Investigation

As discussed, the RA280 has moved on from the four-channel RA180 [HFN Jul '22] not least by offering a far higher +37.6dB gain (vs. +26.1dB) while also benefitting from a 'cleaner' noise floor, somewhat freer – though not entirely free – from (presumably) downstream switching artefacts. The RA180's poor 67.5dB A-wtd S/N ratio is lifted to 75.1dB here which is an improvement but still 10dB behind the 'average' class AB amp. Again, because the inductive filter network lies outside of the PWM feedback loop, there's some modification in HF response with speaker loading. Unloaded, the RA280 has a +18dB peak at 48kHz (coinciding with its peak 20ohm output impedance) while, into 8ohm, the response is flat to within ±0.1dB from 20Hz-20kHz before rolling away to –6.5dB/100kHz. The response also varies with gain (vol. position), particularly at ~2 o'clock where it falls to –0.6dB/20kHz and –11.5dB/100kHz.

High power without heat – and even higher power than achieved by the RA180 – remains the key strength of this GaN FET-based amplifier. Rated at 2x250W it achieves closer to 325W/8ohm and a full 590W/4ohm with some 'safety' built in when addressing very low impedance loads. A true PWM amplifier has no headroom so the RA280 offers 295W, 555W, 340W and 180W into 8, 4, 2 and 1ohm loads, respectively, under dynamic conditions [see Graph 1] where distortion clearly increases into <4ohm loads. Into 8ohm distortion falls to a 0.0015% minimum at ~10W/1kHz but is fairly uniform with power through bass and midrange. At HF the output inductors contribute to an expected rise in continuous THD of 0.055%/20kHz/10W and 0.22%/20kHz/100W [see Graph 2, below]. PM