Outboard DACs

Although the Formula xHD's R-2R DAC concept shares certain technical parallels with MSB's DACs [HFN Aug '19 & Feb '20], its lack of any digital filtering brings it closer to CAD's NOS DAC [HFN Mar '16] in 'real world' performance. Aqua's FET-based analogue stage supports a 3.5V transformer-coupled balanced output, the latter isolating the system from circulating RF interference while blocking DC, but it does cause the output impedance to rise from 29ohm at bass/mid frequencies to 69ohm/20kHz. The 113dB A-wtd S/N ratio is up with MSB's DACs but jitter [see Graph 2, below] is slightly worse than both the MSB and CAD implementations at 910psec (48kHz) and 260psec (96kHz). Low-level linearity is excellent, however, with errors of just ±0.4dB over a full 100dB dynamic range.

The drawback of a filterless DAC is the presence of digital 'images' directly outside the audioband – just 1.4dB down with 44.1/48kHz media. This causes moderate IM distortion within the audioband when treble levels are high (–65dB re. 20kHz/–10dBFs), but is less of an issue with 96kHz files and entirely absent at 192kHz+ sample rates. However, the most intriguing feature of the Formula xHD, and the one most likely to (positively) impact its subjective performance, is the remarkable consistency of its distortion vs. level over the top 50dB of its dynamic range [see Graph 1, below]. Distortion is certainly higher at very low and high frequencies at peak output (8% at 20Hz and 1.1% at 20kHz) but from 200Hz-6kHz it hovers almost unwaveringly at 0.04-0.05% over what amounts to the practical dynamic range of most recordings. The distortion is very extended (10th harmonic and beyond) so is more likely a function of the R-2R DAC modules than the analogue stage. PM