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Equipment Measurements

February 2002

Atma-Sphere MA-1 Mk II.2 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 at 120V AC line voltage.
  • Current adjusted to 750mA when warmed up.
  • Input signal applied to unbalanced RCA connector (pin 1 and 3 of the XLR input connector was bridged and results when taken via the XLR connector were essentially the same)
  • Power output plotted with one channel driven (this is a mono amplifier).
  • Gain: 24.6x, 27.8dB.
  • Output noise, 8-ohm load: wideband 2.1mV, -62.6dBW; A weighted 0.548mV, -74.4dBW.
  • AC line current draw at idle: 4.2A (when operating); 2.7A (in stand-by).
  • Output impedance (measured by an injection of a constant 1A of current at 50Hz): 10.5 ohms.
  • This amplifier does not invert polarity.
Measurements Summary

Power output with 1kHz test signal

  • 8-ohm load at 1% THD: 20W
  • 8-ohm load at 10% THD: 120W

  • 4-ohm load at 1% THD: 8W
  • 4-ohm load at 10% THD: 80W

  • 16-ohm load at 1% THD: 90W
  • 16-ohm load at 10% THD: 130W


This amplifier design has a rather high output impedance. Consequently, the delivered frequency response to the speaker can vary a number of decibels over the frequency range. This can be seen in Chart 1 for open circuit, 8- and 4-ohm loading along with the NHT dummy speaker load. As can be seen in Chart 2, distortion, while sufficiently low at power up to about 10W, does rise to higher percentage numbers as maximum power is approached. Maximum power into 8-ohm loads is about 90-100W and 70-80W into 4-ohm loads. A desirable attribute in power amplifiers and one rarely seen is illustrated in Chart 3 where the amount of distortion is quite constant with frequency above a few watts. The unusually high output impedance of this unit results in a damping factor of less than one as shown in Chart 4. However, as with the distortion, it is essentially flat with frequency. A spectrum of the distortion of a 1kHz signal at 10W into 8 ohms is plotted in Chart 5. The third harmonic is dominant, attesting to the balanced nature of the circuitry. However, there are some 120Hz hum sidebands around the nulled out 1kHz fundamental -- quite a bit more than I have seen in other amplifiers.

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

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

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

Top line: 4-ohm SMPTE IM
Second line: 8-ohm SMPTE IM
Third line: 4-ohm THD+N
Bottom line (red): 8-ohm THD+N

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

8-ohm output loading
Green line: 80W
Blue line: 30W
Magenta line: 10W
Red line: 1W

Chart 4 - Damping Factor as a Function of Frequency

Damping factor = output impedance (measured by an injection of a constant 1A at all frequencies) divided into 8

Chart 5 - Distortion and Noise Spectrum

1kHz signal at 10W into an 8-ohm load


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