VTL MB 175 Signature monoblock power amplifier Measurements

Sidebar 3: Measurements

The VTL MB 175 amplifier developed a glitch about halfway through my first attempt at measuring it, the S/N and THD+noise deteriorating markedly. I suspected a bad tube, but Luke Manley of VTL elected to fly to Santa Fe to repair the amp on the spot. In the process he also made some minor upgrades to the power supply and tweaked the bias. The measurements presented here reflect those changes. I suspect that the initial defect I discovered happened either in transit or on the bench. Had MF experienced the noise level the unit exhibited for me at its worst (it was -45dB), he could not have missed it.

The measurements here reflect tetrode operation except where otherwise stated. After the usual one-hour preconditioning test, the MB 175's operating temperature was no hotter than is typical of a tube amp—perhaps even a little cooler. The VTL's input impedance measured 93.9k ohms (97.5k ohms triode), its voltage gain into 8 ohms 29.9dB (28.7dB triode). The VTL is noninverting, a positive-going input resulting in a positive-going output. DC offset at the output measured 0.3mV. The S/N ratio measured 66.8dB from 22Hz to 22kHz (unweighted), 66dB from 10Hz to 500kHz (unweighted), and 75dB A-weighted (all ref. 1W). (S/N improved slightly in the triode mode, but only by a maximum of 1.5dB.)

The maximum measured output impedance of the MB 175 measured under 1.37 ohms, with slight variations with frequency and load (under 1.25 ohms in the triode mode). We have measured worse (higher values) with other tube amplifiers, but this output impedance will still affect the frequency response somewhat into most real-world loads (see below).

Fig.1 shows the MB 175's frequency response (the triode and tetrode results were virtually identical). The effect of the fairly high output impedance on the response is evident, and should be audible; the actual degree will depend on the loudspeakers.

Fig.1 VTL MB 175, frequency response at (from top to bottom at 20kHz): 1W into 8 ohms, 2W into 4 ohms, and 2.83V into simulated speaker load (0.5dB/vertical div.).

The VTL's reproduction of a small-signal 10kHz squarewave is shown in fig.2. The risetime is slightly slowed, and there is a small, damped overshoot (the 1kHz risetime is quite fast, with the overshoot still visible). The squarewaves in the triode mode were essentially the same.

Fig.2 VTL MB 175, tetrode mode, small-signal 10kHz squarewave into 8 ohms.

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