I spend some more hours on that project, here is what you may do for improvements:
First I have to mention, that I got different Schematics and Layouts of these Projects.
I do not know how much Revisions Gustav made, so some Names might be wrong!
Perhaps Gustav might send me the Final Version! THX
0 PSU
Replace the On-Board-PSU and Replace ist by something less noisy, e.g. 7815, 7915 Regulation with some LOW-ESR Caps behind the Regulators
1 Input section:
R1, R2, R3, R4, R5 10k > 1k
less Johnson Noise!
2 Input Section
add 100k to GND at the Input
e.g. Pos-Left, Neg-Left, Pos-Right, Neg-Right
3 Input VOL POT
R40/R66 1k > ALPS RK27112 10k log
3 Output Driver
C24, C25, C32, C33 100uF/25V > unused
4 GR-Meter-Tracking
TL074 > OPA4131 or MC33079
5 Magnetic Shield
I added a Round MU-Metall Schield to the Power-Torroid and at the Top and Bottom Case too Self-addhesive Mu-Metall!
There should be no space between those Mu-Metal-Shields!
6 Zener
ZD10 > 1N4148
7 Output Driver
D1, D2, D3, D4, D9, D10, D11, D12 > 1N4148
8 Lundahls Output
Add 1k parallel to the Outputs of the Lundahls between Pos and Neg
9 OPA's
NE5532AP seem to be the Best,
I tried OPA2134 and LM4562, Values were worser!
Didn't check MKP's as replacement for the 100u Caps following at the Input stage
10 Pots in General
I simply used ALPS RK27112
Input POT 10k log
Output POT 100k Log
Attack POT 20k Log
Release POT 1M Lin
Those ALPS got Differences up to 20%, If you want real StereoKompression, you will have to match 'em out!!!
If you want extended Release times, try to change R52 via Switch or Relay
11 Fet's or NPDXXXX
I used BF245A, matched 'em on my own,
http://www.masonaudio.org/diy/comp1176
Result:
At 0dBu Input and Output, I got -94dBu Noisefloor on both Channels
-3dB: 9Hz, 100kHz
Good Luck
GrG