Smart Devices 2X150VT power amplifier:
Measurements part 2
Fig.6 confirms the almost pure second-harmonic nature of the 2X150VT's distortion signature—at -63dB (0.07%), this won't be heard by anyone—and intermodulation distortion was also low in level (fig.7). The graph in fig.7 was taken into 8 ohms; halving the load and keeping the output voltage the same didn't result in the 1kHz difference component rising above -66dB (0.05%), but some 60Hz sidebands did appear around the intermod products, suggesting that at the measured power level with this demanding signal (close to clipping at 163W into 4 ohms), the Smart amplifier was starting to break a sweat.

Fig.6 Smart 2X150VT, spectrum of 50Hz sinewave, DC-1kHz, at 200W into 4 ohms (linear frequency scale).

Fig.7 Smart 2X150VT, HF intermodulation spectrum, DC-24kHz, 19+20kHz at 84W into 8 ohms (linear frequency scale).

With continuous 1kHz tones and both channels driven, the 2X150VT comfortably exceeded its specified power (fig.8). It gave 191Wpc into 8 ohms (22.8dBW) at clipping (defined, as usual, as 1% THD+noise), and 275Wpc into 4 ohms (21.4dBW). Into 2 ohms, however, with one channel driven, one of the 8A fuses on the rear panel blew at 282W, which is why the trace in fig.8 stops there.

Fig.8 Smart 2X150VT, distortion (%) vs continuous output power into (from bottom to top at 100W): 8 ohms, 4 ohms, 2 ohms.

I used the Miller Audio Research Amplifier Profiler to assess the 2X150VT's output power with one channel driven, using a toneburst signal more akin to a musical signal (10 cycles of 1kHz followed by 400 cycles of silence). The results are shown in fig.9: the Smart raised 252W into 8 ohms at 1% THD (red trace), 444W into 4 ohms (green), and 652W into 2 ohms (blue). Only into 1 ohm did the output power fall back, to 430W (black trace).

Fig.9 Smart 2X150VT, distortion (%) vs 1kHz burst output power into 8 ohms (red trace), 4 ohms (green), 2 ohms (blue), and 1 ohm (black).

Despite its modest size, price, and appearance, the Smart Devices 2X150VT is quite a powerhouse. Its sonically benign distortion characteristic and generally unflappable measured behavior indicate solid, conservative engineering.—John Atkinson