Bel Canto eVo 200.2 power amplifier Measurements part 2

The active low-pass filter did come in useful for examining the Bel Canto's channel-separation and THD performance, however. Fig.5 shows the separation, adjusted for the filter's insertion loss. It is quite different in the two directions, the R-L trace showing the usual rise at high frequencies due to capacitive coupling, the L-R showing a pronounced minimum at 2.5kHz.


Fig.5 Bel Canto eVo 200.2, channel separation (10dB/vertical div., R-L dashed).

With the output-stage switching noise eliminated by the filter, the true level of measured distortion (fig.6) was low, though better into 8 ohms than into 4 or 2 ohms. It also showed the ubiquitous rise at high frequencies. The waveform of that distortion was primarily third harmonic (fig.7, about 0.04% without switching noise), though higher harmonics are also present (fig.8). However, levels of intermodulation distortion were also low, even with the punishing combination of 19 and 20kHz tones just below visible clipping into 4 ohms (fig.9).


Fig.6 Bel Canto eVo 200.2, THD+noise (%) vs frequency at (from top to bottom at 3kHz): 4W into 2 ohms, 2.83V into simulated loudspeaker load, 2W into 4 ohms, and 1W into 8 ohms.


Fig.7 Bel Canto eVo 200.2, 1kHz waveform at 6W into 4 ohms (top), distortion and noise waveform with fundamental notched out (bottom, not to scale).


Fig.8 Bel Canto eVo 200.2, spectrum of 50Hz sinewave, DC-1kHz, at 62W into 4 ohms (linear frequency scale).

Due to the amplifier's floating (non-grounded) output-stage topology, it wasn't possible to measure the eVo 200.2's dynamic output power using the Miller Audio Research Analyzer, which has a single-ended input. (Interestingly, to even the load on the power supply, the EVo's two channels are internally wired in anti-phase.) But I did assess the amplifier's continuous output power—again using the active low-pass filter to eliminate switching noise—and the results, adjusted for the insertion loss of the filter, are shown in fig.10. The 8 ohm power at the 1% THD+noise clipping point was 120W (20.8dBW) as specified. However, as should be apparent from the truncated traces in this graph, the amplifier turned itself off—with a mighty chattering of relays—well below the actual clipping point into 4 and 2 ohm loads.


Fig.9 Bel Canto eVo 200.2, HF intermodulation spectrum, DC-24kHz, 19+20kHz at 75W into 4 ohms (linear frequency scale).


Fig.10 Bel Canto eVo 200.2, distortion (%) vs continuous output power into (from bottom to top at 1W): 8 ohms, 4 ohms, and 2 ohms.

The eVo 200.2 is the second amplifier I have measured that uses a switching output stage (the first was the Sharp SM-SX100 in July 2000), and I am in no doubt that this kind of amplifier will become increasingly common in the next few years due to their intrinsically high efficiency. Other than its aggressive protection circuitry, the Bel Canto appears to have few anomalies in its measured performance, though I would have liked to have seen less higher-order harmonics in its output, as low in level as these appeared to be.—John Atkinson

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