Outboard DACs

Designed by Robert Watts, the Mojo 2's 16x upsampling, 40,960-tap interpolation filter runs with an eight-element version of his proprietary Pulse Array DAC. There is only one principal digital filter on offer here (unlike the five or six fast/slow roll-off, linear/minimum phase types sent to baffle many enthusiasts and add to my workload on the lab bench – see p66). Thus there is only one 'response' for the Mojo 2, reaching out to within ±0.1dB from 20Hz-20kHz with CD/48kHz media and rolling away to –3dB/35kHz with any input at or above a 96kHz sample rate. Naim Audio offers a similar response shaping via its streaming platform [HFN Aug '21] although its combined brickwall IIR filter and two series-connected 3rd-order analogue filters is very different to Chord's WTA technology. For commercial reasons, the time domain behaviour of Watts' filter remains a closely-guarded secret and any 'probing' with a standard sinx/x impulse reveals a suspiciously idealised pulse [HFN Sep '18, for examples]!

While the exact WTA coefficients are left to musing and mystery there's no concealing the exceptionally 'clean' outcome. Maximum output, prior to clipping, is 4.7V with distortion a very low 0.00013-0.00075% at 0dBFs [note the very linear increase in THD at 1kHz with reducing digital level, Graph 1]. The A-wtd S/N is a wide 114dB, channel balance good to 0.01dB and separation >80dB from 20Hz-20kHz. Correlated jitter is also fabulously low at ~10-12psec (all sample rates) while phase noise is almost entirely absent [when used with Melco's N10/2]. The effect of the four 'tone bands' falls within the solid traces on Graph 2 which show the maximum ±9dB contouring at each frequency (20Hz, 125Hz, 3kHz and 20kHz). The dashed traces show the effect at the ±4dB setting. PM