Dac with only usb input

That's tough, but in principle like this:

-Try to find service documentation somewhere, or see what ICs are inside
-Figure out whether the USB interface communicates with the DAC section via I2S or some other standard serial interface and whether the DAC works in slave mode
-If not, forget about it
-If so, disconnect the DAC part's I2S bus from the USB interface part and connect it to an S/PDIF receiver instead, and provide a 5 V supply if the thing is also powered from the USB
 
Interesting... The CS4398 can handle PCM in I2S format, see https://statics.cirrus.com/pubs/proDatasheet/CS4398_F2.pdf , but then you don't use the software PCM to DSD conversion that comes with the DS-DAC-100. Besides, you would have to force the M1...M4 pins to the correct values as described in section 4.6 of the datasheet, or connect your own microcontroller to the I2C or SPI bus and program it as described in section 5.

In any case, the special features of this DAC are its support for raw DSD and the Korg PCM-to-DSD conversion software that comes with the DAC, and with this change you would use neither of those.
 
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