VTL MB-450 Series III Signature monoblock power amplifier Measurements

Sidebar 3: Measurements

To perform the measurements on the VTL MB-450 Series III Signature, I mostly used Stereophile's loan sample of the top-of-the-line Audio Precision SYS2722 system (see the January 2008 "As We See It" and www.ap.com); for some tests, I also used my vintage Audio Precision System One Dual Domain.

Though the MB-450 offers just one output transformer tap, it can be operated with single-ended or balanced inputs, in triode or tetrode output mode, and with four different levels of negative feedback/damping factor for each mode. Measurement-wise, it is therefore 16 different amplifiers, and fully characterizing the performance of each one would take more time than I have available to prepare this issue's content. I therefore restricted the testing, except where noted, to the MB-450's balanced input, and took a complete set of measurements in both tetrode and triode modes with the negative feedback set to the highest (Max) and lowest (Low) levels. However, I did do spot checks with the negative feedback set to Med(ium), the setting Michael Fremer used.

Both the unbalanced and balanced inputs preserved absolute polarity; ie, were non-inverting. The XLR is wired with pin 2 hot. The unbalanced input impedance was close to the specification, at 47k ohms at low and middle frequencies, dropping to 38k ohms at 20kHz. The balanced input impedance was twice these values, as expected. The voltage gain depended on both the output mode and the amount of negative feedback. Measured into 8 ohms in triode mode, the four feedback settings gave gains of 33.3dB (Low), 31.9dB (Med), 29.6dB (Hi), and 29.25dB (Max). The equivalent gains in tetrode mode were 36.25, 34.3, 31.3, and 30.9dB, respectively. All of these gains are somewhat higher than the 27 or 28dB typical of a solid-state design.

The output impedance also depended on the amount of negative feedback and the operating mode. The lowest impedance was in triode mode with the maximum amount of feedback, where it measured 0.87 ohm in the midrange, rising very slightly to 0.89 ohm in the low bass and 0.9 ohm at the top of the audioband. With the lowest amount of feedback, these figures rose to 1.45, 1.5, and 1.53 ohms, respectively. Operating the MB-450 in tetrode mode gave a slightly higher source impedance, ranging from 0.95 ohm at 1kHz with the maximum feedback, to 1.95 ohms at 1kHz with the lowest feedback. These source impedances are on the low side for a tubed design, meaning that the variation in frequency response when the VTL drives a loudspeaker will be relatively mild, especially with the maximum amount of feedback. Fig.1 shows the response family in tetrode mode with maximum feedback; the variation in response with our standard simulated loudspeaker (gray trace) remained within ±0.7dB limits. The response is extended at both frequency extremes, the output into 8 ohms (blue) being down just 1.5dB at the 200kHz limit of this graph, which is unusual for a tubed, transformer-coupled amplifier.

Fig.1 VTL MB-450 Series III, tetrode mode, maximum feedback, frequency response at 2.83V into: simulated loudspeaker load (gray), 16 ohms (green), 8 ohms (blue), 4 ohms (magenta), 2 ohms (red). (1dB/vertical div.)

You can also see in fig.1 that, with the maximum feedback in tetrode mode, the ultrasonic response into 16 ohms (green) is just starting to peak before beginning the rolloff. This peak is absent into lower impedances, and in triode mode, especially with the lowest amount of feedback (fig.2). Even so, there is just a hint of overshoot with a 10kHz squarewave in this mode, though there is no ringing (fig.3). The 1kHz waveform (fig.4) is superbly square.

Fig.2 VTL MB-450 Series III, triode mode, minimum feedback, frequency response at 2.83V into: simulated loudspeaker load (gray), 16 ohms (green), 8 ohms (blue), 4 ohms (magenta), 2 ohms (red). (1dB/vertical div.)

Fig.3 VTL MB-450 Series III, triode mode, maximum feedback, small-signal 10kHz squarewave into 8 ohms.

Fig.4 VTL MB-450 Series III, triode mode, maximum feedback, small-signal 1kHz squarewave into 8 ohms.

The VTL is relatively quiet for a tubed design, though the noise level depended on the feedback level and output-stage mode. The unweighted, wideband signal/noise ratios (ref. 2.83V into 8 ohms with the input shorted) were: 74dB, tetrode, maximum feedback; 78dB, triode, minimum feedback; 80.5dB, tetrode, minimum feedback; and 82.5dB, triode, maximum feedback. A-weighting these measurements gave figures around 6dB greater.

Even with the internal switches set to give the maximum amount of feedback, the MB-450 is not a high-feedback design. As can be seen from the plots of THD+noise against output power (fig.5, tetrode; fig.7, triode), the gentle rise in THD+N with increasing power above a few hundred milliwatts suggests that the level of negative feedback is still fairly low. With the feedback set to its minimum level (fig.6, tetrode; fig.8, triode), the manner in which the THD+N percentage changed with power was similar to the traces in figs.5 and 7, but with about one and one-half times as much distortion at the same levels into the same impedances.

