Musical Fidelity Tri-Vista kWP preamplifier & Tri-Vista kW Monobloc power amplifier Tri-Vista kW Measurements

Sidebar 4: Tri-Vista kW Measurements

I measured the kW using a separate power supply for each channel. The amplifier offered a slightly higher-than-usual voltage gain of 30.1dB into 8 ohms. It preserved absolute polarity, and its input impedance at 1kHz was a very high 230k ohms. Though this dropped to 166k ohms at 20kHz, the kW's input impedance is so high that it will not load the preamp to any appreciable extent.

The amplifier's output impedance was 0.11 ohm over most of the audioband, rising slightly, to 0.18 ohm, at 20kHz. (Both figures include the series resistance of 6' of speaker cable.) The modification of the amplifier's frequency response by the Ohm's Law interaction between its source impedance and the impedance of the loudspeaker will therefore be just ±0.1dB, as shown by fig.1. This graph also shows that while the amplifier has a wide small-signal bandwidth into 8 ohms—3dB down at 130kHz—this bandwidth decreases into lower loads. Into 2 ohms, for example (fig.1, bottom trace), the -3dB point drops to 55kHz, the kW's output now -1dB at 20kHz. Even so, the amplifier's reproduction of a 10kHz squarewave (fig.2) was clean, with only a faint hint of incipient overshoot evident on its leading edge.

Fig.1 Musical Fidelity kW, frequency response at (from top to bottom at 2kHz): 2.83V into simulated loudspeaker load, 1W into 8 ohms, 2W into 4 ohms, 4W into 2 ohms (0.5dB/vertical div.).

Fig.2 Musical Fidelity kW, small-signal 10kHz squarewave into 8 ohms.

The amplifier's unweighted, wideband signal/noise ratio with its input short-circuited was a moderate 75.5dB (ref. 1W/8 ohms). The figure improved to 88dB when A-weighted, which is equivalent to 118dB ref. full power. As with earlier Musical Fidelity power amplifiers, distortion was extraordinarily low and even across the audioband, though it did increase as the load impedance dropped (fig.3). Figs.4 and 5 show that the third harmonic is predominant at high power levels. Though a smattering of higher-order harmonics is present, these are, again, all at very low levels. The subjectively benign second harmonic was the highest in level at low powers (not shown), but was buried beneath the kW's noise floor. Intermodulation distortion was also very low, though driving the amplifier at high power into low impedances brought the 1kHz difference product up to a still minimal -90dB (0.003%) (fig.6).

Fig.3 Musical Fidelity kW, THD+N (%) vs frequency (from bottom to top): 10V into 8 ohms, 4 ohms, 2 ohms.

Fig.4 Musical Fidelity kW, 1kHz waveform at 250W into 4 ohms (top), 0.0047% THD+N; distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.5 Musical Fidelity kW, spectrum of 50Hz sinewave, DC-1kHz, at 1000W into 4 ohms (linear frequency scale).

Fig.6 Musical Fidelity kW, HF intermodulation spectrum, DC-24kHz, 19+20kHz at 900W into 4 ohms (linear frequency scale).

Finally, the kW is aptly named (and specified), delivering just over one continuous kilowatt of power into an 8 ohm load: 1100W, or 30.4dBW! This increased to 1800W into 4 ohms (29.5dBW) and 2.5kW into 2 ohms (28dBW) (fig.7). However, it should be noted that my wall voltage drooped during this final measurement, from 126.5V to 121V. The resistance of the owner's AC wiring will be what limits the kW's ability to deliver prodigious current into low-impedance loudspeakers.—John Atkinson

Fig.7 Musical Fidelity kW, distortion (%) vs 1kHz continuous output power into (from bottom to top): 8 ohms, 4 ohms, 2 ohms.

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