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Equipment Measurements

September 2006

Belles 350A 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 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 the unbalanced inputs in stereo mode on the left channel unless otherwise noted. "Bridge mode" power would be double the stereo mode power of both channels into twice the stereo load impedance.
  • This amplifier does not invert polarity.
  • AC line current draw at idle: 1.25A.
  • Input impedance @ 1kHz: 44k ohms.
  • Output impedance at 50Hz: 0.004 ohms.
  • Gain (8-ohm load): 21.1X, 26.5dB.
  • Output noise, 8-ohm load, unbalanced input, 1k-ohm input termination, Lch/Rch:
    wideband - 0.192mV, -83.4dBW / 0.261mV, -80.7dBW
    A weighted - 0.116mV, -87.7dBW / 0.171mV, -84.4dBW
Measurements Summary

Power output with 1kHz test signal

  • 8-ohm load at 1% THD: 413W
  • 8-ohm load at 10% THD: 506W

  • 4-ohm load at 1% THD: 658W
  • 4-ohm load at 10% THD: 821W


The Belles 350A is a high-power linear solid-state amplifier representing the culmination of David Belles’ 20-year amplifier research and design experience.

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 over the audio range, and only above perhaps 50kHz can one see any sign of a change with loading. The NHT dummy speaker load is not plotted here as the variations with frequency would not be observable with this vertical scale factor. Needless to say, difficult, impedance-varying speaker loads are not going to affect the flat-delivered response from this amplifier.

Chart 2 illustrates how total harmonic distortion plus noise vs. power varies for 1kHz and SMPTE IM test signals and 4- and 8-ohm loads. As can be seen, attainable power is greater for the 4-ohm load, as is usual for most power amplifiers. Furthermore, the attainable power is quite a bit more than the rated power for this amp. I contacted Mr. Belles to check on this and he said something to the effect that he "likes to be conservative." Well, that is the most conservative amplifier power output rating I have seen! Unusual for these measured data is that the IM distortion vs. power is so close to the 1kHz harmonic distortion vs. power. This is actually indicative of the very low distortion of this design, with actual distortion rising above the noise only at the onset of output clipping.

Total harmonic distortion plus noise as a function of frequency at several different power levels is plotted in Chart 3. There is a moderate rise in distortion with frequency at the higher power levels.

Damping factor vs. frequency is shown in Chart 4 and is very high at low frequencies, but as is typical for most amplifiers, it starts to decline above a few hundred Hertz. But note, since the damping factor is so high, it is still greater than 100 at 20kHz!

A spectrum of the harmonic distortion and noise residue of a 10W 1kHz test signal for 4-ohm loading is plotted in Chart 5. Even though the measured output noise of this amp was low, the nature of the noise is mostly pulses when the power-supply rectifiers conduct and, consequently, there are a lot of higher harmonics of the AC-line frequency in this spectrum. The 1kHz signal distortion consists of a quickly declining series of even and odd harmonics.

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 100W 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

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

4-ohm output loading
Cyan line: 500W
Blue line: 100W
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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