Pre/Power Amplifiers

The designer of this amplifier has gone to great lengths to develop a very 'clean' implementation of Infineon Technologies' Merus AUDAMP24 Class D solution. Levels of switching noise and other spuriae are extremely low, typically <–120dB below full output, while the A-wtd S/N ratio is some 15dB wider than achieved by the older, MOSFET-based IRAUDAMP solutions at 95.3dB (re. 0dBW). Overall gain, via the balanced inputs, is a sensibly low +23dB requiring ~2.8V for full output while efficiency (power out versus AC mains drawn) is ~90%. AGD rates the Gran Vivace variously as 350W/4ohm and 200W/8ohm while, on the lab bench, it achieved closer to 380W and 195W, respectively, at 1% THD. The near-doubling of output between 8 and 4ohm loads is illustrative of the 'stiff rail' Class D architecture but it also explains why there's little or no dynamic headroom. Figures of 205W, 400W and 335W under dynamic conditions into 8, 4 and 2ohm reflect this [see Graph 1, below] while the drop to 150W/1ohm is due to over-current protection.

Graph 1 also indicates a marked increase in THD into loads below 4ohm – typically >0.1% from 1-100W/2ohm versus 0.002-0.08% into 8ohm. Under continuous conditions, distortion increases from 0.004%/1W, 0.005%/10W and 0.09%/100W (all at 1kHz). Distortion also increases with frequency [see Graph 2, below] from 0.005%/1kHz to 0.07%/10kHz and 0.3%/20kHz (all at 10W/8ohm) in line with the response of the reactive output filter network. Into an open load the Gran Vivace's frequency response rises to +2.5dB/20kHz (peaking at +27dB/43kHz) and is tempered under load to +1.3dB/20kHz and –0.35dB/20kHz into 8 and 4ohm respectively. PM