November 2004

Accustic Arts Amp II-AC High Performance Stereo 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.

• Measurements were made with 120V AC line voltage with both channels driven using the balanced inputs (there are only balanced inputs present on this amplifier).
• Gain: 17.6x, 24.9dB.
• Output noise, 8-ohm load, balanced input, 600-ohm input termination: wideband 0.440mV, -76.2dBW; A weighted 0.130mV, -86.8dBW.
• AC line current draw at idle: 1.72A.
• Output impedance at 50Hz: 0.0049 ohms.
• This amplifier inverts polarity.

Power output with 1kHz test signal

• 8-ohm load at 1% THD: 200W
  
  
• 4-ohm load at 1% THD: 350W

General

The Accustic Arts Amp II-AC High Performance is a very large and heavy but medium-/high-power solid-state design with typically wide bandwidth and very low output impedance.

Chart 1 shows the frequency response of the amp with varying loads. As can be seen, the output impedance, as judged by the closeness of spacing between the curves of open-circuit, 8-ohm and 4-ohm loading is quite low. The variation with the NHT dummy load in the audio range is negligible. What is unusual about this design is that the curves stay virtually overlaid all the way out to 200kHz.

Chart 2 illustrates how total harmonic distortion plus noise versus power varies for 1kHz and SMPTE IM test signals and amplifier output load. As can be seen, attainable power is greater for the 4-ohm load, as is usual for most power amplifiers.

Total harmonic distortion plus noise as a function of frequency at several different power levels is plotted in Chart 3. The amount of rise in distortion at high frequencies is fairly pronounced, and is similar to that of many amplifiers measured. Note that the curves for the higher powers of 100W and 300W don't extend all the way to 20kHz as the amp's protection circuit came into play above the highest frequencies shown on these curves.

Damping factor vs. frequency is shown in Chart 4 and is unusually high.

A spectrum of the harmonic distortion and noise residue is plotted in Chart 5. The magnitude of the AC-line harmonics is unusually low in this design. At the test level of 10W into 4 ohms, the signal harmonics are admirably low, with only the second harmonic showing above the noise level. Note that from the additional data the noise level is a bit higher than that of some other recent amplifiers measured and some of the higher signal harmonics may be under this noise level -- but also note that they would be less than 0.0001%.

Red line: open circuit
Magenta line: 8-ohm load
Blue line: 4-ohm load

(line up at 20W 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

4-ohm output loading
Cyan line: 300W
Blue line: 100W
Magenta line: 20W
Red line: 2W

Damping factor = output impedance divided into 8

1kHz signal at 10W into a 4-ohm load

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