Balanced Audio Technology VK-55SE power amplifier Measurements
To perform the measurements on the Balanced Audio Technology VK-55SE, 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.
The VK-55SE has four output terminals for each channel, various pairs of which can be used to provide a feed from three different output-transformer taps: these are labeled "High (68 ohms)," "Med (46 ohms)," and "Low (34 ohms)." I performed a complete set of measurements from each tap, but have concentrated on the High and Low outputs in this discussion, as the Med(ium) output was not that different from the High in most respects.
The amplifier's maximum voltage gain was lower than usual, at 24dB into 8 ohms from the High and Med outputs and 23dB from the Low, though there was a little more experimental error in these figures than usual because of a slight history effect; ie, the actual gain somewhat depended on the signal behavior. When I increased the signal level of a sinewave tone, for example, the level of the output signal drifted down by a few millivolts over a period of several seconds before stabilizing at the new steady-state value. Each output tap was non-inverting (ie, it preserved absolute polarity), with pin 2 of the XLR jacks wired hot. The input impedance was very higha good thingat 250k ohms at 1kHz, increasing slightly at the frequency extremes.
As with the standard VK-55 (see the measurements here), the output impedance from the VK-55SE's High and Med taps was high, at 2.7 ohms in the midrange, rising to almost 3 ohms at high and low frequencies. As expected, the output impedance was somewhat lower from the Low tap, at just under 2 ohms at high and midrange frequencies, with only a slight rise, to 2.1 ohms, at 20Hz. As a result of the Ohm's Law interaction between the high output impedances with the impedance of our standard simulated loudspeaker, the amplifier's frequency response into this load varied by ±1.7dB from the Med and High taps (fig.1, gray trace) and ±1.2dB from the Low tap (fig.2, gray trace). These variations in response will be audible. Into resistive loads, the VK-55SE offered a flat response in the audioband, with excellent channel matching and a wide small-signal bandwidth. Even into 2 ohms (figs.1 and 2, green trace), the ultrasonic response didn't reach 3dB until 90kHz, which results in a very square squarewave response (fig.3). The waveform's risetimes are very short and, commendably, there is only a slight hint of overshoot and no ringing. The VK-55SE uses well-made output transformers.
Fig.1 Balanced Audio Technology VK-55SE, High output tap, frequency response at 2.83V into: simulated loudspeaker load (gray), 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), 2 ohms (green). (1dB/vertical div.)
Fig.2 Balanced Audio Technology VK-55SE, Low output tap, frequency response at 2.83V into: simulated loudspeaker load (gray), 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), 2 ohms (green). (1dB/vertical div.)
Fig.3 Balanced Audio Technology VK-55SE, High output tap, small-signal 10kHz squarewave into 8 ohms.
Channel separation (not shown) was superb at >100dB in both directions below 3kHz. The unweighted, wideband signal/noise ratio, taken with the input shorted, was good at 78.7dB ref. 1W into 8 ohms, improving to 96.5dB when A-weighted. The figures were taken from the Low output; the Med and High outputs were about 0.8dB worse.
Specified as having a maximum output power of 55Wpc into 8 ohms (17.4dBW), the VK-55SE delivered exactly this power into 8 ohms from its High tap at 1% THD+noise, as can be seen in fig.4, which plots the THD+N percentage against output power into 8, 4, and 2 ohms from this tap. The 4 ohms delivery was the same, while into 2 ohms, this tap delivered 38W (9.8dBW). The steady rise in THD with power before actual waveform clipping is reached suggests that this amplifier uses only a modest amount of loop negative feedback. The Med tap delivered slightly more power at 1% THD+N into 8 ohms (60.3W, 17.8dBW) and 4 ohms (64W, 15dBW), but the same 38W into 4 ohms. Fig.5 plots the THD+N percentage against output power from the Low tap. The low-level distortion is lower than from the other two taps, and though less power is available at clipping into 8 ohms40Wpc, 16dBWthis tap does behave a little more like a voltage source, with 64W available into 4 ohms and 59W into 2 ohms (11.7dBW). The Low tap will be the one to use with loudspeakers that offer an awkward impedance, such as the Thiel CS3.7s used by Wes Phillips for some of his auditioning.
Fig.4 Balanced Audio Technology VK-55SE, High output tap, distortion (%) vs 1kHz continuous output power into (from bottom to top): 8, 4, 2 ohms.
Fig.5 Balanced Audio Technology VK-55SE, Low output tap, distortion (%) vs 1kHz continuous output power into (from bottom to top): 8, 4, 2 ohms.
The two channels differed in their small-signal linearity and noise level, perhaps because the review sample's tubes had much mileage on them. For example, figs. 6 and 7 plot the THD+N percentage against frequency from the High and Low taps, respectively. The right channel (red and magenta traces) offered lower distortion+noise in the midrange and treble than the left (blue and cyan). However, while the left and right channels were not significantly different at low frequencies from the High tap, the left channel offered significantly less distortion below 100Hz than the right from the Low tap.
Fig.6 Balanced Audio Technology VK-55SE, High output tap, THD+N (%) vs frequency at 2.83V into: 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), 2 ohms (green).
Fig.7 Balanced Audio Technology VK-55SE, Low output tap, THD+N (%) vs frequency at 2.83V into: 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), 2 ohms (green).
This can be seen in fig.8, which shows the spectrum of the VK-55SE's Low-tap output at a modest power into 8 ohms. A regular series of both odd- and even-order harmonics can be seen, but these are each about 12dB lower in the left channel (blue trace) than the right (red). If this spectrum had been predominantly odd-order harmonics, I would have suspected an undersized core in the right-channel transformer, but as the highest-level harmonic is the second, perhaps it was due to a tube mismatch. At higher frequencies and lower levels, the third harmonic dominates (fig.9).
Fig.8 Balanced Audio Technology VK-55SE, Low output tap, spectrum of 50Hz sinewave, DC10kHz, at 10W into 8 ohms (left channel blue, right red; linear frequency scale).
Fig.9 Balanced Audio Technology VK-55SE, High output tap, 1kHz waveform at 3.4W into 4 ohms (top), 0.248% THD+N; distortion and noise waveform with fundamental notched out (bottom, not to scale).
Finally, as implied by the fact that the distortion percentage doesn't rise at the top of the audioband in figs. 6 and 7, the VK-55SE did quite well on the demanding twin-tone high-frequency intermodulation test (fig.10). At a level from the Low tap just below visible clipping on the oscilloscope screen, the 1kHz difference tone was the highest in level, at 57dB (0.14%), though a series of higher-order products can also be seen at lower levels. The higher-frequency tones were surrounded by sideband pairs at ±120Hz and its harmonics, suggesting that the amplifier's power supply is being stressed under these conditions, even though the actual 120Hz component lies at a low 95dB. (This behavior was not affected by experimenting with the grounding between the VK-55SE and the Audio Precision test set.)
Fig.10 Balanced Audio Technology VK-55SE, Low output tap, HF intermodulation spectrum, DC24kHz, 19+20kHz at 20W peak into 8 ohms (linear frequency scale).
Overall, the VK-55SE is as well engineered as I have come to expect from Balanced Audio Technology. Certainly it sounds as good as WP describes, as I found out when I drove, from its Low output-transformer tap, the Aerial Model 20Tv2 loudspeakers currently residing in my listening room. But some aspects of the VK-55SE's measured performance bothered me, particularly the difference between the channels' low-frequency linearity from the Low tapand it is only fair to point out that the SE measures no better than the plain-Jane VK-55.John Atkinson