Here is an interesting schematic diagram for USB DAC based on the famous PCM2706 chip by Texas Instruments.

The design incorporates opto isolators to reduce the high-frequency garbage and prevent it from entering the analog part of the DAC. Like most of the DIY minimalistic designs, this one uses passive low-pass analog stage at the output of the DAC chips. The current to voltage conversion is done passively (duh) thanks to R19 and R17. Those are subject to experimentation to obtain the optimum value. Too high the resistor value and you get increased THD. Too low means you get lower output level.

PCM2706 + 4 x TDA1543 – LINK