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Equipment Measurements

September 2009

Belles Statement SA-30 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.

Additional Data
  • Measurements were made at 120V AC line voltage with both channels being driven. Measurements made on left channel through unbalanced inputs unless otherwise noted.
  • This amplifier does not invert polarity.
  • AC line current draw at idle: 2.17A, 0.8PF, 208W (thoroughly warmed up).
  • Input impedance @ 1kHz: 90k ohms.
  • Output impedance at 50Hz: 0.0032 ohm.
  • Input sensitivity for 1W output in 8 ohms, Lch/Rch: 133.4mV/134.8mV
  • Gain (8-ohm load), Lch/Rch: 21.2X, 26.5dB/21.0X, 26.4dB
  • Output noise, 8-ohm load, 1k-ohm input termination:
    • Wideband, Lch/Rch: 180uV, -83.9 dBW/206uV, -82.8 dBW
    • A weighted, Lch/Rch: 51 uV, -94.9 dBW/61 uV, -93.3 dBW
Measurements Summary

Power output with a 1kHz test signal

  • 16-ohm load at 1% THD: 24.7W
  • 16-ohm load at 10% THD: 32.4W

  • 8-ohm load at 1% THD: 43.3W
  • 8-ohm load at 10% THD: 56.9W

  • 4-ohm load at 1% THD: 76.0W
  • 4-ohm load at 10% THD: 93.2W

General

Here we have a very rare animal: an honest-to-God true class-a amplifier. True in the classic sense that the AC input power stays constant up to output clipping into 8-ohm loads. For lower loads, the mode of operation does change to class AB, but at reasonably high powers -– higher than where the kernel of most of the music resides at reasonable playing volumes.

This amp gets quite HOT when warmed up. I measured about 140 F at the center of the heat sinks and about 120 F on the front panel, top plate, and rear panel. Surprisingly, when the top cover was removed, the internal temperature of the components was less -- a good thing for the life of these components.

Chart 1 shows the frequency response of the amp with varying loads. The high-frequency response is moderately wide with an approximate 3dB down point of 100kHz. Output impedance, as judged by the closeness of spacing between the curves of open-circuit, 8-ohm, and 4-ohm loading, is quite low in the audio band and even up to 200kHz. The usual NHT dummy-load curve is not shown as the variations in the response would not show. The variation with the NHT dummy load in the audio range is of the order of +/-0.005dB -– a truly negligible amount.

Chart 2 illustrates how THD+N (total harmonic distortion plus noise) vs. power varies for 1 kHz and SMPTE IM test signals and amplifier output load. Amount of distortion is noise dominated up to about 1W and starts to rise smoothly up to clipping.

THD+N as a function of frequency at several different power levels is plotted in Chart 3 for 4-ohm loads. Typical of most power amplifiers, the THD+N rises with frequency above 500–1000 Hz. In this design, the rise is quite pronounced. Results for this test were similar for 8-ohm loading.

Damping factor vs. frequency is shown in Chart 4 and is of an unusually high value up to about 400Hz and, again, typically, falls off above this frequency. Still at 20kHz, it is greater than 170.

A spectrum of the harmonic distortion and noise residue of a 10W 1kHz test signal is plotted in Chart 5 for 4-ohm loading. The AC-line harmonics are complex but reasonable in magnitude and caused mostly by filter-capacitor-charging current pulses getting into the signal-ground circuitry. The signal harmonics fall off nicely with order at this power level and test frequency.

Chart 1 - 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 10W to determine lines)
Top line: 8-ohm SMPTE IM
Second line: 4-ohm SMPTE IM
Third line: 8-ohm THD+N
Bottom line: 4-ohm THD+N


(line up at 10W to determine lines)
Top line: 16-ohm SMPTE IM
Bottom line: 16-ohm THD+N

Chart 3 - Distortion as a Function of Power Output and Frequency


4-ohm output loading
Green line: 60W
Blue line: 30W
Magenta line: 10W
Red line: 1W

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 4-ohm load

 

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