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The filter input is B.A.D. (Broken As Designed)
It has DC coupled inputs for the VCO outputs . And 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 build built in, the best setting depends on the patch...
So: have
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´ll go to 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. )
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