VTL MB-450 Series II Signature monoblock power amplifier Measurements
Although it has only one output transformer tap, VTL's MB-450 Series II Signature can be used with its output stage configured in either its Tetrode or Triode modes. I performed a full set of tests in each mode, using both balanced and unbalanced input connections, but discuss only a representative selection here.
The voltage gain into 8 ohms was the same for both balanced and unbalanced inputs, but varied with the mode. Tetrode mode gave 26.4dB gain, Triode 23.7dB. All combinations of mode and input preserved absolute polarity; ie, were non-inverting. The MB-450's balanced XLR jacks are wired with pin 2 hot, the AES standard. The balanced input impedance was lower than specified, at 78k ohms, and half that figure for unbalanced sources.
The output impedance was a fairly high 2.1 ohms across the audioband in Tetrode mode, a lower 1.55 ohms in Triode mode. As a result, there will be a moderately high modification of the amplifier's frequency response by the manner in which the partnering loudspeaker's impedance changes with frequency. The magenta trace in fig.1, for example, shows the amplifier's frequency response driving our standard simulated loudspeaker. It fluctuates by an audible ±1.2dB. Into an 8 ohm resistive load (red trace), the MB-450's small-signal response is commendably flat from 10Hz to 30kHz, and doesn't reach –3dB until 100kHz. VTL's Signature output transformer is obviously a high performer, and the wide bandwidth correlates with the excellent high-frequency squarewave reproduction (figs.2 and 3). Though there is a slight trace of overshoot in Tetrode mode (fig.2), this is absent in Triode (fig.3). The flat tops and bottoms of the MB-450's 1kHz squarewave response (fig.4) indicate its extended low-frequency performance.
Fig.1 VTL MB-450 Series II Signature, Tetrode mode, frequency response at 2.83V into: simulated loudspeaker load (magenta), 8 (red), 4 (blue), 2 (green) ohms (0.5dB/vertical div.).
Fig.2 VTL MB-450 Series II Signature, Tetrode mode, small-signal 10kHz squarewave into 8 ohms.
Fig.3 VTL MB-450 Series II Signature, Triode mode, small-signal 10kHz squarewave into 8 ohms.
Fig.4 VTL MB-450 Series II Signature, Triode mode, small-signal 1kHz squarewave into 8 ohms.
Noise levels were relatively low, with the A-weighted signal/noise ratio in Tetrode mode measuring 79.5dB ref. 1W into 8 ohms. The MB-450's parsimonious use of loop negative feedback means that its THD+noise percentage rises with increasing power. Figs.5 and 6 plot the THD+N figure against output power into loads ranging from 2 to 16 ohms for the Tetrode and Triode modes, respectively. The traces in these graphs cross the 1% THD line before the level where there is a true discontinuity. The MB-450 does meet or almost meet its specified output powers into 8 and 4 ohms at a slightly relaxed 3% THD point, delivering into 8 ohms 350W (Tetrode, 25.4dBW) or 195W (Triode, 22.8dBW); and, into 4 ohms, 510W (Tetrode, 24.1dBW) or 225W (Triode, 20.5dBW).
Fig.5 VTL MB-450 Series II Signature, Tetrode mode, distortion (%) vs 1kHz continuous output power into (from bottom to top at 10W): 4, 8, 2, 16 ohms.
Fig.6 VTL MB-450 Series II Signature, Triode mode, distortion (%) vs 1kHz continuous output power into (from bottom to top at 100W): 8, 16, 4, 2 ohms.
At small-signal levels, the MB-450 is definitely a low-distortion design. Figs.7 and 8 plot the THD+N percentage against frequency at a level of 2.83V in Tetrode and Triode modes, respectively. While a rise in distortion can be seen in the top audio octave, the distortion is below 0.1% at lower frequencies into almost all loads, especially in Tetrode mode. Interestingly, the lowest distortion was not delivered into the highest impedance, as is the case with solid-state designs, but varied according to output mode. In Tetrode, the MB-450 was at its most linear into 4 ohms (fig.7, blue trace), while in Triode it was most linear into 8 ohms (fig.8, red).
Fig.7 VTL MB-450 Series II Signature, Tetrode mode, THD+N (%) vs frequency at 2.83V into: 16 (magenta), 8 (red), 4 (blue), 2 (green) ohms.
Fig.8 VTL MB-450 Series II Signature, Triode mode, THD+N (%) vs frequency at 2.83V into: 16 (magenta), 8 (red), 4 (blue), 2 (green) ohms.
The distortion content also varied with the mode. At low levels and midrange frequencies, Tetrode mode gave almost pure second-harmonic distortion (fig.9), while in Triode mode, the third harmonic predominated (fig.10). The pattern was the same at higher powers and low frequencies (fig.11), though the amplifier was clearly working harder. The VTL MB-450 really stumbled only when driving high frequencies at very high levels. Fig.12, for example, shows the spectrum of the amplifier's output just below visible clipping on the oscilloscope screen as it drove an equal mix of 19 and 20kHz tones into 4 ohms. I conjecture that it is this behavior that correlates with Michael's finding the VTLs to sound a little "splashy" quality on musical transients.
Fig.9 VTL MB-450 Series II Signature, Tetrode mode, 1kHz waveform at 7.6W into 4 ohms (top), 0.061% THD+N; distortion and noise waveform with fundamental notched out (bottom, not to scale).
Fig.10 VTL MB-450 Series II Signature, Triode mode, 1kHz waveform at 4W into 4 ohms (top), 0.106% THD+N; distortion and noise waveform with fundamental notched out (bottom, not to scale).
Fig.11 VTL MB-450 Series II Signature, Tetrode mode, spectrum of 50Hz sinewave, DC–1kHz, at 41W into 8 ohms (linear frequency scale).
Fig.12 VTL MB-450 Series II Signature, Tetrode mode, HF intermodulation spectrum, DC–24kHz, 19+20kHz at 250W peak into 4 ohms (linear frequency scale).
In common with the other VTL amplifiers reviewed by Stereophile, the MB-450 Series II Signature offers generally excellent measured performance, compromised only by the design decision (on sound-quality grounds) to use a low level of loop negative feedback. It is indeed a powerhouse. And I continue to be impressed by the wide bandwidth of VTL's Signature output transformer.—John Atkinson