July 2005

Blue Circle Audio BC202 Stereo Amplifier: Measurements

All amplifier measurements are performed independently by BHK Labs. Please click to learn more about how we test amplifiers there. All measurement data and graphical information displayed below are the property of SoundStage! and Schneider Publishing Inc. Reproduction in any format is not permitted.

• Measurements were made at 120V AC line voltage with both channels being driven, test signals driving the balanced inputs unless otherwise noted. Measurements made on left channel unless otherwise noted
• Gain: 12.6x, 22dB unbalanced input; 12.4x, 21.8dB balanced input.
• Output noise, 8-ohm load, unbalanced input, 1k-ohm input termination: wideband 0.363mV, -77.8dBW; A weighted 0.038mV, -97.4dBW.
• Output noise, 8-ohm load, balanced input, 600-ohm input termination: wideband 0.361mV, -77.9dBW; A weighted 0.038mV, -97.4dBW.
• AC line current draw at idle: 0.86A.
• Output impedance at 50Hz: 0.12 ohms.
• This amplifier does not invert polarity on the unbalanced input; however, it does invert polarity on the balanced input.

Power output with 1kHz test signal

• 8-ohm load at 1% THD, unbalanced input: 85W
• 8-ohm load at 1% THD, balanced input: 145W
• 8-ohm load at 10% THD, unbalanced input: 183W
• 8-ohm load at 10% THD, balanced input: 183W
  
  
• 4-ohm load at 1% THD, unbalanced input: 160W
• 4-ohm load at 1% THD, balanced input: 209W
• 4-ohm load at 10% THD, unbalanced input: 270W
• 4-ohm load at 10% THD, balanced input: 270W

General

The Blue Circle BC202 is a medium-/high-power hybrid design with very wide bandwidth and of very unusual design. It utilizes a tube front-end, which is not that unusual, and an output stage composed of power op-amps, which is most unusual.

Chart 1 shows the frequency response of the amp with varying loads. As can be seen, the output impedance, as judged by the closeness of spacing between the curves of open circuit, 8-ohm, and 4-ohm loading, is quite low. The variation with the NHT dummy load in the audio range is of the order of +/-0.1dB. Note the very wide high-frequency bandwidth and a trace of rising response suggestive of a possible peak in the response beyond 200kHz.

The distortion behavior of the BC202 is very different with the unbalanced and balanced inputs. With the unbalanced inputs, the dominant distortion is second harmonic and of relatively high value. Use of the balanced inputs causes cancellation of most of the second-harmonic-distortion component and results in much lower measured distortion. Chart 2 illustrates how total harmonic distortion plus noise vs. power varies for a 1kHz and SMPTE IM test signals and amplifier output load. The strange dip in distortion between 1W and 10-20W is a phenomenon that I can't explain. As can be seen, attainable power is greater for the 4-ohm load, as is usual for most power amplifiers.

Total harmonic distortion plus noise as a function of frequency at several different power levels is plotted in Chart 3. The amount of rise in distortion at high frequencies is admirably low. There are some unusual changes in the shape of the distortion curves below about 500Hz for the lower powers.

Damping factor vs. frequency is shown in Chart 4 and is of a good effective value and is reasonably constant with frequency.

A spectrum of the harmonic distortion and noise residue of a 10W 1kHz test signal is plotted in Chart 5. The magnitude of the AC-line harmonics is quite numerous and intermodulation components of line harmonics with signal harmonics are also very numerous and visible. The test signal harmonics are both even and odd and decline reasonably quickly with frequency.

Red line: open circuit
Magenta line: 8-ohm load
Blue line: 4-ohm load

(line up at 20W to determine lines)
Top line: 8-ohm SMPTE IM
Second line: 4-ohm SMPTE IM
Third line: 8-ohm THD+N
Bottom line: 4-ohm THD+N

4-ohm output loading
Cyan line: 200W
Blue line: 70W
Magenta line: 10W
Red line: 1W

Damping factor = output impedance divided into 8

1kHz signal at 10W into an 8-ohm load

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