* http://worldofspectrum.org/ - has a review about the Spectrum version - which conflicts with the DIY info (the Spectrum review implies it contains a mono 1-bit A/D converter, not a stereo connection without A/D converter)
* http://www.olivertwins.com/ - according to this webpageswebpage, the Oliver Twins created "The CD Games Pack" in December 1989, for Amstrad CPC, Spectrum, and Commodore 64, published by Codemasters
Whether or not the Amstrad CPC version was actually released is a bit unclear. Did anybody ever see it for real?