Balanced Audio Technology VK-5 preamplifier & VK-60/75 power amplifiers VK-60 Measurements
All of the 8 ohm measurements on the VK-60 were made using its high-impedance output tap. Two and 4 ohm measurements were made at the low tap. Bridging was performed at the low impedance tap.
Following the 1/3-power, one-hour preconditioning test, the VK-60 ran typically hot for a tube amplifier. Its input impedance was too high to measure with precision on our Audio Precision test set using normal procedures, but was clearly well above 100k ohms. The output impedance measured a very high 3.5 ohms, or marginally less at 20Hz and 1kHz, increasing to 3.9 ohms at 20kHz. In the bridged mode, the 1kHz output impedance decreased to a more tolerable 1.5 ohms. In either mode, but especially in the standard stereo configuration, the sound of the VK-60 will vary with loudspeaker impedance. An amplifier like this is, to a certain extent, review-proof; the reviewer's observations will only be transportable if the reader listens over the same loudspeakers.
The voltage gain of the VK-60 measured 26dB into 8 ohms. DC offset measured 1.4mV in the left channel, 0.8mV in the right. Signal/Noise ratio (ref. 1W into 8 ohms) measured 77dB over a 22Hz-22kHz bandwidth, unweighted, and 83dB over a 10Hz-500kHz bandwidth, A-weighted. The VK-60 has pin 2 of its input configured as the positive leg, pin 3 as the negative.
Fig.1 shows the frequency response of the VK-60 in both its standard and bridged configurations. The response into resistive loads is good, though the HF rolloff into a 4 ohm load will likely be marginally audible. More significant, however, is the unbridged response into our simulated real-world load (see Stereophile, August 1995, Vol.18 No.8. p.168). The deviations here, caused by the amplifier's high output impedance, will be audible. The 10kHz squarewave response in fig.2 is good, however, with only a slight rounding at the leading edge reflecting the HF rolloff. The 1kHz squarewave, not shown, is near-perfect.
Fig.1 BAT VK-60, frequency response (from top to bottom): at 1W into 8 ohms in bridged mode; 2W into 4 ohms in bridged mode; 1W into 8 ohms in stereo mode; 2W into 4 ohms in stereo mode; 1W into simulated load (right channel dashed, 0.5dB/vertical div.).
Fig.2 BAT VK-60, 10kHz squarewave at 1W into 8 ohms.
The crosstalk shown in fig.3 is a very respectable result. It's about 10dB better in one direction than in the other, but even in the worst case the channel separation is better than 65dB—a figure unlikely to result in any audible performance compromise.
Fig.3 BAT VK-60, crosstalk, from top to bottom: L-R, R-L (10dB/vertical div.).
The THD+noise vs frequency results are plotted in fig.4. The results are satisfactory above about 200Hz, but there's a notable increase in distortion at low frequencies and into lower impedance loads. The right channel is noticeably less good than the left at low frequencies. The Balanced Audio Technology's distortion waveform (fig.5) indicates primarily 3rd-harmonic content, with some additional components.
Fig.4 BAT VK-60, THD+noise vs frequency at (from top to bottom at 10kHz): 4W into 2 ohms, 2W into 4 ohms, and 1W into 8 ohms, stereo mode; 1W into simulated load; 2W into 4 ohms, and 1W into 8 ohms, bridged mode.
Fig.5 BAT VK-60, 1kHz waveform at 1W into 8 ohms (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).
The spectrum of the VK-60's output, reproducing a 50Hz input tone at 24W into 4 ohms (2/3 rated power) is shown in fig.6. This is a fairly poor result relative to the best we've measured, and is indicative of the relatively high distortion at low frequencies into lower impedances. The largest artifacts are at 100Hz (-35dB, or about 1.7%) and 150Hz (-37dB, or about 1.5%). Interestingly, the results for the same output into our simulated real-world load (fig.7) are better: -46.5dB (just under 0.5%) at 100Hz, -44.6 (about 0.6%) at 150Hz, and visibly lower at higher frequencies as well.
Fig.6 BAT VK-60, spectrum of 50Hz sinewave, DC-1kHz, at 24W into 4 ohms (linear frequency scale). Note that the second harmonic at 100Hz is the highest in level, at -35dB (about 1.7%).
Fig.7 BAT VK-60, spectrum of 50Hz sinewave, DC-1kHz, at 24W into simulated load (linear frequency scale). Note that the third harmonic at 150Hz is the highest in level, at -46.5dB (just under 0.5%).
Fig.8 shows the VK-60's output spectrum driving a combined 19+20kHz signal at the same indicated 24W into 4 ohms. At -37.7dB (about 1.3%) at 1kHz and -32dB (about 2.5%) at 18kHz and 21kHz, along with a considerable amount of noise, this is not a particularly good result.
Fig.8 BAT VK-60, HF intermodulation spectrum, DC-22kHz, 19+20kHz at 24W into 4 ohms (linear frequency scale).
The 1kHz, THD+noise vs output power performance for the VK-60 is shown in fig.9. This amplifier is clearly of moderate power, even in its bridged mode. The curves are typical of a tube amplifier: rising gradually to the 1% distortion point. The VK-60's discrete clipping levels (at 1% THD+noise) are shown in Table 1.
Fig.9 BAT VK-60, distortion (%) vs output power into (from bottom to top at 10W): 8 ohms and 4 ohms, bridged mode; 8 ohms, 4 ohms, and 2 ohms, stereo mode.
The test-bench results of the VK-60 were not atypical for a tube amplifier. But neither were they particularly impressive, especially considering the effort expended to build a balanced design (relatively unusual in a tube amp).—Thomas J. Norton
TABLE 1: BAT VK-60 Clipping (1% THD+N at 1kHz)
|Both Channels Driven||One Channel Driven|
|Load||W (dBW)||W (dBW)|
|8||34 (15.3)||29.5 (14.7)||33.7 (15.3)|
|4||29.4 (11.7)||24.2 (10.8)||30 (11.8)|