GitHub - tebl/C64-Stereo-SID: There are many Stereo SID boards, but this one in mine... Add a second SID of the same type or newer substitutes for adding stereo audio to your system...
The SIDFX Sound Mod – breadbox64.com
SID2SID & Stereo SID Project – Vintage is The New Old
The SIDFX Sound Mod – breadbox64.com
Stereo SID with VICE C64 emulator | ezContents blog
Stereo to C64 with DualSID | ilesj's blog
Stereo to C64 with DualSID | ilesj's blog
SID2SID & Stereo SID Project – Vintage is The New Old
FPGASID - The better SID, KryoFlux Web Store
Mingos Commodorepage
Stereo SID - Internal Board for the Commodore 64 | CommodoreServer.com