Some USB ready to use cards for high Q usb interface to SPDIF dac
400$ exaU2I Asynchronous USB to I2S Interface www.exadevices.com > Home
300$ Edel USB to I2S/SPDIF. http://www.maison-passive-nice.fr/pdf/1102-USB-price-list.pdf]
USB Audio interface | ABC PCB
The sites provide the drivers for these cards free.
400$ exaU2I Asynchronous USB to I2S Interface www.exadevices.com > Home
300$ Edel USB to I2S/SPDIF. http://www.maison-passive-nice.fr/pdf/1102-USB-price-list.pdf]
USB Audio interface | ABC PCB
The sites provide the drivers for these cards free.