Outboard DACs

Like MSB [HFN Aug '19 & Feb '20], Aqua employs a precision-matched resistor ladder network to convert 'bits' into sequential steps of current. In an R-2R DAC the LSB (Least Significant Bit) is represented by the smallest current source, with each subsequent 'bit' twice the output of the last (a ratio of 1:65,536 over 16-bits). Burr-Brown's Sign Magnitude DACs were a popular commercial example of this technique, employing two R-2R ladder networks with resistors laser-trimmed to achieve a claimed 19-bit accuracy. Indeed, Aqua itself used Burr-Brown PCM1704 DACs in its previous La Voce S2 [HFN Aug '16] but, for its Formula xHD, has implemented its own modules, each with 68 chip resistors sequenced through a bank of four 8-bit shift registers [see inset picture].

Aqua's added 'twist' involves running these DACs at the incoming sampling rate, ie, with no up- or oversampling or digital filtering. With no time domain distortion, impulse tests [see inset Graph] appear entirely 'natural' with no artificial pre or post ringing (echo). Payback comes with aliasing distortions – particularly troublesome with lower (44.1/48kHz) sample rates where the frequency response also rolls away through the treble from –0.8dB/10kHz to –3.1dB/20kHz [black trace, inset Graph]. The best of both worlds is realised at higher sample rates where the response reaches out to –0.6dB/20kHz and –3.2dB/45kHz with 96kHz files [red trace] and –0.1dB/20kHz, –0.6dB/45kHz and –3.0dB/90kHz (192kHz media) and where aliasing distortions are also pushed well out of the audio band. PM