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
- 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
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
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.
- Frequency Response of Output Voltage as a Function of Output Loading
Red line: open circuit
Magenta line: 8-ohm load
Blue line: 4-ohm load
|Chart 2 - Distortion as a Function
of Power Output and Output Loading
(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
|Chart 3 - Distortion
as a Function of Power Output and Frequency
4-ohm 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
1kHz signal at 10W into an 8-ohm load