VTL MB 175 Signature monoblock power amplifier Measurements part 2

The distortion curves in figs.3 and 4 show a rising distortion at higher frequencies but a reasonable performance, particularly up to 10kHz. The triode distortion in fig.4 is actually slightly higher than that for tetrode operation, though the differences are not huge. The THD+noise waveform is shown in fig.5. The triode result (shown here) is marginally worse than the tetrode. As MF conjectured, it is heavily low-order, but it is mainly third- rather than second-order. There are also unmistakable higher harmonics present. The 2 ohm result (not shown) transitions to a dominant third harmonic.

Fig.3 VTL MB 175, tetrode mode, THD+noise vs frequency at (from top to bottom at 10kHz): 4W into 2 ohms; 2W into 4 ohms; 2.83V into simulated speaker load; and 1W into 8 ohms.

Fig.4 VTL MB 175, triode mode, THD+noise vs frequency at (from top to bottom at 10kHz): 4W into 2 ohms; 2W into 4 ohms; 2.83V into simulated speaker load; and 1W into 8 ohms.

Fig.5 VTL MB 175, tetrode mode, 1kHz waveform at 2W into 4 ohms (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).

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