tomg vcf9a_sd

[jorgen.bergfors at idg.se]

>The problem is : I have found an offset at the lowpass output
>which is dependent on the CV input (filter frequency control) >Any ideas ?

You must have a trimmer to cancel out the DC offset in the OTAs. See my schematic
http://www.idg.se/personal/bergfors/bergfotron/vca/lm13600%20vca%202.htm
This is a VCA, but  you can use the same principle in an ota-based filter.
I have also made a 13600-based phaser with this type of offset trimmer.

/Jorgen