September 2008
Blue Circle
Audio DAR Integrated 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 driven; measurements made on left channel with unbalanced input unless otherwise
noted.
 This integrated amplifier does not invert polarity.
 AC line current draw: 0.32A
 Input sensitivity for 1W output into 8 ohms, volume at
maximum, Lch/Rch: 174mV / 175mV
 Input impedance @ 1kHz: 95.0k ohms
 Output impedance at 50Hz: 0.12 ohms
 Gain, output voltage divided by input voltage, volume at
maximum, Lch/Rch: 16.26X, 24.2dB / 16.20X, 24.2dB
 Output noise, 8ohm load, 1kohm input termination, Lch/Rch
 Volume control at reference position
 wideband: 0.62mV, 73.2dBW / 0.57mV, 73.9dBW
 A weighted: 0.36mV, 77.9dBW / 0.33mV, 78.7dBW
 Volume control full clockwise
 wideband: 0.83mV, 70.7dBW / 0.75mV, 71.5dBW
 A weighted: 0.49 mV, 75.2 dBW / 0.47 mV, 75.6 dBW
 Volume control set for 30dB attenuation
 wideband: 0.21mV, 82.6dBW / 0.19mV, 83.4dBW
 A weighted: 0.062mV, 93.2dBW / 0.058mV, 93.8dBW
 Volume control full counterclockwise
 wideband: 0.079mV, 91.1dBW / 0.077mV, 91.3dBW
 A weighted: 0.036mV, 97.9dBW / 0.034mV, 98.4dBW
Power output with 1kHz test signal
 8ohm load at 1% THD: 102.0W
 8ohm load at 10% THD: 122.5W
 4ohm load at 1% THD: 130.8W
 4ohm load at 10% THD: 163.6W
General
The Blue Circle DAR is a mediumpower hybrid integrated
amplifier. Overall gain of this unit is about right for a modern integrated amplifier 
just a bit lower than the average poweramponly gain.
Chart 1 shows the frequency response of the amp with
varying loads. This plot was made with the reference volumecontrol position set for 0.5V
input to produce 5W output into an 8ohm load. The highfrequency response at the
reference setting of the volume control is somewhat rolled off, with an approximate 3dB
point of about 50kHz. The attenuation at this setting of the volume control is about 2.7dB
down from full gain. In contrast, the highfrequency response with the volume control full
up is much wider, with a 3dB point of about 200kHz. This is shown in Chart 1A. The
highfrequency rolloff shape of Chart 1 more or less holds down to 20dB attenuation,
where the bandwidth is starting to widen again. At 30dB attenuation, a setting that could
well be a typical listening level, bandwidth is about 80kHz and is shown in Chart 1B.
Below this setting, bandwidth continues to increase to what's shown in Chart 1A. This
general behavior of varying highfrequency response with volumecontrol setting is usually
caused by too much capacitance to ground following the volume control. Tracking between
channels was within about 0.1dB from full up to 0dB of attenuation, where tracking began
to diverge to about 1.2dB at 50dB and about 5dB at 60dB of attenuation.
Output impedance, as judged by the closeness of spacing
between the curves of opencircuit, 8ohm, and 4ohm loading, would be a bit high for a
solidstate power amplifier. Still, the variation with the NHT dummy speaker load is only
on the order of +/0.15dB  not much of an audible difference with most speakers.
Chart 2 illustrates how total harmonic distortion plus
noise vs. power varies for 1kHz and SMPTE IM test signals and amplifier output load. The
relatively high values of distortion indicate that there is probably not much negative
feedback taken around the output stage of this design.
Total harmonic distortion plus noise as a function of
frequency at several different power levels is plotted in Chart 3 for 8ohm loading. A
good result is that the amount of rise in distortion at high frequencies is very low.
Damping factor vs. frequency is shown in Chart 4 and is of
a reasonable value at low frequencies and, as typical of most power amplifiers, it begins
to fall off rapidly with frequency at about 500Hz.
A spectrum of the harmonic distortion and noise residue of
a 10W 1kHz test signal into 8 ohms is plotted in Chart 5. The magnitudes of the ACline
harmonics are quite high and complex in nature, as are the signal harmonics.
Intermodulation components of line harmonics with signal harmonics are quite numerous and
extend up to about 10kHz.
Chart 1
 Frequency Response of Output Voltage as a Function of Output Loading 
Chart 1A
Red line: open circuit
Magenta line: 8ohm load
Blue line: 4ohm load
Cyan line: NHT dummyspeaker load
Chart 1B
Response with volume control full up, 8ohm load.
Chart 1C
Response with volume set for 30dB attenuation, 8ohm load.
Chart 2  Distortion as a Function
of Power Output and Output Loading 
(line up at 10W to determine lines)
Top line: 4ohm SMPTE IM
Second line: 8ohm SMPTE IM
Third line: 4ohm THD+N
Bottom line: 8ohm THD+N
Chart 3  Distortion
as a Function of Power Output and Frequency 
8ohm output loading
Cyan line: 100W
Blue line: 30W
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
