Outboard DACs

Our boxout includes a full exploration of the xDSD Gryphon's headphone amplifier while this lab report is devoted to its USB input/line output performance. In practice, the xDSD Gryphon clips at volume setting '103' (re. 0dBFs), the optimum maximum being '101' which yields a 2.28V output from a 144ohm source impedance with a usefully wide 106.7dB A-wtd S/N ratio. The underlying digital/line performance is very good, the DSD1793 DAC delivering a low 0.00045% distortion at –20dBFs/1kHz, increasing to just 0.0025%/20kHz at the same level [see Graph 1, below]. Jitter is as low as we've seen in any high-end DAC solution – less than 10psec at all sample rates via the USB-C input – while low-level resolution is good to within ±0.1dB over a full 100dB dynamic range. One oddity – in line output, 'XBass II' brings about a +4.3dB boost at 1.2kHz...

As ever, the selection of digital filters is a trade-off between stopband rejection, frequency response and time domain distortions. There are four rather than the three filters mentioned in iFi Audio's manual – the 'Std' filter [black traces, Graph 2] is a linear phase type offering a stopband rejection of 53dB and the flattest response here at –0.2dB/20kHz with CD/48kHz media and –0.7dB/90kHz with 192kHz media. 'GTO' is a minimum phase type [blue traces] dropping to –5.4dB/90kHz and offering a mere 12dB stopband rejection while 'Min' [red traces] is actually a very slow roll-off linear phase filter reaching –3.2dB/20kHz and –4.4dB/90kHz with minimal time domain distortion but a poor 7dB stopband rejection. The 'BP' option [green traces] offers no upsampling or digital filtering at all and rolls-away early to –3.2dB/20kHz with 48kHz media but is flatter to –3.9dB/90kHz with 192kHz files. PM