VTL MB750 monoblock power amplifier Measurements

Sidebar 4: Measurements

A complete set of measurements was made on the VTL MB-750 in the unbalanced triode mode, with selected measurements repeated in the balanced triode and unbalanced tetrode configurations. If not noted otherwise, the result shown is for the unbalanced triode mode. Because of the amplifier's high rated power, it was warmed up at 50W in triode mode for one hour—the 1/3-power test is of limited usefulness for tube amplifiers, in any event. High-power tests were not run on our simulated real load.

The MB-750's input impedances measured 550k ohms and 440k ohms in triode unbalanced and balanced modes, respectively, and 420k ohms tetrode unbalanced. (Though the accuracy of our input impedance measurements is reduced at such high values, the important point here is that such high impedances mean there should be no matching problems with any realistic source component.) The output impedance measured between 1.3 and 1.4 ohms in triode mode, depending on frequency and load value, and between 1.7 and 1.82 ohms in tetrode mode. Voltage gain measured 29.1dB triode, unbalanced; 23.7dB triode, balanced; and 31.2dB tetrode, unbalanced. DC offset measured 19mV triode and 13.4mV tetrode, though these are approximate maximum values; the readings varied radically (not unusual in a tube product). The amplifier is noninverting in its unbalanced configuration, with pin 2 positive in balanced mode. S/N ratio measured 66.9dB from 22Hz to 22kHz, 66.2dB from 10Hz to 500kHz, and 78.5dB A-weighted.

The MB-750's small-signal frequency response is shown in fig.1. Note the wide variation with the simulated loudspeaker load caused by the amplifier's output impedance. While this will vary depending on the exact loudspeaker load, it should generally be somewhat (though not grossly) audible. The 10kHz squarewave for the triode mode is shown in fig.2 and is nearly perfect, with only the barest hint of overshoot. (The 1kHz squarewave, triode, is similarly excellent and is not shown.) The 10kHz squarewave shown in fig.3 for tetrode operation does indicate a clear overshoot on the leading edge, but this is damped immediately, with no ringing. This overshoot is also visible on the 1kHz squarewave, tetrode (not shown), as a single spike on the leading corner.

VTL MB-750, frequency response at (from top to bottom at 20kHz): 1W into 8 ohms (tetrode), 2W into 4 ohms (tetrode), 1 W into 8ohms (triode), 2W into 4 ohms (triode), and 2.83V into simulated speaker load (triode) (0.5dB/vertical div.).

VTL MB-750, triode mode, small-signal 10kHz squarewave into 8 ohms.

VTL MB-750, tetrode mode, small-signal 10kHz squarewave into 8 ohms.