Integrated Amplifiers

Tested via its analogue line input, the M33's power output and distortion rather depend on the position of the volume control because the input ADC (sampling at 48kHz-192kHz/24-bit) can clip before the output. At a volume setting between 0dB and –6dB distortion is 0.0005% (very close to NAD's specification) but falls still further to 0.0003% between –6dB and –12dB on the numeric readout (all for the same 1kHz/1W/8ohm output). However, with the volume at –12dB the max. power output is just 2x77W/8ohm because the input clips first, while between –6dB and 0dB it is 2x220W/8ohm and 2x430W/4ohm. True to tradition, and despite being a Class D amp, NAD has engineered some useful headroom into the M33 which supports peaks of 261W, 515W and 815W into 8, 4 and 2ohm loads with 502W/1ohm limited by current protection (22.4A). Note how there is no abrupt clipping point but, instead, maximum power is 'feathered' as it approaches 1% THD [Graph 1]. This is the modern-day equivalent of NAD's 'Soft Clipping'.

The digital performance is equally impressive with vanishingly low 0.00004-0.0003% distortion from 20Hz-20kHz [see Graph 2] and jitter suppressed to <10psec across all sample rates. Frequency response(s) via the digital inputs necessarily scale with input sampling rate reaching ±0.02dB/20kHz, –0.35dB/45kHz and –1.3dB/90kHz with 48kHz, 96kHz and 192kHz media, respectively. These are exceptionally flat and extended responses even though the 83dB stopband rejection suggests the linear phase digital filtering is not overly aggressive. Moreover, low-level linearity is good to within ±0.1dB over a 100dB dynamic range while the A-wtd S/N is a very fine 110dB. PM