Audio Research VS110 power amplifier & SP16L line preamplifier Measurements part 2

Distortion levels depended on both frequency and the load impedance. From the 4 ohm tap into 8 ohms (fig.9, lower pair of traces), the THD+N figure hovered around 0.025% for much of the audioband, rising only at the frequency extremes. Halving the load (doubling the current) doubled the midband THD, while changing to the 8 ohm tap and keeping the output voltage the same (not shown) also increased the THD+N, from 0.025% to 0.035%, with similar but smaller rises seen below 100Hz and above 5kHz. Fig.10, however, shows that the spectrum of that distortion is heavily second-harmonic in nature, which tends to be subjectively benign.

Fig.9 Audio Research VS110, 4 ohm tap, THD+N (%) vs frequency (from bottom to top at 2kHz): 2.83V into simulated loudspeaker load, 8 ohms, 4 ohms, 2 ohms.

Fig.10 Audio Research VS110, 4 ohm tap, 1kHz waveform at 2W into 4 ohms (top), 0.049% THD+N; distortion and noise waveform with fundamental notched out (bottom, not to scale).

At high levels and low frequencies, whether the second or third harmonic was higher depended on the output tap used (figs.11 and 12). But, as with the SP16L, the ordered way in which harmonics decrease in level as they increase in order is apparent, at least with the 4 ohm tap (fig.12). Only on the punishing HF intermodulation test did the VS110 stumble, the 1kHz difference component lying at -60dB (0.1%) even at 1W output (fig.13). Increasing the output power to just below visible clipping on the 'scope screen with this signal—90W into 8 ohms from the 8 ohm tap—increased the 1kHz component to -40dB (1%). And even at 1W, I was bothered by the sidebands that appeared around the primary spectral components, spaced at power-supply-related frequencies.

Fig.11 Audio Research VS110, 8 ohm tap, spectrum of 50Hz sinewave, DC-1kHz, at 69W into 8 ohms (linear frequency scale).

Fig.12 Audio Research VS110, 4 ohm tap, spectrum of 50Hz sinewave, DC-1kHz, at 69W into 4 ohms (linear frequency scale).

Fig.13 Audio Research VS110, 8 ohm tap, HF intermodulation spectrum, DC-24kHz, 19+20kHz at 1W into 8 ohms (linear frequency scale).

Finally, figs.14 and 15 show that the VS110 more than meets its specified output power as long as the transformer tap is matched to the load. The 1% THD clipping point was reached at 135W into 8 ohms from the 8 ohm tap (21.3dBW), and at 120W into 4 ohms from the 4 ohm tap (17.8dBW). (The AC wall voltage was 124.1V for these measurements.) But into loads that are significantly lower in impedance than the transformer tap, the distortion increases dramatically at powers well below the actual clipping point.

Fig.14 Audio Research VS110, 8 ohm tap, distortion (%) vs 1kHz continuous output power into (from bottom to top): 8 ohms, 4 ohms, 2 ohms.

Fig.15 Audio Research VS110, 4 ohm tap, distortion (%) vs 1kHz continuous output power into (from bottom to top): 8 ohms, 4 ohms, 2 ohms.

The VS110's measured performance strongly indicates that it is very important that the correct output transformer tap be used for its owner's speakers. When that is done, and HF intermodulation apart, the amplifier gets a clean bill of health.—John Atkinson

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Audio Research
3900 Annapolis Lane North
Plymouth, MN 55447-5447
(763) 577-9700
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