Fig.5 VTL MB-450 Series III, tetrode mode, maximum feedback, distortion (%) vs 1kHz continuous output power into (from bottom to top): 16, 8, 4, 2 ohms.

Fig.6 VTL MB-450 Series III, tetrode mode, minimum feedback, distortion (%) vs 1kHz continuous output power into (from bottom to top): 16, 8, 4, 2 ohms.

Fig.7 VTL MB-450 Series III, triode mode, maximum feedback, distortion (%) vs 1kHz continuous output power into (from bottom to top): 16, 8, 4, 2 ohms.

Fig.8 VTL MB-450 Series III, triode mode, maximum feedback, distortion (%) vs 1kHz continuous output power into (from bottom to top): 16, 8, 4, 2 ohms.

These graphs show that the maximum power is obtained into 4 ohms—200W in triode (20dBW), 405W in tetrode (23.1dBW)—but the power at clipping (defined as 1% THD+N) didn't change significantly with feedback level. The distortion at lower powers, however, drops significantly into higher impedances. So while 113W in triode and 155W in tetrode was available into 16 ohms, the THD+N percentage was less than 0.03% below 1W.

Figs.9–12 show how the THD+N percentage changes with frequency in the four different modes tested: tetrode, maximum feedback; tetrode, minimum feedback; triode, maximum feedback; triode, minimum feedback. Overall, the differences between these graphs are what you would expect from figs.5–8, with tetrode offering lower distortion than triode, and the maximum feedback setting offering lower distortion than the minimum. The disruption is also respectably low at low frequencies, suggesting excellent output-transformer design. But all four graphs demonstrate that the MB-450 rapidly loses linearity in the treble, with at least 1% THD apparent at 20kHz in all four modes.

Fig.9 VTL MB-450 Series III, tetrode mode, maximum feedback, THD+N (%) vs frequency at 2.83V into: 16 ohms (green), 8 ohms (blue), 4 ohms (magenta), 2 ohms (red).

Fig.10 VTL MB-450 Series III, tetrode mode, minimum feedback, THD+N (%) vs frequency at 2.83V into: 16 ohms (green), 8 ohms (blue), 4 ohms (magenta), 2 ohms (red).

Fig.11 VTL MB-450 Series III, triode mode, maximum feedback, THD+N (%) vs frequency at 2.83V into: 16 ohms (green), 8 ohms (blue), 4 ohms (magenta), 2 ohms (red).

Fig.12 VTL MB-450 Series III, triode mode, minimum feedback, THD+N (%) vs frequency at 2.83V into: 16 ohms (green), 8 ohms (blue), 4 ohms (magenta), 2 ohms (red).

Fortunately, the harmonic content of the distortion is predominantly low-order (fig.13), and this doesn't change much when the amount of feedback is varied. Fig.14, for example, shows the spectrum of the VTL's output in tetrode mode with maximum feedback (red trace) and minimum feedback (blue). With maximum feedback, the second and third harmonics are the highest in level, at –64dB (0.06%) and –74dB (0.05%), respectively. Reducing the feedback to its minimum raised both harmonics by 4dB, but other than the fourth harmonic making an appearance, the distortion signature remained the same. The same was true for triode mode (fig.15), except that the 1kHz fundamental is now surrounded by many sidebands spaced at the supply-related frequency of 120Hz. The presence and absence of these sidebands can also be seen in the plots of the VTL's behavior driving an equal mix of 19 and 20kHz tones into 8 ohms (fig.16, tetrode; fig.17, triode), though the circuit's poor top-octave linearity gives a disappointing result on this test, even with the maximum amount of feedback. The 1kHz difference component lies at –42dB in tetrode mode, –46dB in triode.

Fig.13 VTL MB-450 Series III, triode mode, minimum feedback, 1kHz waveform at 5W into 8 ohms (top), 0.128% THD+N; distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.14 VTL MB-450 Series III, tetrode mode, spectrum of 1kHz sinewave, DC–1kHz, at 5W into 8 ohms (minimum feedback, blue; maximum, red; linear frequency scale).

Fig.15 VTL MB-450 Series III, triode mode, spectrum of 1kHz sinewave, DC–1kHz, at 5W into 8 ohms (minimum feedback, blue; maximum, red; linear frequency scale).

Fig.16 VTL MB-450 Series III, tetrode mode, maximum feedback, HF intermodulation spectrum, DC–24kHz, 19+20kHz at 50W peak into 8 ohms (linear frequency scale).

Fig.17 VTL MB-450 Series III, triode mode, maximum feedback, HF intermodulation spectrum, DC–24kHz, 19+20kHz at 50W peak into 8 ohms (linear frequency scale).

With its four levels of negative feedback and two output-stage modes, the VTL MB-450 Series III's behavior can be fine-tuned to match the speaker with which it is to be used. Though the amplifier gives its maximum power into 4 ohms and has a usefully low output impedance for a tubed design, it does give lower distortion into higher impedances, suggesting that higher-impedance speakers are to be preferred.—John Atkinson

COMPANY INFO
VTL Amplifiers Inc.,
4774 Murrieta Street, Suite 10
Chino, CA 91710
(909) 627-5944
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