Pre/Power Amplifiers

In common with Pass Labs’ previous-generation XA60.5 amplifier [HFN Apr ’10], this latest ‘entry-level’ stereo power amp vastly exceeds its rated output power, the 30W specification bested to the tune of 2x94W/8ohm and 2x156W/4ohm. Moreover, there’s headroom to accommodate 105W, 205W, 357W and 470W (21.7A) into 8, 4, 2 and 1ohm loads, respectively, up to 1% THD under dynamic conditions [see Graph 1, below]. In practice the 30W specification better reflects the envelope of its claimed Class A operation – and with an idle power draw of 265W and ambient heatsink temperature of 45oC this is not unrealistic.

Nevertheless, also in common with earlier Pass Labs amps and also visible from Graph 1, the XA30.8’s distortion trend increases gently with power output from 0.01%/1W to 0.038%/ 10W and up to 0.078% at the rated 30W, subsequently lifting to 0.09%/45W above which there’s a slight ‘knee’ in THD to 0.12%/50W and 0.8%/80W (all re. 1kHz/8ohm). That kink at 45W probably represents the true Class A to A/B transition. Versus frequency this same gentle increase in THD with increasing output is maintained [see Graph 2, below]. From 0.0029-0.009% at 100mW/8ohm to 0.058-1.26% at 50W/8ohm (all 20Hz-20kHz) this colour, favoured by many audiophiles, is seemingly a hallmark of Nelson Pass’s proven complementary, current feedback power amp topology.

The A-wtd S/N is slightly above average at 88.5dB (re. 0dBW) but output impedance is not especially low at 0.185-0.21ohm (20Hz-20kHz) so there may be some modification of response depending on the attached speaker load. Into a non-inductive 8ohm load the XA30.8 has –1dB points at <1Hz-76kHz. PM