Back in the days when tomb and I were working on bringing the grubDAC to production, we were looking at the kitting process and lamenting the fact that the PCM2706/7 were in such short supply and that the PCM2704/5 had huge quantities in stock. We discussed the fact that it would be convenient to have a DAC built on one of those chips. Spring and Summer came and being outside mountain biking seemed like a much better idea than sitting inside designing a DAC.
However, it has been winter. I'm not as young as I used to be and riding in the snow is not quite my thing. So I designed the SkeletonDAC:
The idea was to build a "bare-bones" (i.e skeleton) DAC based upon the PCM2704/5. No external regulator and low part count. However it did morph a little bit due the fact that after laid the PCB out, I kept looking at the fact that there was this expanse of board with nothing on it. With a little bit of part shoving and some choices of parts, I managed to incorporate the SPDIF output of the PCM2704 onto the board, complete with output transformer. As well, given that the PCM2704 is spec'ed at being able to drive headphones, there is the option to put larger output caps - basically anything with 2.5mm or 5mm lead spacing. 2.5mm caps positions are offset so that it is possible to use larger size electrolytics and drive headphones right off the dac.
The board dimensions are exactly the same size as the grubDAC. The USB input and audio output are in exactly the same place. The same Hammond case can be used (even with SPDIF), so it should be a great versatile little dac. From a build perspective, the only challenge to this should be that most of the surface mount parts are 0805. However, if you can get the PCM2704 down reasonable clean, you should have a very high chance of success building the DAC.
Board layers top and bottom:
The BOM is to come. I need to clean it up to show the three versions that can be built - full, audio out only, SPDIF out only.
Since BatchPCB's shipping to Canada is really sucky, the boards took a detour through Beezar as it is a lot cheaper for me to have tomb ship them to me than to have BatchPCB ship them to me. As a result tomb has built two already and has already provided me with some really useful feedback on what to do with future versions of the board. He has reported to me quite reasonable results driving headphones direct, but I'll let him report on that.
At the moment, I'm looking into the cost of running a bit larger prototype run and was curious if there was anyone out interested in building some of these.