Source: http://www.muffwiggler.com/forum/viewtopic.php?t=98954&postdays=0&postorder=desc&start=100
The filter input is B.A.D. (Broken As Designed)
It has DC coupled inputs for the VCO outputs and the pulse, square and saw waves have a 5V DC offset.
There are no coupling caps inside the filter.
So it is *designed* to give a massive *THUMP* when the VCA opens.
If you trim the Filter offsets for minimal thump with these offsets still built in, the best setting depends on the patch...
Have a look a the PCB, the TTSH has one single mod pre-thought: adding caps to the filter input. Thanks Jon 
Values can be in range of 470nF to 1uF bilpolar - I use polyester
(Speaking of "making the thing usable": this is one of the changes I would consider mandatory if you want to use an ARP2600 for more than fx sounds. )
