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Equipment Measurements

November 2002

Opera Consonance Reference 9.9A Mono Amplifiers: 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 with 120V AC line voltage.
  • Gain: 14.9x, 23.5dB.
  • Output noise, 8-ohm load: wideband 1.33mV, -66.5dBW; A weighted 0.110mV, -88.2dBW.
  • AC line current draw at idle: 1.36A.
  • Output impedance (measured by an injection of a constant 1A of current at 50Hz): 2.86 ohms.
  • This amplifier inverts polarity.
Measurements Summary

Power output with 1kHz test signal

  • 8-ohm load at 1% THD: 10W
  • 8-ohm load at 10% THD: 22W

  • 4-ohm load at 1% THD: 1.8W
  • 4-ohm load at 10% THD: 17W

  • 16-ohm load at 1% THD: 13W
  • 16-ohm load at 10% THD: 18W


The Reference 9.9A SET (single-ended triode) amplifier measured here is typical of a design utilizing the venerable 845 output tube. Frequency response, as seen in Chart 1, rolls off at both ends of the audio range and there is some pronounced ultrasonic aberrations, likely due to the characteristics of the output transformer. Output impedance is moderate giving an approximate plus and minus 2dB frequency-response variation on the NHT dummy speaker load. Total harmonic distortion plus noise as a function of power output for a test frequency of 1kHz is plotted in Chart 2. It shows that the amount of distortion at the 1kHz test frequency is reasonably low at less than 1% up to 10W output for the 8-ohm load on the 8-ohm output (and measures similarly for a 4-ohm load on the 4-ohm output). However, loading the 8 ohm output with 4 ohm or 16 ohm load raises or lowers the distortion considerably. Total harmonic distortion plus noise as a function of frequency at several power levels is plotted in Chart 3. Amount of distortion over the main midrange energy band is less than 1-2% for power outputs of 10W or less. However, distortion does rise considerable at both ends of the audio range. Damping factor versus frequency shown in Chart 4 is a modest 2.5 over the middle range of frequencies, and actually increases a bit at the frequency extremes. Not surprisingly, on the 4-ohm output, the damping factor was about double. In the spectral plot of distortion and noise for a 10W 1kHz signal into an 8-ohm load on the 8-ohm output (Chart 5), the signal distortion components are dominated by the second and third harmonics with the higher-order products tailing off rapidly. There are some 120Hz sidebands around the nulled out fundamental along with a fairly high amount of AC line hum harmonic frequencies. The amount of wideband noise of over 1mV would likely be audible on speakers with efficiencies of 90dB or more.

Chart 1 - Frequency Response of Output Voltage as a Function of Output Loading

Magenta line: open circuit
Red line: 8-ohm load
Blue line: 4-ohm load
Cyan line: NHT dummy speaker load

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

(line up at 10W to determine lines)
Top line: 8-ohm SMPTE IM
Second line: 4-ohm THD+N
Third line: 8-ohm THD+N
Bottom line (red): 16-ohm THD+N

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

8-ohm output loading
Green line: 20W
Cyan line: 15W
Blue line: 10W
Magenta line: 5W
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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