Outboard DACs

Co-developed by Electronic Design Director, Steve Sells, the custom brickwall IIR filter employed in the NDX 2 is executed on a SHARC DSP and combined with a gentle 6th-order analogue filter at the output. This digital filter type suffers no pre-ringing, but does exhibit extended post-ringing [see inset Graph] and, therefore, very little acausal distortion – just like a pure analogue filter. All incoming sample rates are increased to one of two elevated base rates – 768kHz (for 48k/96k/192kHz media) and 705.6kHz (for 44.1k/88.2k/176.4kHz media and DSD64 files). The Burr-Brown PCM1792A DAC, used here in current output mode with discrete I-to-V conversion, may handle 768kHz/24-bit data natively but the NDX 2’s response does not stretch out to 45kHz (with 96kHz media) or 90kHz (with 192kHz media). Instead, Naim’s custom IIR filter coefficients cut in earlier, delivering a 60th-order roll-off at 25kHz [see Lab Report]. PM