The MixSID is a stereo SID board for the Commodore 64 that can be used with any combination of SID models on a wide range of C64 mainboards.
Features include an integrated stereo channel mixer, pragmatic stereo addressing scheme, flexible handling of io areas, onboard audio input, output and digiboost circuits, dedicated stereo output, full paddle compatibility and versatile digital control.
For more info check the website below ;)
Website: http://henning-bekel.de