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, 8ohm load, unbalanced input, 1kohm input
termination: wideband 0.363mV, 77.8dBW; A weighted 0.038mV, 97.4dBW.
 Output noise, 8ohm load, balanced input, 600ohm 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
 8ohm load at 1% THD, unbalanced input: 85W
 8ohm load at 1% THD, balanced input: 145W
 8ohm load at 10% THD, unbalanced input: 183W
 8ohm load at 10% THD, balanced input: 183W
 4ohm load at 1% THD, unbalanced input: 160W
 4ohm load at 1% THD, balanced input: 209W
 4ohm load at 10% THD, unbalanced input: 270W
 4ohm load at 10% THD, balanced input: 270W
General
The Blue Circle BC202 is a medium/highpower hybrid design
with very wide bandwidth and of very unusual design. It utilizes a tube frontend, which
is not that unusual, and an output stage composed of power opamps, 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, 8ohm, and 4ohm 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
highfrequency 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 secondharmonicdistortion 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 1020W is a phenomenon that I can't explain. As can be
seen, attainable power is greater for the 4ohm 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 ACline 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.
Chart 1
 Frequency Response of Output Voltage as a Function of Output Loading 
Red line: open circuit
Magenta line: 8ohm load
Blue line: 4ohm load
Chart 2  Distortion as a Function
of Power Output and Output Loading 
(line up at 20W to determine lines)
Top line: 8ohm SMPTE IM
Second line: 4ohm SMPTE IM
Third line: 8ohm THD+N
Bottom line: 4ohm THD+N
Chart 3  Distortion
as a Function of Power Output and Frequency 
4ohm output loading
Cyan line: 200W
Blue line: 70W
Magenta line: 10W
Red line: 1W
Chart 4  Damping Factor
as a Function of Frequency 
Damping factor = output impedance divided into 8
Chart 5  Distortion and
Noise Spectrum 
1kHz signal at 10W into an 8ohm load
