Integrated Amplifiers

With so much functionality on offer, where to begin? I discuss the MM/MC phono stage on p47 so, here, let's cut to the heart of these Class D modules. The quartet are identical, one of each pair having the option of a HPF set between 600Hz-6kHz [orange and purple traces, respectively, Graph 2], ostensibly for driving the HF arm of a speaker crossover or supertweeter. Overall gain is a sensibly low +26.1dB, requiring ~2V for the rated 200W output, but this drops by 0.5dB when the HPF option is engaged. Also, because the inductive filter network lies outside of the PWM stage's feedback loop, there's a modification in HF response with speaker loading. Unloaded, the RA180 has a +22dB peak at 80kHz [grey trace] while, loaded, the response reaches ±0.1dB/20kHz into 8/4ohm [black/red] but –0.95dB/2ohm [blue] and –1.4dB/1ohm [green]. There's a further shift in response that varies with gain (volume position). This is most exaggerated at +17.6dB gain where a broad +1.35dB bass lift meets a –0.8dB presence/treble shelf [dashed trace, Graph 2].

Power output is comfortably over-specification at ~270W into 8/4ohm per channel. Under dynamic conditions this lifts to 255W and 483W into 8/4ohm before dropping back to 296W and 167W into 2/1ohm [Graph 1, below]. Maximum current is limited to 8.6A. Distortion is impressively low at 0.0008-0.0015% through bass and midrange at 10W/8ohm, increasing to 0.045%/20kHz. At 100W/8ohm, THD is 0.003% midband and 0.4%/20kHz. Unfortunately, idle tones contaminate the noise floor at ~8.5kHz and 17kHz without which the poor 67.5dB A-wtd S/N ratio (re. 0dBW) would be closer to today's 85-86dB average. PM