Pre/Power Amplifiers

This is not ARC's first outing with KT150 tubes, nor the first to incorporate auto-biasing, but it is the first Reference series model to utilise both [see The Family Tree sidebar]. Its 140W rating is met into 8 and 4ohm with 165W being delivered at up to 1% THD via the 8ohm and 4ohm taps. This improves very slightly under dynamic conditions to 170W/8ohm and 175W/4ohm with 200W/2ohm (<2% THD) and 165W/1ohm (<3% THD) all in the REF160M's 'Ultralinear' configuration [see Graph 1]. In 'Triode' mode the output falls to 82W/8ohm and 85W/4ohm with a very marginal reduction in distortion at very high frequencies [see black vs. green trace, Graph 2]. The meter is decorative only – 1W/8ohm (8ohm tap) reads 0.015W, 10W reads 0.15W and 100W reads 1.5W on the 'Ultralinear' scale.

The 'shaped' frequency response with its mild +0.1dB bass shelf from 300Hz down to 10Hz allied to a gentle mid/presence roll-off of –0.2dB/5kHz to –0.7dB/20kHz (and –7.1dB/100kHz) is a mildly exaggerated version of that offered by the REF150SE and GS150 as is the magnification of the trend with reducing load impedance (–1.3dB/20kHz into 4ohm and –2.3dB/20kHz into 2ohm). The 0.6-1.5ohm output impedance (20Hz-20kHz) is also shared between all three models. Distortion increases gently with output level from 0.035%/1W to 0.11%/10W and 0.5%/100W (all 1kHz/8ohm). Versus frequency, the minimum is at 100Hz (0.06% at 10W), increasing to 0.19%/20kHz. Noise is very low, extremely so for a substantial tube amp and some 13dB better than the GS150 at –83dBV (unweighted). The A-wtd S/N ratio is correspondingly wide and up with the quietest solid-state amplifiers at 101dB (re. 0dBW). PM

Power output (<1% THD, 8/4ohm): 165W / 165W
Dynamic power (<1% THD, 8/4/2/1ohm): 170W / 175W / 200W / 165W
Output impedance (20Hz–20kHz): 0.69–1.51ohm
Freq. resp. (20Hz–20kHz/100kHz): +0.1dB to –0.7dB/–7.1dB
Input sensitivity (for 0dBW/140W): 179mV / 2105mV (balanced)
A-wtd S/N ratio (re. 0dBW/140W): 100.5dB / 122.0dB
Distortion (20Hz-20kHz, 10W/8ohm): 0.06–0.19%
Power consumption (Idle/Rated o/p): 220W / 380W
Dimensions (WHD) / Weight: 438x254x457mm / 25.5kg
Price: £28,998 (per pair)