August 2004

Belles 150A Reference 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, including graphical information displayed below, is the property of SoundStage!. Reproduction in any format is not permitted.

• Measurements were made with 120V AC line voltage with both channels driven.
• Output noise, 8-ohm load, unbalanced input, 1k-ohm input termination: wideband 0.116mV, -87.7dBW; A weighted 0.048mV, -95.4dBW.
• AC line current draw at idle: 0.68A.
• Output impedance at 50Hz: 0.0028 ohms.
• This amplifier does not invert polarity.

Power output with 1kHz test signal

• 8-ohm load at 1% THD: 200W
  
  
• 4-ohm load at 1% THD: 337W

General

The Belles 150A is a medium-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 (not plotted) is negligible. Chart 2 illustrates how total harmonic distortion plus noise vs. 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. Amount of rise in distortion at high frequencies is typical of many power amplifiers. Damping factor vs. frequency is shown in Chart 4. Unusual is how high the damping factor is at low to medium frequencies. 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 with the odd harmonics dominating the total harmonic sum.

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

(line up at 20W to determine lines)
Top line: 4-ohm THD+N
Second line: 4-ohm SMPTE IM
Third line: 8-ohm THD+N
Bottom line: 8-ohm SMPTE IM

8-ohm output loading
Cyan line: 125W
Blue line: 60W
Magenta line: 10W
Red line: 1W

Damping factor = output impedance divided into 8

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

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