Pre/Power Amplifiers

The REF 330M may not be the most powerful amp developed by Audio Research (that’s the limited edition 750SEL), but it’s still the most powerful tube amp I’ve tested over a period of 40 years! Previously, ARC’s REF 160M delivered 110W and the GS150 170W [HFN Aug ’18 & Jan ’15] into 8 and 4ohm loads. However, the new 330W-rated monoblock not only buries these predecessors, it smashes ARC’s specification by delivering no less than 410W into both 8ohm and 4ohm via its 8/4ohm taps, respectively. Under dynamic conditions [see Graph 1] the REF 330M supports 500W into 8ohm (8ohm tap) and 490W, 580W and 350W into 4, 2 and 1ohm (all via the 4ohm tap). With an 18.7A current capacity, and more if the 1% THD limit is relaxed, the REF 330M will clearly drive all likely partnering loudspeakers with ease.

Distortion rises gently with power output, from 0.025%/1W to 0.11%/10W, 0.24%/100W and 0.64% at the rated 330W (all re. 1kHz/8ohm). Versus frequency there’s the inevitable and sharper increase in THD at low frequencies – 0.03%/1W and 0.12%/10W and 3.1%/50W (all re. 20Hz/8ohm) – as the output transformer cores reach saturation [see Graph 2, below]. Bass performance, and overall system response, is necessarily modified by the 330M’s ~0.7ohm output impedance but this value is still lower than achieved by many other tube amps. Into a non-reactive load, the 330M’s response has –3dB limits of 5Hz-105kHz/8ohm, falling slightly more quickly to –0.2dB, –0.5dB and –1.0dB/20kHz into 4, 2 and 1ohm loads (all re. 4ohm tap). Noise is well managed for such a large and powerful tube amp, the 86dB A-wtd S/N ratio (re. 0dBW) being a very respectable figure. PM