How does the internal Wolfson DAC compare to the standalone PCM1794-based VDA-2?
They sound very similar but completely different designs.
VDA-2 uses a PCM1794 differential current output DAC, fed to a discrete analog stage, powered by a higher voltage external supply.
The Transient design requires a DAC IC that can produce a 2V analog output from 5V supplied by the USB. There are a small number of IC's that can do this.
We built prototypes using the Wolfson, ESS, and TI parts, and found the Wolfson to be the most musical. It also had the benefit of a built-in digital volume control, which we felt was a big benefit for a portable device.
Being that these parts use a charge pump to generate the negative rail, and have the output stage built-in, they take only a handful of surrounding parts. We optimized the design by keeping the pcb layout compact and using very high grade parts that were chosen through many listening tests.
The other advantage to the Transient is that it is asynchronous, meaning the data is fed into the DAC IC by a precision clock within the DAC(< 1 psec master clock jitter). The VDA-2 accepts s/pdif signals (via coax or toslink) using the clock of the source, which is much more prone to jitter.