Outboard DACs

While the La Scala MkIII’s novel R-2R DAC concept is an evolution of that seen in Aqua’s Formula xHD [HFN Apr ’20], and while both models feature a transformer-coupled balanced XLR output, it’s the MkIII’s triode tube buffer that adds its own ‘presence’ to the DAC’s analogue distortion and noise. The 3.7V output is similar to the Formula xHD’s 3.5V but the 114ohm (20Hz-1kHz) output impedance is higher and increases still further to 549ohm/20kHz. The latter’s 113dB A-wtd S/N ratio drops down to 100dB in the tube-based model too, while peak level distortion is moderate at ~0.25% through mid and treble but increases to 0.95% at 20Hz. However, THD is now dominated by a 2nd harmonic, rather than higher harmonics, and remains very much more steady with frequency above 300Hz [see Graph 1]. THD is lowest at 0.011-0.015% (re. 20Hz-20kHz) over the top 30dB of its dynamic range while low-level linearity, necessarily influenced by noise, is good to ±0.7dB over a 100dB range.

While side-stepping time-domain distortions, the drawback of a filterless DAC [see boxout], is the presence of digital ‘images’ directly outside the audioband – just –1.6dB/26kHz with 48kHz media. This causes moderate IMD within the audioband when treble levels are high (–75dB re. 20kHz/–10dBFs), but is almost entirely absent with 96kHz, and higher, sample rates. While the responses roll away earlier than anticipated with high sample rate files (–3dB/30kHz) – no bad thing – it’s the digital jitter that, arguably, reveals the ‘uniqueness’ of Aqua’s R-2R DAC’s operation. While the jitter pattern looks complex [see Graph 2], and moderate at <1000psec depending on sample rate, this is resolved into data- and PSU-induced sidebands leaving the main signal free of any very low-rate/phase-related broadening. PM