SID buffer mod
The mod involves adding two 74x245 buffer chips to isolate the SID from the C64 address and databuses. The main reason for the background noise is that a common ground is used for both analog and digital parts inside the chip. When the chip is not accessed, the high speed data signals leak from the digital side to the analog through the common ground and build up noise.
I've drawn the schematics and a board for this mod with the help off instructions provided by Grue / Beyond Force (thanks!). The board has an option of adding also a 7805 regulator to power up the digital side of SID. This lowers the background noise even more but not as drastically as the buffers do. If you don't want to use a regulator, you can just bridge pins 2 and 3 on J3. The board can also carry the filter caps for the SID; this was mainly added for testing different cap values. If used as such, mount a socket so that there's no need to solder the new caps in all the time. A socket can be made for instance by cutting a few pins off a regular round-holed DIP-socket. If not using this, just bridge the cap pins from U1 to U2 with a length of wire.
I recorded short clips using a Wizard of Wor -cartridge, here's the results: Default SID and Buffered SID. As the filename suggests, a R4 6581 SID was used.
from the Prophet64 Forum: