Outboard DACs

The Pro iDSD Signature's 'Pro' output setting doubles the 'Hi-Fi' output from 4.5V to 9V but there's no significant improvement in the 112dB A-wtd S/N ratio. There's also a small 'bug' detected when switching to the 'Tube' or 'Tube+' output modes then back to 'solid-state'. The Tube modes fire up an internal orange LED and the GE5670 tubes, but this stays illuminated if the Pro iDSD Sig is switched back to 'solid-state' mode where THD sticks at a higher 0.003-0.037%. Only by de-powering the unit and re-booting in 'solid-state' mode does it achieve the low 0.001-0.002% distortion (falling to 0.0002% over the top 30dB of its range) recorded here [see Graph 1, below]. Live, A/B comparisons between 'solid-state' and 'tube' are meaningless as a result.

IFi Audio has deleted the minimum phase filter offered on the original DAC [HFN Sep '18] although the 'Apodising' and 'GTO' filters behave like fast and slow roll-off minimum phase types offering 65dB and 12dB stopband rejection, respectively, with responses that reach –0.1dB and –1.2dB/20kHz, –0.5dB and –2.5dB/45kHz, out to –3.5dB and –5.9dB/90kHz with 48kHz, 96kHz and 192kHz files. The 'Transient Aligned' filter with its 58dB stopband rejection and flattest response (dipping only to –3.4dB/90kHz) is a linear phase type while the 'Bit Perfect' filters offer almost no time domain ripple/distortion but suffer maximum alias distortion and the most rolled-off HF response (–3.1dB/20kHz with 48kHz files, peaking at +0.3dB/11kHz to –0.6dB/20kHz in 'Bit Perfect +' mode). Selecting the 'DSD Remaster' mode introduces extra UHF noise but the reclocking/data formatting has a positive impact on jitter, clearly reducing the system phase noise [see Graph 2, below]. PM