First we had CD players, then separate DAC's came along allowing us to use a DAC of our choice. In the same way the USB DAC is now available as a combined unit with the USB converter and DAC in one box, or we can buy a USB converter, and combine that with a standalone DAC. There haven't been too many USB converters but that appears to be changing, more so at the higher end of the market.
John Kenny has built up a good reputation with his products, first for his Sabre DAC (now JKDAC), and then with the JKDAC32. He also produces a USB converter called the SPDIF Mk3 (that you can use with your existing DAC). Like his DAC's, the Mk3 relies on battery power, more precisely a pair of LiFeP04 very low noise and low ripple batteries. The batteries require recharging of course, and that is done using a 5 volt power supply, or via a USB lead, but not the USB lead that supplies the signal from the computer. Instead, the charger lead (supplied with the Mk3) connects to a different USB port, and has a plug on the other end that connects to the Mk3.
Now things start to get a bit more complicated! John Kenny supplies an RF attenuator with the SPDIF Mk3 which can be inserted at the end of the SPDIF cable between the converter and DAC. Fitting the attenuator produced an audibly different sound, lighter, more airy is how I would describe that difference. With the attenuator fitted, I still slightly preferred the Tabla which still appeared to have a marginally firmer bottom end. The Tabla is over twice the price of the Mk3 though so perhaps we should expect it to come out on top. And the difference in sound between them is nowhere near 200%, more like 5%!
There are so many variables involved with reviewing any piece of computer audio equipment but I have drawn my conclusions having tried as many of them as I can. In conclusion, if you are looking for the best sounding USB converter you will have to pay over double the price of the SPDIF Mk3. I would suggest that you would only hear the miniscule difference in an AB comparison so I have no hesitation in very strongly recommending this USB converter at its significantly lower price. Also consider that the Mk3 will convert files up to 32 bit/338 hz (with the correct driver installed) compared to the 24 bit/192 hz of the Tabla! Conclusion - a very impressive piece of gear for the money, and highly recommended!