Another designer tried my mosfet compressor using a darlington pair as the gain stage device Q1. He described some improvements in performance – so I tried this as well. I love it! Using a darlington such as a MPSA14 (or a homemade one) is a drop in replacement for the MOSFET Q1. It gives a little more gain and reduces noise in the control loop slightly (allowing more freedom in changing and or eliminating C7, c8, C9). I definitely can reccomend it.
New Schematic:
https://circuitsaladdotcom.files.wordpress.com/2012/11/mosfet-compressor1.gif
Circuit Salad 
Hi!
I’ve just a general question about the design : In the audio path, why you choose to use a darlington transistor instead of an OpAmp with gain? Simple matter of hype around this type of practice or real technical consern?
I ask it because I’ve swapped the darlington with an non-inverting opamp gain stage (in simulation only) and adjusting some resistor values to match the gain ceil. It seems to work great with lots of flexibility and no distortion, actually better than the simulated darlington.
Anyway, great design, great job!
Bye