February 2002
AtmaSphere MA1
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.
 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, 8ohm 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 standby).
 Output impedance (measured by an injection of a constant 1A
of current at 50Hz): 10.5 ohms.
 This amplifier does not invert polarity.
Power output with 1kHz test signal
 8ohm load at 1% THD: 20W
 8ohm load at 10% THD: 120W
 4ohm load at 1% THD: 8W
 4ohm load at 10% THD: 80W
 16ohm load at 1% THD: 90W
 16ohm load at 10% THD: 130W
General
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 4ohm 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 8ohm
loads is about 90100W and 7080W into 4ohm 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: 8ohm load
Blue line: 4ohm load
Chart 2  Distortion as a Function
of Power Output and Output Loading 
Top line: 4ohm SMPTE IM
Second line: 8ohm SMPTE IM
Third line: 4ohm THD+N
Bottom line (red): 8ohm THD+N
Chart 3  Distortion
as a Function of Power Output and Frequency 
8ohm 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 8ohm load
