Blue Circle BC21 preamplifier & BC22 power amplifier Measurements part 2

The unweighted, wideband S/N ratio (ref. 1W into 8 ohms) was only fair at 58.5dB, due to the presence of some 60Hz hum and some ultrasonic noise. This improved to 83.3dB when A-weighted, but I would have preferred a lower noise floor. Distortion levels were also generally low, at around 0.03% in the midrange and bass (fig.9), but the THD rose precipitously in the treble, due to a low level of negative feedback, that lack of proper compensation, or both.

Fig.9 Blue Circle BC22, 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.

However, the harmonic content of that distortion was predominantly second-order (fig.10), even at high powers into 8 ohms (fig.11). With the torture 19+20kHz test, the difference component at 1kHz was low in level (fig.12). However, the poor HF linearity can be seen to generate some higher-order intermodulation components.

Fig.10 Blue Circle BC22, 1kHz waveform at 1W into 8 ohms (top), distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.11 Blue Circle BC22, spectrum of 50Hz sinewave, DC-1kHz, at 80W into 8 ohms (linear frequency scale).

Fig.12 Blue Circle BC22, HF intermodulation spectrum, DC-24kHz, 19+20kHz at 50W into 4 ohms (linear frequency scale).

Even with continuous drive and both channels operating (fig.13), the BC22 exceeded its specified power rating, offering about 156W (21.9dBW). But the clipping power dropped dramatically into lower impedances, with 55W available into 4 ohms (14.4dBW) and just 16W into 2 ohms (6dBW), implying a current-limited design. The picture was a little better when just one channel of the amplifier was driven with a low-duty-cycle 1kHz toneburst (fig.14), with now 163W available into 8 ohms at the 1% limit (black trace) and 270W into 4 ohms (red). But the 2 ohm delivery dropped back to 151W (blue), and just 53W were available into 1 ohm (green).

Fig.13 Blue Circle BC22, distortion (%) vs continuous output power into (from bottom to top at 10W): 8 ohms, 4 ohms, and 2 ohms.

Fig.14 Blue Circle BC22, distortion (%) vs 1kHz burst output power into 8 ohms (black trace), 4 ohms (red), 2 ohms (blue), and 1 ohm (green).

While I was impressed by the technical performance of Blue Circle's BC21 preamplifier, the BC22 power amp strikes me as only adequately engineered. Its measured performance is reminiscent of run-of-the-mill solid-state amplifiers you could buy 20 years ago. Yet KR was impressed with its overall sound quality, provided it was matched with appropriate loudspeakers. That's the ongoing paradox with audio components—do they sound they way they do because of their measured performance, or despite it?—John Atkinson

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