Outboard DACs

The South Korean Citech Company has swapped out AKM's AK4499EQ DAC from the older RS150 network streamer [HFN Jun '21] for the ubiquitous ESS ES9028PRO in this latest RS250A model, adding ESS's raft of user-selectable digital filters to the product's roster of features. However, these seven filters are only correctly applied if the 'maximum sample rate' is set to 'auto' and the 'resample rate' is disabled. Moreover the RS250A's S/PDIF inputs do not recognise rates above 48kHz, although the LAN and USB connections are good to the specified 768kHz/DSD512. In general the 'Slow' filters offer a gentler but earlier HF roll-off and reduced time domain distortion (ringing), traded against poorer stopband rejection (~14dB versus ~55dB).

Tested with the default 'Linear Phase Fast' filter, the RS250A offers a maximum 2.28V phase-inverted output and wide 110.4dB A-wtd S/N ratio from a moderate 142ohm impedance. Low-level linearity is good to ±0.1dB over a 100dB dynamic range while distortion rises to 0.002-0.004% at peak output (20Hz-20kHz) and falls to just 0.00045% over the top 20dB of its dynamic range [see Graph 1, below]. Responses vary from very flat with the 'Fast' filters (typically –0.6dB/45kHz and –1dB/90kHz) and slightly 'sweeter' with the 'Slow' types (–5dB/20kHz, –7dB/45kHz and –8dB/90kHz). Interestingly, while the RS250A's performance is generally uneventful, it does show a design-specific jitter [see Graph 2]. The RS150 suffered from an extensive, very low-rate ±6.25Hz jitter while the RS250A shows clusters of low-rate jitter signals that are modulated on top of ±500Hz sidebands [red spectrum]. These only amount to ~250psec and are largely eliminated by the 'Resampling' mode [black spectrum], leaving a hint of residual phase noise. PM