February 2002Atma-Sphere MA-1 Mk II.2 Mono Amplifiers: Measurements
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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.
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