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Equipment Measurements

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.

Additional Data
  • 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, 8-ohm load, 1k-ohm 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
Measurements Summary

Power output with 1kHz test signal

  • 8-ohm load at 1% THD: 102.0W
  • 8-ohm load at 10% THD: 122.5W

  • 4-ohm load at 1% THD: 130.8W
  • 4-ohm load at 10% THD: 163.6W


The Blue Circle DAR is a medium-power hybrid integrated amplifier. Overall gain of this unit is about right for a modern integrated amplifier -- just a bit lower than the average power-amp-only gain.

Chart 1 shows the frequency response of the amp with varying loads. This plot was made with the reference volume-control position set for 0.5V input to produce 5W output into an 8-ohm load. The high-frequency 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 high-frequency response with the volume control full up is much wider, with a -3dB point of about 200kHz. This is shown in Chart 1A. The high-frequency roll-off 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 high-frequency response with volume-control 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 open-circuit, 8-ohm, and 4-ohm loading, would be a bit high for a solid-state 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 8-ohm 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 AC-line 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: 8-ohm load
Blue line: 4-ohm load
Cyan line: NHT dummy-speaker load

Chart 1B

Response with volume control full up, 8-ohm load.

Chart 1C

Response with volume set for -30dB attenuation, 8-ohm load.

Chart 2 - Distortion as a Function of Power Output and Output Loading

(line up at 10W to determine lines)
Top line: 4-ohm SMPTE IM
Second line: 8-ohm SMPTE IM
Third line: 4-ohm THD+N
Bottom line: 8-ohm THD+N

Chart 3 - Distortion as a Function of Power Output and Frequency

8-ohm 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 8-ohm load


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