Simaudio Moon Evolution i-7 integrated amplifier Measurements

Sidebar 3: Measurements

Art Dudley is a rapscallion. When I hoisted the Simaudio Moon Evolution i-7 onto my test bench, plugged it in, and switched it out of standby, its display lit up with the message "HELLO JOHN." Switching it from Input 1 to Input 2 brought forth the command to "BITE ME," while Input 3 exclaimed "OUCH." Stuck up there in the wilds of upstate New York with time weighing heavily on his hands, Artie obviously had time to discover a new use for the amplifier's programmable input names!

I selected "HELLO JOHN" with the chunky metal remote and began my test routine. With an amplifier, this always starts with my preconditioning it by running both channels for 60 minutes at one-third the specified power into 8 ohms, which is the worst case for an amplifier with a class-B output stage. At the end of that hour, the Moon i-7's side-mounted heatsinks were just too hot to touch, while the chassis was warm. At the start of the hour, the THD+noise measured 0.019%; after 60 minutes it had dropped inconsequentially, to 0.018%.

The maximum voltage gain into 8 ohms was the same for both the unbalanced and balanced inputs, at a moderate 36.3dB, and all the inputs preserved absolute polarity; ie, were non-inverting. The gain at the preamp output jacks was 5.5dB, and these had a source impedance of 50 ohms across the band as specified. The balanced input is wired according to the AES convention, with pin 2 hot and pin 3 cold. At low and midrange frequencies the input impedance with the volume control at its maximum was a reasonably high 19k ohms unbalanced and 32k ohms balanced, but dropped at 20kHz to 5k ohms and 14k ohms, respectively. (It didn't change with the volume control setting.) Sources with tubed output stages might sound a little rolled-off due to the Ohm's Law interaction between the high source impedance and the change in the Moon's input impedance.

The i-7's output impedance was a very low 0.05 ohm in the bass and midrange, rising to a still low 0.07 ohm at 20kHz (both figures include the contribution of 6' of speaker cable). As a result, the modification of the amplifier's frequency response with changing load impedances was minimal (fig.1). With the volume control at its maximum setting ("80.0"), the Moon's ultrasonic bandwidth extended to –3dB at 91kHz. Reducing the gain to unity ("44.0") raised the 3dB-down frequency very slightly, to 95kHz, but the audioband response was just as flat. With its wide bandwidth, the i-7 reproduced a 10kHz squarewave with short risetimes but no hint of overshoot or ringing (fig.2). The 1kHz squarewave (not shown) was essentially perfect.

Fig.1 Simaudio Moon i-7, frequency response at 2.83V into (from top to bottom at 2kHz): simulated loudspeaker load, 16, 8, 4, 2 ohms (0.5dB/vertical div., right channel dashed).

Fig.2 Simaudio Moon i-7, small-signal 10kHz squarewave into 8 ohms.

Channel separation (not shown) was superb in the midrange and below, at better than 100dB, but it did decrease to a still good 64dB at 20kHz, due to the usual capacitive coupling. Assessed under the worst-case situation, with the input shorted but with the volume control set to its maximum, the i-7's signal/noise ratio was a good 65.2dB (wideband, unweighted, ref. 2.83V into 8 ohms), and improved to 74.8dB when A-weighted.

The Moon amplifier easily exceeded its specified 150Wpc output power, reaching the 1% THD clipping point at 200W into 8 ohms (23dBW) and 326W into 4 ohms (22.1dBW). The manner in which the THD+N percentage changes with output power into these two loads is shown in fig.3. Note that the 4-ohm trace stops at 326W; this is because the 8A rear-panel fuse blew at that point. Replacing it brought the amplifier back into operation, but the incident suggested that it would be unwise for me to try sustained high-power testing into loads lower than 4 ohms.

Fig.3 Simaudio Moon i-7, distortion (%)vs 1kHz continuous output power into (from bottom to top at 100W): 4, 8 ohms.

The shape of the traces in fig.3 suggests that the actual distortion rises out of the amplifier's noise floor at around 20W, so I plotted how the THD+N percentage changes with frequency at 13V, equivalent to 21W into 8 ohms. The results are graphed in fig.4: other than the high treble, where there is a slight rise, there is very little change as the load impedance halves. Adding to that good result, the distortion content is heavily third-harmonic (fig.5), with some second harmonic becoming apparent at high powers (fig.6). Both of these harmonics are subjectively innocuous, provided the underlying nonlinearity doesn't give rise to excessive intermodulation distortion—and, as is shown by fig.7, even at a very high output into 4 ohms, the distortion products associated with an equal mix of 19kHz and 20kHz tones are very low in level.

Fig.4 Simaudio Moon i-7, THD+N (%)vs frequency at 13V into (from bottom to top): 2, 4, 8, 16 ohms (right channel dashed).

Fig.5 Simaudio Moon i-7, 1kHz waveform at 21W into 8 ohms (top), 0.0136% THD+N; distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.6 Simaudio Moon i-7, spectrum of 50Hz sinewave, DC–1kHz, at 200W into 4 ohms (linear frequency scale).

Fig.7 Simaudio Moon i-7, HF intermodulation spectrum, DC–24kHz, 19+20kHz at 200W peak into 4 ohms (linear frequency scale).

Simaudio's Moon Evolution i-7 amplifier offers excellent measured performance, as I have come to expect from this Montreal-based company.—John Atkinson

95 Chemin du Tremblay Street, Unit 3
Boucherville, Quebec J4B 7K4
(877) 980-2400
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