The project, called the 503HTA, packs a single 12AU7 or 6922 tube and a solid-state IRL510 Mosfet circuit into a hybrid amplifier that mounts to 40-pin Raspberry Pi boards, including the Pi 2 and Pi 3. It includes a 24-bit DAC, a gain select switch, and a 3.5mm headphone jack. Putting out just 500mw, it’s designed to power headphones from 32–300 ohms, or act as a preamp for powered stereo and speaker systems. Just be careful when doing that to keep the gain switch at the lowest level to not burn out your amp.
he design is derived from the popular Bravo V2 architecture with all the modifications recommended such as: IRL510 Output Mosfet; Regulated Heater Voltage to reduce cross-talk; Adjustable Plate Bias for Tube Rollers; Nichicon MUSE Output Capacitors; and Jumper-able output impedance of 1.5, 32 and 100 ohm and switchable gain selection to match your headphones exactly.
They launched a Kickstarter campaign successfully funded on June 6th, with a 500% result!
Source: Warm Tube Tone Is Just What the Raspberry Pi Has Always Been Missing