Project status: Prototyping
2020-06-04: Layout cleaned up. Prototype ordered from Aisler.
2020-06-02: Layout mostly done.
2020-05-28: Component selection done. Drawing schematics and layout.
2020-05-27: Redefined and redesigned. Earlier plans turned into an advanced unit to be developed at a later point.
2020-05-08: Having trouble with the code. Focusing on other projects while I figure it out.
In short: USB-input, 48/24 DAC in Nanostack form factor. MCU control.
My goal with this design was first and foremost to keep it simple. Thus I chose on a simple commercially available USB audio bridge IC. This choice limits the unit to maximum of 48 kHz sampling rate, but that's acceptable, as benefits from higher rates are marginal, especially in home listening contexts (studio is another matter, but more about that elsewhere).
Problems arose when it became clear that the bridge IC used 12 MHz clock for the I2S bus which carries the sound to the DAC chip. This meant most DAC chips would not work, since they prefer 12.288 MHz master clock rate (operating at 256 times the sampling rate, while the bridge IC operates at 250 times the 48 kHz rate). Thus I needed to find a DAC IC that would not be tied down by an unconventional clock speed. One promising candidate is PCM5121 (or 5122 for slightly better performance at a higher cost. For this project 5121 is good enough).
Because these ICs are quite flexible, they need to be programmed through I2C bus. To do this, I added a STM32 microcontroller, which allowed not only to control the audio ICs, but also external control of the system. I have not yet decided on the full set of features, but made connections with future extensibility in mind. This did increase complexity a little, making some of the signal routings challenging.