Pre/Power Amplifiers

BAT's REX 500 is the only model in its range that's not a tube or tube-hybrid design [HFN Sep & Dec '21] but neither is it a cool-running exemplar of solid-state modernism. Instead this amplifier gets cookin', its side-mounted heatsinks showing hot spots up to 65oC after one hour at idle where both sides are pulling 235-260W from the wall. While richly biased power output stages – a novel balanced/bridged N-channel MOSFET stage in this instance – generally enjoy some subjective uplift, in combination with low levels of compensation (feedback) they can exhibit a degree of thermal drift. In this case the REX 500's harmonic distortion increases with increasing temperature, as illustrated in the inset Graph which shows a change from 0.095-0.12% between 20oC-60oC over 30mins (re. 1kHz/10W/8ohm over one hour, and note the linear Y scale here).

In our review of the far smaller, though equally toasty Musical Fidelity A1 amplifier [HFN Jan '24], I discussed the basic calculations for a conventional Class A push-pull output configuration. That little integrated drew ~80W from the AC mains for 2x10W/8ohm of Class A power which illustrates how practical implementations almost always fall well short of the theoretical 50% efficiency. By the same measure, the BAT REX 500 is operating in Class A to at least 50W/8ohm per channel even though the brand makes no boast, idle or otherwise, about this very generous envelope. With hindsight, and bearing in mind it's pulling half a kilowatt from the wall, perhaps 'greedy' might be a better adjective... But it offers a glorious sound, and it keeps the listening room warm on those chilly winter evenings! PM