This build came out of a double failure: the maker's existing DAC began oscillating, giving intermittent noise and at times distortion, while the Electrocompaniet ECI-3 doing preamp duty also developed an intermittent fault around the same time. With lockdown freeing up hours that used to go on weekly drives to Germany, garbz01 set out to replace both boxes with one unit, and to tidy the living room by folding the preamp into the DAC and dropping the separate phono stage.
The result is the Pandemic DAC, a combined streamer, DAC and preamp. Playback runs from a Raspberry Pi 3B on Volumio, with a Digi+ HAT as the clock source, currently over S/PDIF and with I2S still listed as the end goal. A hacked off-the-shelf Bluetooth S/PDIF module adds wireless streaming. For the analogue side, DS1882 digital potentiometers feed OPA1632 fully balanced amplifiers, so the outputs stay balanced. Relays switch every input and output independently in any combination, and digital lines to and from the DAC core are isolated through transformers or galvanic bus isolators.
The construction story is just as good. It started as a hand-drawn block diagram, moved through a home-etched prototype made with laser-printed masks, UV exposure, chemical etching and drilling, then onto commercial double-sided green PCBs with Mouser parts, hand-soldered and stacked on headers. Schematics and layouts were drawn in Altium Designer, firmware written in Visual Studio Code with PlatformIO, and internal brackets printed on an Original Prusa i3. Two motherboards keep digital and analogue ground planes apart with separate power supplies, backed by a dual tracking design with 10V and inverted -10V rails. The faceplate was machined on a drill press, and the temporary aluminium-and-acrylic panel looked good enough to stay permanently.
One open point: the core is named as the AMB Gamma 3 in one place and as a Gamma 24 dual WM8742 balanced differential DAC in another, so the exact version is unclear. Also, keeping the DS1882 in sync with the WM8742 digital volume was apparently not trivial, and the plan to be entirely DC coupled ran into overlooked input characteristics.
Project by garbz01. Watch it on their YouTube channel: