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Unless otherwise noted, measurements were taken at the balanced input and at 120V AC line voltage, with the Audio Precision AUX-0025 measurement filter.

Power output

  • Power output at 1% THD+N: 358.4W @ 8 ohms, 658.4W @ 4 ohms
  • Power output at 10% THD+N: 452.7W @ 8 ohms, 852.4W @ 4 ohms

Additional data

  • Input/output polarity: noninverting
  • AC line current draw at idle: 16.0W, 0.32A, 0.44PF
  • Gain: output voltage divided by input voltage, 8-ohm load
    •      Unbalanced input: 22.5X, 27.0dB
    •      Balanced input: 22.3X, 27.0dB
  • Input sensitivity for 1W output into 8 ohms
    •      Unbalanced input: 125.7mV
    •      Balanced input: 126.9mV
  • Output impedance @ 50Hz: 0.03 ohm
  • Input impedance @ 1kHz
    •      Unbalanced input: 92.5k ohms
    •      Balanced input: 92.5k ohms
  • Output noise, 8-ohm load, unbalanced inputs terminated with 1k ohms
    •      Wideband: 0.471mV, -75.6dBW
    •      A weighted: 0.022mV, -102.2dBW
  • Output noise, 8-ohm load, balanced inputs terminated with 600 ohms
    •      Wideband: 0.476mV, -75.5dBW
    •      A weighted: 0.0206mV, -102.8dBW
  • Output noise, 8-ohm load, unbalanced inputs terminated with 1k ohms, without the AP AUX-0025 filter
    •      462kHz: 423.0mV, -16.5dBW
  • Output noise, 8-ohm load, balanced inputs terminated with 600 ohms, without the AP AUX-0025 filter
    •      462kHz: 423.0mV, -16.5dBW

Measurements summary

The Bel Canto Design REF600M mono power amp is a switching design using the latest Hypex Ncore technology.

Chart 1 shows the REF600M’s frequency response with varying loads. One of the interesting aspects of the Ncore technology, in contrast to many other switching-amplifier designs, its amazing independence from load in the high frequencies. Here, though, there is a little anomaly between 50 and 60kHz. The output impedance is low enough that there was negligible variation with the NHT dummy speaker load.

Chart 2 illustrates how the REF600M’s total harmonic distortion plus noise (THD+N) vs. power varies for 1kHz and SMPTE intermodulation test signals and amplifier output for loads of 8 and 4 ohms. The amount of distortion visible in this plot is quite low.

The Bel Canto’s THD+N as a function of frequency at different power levels is plotted in Chart 3. The rise in distortion with increasing frequency is quite pronounced, and the low-frequency region also shows more distortion at higher power levels.

The REF600M’s damping factor vs. frequency (Chart 4) is somewhat lower than Bel Canto’s specified >1000. My measurements of some other amplifiers have resulted in values higher than 1000, so I know the measurement technique is valid. However, of considerable interest is that the damping factor’s “bandwidth” is quite a bit wider than with most amplifiers.

Chart 5 plots a spectrum of the Bel Canto’s harmonic distortion and noise residue of when fed a 10W, 1kHz test signal. The AC line harmonics are extremely low and relatively simple. The signal harmonics are dominated by the third and second harmonic, with higher harmonics of decreasing magnitude.

Chart 1 - Frequency response of output voltage as a function of output loading

Chart 1

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

Chart 2

(Line up at 100W to determine lines)
Top line = 8-ohm SMPTE IM distortion
Second line = 4-ohm SMPTE IM distortion
Third line = 8-ohm THD+N
Bottom line = 4-ohm THD+N

Chart 3 - Distortion as a function of power output and frequency

Chart 3

(4-ohm loading)
Red line = 1W
Magenta line = 10W
Blue line = 70W
Cyan line = 150W
Green line = 300W
Yellow line = 500W

Chart 4 - Damping factor as a function of frequency

Chart 4

Stereo mode
Damping factor = output impedance divided into 8

Chart 5 - Distortion and noise spectrum

Chart 5

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

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