This device is fully functional sound card for PC. The main advantage of using PCM2704 against PCM2702 is much easier construction. As you can see on the block diagram it has built-in 5V and 3.3 voltage regulator, HID interface (MUTE, VOL+, VOL-), S/PDIF output. The circuit can be powered directly from USB port. Next advantage is that the output DAC is able to drive directly 32ohms headphones, but the output power is only 12mW. For all details please refer to the PCM2704 datasheet.
The schematic is very simple. It is almost copy of the datasheet circuit diagram. You can see the core IC PCM2704 (U1), crystal with supporting parts (X1, C1, C2, R1), connection to the USB (USB connector, R2, R3,R4, L1), a lot of blocking capacitors for all voltages (C3, C4, C5, C6, C7, C8), S/PDIF output header (J1), HID header (J2), output filter (R5, C11, R6, C12, R7, R8), coupling capacitors (C9, C10) and output 3.5mm Jack connector (J3).
I designed my own PCB. The dimensions of the PCB are 55x18mm, including USB and Jack connector it is 73x18mm. To keep the size as small as possible 0805 size of capacitors and resistors were used. The L1 is ferrite bead which reduce high frequency hum. With good soldering iron and litle experience it is possible to assembly the PCB by hand.
I built two pieces of this sound card. I have tested the sound card under Windows XP and Windows Vista and it works without any problem. I wanted to use this device for my Internet radio receiver, which I'm building according to the project published on mightyOhm.com. The receiver is based on Asus WiFi router WL-520GU with OpenWrt Linux distribution. The sound card works under the Linux as well, but it stop and start playing the sound periodically (the period varied from few minutes to one hour). I guess it is driver problem (if somebody has any idea how to fix it I will be very grateful).
You can ask me why to build this sound card when you can buy similar device for few bucks. I have also one sound card from China which cost around 5$ but the quality of the sound is very bad, so the main difference is sound quality.