BFE BKE4 compressor limiter

Hello Everybody,
I recentrly aquired two beautiful BFE BKE4 compressors whic work just perfectly.
I have a question: I would like to hook up an external filter (for the sidechain) and from the schematic there is a "send and Return" chain made with opamps. (IC9 and IC10)

Can anyone suggest an approach on hokking up an external filter?
is it meant to be purely passive (a cap and off you go?) or shall I implement something else?

I am attaching the circuit, an help will be much appreciated.
( I also have a PDf which is more readable, but could not upload it)


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Best,
Mattia.

Anyone?!

This is clearly meant to connect to an external equalizer. The connections are balanced and the output is non-floating (you can't connect it directly to an unbalanced input).
Unless you want to produce very specific effects, I've always found that the best option for me is inserting a graphic EQ in the side-chain. You don't need much resolution, so you don't need a 1/3 octave graphic, a typical 15-band 2/3 oct EQ is generally sufficient.

Many many thanks!!

Mattia.