November 2004
Accustic Arts
Amp IIAC 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, 8ohm load, balanced input, 600ohm 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
 8ohm load at 1% THD: 200W
 4ohm load at 1% THD: 350W
General
The Accustic Arts Amp IIAC High Performance is a very
large and heavy but medium/highpower solidstate 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 opencircuit, 8ohm and 4ohm 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 4ohm 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 ACline 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%.
Chart 1
 Frequency Response of Output Voltage as a Function of Output Loading 
Red line: open circuit
Magenta line: 8ohm load
Blue line: 4ohm load
Chart 2  Distortion as a Function
of Power Output and Output Loading 
(line up at 20W to determine lines)
Top line: 4ohm SMPTE IM
Second line: 8ohm SMPTE IM
Third line: 4ohm THD+N
Bottom line: 8ohm THD+N
Chart 3  Distortion
as a Function of Power Output and Frequency 
4ohm output loading
Cyan line: 300W
Blue line: 100W
Magenta line: 20W
Red line: 2W
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 a 4ohm load
