Outboard DACs

High-end outboard converters featuring bespoke DAC architectures – the dCS Vivaldi One [HFN Feb '18] and MSB Premier [HFN Aug '19] being good examples – will typically exhibit a distinct technical behaviour, all of which can inform how the product will sound. This is not entirely true of the REX 3 because, although the adaptive nature of the digital filter can be examined in the lab, other 'features' of Andreas Koch's AKDAC-3 DAC module are masked by BAT's 6C19 tube-based output stage. For example, while the DAC's low-level linearity is good to within ±0.2dB over a 100dB dynamic range, the overall A-wtd S/N ratio is reduced to just 95dB because of a prominent 100Hz PSU noise component. PSU noise does influence the REX 3's jitter spectrum but the overall figures of 280psec (48kHz/24-bit data) and 155psec (96kHz data) are still very low [see Graph 2, below]. Furthermore, the sharpness of the main test signal suggests very low phase noise on the part of the AKDAC-3 design – always a good indicator of stereo focus.

The moderate 530ohm output impedance, decreasing to 315ohm/20Hz, is a function of the transformer-coupled balanced output, as is the loss in sub-100Hz bass which falls to –0.7dB/50Hz, –2.9dB/20Hz and –6dB/12Hz. With non-transient signals the response reaches out to –1.1dB/20kHz, –16.5dB/45kHz and –23.7dB/90kHz [48kHz, 96kHz and 192kHz media]. In all likelihood, the tube output pushes up (2nd harmonic) distortion uniformly higher than might be incurred by the AKDAC-3, but the unusual pattern of distortion vs. digital level is indicative of the DAC's digital processing and final bitstream mapping. At its lower-than-typical 1.55V maximum output, the tube stage incurs some 0.003-0.006% THD. PM