Tube Power Amp Reviews

The KR Enterprise VT8000 was warmed up for one hour at a third of its rated maximum power. For an amplifier with a tube output stage, it runs relatively cool.

The VT8000's input impedance measured 62.6k ohms. Its measured minimum output impedance is 4.8 ohms at 20Hz; the maximum measured value is 6.6 ohms at 20kHz; and there was some dependence on the exact figure and the load impedance used to make this measurement. This amplifier's frequency response will be very dependent on the loudspeaker with which it is used. Voltage gain into 8 ohms was 26.8dB in the 1V input sensitivity setting (used for the remainder of the tests), 21dB in the 500mV setting, and 35.5dB in the 2V setting. S/N ratio measured 72.4dB over a bandwidth of 22Hz–22kHz, 71dB over 10Hz–500kHz, and 75.9dB A-weighted (all ref. 1W into 8 ohms). DC offset was 0.0mV; ie, unmeasurable.

Fig.1 shows the VT8000's frequency response (note the scale), which has a significant rolloff at the top end. There were serious frequency-response deviations with our simulated speaker—as with all amplifiers with high output impedances, the response heard from any given set of loudspeakers will not be the speaker's inherent response, but that response as modified by the "equalization" resulting from the amp's output impedance as it reacts with the speaker's own impedance. This equalization may be serendipitous—or not.

Fig.1 KR VT8000, frequency response at (from top to bottom): 1W into 8 ohms, 2W into 4 ohms, and 2.828V into simulated loudspeaker load (1dB/vertical div.).

Fig.2 shows the 10kHz squarewave response. The risetime is fairly slow, as expected from the seriously nonflat treble response. There is also a small amount of overshoot and ringing, though the latter is not clearly evident in the figure. (It's more obvious on an analog 'scope.) The 1kHz squarewave, not shown, also shows a small overshoot on its leading edge.

Fig.2 KR VT8000, small-signal 10kHz squarewave into 8 ohms.

The VT8000's variation in THD+noise percentage with frequency is plotted in fig.3. It was in measuring this that I discovered a serious glitch in the KR's performance. With the first sample I tried, the measurement would hang up at frequencies above 10kHz, with much chattering and complaining from our Audio Precision test set as the amplifier went into intermittent oscillation. The second sample—from which the measurements presented here were taken—was much more stable, but it, too, hung up briefly above around 10kHz before continuing. In both cases I was able to defeat the problem by grounding the negative speaker output lead to the Audio Precision's reference ground terminal. Still, I have never seen such an anomaly before. Fig.3 shows the THD+noise with this ground attached. Fig.4 shows the 2 ohm result with the ground attached and detached. Perhaps associated with the overshoot noted on its squarewave response, the amplifier appears to be marginally unstable at high frequencies, our samples differing from each other only in degree of instability.

Fig.3 KR VT8000, THD+noise (%) vs frequency at (from top to bottom at 2kHz): 4W into 2 ohms, 2W into 4 ohms, 1W into 8 ohms, and 2.83V into simulated loudspeaker load.

Fig.4 KR VT8000, THD+noise (%) vs frequency at 4W into 2 ohms with the output transformer floating (top) and grounded (bottom).

Fig.5 shows the waveform of THD+noise into a 4 ohm load at 1kHz. It is heavily third-harmonic, with a trace of irregularity indicating the presence of higher-order components. Fig.6 shows the spectrum of the KR Enterprise's output in response to a 50Hz input, taken at a 21.5W output into a 4 ohm load. The distortion here is relatively high: –36.8dB (about 1.5%) at the third harmonic (150Hz).

Fig.5 KR VT8000, 1kHz waveform at 2W into 4 ohms (top), distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.6 KR VT8000, spectrum of 50Hz sinewave, DC–1kHz, at 21.5W into 4 ohms (linear frequency scale).

Feeding a combined 19kHz+20kHz signal into the VT8000 resulted in the spectral content shown in fig.7, for 8.5W into 4 ohms. (Visible signs of clipping start to appear on the 'scope at higher power with this input signal.) Distortion levels are –36.4dB at 1kHz (again, about 1.5%) and –33.1dB at 18kHz (just over 2%).

Fig.7 KR VT8000, HF intermodulation spectrum, DC–22kHz, 19+20kHz at 8.5W into 4 ohms (linear frequency scale).

Compared to the VT8000's power rating, its actual power output on continuous tones (fig.8) is a little disappointing. Even at 3% THD+noise, I measured discrete clipping at 39.7W (16dBW) into 8 ohms, 32.1W (12dBW) into 4 ohms, and 16.9W (6.3dBW) into 2 ohms (115V line in all cases). These figures are all lower than the 75W specified.

Fig.8 KR VT8000, distortion (%) vs continuous output power into (from bottom to top at 1W): 8 ohms, 4 ohms, and 2 ohms.

John Atkinson used the Miller Audio Research Amplifier Profiler to investigate the KR VT8000's distortion performance on a low-duty-cycle toneburst. This is more indicative of how an amplifier behaves with a music signal rather than continuous tones. Following some experimentation with grounding to reduce the measured level of distortion, his result for a 1kHz toneburst is shown in fig.9. The VT8000's distortion percentage is shown as a function of output power into 8 ohms (black trace), 4 ohms (red), 2 ohms (blue), and 1 ohm (green). No more power was delivered on the toneburst signal than with continuous tones, which suggests a tightly regulated power supply. The 8-ohm figure, for example, was 38.9W, with less power available as the load impedance dropped. The output basically seemed limited to an RMS current of around 2.8A at the 3% THD+N limit. The amplifier is certainly not suited for use with low-impedance loudspeakers.

Fig.9 KR VT8000, distortion (%) vs 1kHz burst output power into 8 ohms (black trace), 4 ohms (red), 2 ohms (blue), and 1 ohm (green).

What is most unusual in fig.9 is the sawtooth shape of the actual plots. At some output levels, the amplifier's measured spuriae suddenly increased, only then to fall slowly as the power level increased. Looking at the 'scope screen while this test was being run indicated the presence of some extra very-high-frequency oscillations at these points in the plots.

After I informed him of the apparent instability problem with the first sample of the VT8000, JA wondered if the amplifier was broken. But other than a reduced tendency to oscillate, the second amplifier did not behave in a significantly different manner, which led us to believe that we were measuring representative samples. Given the VT8000's low power, high source impedance, and potential instability, its test-bench results are disappointing, particularly in view of its high price.—Thomas J. Norton