Pre/Power Amplifiers

The V-400’s rich biasing and compact casework results in an increase in temperature from RT to 50°C over the first hour – distortion adapts, in this case falling from 0.008% to 0.005% (all re. 1kHz at 10W/8ohm). Furthermore, power consumption also falls from 100W to 75W at idle over the first one hour, but then increases to 93W at 10W/8ohm output – behaviour that’s not typical for a conventional Class A amp... That said, in common with other Class A designs [see p68] distortion falls from its 8ohm value when driving a tougher 4ohm load (from 0.016% down to 0.004% at, for example, 1kHz/50W). This is illustrated in the dynamic profile plot [red vs. black traces, Graph 1] that also shows the V-400 MAX’s 155W, 281W, 460W and 542W (23.3A) output into 8, 4, 2 and 1ohm loads. This is substantial for such a physically compact Class A-AB amplifier but note that the very ‘stiff’ switchmode PSU regulation means there’s almost no difference in continuous or dynamic power output (the former comfortably higher than Burson’s 100W/8ohm specification).

Burson offers two gain settings and the higher +24.4dB mode will be more useful for most users (a 1.74V balanced input is required for the rated 100W output). There is no change in distortion or power output, but the A-wtd S/N ratio is 100.0dB in low gain mode and 96dB in the high gain setting (all re. 0dBW and both well above average). Looking at THD in more detail, this increases with output level from 0.0016%/1W to 0.005%/10W and 0.04% at the rated 100W (all re. 1kHz/8ohm) and, again unusually, increases markedly at very low, sub-20Hz freqs [see Graph 2]. Across the 20Hz-20kHz audio range, distortion is usefully ‘flat’ at 0.004-0.01% [re. 10W, pink trace, Graph 2]. PM