This user port sound cart was designed by my friend and long time ally in pushing forward Commodores in digital media. Vanessa Ezekowitz, of Digital Audio Concepts. There was a time when I counted myself amoung the group members of Digital Audio Concepts. A quick web search of Greg/DAC will surface my name with the DAC label attached to it. In fact, I am apparently still listed as a group member according to the Commodore Scene Database. That was a lifetime ago. But I still feel a special connection to these fine folk, and to this particular board.
Of course, I own one of the official boards, distributed by Shareware Plus, it along with the 8-Bit Stereo Sampler were some of the first pieces of C64 kit that I purchased upon my return to the scene in 2016. Before that, I have a long history with early prototypes of the board. The one I always brought with me to demo my WiNGs digital media software projects was literally a prototyping breadboard with the original chip, the MAX505 from Maxim. This chip has since been replaced in the official product by the similar but less expensive TLC7226CN from Texas Instruments.
This little board plugs into the User Port, and hooks up a quad 8-Bit DAC (Digital to analog converter) and outputs stereo audio to a standard mini jack. When we were working with prototypes back in the early 2000s it was common wisdom that the SID chip was only capable of producing 4-bit mono digis. That always left quite a lot to be desired. I was pleasantly surprised when I showed up at World Of Commodore 2016 to see the Wonderland XIII demo playing, and blaring amazingly clear digital audio. In the intervening years, those clever demo coders have figured out techniques to coax 8-bit mono digis out of the SID. However, doing so with the DigiMax is much more straightforward, plus you get stereo output, and this board can be used in conjunction with the SID.
Software support is pretty good, and I've provided here links to download a number of programs that make use of it: