Ok, so this here is a circuit that can be used to lower the background noise generated by a SID chip quite dramatically. It works for both SID models. The basic idea is to separate the SID from the data/address buses of the C64 when the chip is not accessed. As the digital and analog sides use the same ground, the more rapidly changing digital signal bleeds to the analog side through the common ground (hence the background noise).
(Click the image to download the pdf)
Parallel to building my TB 3k3 I had been, to some extent, getting acquainted with various sequencer designs. After taking my time searching and evaluating these, I ended up with a slightly modified Superseque schematic. This design, by Thomas Henry, is about as simple as it gets whilst still having a formidable set of features like sequence playing direction. I modified the schematic to include a second 555 timer chip as the original design relied on external sync alone. The sync source is simply selected with a switch. I also opted to use SMD parts where ever possible as this simplifies the board manufacturing procedure a lot, I get my share of drilling through-holes with the Vocoder project anyway..