It’s amazing the amount of people willing to spend insane amounts of money on asynchronous USB audio – either direct to DAC or an asynchronous USB to S/PDIF adapter. Though to be fair, I can say from experience that asynchronous USB is absolutely necessary over standardised USB audio, unless you get extremely lucky with your hardware and driver stack. But there is no point in throwing money at the problem (async USB is invariably much more expensive) – there is a much cheaper solution.
At this point it is assumed that your laptop or PC has an HDMI output with audio support, and that your DAC has a S/PDIF coaxial input (if you have bought a DAC which only supports USB, you’ve made a terrible mistake).
What you need is a chinese-made HDMI switch, labelled ‘HDMI 4 Ports Switcher With Audio Outputs‘ (in big letters across the top of the device itself). This is available on ebay and countless import sites. Inside you will find a EP94A1 chip, which not only switches 4 HDMI inputs to 1 output, but provides a raw audio stream in all the usual compressed formats (actually more than I have ever seen reported by a device), as well as up to 192KHz 24-bit PCM. Unlike some similar boxes with inferior chips, it overrides the audio capabilities reported by the HDMI video device attached to the output port. It is also capable of downconverting 5.1 channel audio to 2 channel for the digital outputs, but I think you would be much better off doing this on the software end.
Sadly the optical output is of poor quality, and can barely output 24-bit at any sample rate – from subjective testing I found the highest quality available was 16-bit 88.2KHz. For this reason you want to use the coaxial output, which unless I am mistaken, is a perfect bitstream from the HDMI audio signal, and is capable of full 24-bit 192KHz PCM. The stereo 3.5mm output is powered by a CSC4344C DAC, which is common in basic S/PDIF to analogue conversions, for example the FiiO D3. Sadly it is far from audiophile quality, so you will want to steer clear of it.
The only major flaw in the device is that it is extremely susceptible to electrical interference – I am forced to keep my smartphone a minimum of 1 metre away to stop the audio cutting out entirely, replaced by the typical GSM noise.
If you are a mad bastard like me and feel the need to overclock every HDMI chip you see in order to drive high resolutions or 120Hz monitors, you will be pleased (or possibly disappointed) to know that the EP94A1 can pass through any obscure signal (I’m using 1920×1080 72+85Hz and 1440×1080 120Hz), as long as the pixel clock remains under 200MHz. Once you cross the 200MHz threshold, the audio cuts out for 5 seconds every 30-60 seconds, but the video signal remains intact. I have not tested if this holds true once you go significantly past 200Mhz due to monitor limitations.
Basically it’s an excellent little device (especially considering the cost of only £28, a tiny fraction of what async USB to S/PDIF adapters cost), perfect for bypassing the awful USB system in the FiiO E17 amp and tapping the true potential of the E17’s DAC chip, which, if I remember the specifications correctly, is connected directly to the S/PDIF inputs.