Balanced Audio Technology VK-3iX preamplifier & & VK-55 power amplifier VK-55 Measurements

Sidebar 4: VK-55 Measurements

This amplifier's two output transformer taps are labeled "4–6" and "6–8"; I assume these numbers refer to the recommended load impedance in ohms. The VK-55 offered a fairly low voltage gain into 8 ohms from either tap: 22.1dB and 23.67dB, respectively. It also inverted signal polarity from both taps. The input impedance was usefully high, at 290k ohms at 1kHz, this dropping slightly to around 160k ohms at the band edges (footnote 1). The amplifier will therefore be a perfect match for the VK-3iX preamp.

The VK-55's source impedance was very high from the "6–8" tap, at an estimated 3.5 ohms. It dropped to 2 ohms from the "4–6" tap, but in both cases there will be quite a large modification of the amplifier's frequency response by the Ohm's Law interaction between this source impedance and the loudspeaker's impedance curve. The top trace in fig.7, for example, shows that with the magazine's simulated loudspeaker load, the response variations with the "6–8" tap are as large as ±1.8dB, which will be very audible. These variations drop to ±1.1dB from the "4–6" tap (not shown).

Fig.7 Balanced Audio Technology VK-55, "6–8" tap, balanced frequency response at 2.83V into (from top to bottom at 2kHz): simulated loudspeaker load, 8, 4, 2 ohms (1dB/vertical div., right channel dashed).

The VK-55 offers a wide bandwidth into resistive loads, though this does decrease a little with decreasing load impedance. Even so, in the worst case—"6–8" tap into 2 ohms—there is virtually no rolloff in the audioband. As a result of this wide bandwidth, the VK-55's reproduction of a 1kHz squarewave is essentially perfect (fig.8), though the 10kHz squarewave showed a hint of overshoot into higher load impedances (fig.9).

Fig.8 Balanced Audio Technology VK-55, "6–8" tap, small-signal 1kHz squarewave into 8 ohms.

Fig.9 Balanced Audio Technology VK-55, "6–8" tap, small-signal 10kHz squarewave into 8 ohms.

Midband channel separation (not shown) was better than 100dB in both directions, which is excellent. The S/N ratio, ref. 1W into 8 ohms from the "6–8" tap, was also excellent, at 83.9dB (wideband, unweighted). The A-weighted figure is even better, at 97dB, suggesting that despite its relatively low output power, the VK-55 is still a wide–dynamic-range design.

Plotting the percentage of distortion and noise in the VK-55's output against output power for the "6–8" and "4–6" taps (figs.10 and 11, respectively) revealed that the amplifier is not a particularly low-distortion design, perhaps due to the limited use of loop negative feedback. BAT specifies the amplifier as offering its rated 55W power at 3% THD; these graphs show that with the load matched to the tap, the VK-55 offers more than this power at the point in the curve between 1.2% and 1.5%, where there is a sharp "knee" that indicates where true clipping begins. With the "6–8" tap driving 8 ohms, 66.67W are available (18.2dBW), while with the "4–6" tap driving 4 ohms, 64W are available (15dBW).

Fig.10 Balanced Audio Technology VK-55, "6–8" tap, distortion (%)vs 1kHz continuous output power into (from bottom to top at 1W): 8, 4, 2 ohms.

Fig.11 Balanced Audio Technology VK-55, "4–6" tap, distortion (%)vs 1kHz continuous output power into (from bottom to top at 1W): 8, 4, 2 ohms (right channel dashed).

As can be seen from figs.10 and 11, the distortion rises into lower impedances; this can also be seen in the graph of small-signal THD+N against frequency (fig.12). This graph was taken from the "4–6" tap; the percentages from the "6–8" tap are approximately twice as high. The distortion in the midrange is predominantly third-harmonic in nature and relatively low in level (fig.13), but at low frequencies it rises to levels that will be audible, even at quite low power levels (fig.14).

Fig.12 Balanced Audio Technology VK-55, "4–6" tap, THD+N (%)vs frequency at 2.83V into (from bottom to top): 8, 4, 2 ohms.

Fig.13 Balanced Audio Technology VK-55, "6–8" tap, 1kHz waveform at 1W into 8 ohms (top), 0.144% THD+N; distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.14 Balanced Audio Technology VK-55, "6–8" tap, spectrum of 50Hz sinewave, DC–1kHz, at 3.5W into 8 ohms (linear frequency scale).

The VK-55 managed well with the high-frequency intermodulation test at small-signal levels. However, at a level just below visible clipping on an oscilloscope screen (fig.15), the 1kHz difference product resulting from an equal mix of 19kHz and 20kHz tones reached –60dB (0.1%), while the higher-order products at 18kHz and 20kHz lay at –50dB (0.3%). Sidebands at the power-supply frequencies can also be seen around the spectral lines in this graph, suggesting that the amplifier is working very hard.

Fig.15 Balanced Audio Technology VK-55, "6–8" tap, HF intermodulation spectrum, DC–24kHz, 19+20kHz at 40W peak into 8 ohms (linear frequency scale).

I am a little concerned by that highish level of THD at low frequencies, perhaps due to an undersized output-transformer core. Otherwise, the VK-55 offers quite respectable measured performance.—John Atkinson



Footnote 1: Because of the very small changes in voltage involved in this measurement, these figures have quite a large margin of error.—John Atkinson
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