Blue Circle BC21 preamplifier & BC22 power amplifier Measurements

Sidebar 3: Measurements

BC21: I first looked at the performance of the Blue Circle BC21 on the test bench. Its maximum voltage gain was a moderate 18.7dB and its input impedance was high at around 50k ohms, the exact figure varying slightly with the position of the volume control. The output impedance was quite low at 453 ohms at 1kHz and 20kHz, this increasing slightly to 727 ohms at 20Hz. The BC21 inverted absolute signal polarity.

The Blue Circle's frequency response at 1V output (fig.1) was flat within the audioband, but there was a greater degree of ultrasonic rolloff with its volume control set to unity gain (11:00) than to its maximum position, when it reached -3dB at 200kHz. Dropping the load impedance to 600 ohms also slightly increased the extent of the ultrasonic rolloff. Note from this graph that, at the unity-gain setting of the volume control, there is a channel imbalance of 0.3dB. Channel separation (fig.2) was excellent, at >100dB below 400Hz, and while capacitive coupling reduced the separation to 70dB at 20kHz, this is still good.

Fig.1 Blue Circle BC21, frequency response at 1V into 100k ohms with volume control at maximum (top) and at unity gain (bottom). (0.5dB/vertical div., right channel dashed.)

Fig.2 Blue Circle BC21, channel separation (10dB/vertical div., R-L dashed).

Noise levels were low, and the distortion+noise reading with a 1V output stayed at around 0.04% across the band. Increasing the output to 2V—the highest level that this preamp will be expected to deliver—raised the level of the second harmonic to -60dB (0.1%), which will be subjectively benign (fig.3). Dropping the test impedance to the punishing 600 ohm load increased the level of the second harmonic to -35dB (1.8%), and the third, fourth, and fifth harmonics all made appearances (not shown). This preamp should not be used with power amplifiers featuring such low impedances. Into 100k ohms, intermodulation distortion was low, the 1kHz difference component with a combined 19kHz+20kHz signal (fig.4) remaining below -66dB (0.05%).

Fig.3 Blue Circle BC21, spectrum of 50Hz sinewave, DC-1kHz, at 2V into 100k ohms (linear frequency scale).

Fig.4 Blue Circle BC21, HF intermodulation spectrum, DC-24kHz, 19+20kHz at 2V into 100k ohms (linear frequency scale).

The BC21's output stage actually appeared to be a little more linear when it was delivering moderate current, as can be seen in fig.5, where the lowest measured distortion occurred with the preamp driving 4k ohms (bottom trace). Even into this load, the maximum voltage (1% THD+N) was a high 13V, well above any real-world need. However, this graph also shows the BC21's poor performance into 600 ohms (top trace), where just 700mV is available for the same 1% THD+N limit. As I said earlier, this well-engineered little tube preamp should not be used into such low loads.

Fig.5 Blue Circle BC21, distortion (%) vs output voltage into (from bottom to top at 1V): 4k ohms, 10k ohms, 100k ohms, and 600 ohms.

BC22: The slimline, solid-state BC22 power amplifier offered a voltage gain of 27.3dB from its unbalanced inputs. (I didn't test the balanced inputs, as these cannot be used with the BC21.) The BC22 didn't invert absolute polarity, and its unbalanced impedance was a moderately high 22.7k ohms, well-suited to the BC21 preamp. After the one-hour preconditioning test at one-third power into 8 ohms, which thermally stresses an amplifier with a conventional class-B output stage to the maximum degree, the BC22's chassis was way too hot to touch, and the amplifier was actually switching off and on about once per second. Reducing the power level prevented this from happening, but the hot chassis and the thermally induced cycling suggest inadequate heatsinking.

Channel separation (not shown) was excellent, at better than 90dB below 2kHz, and still 77dB (L-R) and 87dB (R-L) at 20kHz. The output impedance was a moderately low 0.13 ohm across most of the band, this dropping to 0.1 ohm at 20kHz. As a result, there will be virtually no response interaction between the amplifier and the speaker impedance. However, as can be seen in fig.6, the ultrasonic response peaks by 4dB at 105kHz. This adds a noticeable overshoot spike and ringing to both the 1kHz (fig.7) and 10kHz (fig.8) squarewaves, and suggests either the presence of a parasitic resonance or inadequate HF compensation in the amplifier's main gain stage, or both, which might lead to instability under some circumstances. This behavior is generally regarded as not a good thing, but I note than KR was not bothered by any untoward HF behavior.

Fig.6 Blue Circle BC22, frequency response at (from top to bottom at 2kHz): 2.83V into dummy loudspeaker load, 1W into 8 ohms, and 2W into 4 ohms (0.5dB/vertical div., right channel dashed).

Fig.7 Blue Circle BC22, small-signal 1kHz squarewave into 8 ohms.

Fig.8 Blue Circle BC22, small-signal 10kHz squarewave into 8 ohms.

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