VAC Avatar integrated amplifier Measurements

Sidebar 3: Measurements

Except as noted, these results are for the VAC Avatar's line-level input, and with the amplifier's level control set to a position in which an input of 100mV resulted in a voltage gain of 35dB into 8 ohms. This is approximately the overall gain of a typical preamp of 6dB gain and a power amp of 29dB gain. The physical position of the level control that provided this result was approximately 2 o'clock. As is typical of a tube amplifier, the VAC was hot after its one-hour, 1/3-power preconditioning test.

Most of the tests were performed driving 8 ohm loads from the 8 ohm output tap, and 2 ohm, 4 ohm, and simulated loads from the 4 ohm tap. There was, however, one important exception to this, explained below.

The Avatar's line input impedance measured a usefully high 165k ohms, and its maximum gain (at the full setting of the level control) 46.2dB. Its output impedance was also very high: 3.4 ohms at 20Hz, increasing to 4.1 ohms at 20kHz from its 8 ohm tap into 8 ohms. The output impedance dropped to 1.3 ohms at 1kHz from the 4 ohm tap, and there was some dependence between the measured impedance and the load used to assess that impedance. In any event, these values will have a significant audible effect on the amplifier's performance into most real-world loads.

DC offset at the main outputs measured a negligible 0.2mV in both channels. The S/N ratio (unweighted at 1W into 8 ohms) measured 68.1dB from 22Hz to 22kHz, 66.1dB from 10Hz to 500kHz, and 72dB unweighted. The VAC inverts polarity from its line inputs, a positive-going input resulting in a negative-going output.

Fig.1 shows the Avatar's frequency response. The curves, which are displaced by -3dB, are the equalized curves for the phono stage, measured at the line output. But note the line-level response for the right channel into 4 ohms: It's down 6dB from the left, but there is no such discrepancy into 8 ohms. This suggests something amiss in the amplifier's output transformer. In any event, some of the following 4 ohm measurements, as noted, were taken using the 8 ohm tap if both channels were required. Four-ohm measurements that required only one channel (THD+noise vs output, the 50Hz and 19+20kHz spectra, discrete clipping) were taken from the properly functioning 4 ohm tap on the left channel.

Fig.1 VAC Avatar, frequency response at (from top to bottom at 6kHz): 2W into 4 ohms, 1W into 8 ohms, and 2.828V into simulated loudspeaker load; phono input to tape-out jacks; 2W into 4 ohms, right channel 4 ohm output tap (1dB/vertical div.).

Fig.2, the VAC's output with a 10kHz squarewave input, indicates a relatively slow risetime and some residual ultrasonic oscillation. (This was more obvious on an analog 'scope display.) The same were visible on the 1kHz squarewave (not shown). Fig.3 shows the Avatar's channel separation, an acceptable if unexceptional result. The inconsistency between channels is likely inaudible. The increase at higher frequencies is likely (as usual) due to interchannel capacitive coupling.

Fig.2 VAC Avatar, small-signal 10kHz squarewave into 8 ohms.

Fig.3 VAC Avatar, channel separation: R-L (top) and L-R (10dB/vertical div.).

The THD+noise percentage vs frequency results in fig.4 (taken from the 8 ohm tap for the reasons stated above) are neither particularly notable nor unusual for a modestly powered tube amp. They do imply that the 8 ohm tap is really optimal for driving speakers that do not have impedances that dip much below 8 ohms. Fig.5 shows the THD+N output waveform of the VAC with a 1kHz input at 2W into 4 ohms. It is heavily second-harmonic, but there is some evidence of higher-order harmonics as well; this evidence increases into a 2 ohm load.

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

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

Fig.6 shows the VAC's output spectrum driving a 50Hz tone at 37.3W into a 4 ohm load. The second harmonic is at -35.5dB, or about 1.5%, with then a series of harmonics whose level regularly decreases with increasing order. Fig.7 shows the intermodulation distortion resulting from a combined input of 19+20kHz at 19.4W into 4 ohms. (The amp visibly clips the waveform above this level with this input signal.) The largest artifact here is at 1kHz (-50.8dB, or about 0.3%). The spectrum resulting from an 18.8W output into an 8 ohm load (not shown) is very similar, with the artifacts marginally lower in level at 1kHz, somewhat higher at high frequencies.

Fig.6 VAC Avatar, spectrum of 50Hz sinewave, DC-1kHz, at 37.3W into 4 ohms, 4 ohm tap (linear frequency scale).

Fig.7 VAC Avatar, HF intermodulation spectrum, DC-22kHz, 19+20kHz at 19.4W into 4 ohms, 4 ohm tap (linear frequency scale).

Fig.8 shows the THD+noise percentage vs continuous output power curves for the Avatar, one channel (the left) driven, at 1kHz. With both channels driven in ultralinear mode into matched loads, about the same power was available from both output transformer taps, 42.3W from the 8 ohm tap (16.2W), and 36.6W from the 4 ohm tap (12.6dBW). Even though these power levels were assessed at a relaxed 3% THD+noise level, there are below spec.

Fig.8 VAC Avatar, distortion (%) vs continuous output power into (from bottom to top at 2kHz) 4 ohms, 8 ohms, and 2 ohms (matched impedance taps).

Using the Miller Amplifier Profiler to drive the amplifier with a low-duty-cycle 1kHz sinewave, which better approximates a music signal than a continuous sinewave, we raised 52W from the 8 ohm tap into 8 ohms in ultralinear mode at the traditional 1% THD clipping point. At 3% THD, the ultralinear figures were 57W from the 8 ohm tap into 8 ohms, 54W from the 4 ohm tap into 4 ohms, both closer to the specified power. In triode mode, the available power dropped to 24.1W into 8 ohms (1% THD), 26.2W (3%). But as can be seen from fig.9, the output transformer rating is very much a minimum rating for this amplifier. The 8 ohm tap really needs to be used with speakers having a minimum impedance of 8 ohms, not a nominal impedance.

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

During these tests, it was confirmed that the problem we were noting involved the 4 ohm output transformer tap but also had something to do with the ultralinear/triode switch, as toggling this switch would briefly restore correct operation. However, when I measured the phono stage, the Avatar began blowing fuses: once when I turned the amp back on to finish the measurements after an overnight hiatus, and again when I replaced the fuse. And that second time, the blown fuse was accompanied by the smell of a resistor committing hara-kiri. That ended the measurements. The amplifier's preamp section was not functioning, nor could any output be raised from the right channel.

While a complete set of preamp measurements was thus not conducted, I was able to determine that the phono input impedance was 48.5k ohms and the preamp gain 36.9dB. The phono frequency response is shown in fig.10, the plot of phono THD+noise percentage vs output voltage in fig.9.

Fig.10 VAC Avatar, phono section, distortion (%) vs output voltage at 1kHz into 100k ohms.

All told, the VAC Avatar's measurements are not reassuring for this type and class of amplifier. It's likely, of course, that our sample was defective. But the test-bench results alone provide little cause for enthusiasm. A Follow-Up review is mandatory before Stereophile can unconditionally recommend this amplifier.—Thomas J. Norton

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