Balanced Audio Technology VK-150SE monoblock power amplifier:
Measurements part 2
This behavior is shown somewhat differently in fig.7, the spectrum of a 50Hz tone driven at a fairly low level into 8 ohms from the 4 ohm tap. Both second and third harmonics lie at -64dB with respect to the fundamental (0.06%). As implied by figs.4 and 5, the distortion rose quite rapidly at higher powers. The second harmonic reached 1% at 40W into 8 ohms, for example. Repeating the spectral analysis for a fairly high-level 1kHz tone (fig.8) reveals that the second- and third-harmonic products rise to -50dB (0.3%), and are now joined by higher harmonics.

Fig.7 Balanced Audio Technology VK-150SE, 4 ohm tap, spectrum of 50Hz sinewave, DC-1kHz, at 7W into 8 ohms (linear frequency scale).

Fig.8 Balanced Audio Technology VK-150SE, 4 ohm tap, spectrum of 1kHz sinewave, DC-1kHz, at 40W into 8 ohms (linear frequency scale).

Similarly, using the BAT to drive an equal mix of 19 and 20kHz tones into 4 ohms from the 4 ohm tap to just below visible clipping on an oscilloscope screen (fig.9) indicates that the VK-150SE is not happy with this admittedly demanding test. It produces both a moderately high level of intermodulation products and a higher noise floor around the fundamental tones.}

Fig.9 Balanced Audio Technology VK-150SE, 4 ohm tap, HF intermodulation spectrum, DC-24kHz, 19+20kHz at 57W into 4 ohms (linear frequency scale).

Figs.10 and 11 plot the percentage of THD+noise against continuous output power from the 8 and 4 ohm taps, respectively, into 16, 8, 4, and 2 ohms. It can be seen that the amplifier meets its specified power delivery at 3% THD into a matched load. Below that clipping point, however, the distortion rises with output power in a fairly consistent manner. It can also be seen that the distortion rises more quickly with level and the maximum power is reduced, the more extreme the mismatch between output transformer and load.—John Atkinson

Fig.10 Balanced Audio Technology VK-150SE, 8 ohm tap, distortion (%) vs continuous output power into (from bottom to top at 1W): 16 ohms, 8 ohms, 4 ohms, 2 ohms.

Fig.11 Balanced Audio Technology VK-150SE, 4 ohm tap, distortion (%) vs continuous output power into (from bottom to top at 1W): 16 ohms, 8 ohms, 4 ohms, 2 ohms.