Network Audio Players/Servers

Initially tested via its line inputs, which appear to be sampled at 96kHz, the Axign/TI-based digital Class D amplifier solution matches the M1's 100W specification by delivering a maximum continuous 113W into both 8 and 4ohm loads, with 113W and 145W into 8 and 4ohm under dynamic conditions (1kHz/10msec at <1% THD). The M1 protects its output into lower impedances, offering just 82W and 35W into 2 and 1ohm loads, respectively [see Graph 1], suggesting the amp is best suited to sensitive, 8ohm nominal loudspeakers. That said, the Class D output filter is nestled within the amp's feedback network, delivering an impressively uniform 0.035-0.038ohm (20Hz-20kHz) output impedance and a response that's determined more by your choice of digital filter - the minimum phase Filter 2 being the 'flattest' - than the impedance trend of the attached loudspeaker. Using the slow roll-off linear phase Filter 1, the response reaches out to -0.35dB/20kHz and -6.3dB/45kHz with 48kHz and 96kHz media, respectively, via the digital inputs.

Marantz's USB/network inputs expand its resolution to 192kHz/DSD128 but the shaped requantisation noise that's part-and-parcel of this Axign Class D controller remains unchanged, rising steeply immediately above 20kHz and causing THD+N to rise abruptly above ~10kHz [see Graph 2]. For example, distortion holds to a very low 0.001-0.002% at 1kHz over the M1's full 100W rated power but this leaps to 0.85%/20kHz and 4%/30kHz. This swell of noise may feed the breakup modes of hard-dome tweeters and also cause issues with other audio test gear. Finally, discrete jitter at ±14.5Hz amounts to 1580psec but there is also a skirt of uncorrelated phase noise. PM

Above: Dynamic power output versus distortion into 8ohm (black trace), 4ohm (red), 2ohm (blue) and 1ohm (green) speaker loads. Maximum current is 6.4A

Above: Thd+Noise Versus 48Khz/24-Bit Digital Signal Level Over A 120Db Range (1Khz, Black; 20Khz, Blue)