Tube Power Amp Reviews

The THD+noise percentage vs output power results at 1kHz are shown in fig.8, with one channel driven. The discrete clipping measurements are shown in Table 1. The results are shown for 3% THD+noise, since that is the distortion level at which VTL rates the ST-85's power. The output is just a little short of its rating with both channels driven into 4 ohms, but exceeds the spec with one channel driven.

Fig.8 VTL ST-85, distortion (%) vs continuous output power into (from bottom to top): 8 ohms, 4 ohms, 2 ohms.

	Table 1 VTL ST-85 Clipping (3% THD+noise at 1kHz)
	Both Channels Driven W (dBW)		One Channel Driven W (dBW)
Load/ohms	(L)	(R)	(L)
8	63.3 (18)	64 (18.1)	72 (18.6)
(line)	119V	119V	120V
4	78.6 (15.9)	77.5 (15.9)	95 (16.8)
(line)	119V	118V	119V
2	75.4 (12.8)
(line)	119V

JA repeated this test using a 20% duty-cycle 1kHz toneburst and the Miller Audio Research Amplifier Profiler. The results are shown in fig.9. The lowest distortion is exhibited into 8 ohms (black trace), though the highest power is developed into 4 ohms (red trace, 50.7W for 1% THD+N, 121.75W for 3% THD+N). The little VTL, however, is not happy driving loads lower than 4 ohms, with high levels of distortion apparent even at low powers in to 2 ohms and 1 ohm (blue and green traces, respectively).

Fig.9 VTL ST-85, distortion (%) vs burst output power into (from bottom to top): 8 ohms (black trace), 4 ohms (red), 2 ohms (blue) and 1 ohm (green).

While none of these results is exceptional, they are respectable for what is, in today's market, a moderately priced tube amplifier.—Thomas J. Norton