Solid State Power Amp Reviews

Sidebar 3: Measurements

I measured the Bel Canto e1X using my Audio Precision SYS2722 system (see the January 2008 "As We See It"). As the Bel Canto is a class-D design, it wasn't necessary to precondition the amplifier by using it to drive a 1kHz tone at one-third power into 8 ohms. Nevertheless, before doing any testing I ran it for an hour at a moderate power level, to ensure that it was fully warmed up. Because class-D amplifiers emit relatively high levels of ultrasonic noise that would drive my analyzer's input into slew-rate limiting, all measurements were taken with Audio Precision's auxiliary AUX-0025 passive low-pass filter, which eliminates noise above 200kHz. Without the filter, there was 398mV of ultrasonic noise present at the loudspeaker terminals.

The voltage gain at 1kHz into 8 ohms was the specified 27dB from both the balanced and single-ended inputs. Internal switches should increase the gain by 6dB; I measured 5.1dB. (Removing the top cover to reset the gain revealed the two Hypex Ncore class-D output modules and the switch-mode power supply.) The amplifier preserved absolute polarity (ie, was noninverting) from both sets of inputs. The single-ended input impedance measured the specified 10k ohms at low and middle frequencies, dropping very slightly to 9.2k ohms at the top of the audioband. The balanced input impedance was exactly twice these values, again as specified.

The Bel Canto's output impedance, including the series impedance of 6' of loudspeaker cable, was a very low 0.06 ohm at 20Hz and 1kHz, increasing very slightly to 0.07 ohm at 20kHz. The modulation of the amplifier's frequency response, due to the Ohm's law interaction between this source impedance and the impedance of our standard simulated loudspeaker, was therefore minuscule, at ±0.1dB (fig.1, gray trace). The response into resistive loads was flat to 10kHz into 8 ohms, then down by 0.6dB at 20kHz and 3dB at 38kHz (fig.1, blue and red traces). The restricted ultrasonic response is responsible for slightly lengthened risetimes with the amplifier's reproduction of a 10kHz squarewave (fig.2). There is also a faint hint of overshoot with the Bel Canto's squarewave response, this presumably due to the class-D output stage's integral low-pass filter.

Channel separation was superb at 105dB in both directions below 1kHz and still 82dB at 20kHz. With the Audio Precision ultrasonic filter, the gain set to 27dB, and the analog inputs shorted to ground, the wideband, unweighted signal/noise ratio (ref. 2.83V into 8 ohms) measured 73.7dB in the left channel and 71.4dB in the right. Restricting the measurement bandwidth to 22kHz increased the ratio to 90.8dB, left, and 85.5dB, right, and an A-weighting filter increased it further, to an excellent 100dB in both channels. Spectral analysis of the Bel Canto's low-frequency noise floor (fig.3) revealed spuriae related to the AC power-line frequency to be very low in level.

The e1X is specified as delivering a maximum output power, at 1% THD+noise, of 250Wpc into 8 ohms, and 500Wpc into 4 ohms,3 both equivalent to 24dBW (footnote 1). I found that the Bel Canto amplifier easily exceeded its specified power, clipping at 340Wpc into 8 ohms (fig.4, 25.3dBW) and at 530Wpc into 4 ohms (fig.5, 24.2dBW), both powers at 1% THD+noise.

Distortion levels in the bass and midrange were low (fig.6) and didn't increase by much into 2 ohms (gray trace). The THD was higher in the treble, especially into lower impedances (cyan, magenta, and gray traces). The shape of the THD+N spuriae waveform at a moderate power into 8 ohms (fig.7, bottom trace) suggests that the distortion signature is primarily third harmonic in nature, though spectral analysis reveals that the second harmonic is also present at a slightly lower level (fig.8). Higher-order harmonics all lie below –110dB (0.0003%). Into lower impedances (not shown), the second harmonic rises to the same level as the third, but at –90dB (0.003%), neither will have any effect on the amplifier's sound. Despite the rise in THD in the top audio octaves, intermodulation distortion with an equal mix of 19kHz and 20kHz tones was very low, even at high powers into 4 ohms (fig.9).

Bel Canto's e1X performed well on the test bench.—John Atkinson

---

Footnote 1: Our dBW figures follow the convention established by Martin Colloms and the late Peter W. Mitchell in the 1980s: they are referred to 2.83V (1W into 8 ohms). Thus an amplifier that behaves as a perfect voltage source (the ideal) offers the same dBW rating into all loads, an easier paradigm to grasp than having to remember to add 3dB every time the load impedance halves.