Outboard DACs

Having designed his own high-end DSD-based DAC in 2008, before joining PS Audio in 2011, Ted Smith continues to define the brand's 'digital path'. So while the original PerfectWave DAC was launched nearly 15 years ago [HFN Aug '09], Ted Smith's oversampling/DSD DAC technology was only formally unveiled in the first DirectStream DAC [HFN Aug '14]. The latest version of his FPGA-based DAC will accept inputs up to 705.6kHz (14x 44.1kHz) and DSD256 before upsampling and truncating 24-bit LPCM data to 56.448MHz/1-bit (20x 'DSD64') followed by 'noise-shaping' the bitstream to 5.6MHz/1-bit (DSD128). By crunching all its inputs into a 'pure' bitstream, PS Audio is hoping to leverage the potential of purely monotonic conversion, realising improved low-level linearity and reduced distortion.

Otherwise, the DirectStream DAC's analogue stage includes high-speed op-amps and a simple low-pass, transformer-coupled filter operating at DSD256. Hence the DirectStream DAC MK2's output betrays the same sweep of ultrasonic requantisation noise we'd expect to see from most conventional DACs handling a DSD128 input. Here, of course, the output 'looks like' DSD128 regardless of whether it's fed an LPCM or DSD input [see inset Graph showing DSD128 output from a 20kHz/–30dBFs LPCM input]. PM