The BBB has an on-board I2S interface, which makes interfacing an external DAC easy. The I2S interface is connected from the AM3359 processor to the HDMI chip (TDA19988) but it is not required for HDMI to be used in order to make use of the I2S interface. The I2S interface is brought out to some pins on port P9 (see here for details). It was encouraging that Technoshaman and Donald Miller had some success with the interface, so I wanted to try it out and I got some time today.
Since the interface is capable of 24-bit/192kHz audio, it was worthwhile picking a nice DAC. A good but not over-the-top device seemed to be the Texas Instrument’s PCM5101 which is nice because it has reasonable specs and also provides a built-in charge pump so that it can function from a single supply. A headphone amp was connected, and this was ON Semi’s NCP2811A. The combination of PCM5101 (actually my prototype used PCM5101A which can also function from 1.8V, but this was not needed) and NCP2811A was chosen so that the entire circuit could operate from the BBB supply if desired, even if it was from a Li-Ion battery that was powering the BBB, and for low noise and distortion. The only problem is that the PCM5101 has quite a high output (clearly a good thing!) but the audio amp minimum recommended gain is 1 and it cannot meet the required output voltage, and it means that it will distort at high volume. So, at a lower volume (and lower SNR) it will be fine, and I prefer this particular amp because of the very low distortion. The ON semi datasheet was sparse though.