I found this a while ago on Gearslutz:
http://www.gearslutz.com/board/1216798-post6.htmlRun that AP with a frequency/phase plot. See that phase lead shift in the low end? Not too pretty, is it? That's a result of simple 2pi /rc/sq of selecting too small of a coupling cap. A 10 uf in front of the VCA is one sore spot with me. I would increase that a decade and call it a day. Remember, each coupling cap stage is a 1 pole high pass filter. A few stages, no problems, run hundreds and you will find a 10 or 12 pole filter with rather poor phase response and some non linear phase shift from the fact that all the roll-offs are not tuned to each other but are set somewhat randomly.
Fix them and then run that phase plot and see 1~2 degrees at 20 hz. Don't forget the polyprop film bypasses across them. A nicely fixed up 4k will shake your world if that low end is fixed.
Warning! You will find the high pass filter in regular use afterwards. A couple of nice poly film caps there will allow it to become your friend.
Jim Williams
Audio Upgrades
Full thread with more details:
http://www.gearslutz.com/board/high-end/117824-ssl-4000-e-g-modifications.html
