- Written by John E. Johnson, Jr.
- Published on 10 May 2012
The Design of the Rein Audio X-DAC
Although USB DACs may be on you mind because there are so many coming out on the market right now, you should also consider how the unit performs as a regular DAC using the S/PDIF inputs, and is more on this below.
The Rein X-DAC uses Wolfson CS8416 chipset for digital receiving and the WM8740 as the decoder. The analog output stage consists of AD797 op-amps, run in balanced configuration for the XLR output.
The front panel has an on/off knob on the left, an indicator LED when using the USB input, and the input selector dial on the right. It selects Coax 1, Coax 2, USB, and Optical.
The rear panel, shown below, has the analog outputs on the left (the RCA outputs use a different output op-amp), followed by the optical digital input, the two digital coax inputs, and the USB input.
There are three basic types of USB digital audio interfaces used with DACs. One is called "Synchronous", and this means that the USB DAC uses the computer's digital clock that times the bitstream. As you can imagine, the quality of the digital audio circuitry in a typical computer is not very high quality, so there is a lot of jitter in the bitstream, which results in significant amounts of harmonic distortion in the analog output. I say significant in the relative sense, in that there is more distortion than if the USB DAC utilizes an "Asynchronous" interface. The asynchrounous USB interface has its own clock, and the bitstream is fed into a buffer, reclocked at a much lower jitter rate, and then decoded, with the result (presumably) being lower distortion than it would have been with a synchronous interface. The less expesive USB DACs are synchronous, due to cost. The third type of interface is called "Adaptive", which is somewhere in between Synchronous and Asynchronous. It looks at the incoming data and adjusts the original timing rather than reclocking it altogether.
The Rein X-DAC utilizes the adaptive interface. That does not mean it is not as good a USB DAC as any DAC that has a synchronous USB interface, because it is the sum total of the entire circuit that gives you good or not-so-good sound, not any one individual circuit in the signal path.
Because the Rein X-DAC has coax and optical inputs as well as the USB, I would consider this product to be a DAC, period. It has, as a feature, an adaptive USB digital input, along with conventional digital inputs.