trigger compression in the bass audio band?

There isn't really a GOOD simple way to do it. For instance, I thought about making a Bearhug with two envelope paths, connected to two different source bypass caps. I ditched the idea because I couldn't isolate the crossover frequencies enough in either the audio or envelope circuits.

The other option is simply to use switchable a treble bypass. (Like in the Engineer's Thumb.) You lose the ability to compress just the treble frequencies, but I think that usually people want the opposite.

EQ circuits have decent isolation, but it's still not as isolated as you'd really want. However, I can think of a really good tool for the job: An 8th order filter chip. They're still available in through-hole chips and would be able to provide the steepest possible cutoff frequency to split a bass and treble band. From there you just have to deal with frequency cancellations in the summing to avoid a dip in the midrange, but that's been discussed to death in mixer circuits.

This would NOT be a small compressor, but I think it could be done in a 1590BB without too many bells and whistles.

EDIT: You could add an adjustable frequency control to the envelope section on a host of compressors. Link to a schematic for a compressor you like and I could make a suggestion.