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
- 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
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
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
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%.
- Frequency Response of Output Voltage as a Function of Output Loading
Red line: open circuit
Magenta line: 8-ohm load
Blue line: 4-ohm load
|Chart 2 - Distortion as a Function
of Power Output and Output Loading
(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
|Chart 3 - Distortion
as a Function of Power Output and Frequency
4-ohm 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
1kHz signal at 10W into a 4-ohm load