Dac19 series Dual WM8741 D/A converter providing an USB and SPDIF input (coax and optical). It has a fixed level, single ended, output with regular RCA connectors and Audio-GD's propriatory ACSS connection. The latter is current instead of voltage based.
The DAC is characterized by extensive power regulation, discrete analog stage and high quality components. The difference between the NFB-2 and NFB-3 are well described on the Audio-GD website. It boils down to lesser quality power supply and capacitors for the NFB-3.
Briefly after release the standard digital input IC was changed from WM8416 to WM8805. This newer Wolfson design provides much better jitter rejection. Still the DIR9001 IC, which some regards as still better than the WM8805, is available as an option.