September 2004

Audio Research 150.2 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.
• All measurements, except for frequency response, were made with the new Audio Precision Aux-0025 filter, that is especially designed to keep the out-of-band switching noise out of the Audio Precision measurement system and thus prevent inaccurate measurements due to input overload.
• Gain, unbalanced input: 23.3x, 27.3dB.
• Output noise, 8-ohm load, 1k-ohm input termination: wideband 29.0mV, -39.8dBW; A weighted 0.72mV, -71.9dBW.
• AC line current draw at idle: 0.56A.
• Output impedance at 50Hz: 0.047 ohms.
• This amplifier does not invert polarity.

Power output with 1kHz test signal

• 8-ohm load at 1% THD: 166W
  
  
• 4-ohm load at 1% THD: 300W

General

The Audio Research 150.2 is yet another of the new breed of switching amplifiers utilizing the TriPath technology and controller chipset.

Measurements were made through the unbalanced inputs except otherwise noted. Performance through the balanced inputs was essentially the same. 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. However, the ultrasonic response past 20 kHz is a strong function of the load, with strong peaking near 100kHz with an open circuit load. The response with the NHT dummy load is reasonable, with a variation of about +0.6/-0dB over the audio range. Some speakers utilizing dome tweeters, due to the high frequency inductive nature of these types of drivers, may exhibit some fractional part of this resonant rise above the audio range. 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 TriPath system has a peculiar behavior at power levels approaching clipping. The output waveform starts to exhibit some switching noise near the peaks that will get through the Aux-0025 filter and into the distortion measurement when the bandwidth of the measurement is opened up to 80kHz. Generally, I use the 80 kHz measurement bandwidth to more accurately let some of the actual signal harmonics get through at frequencies at the high end of the audio band. This shows up as the higher distortion at the 200W and 300W levels on the chart, and does not indicate distortion that would necessarily be audible. Damping factor versus frequency is shown in Chart 4. A spectrum of the harmonic distortion and noise residue is plotted in Chart 5. The magnitude of the AC line harmonics is reasonably low for this amplifier. The test signal harmonics are both even and odd harmonic, with the second and third harmonics dominating the total distortion makeup.

Red line: open circuit
Cyan line: NHT dummy-speaker load
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: 4-ohm THD+N
Third line: 8-ohm SMPTE IM
Bottom line: 8-ohm THD+N

4-ohm output loading
Green line: 300W
Cyan line: 200W
Blue line: 140W
Magenta line: 20W
Red line: 2W

Damping factor = output impedance divided into 8

1kHz signal at 10W into an 8-ohm load

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