Simaudio Moon i-5 integrated amplifier Measurements

Sidebar 3: Measurements

The Moon i-5's heatsinks were too hot to touch after the one-hour preconditioning period at one-third power into 8 ohms. While I could just keep my hand on the top plate, the amplifier should still be given plenty of ventilation. The amplifier was non-inverting and the maximum voltage gain was 38.8dB into 8 ohms. The volume-control position closest to unity gain was an indicated "23" on the red numeric display, which goes up to "50," and the volume control operated in 1dB steps at the top of its range, coarse 2.5dB steps in the bottom half. The input impedance was a fairly low 13.2k ohms at 1kHz.

The output impedance was a very low 0.035 ohm across most of the audioband, rising inconsequentially to 0.05 ohm at 20kHz. As a result, there will be very little modification of the amplifier's frequency response by the voltage-divider action of this source impedance with the speaker's impedance. The i-5's response is shown in fig.1: flat in the audioband, it rolls off to a sensible -3dB at 122kHz. This response was taken via the "CD" input with the volume control set to its unity-gain position. There was no change in the response as the volume-control setting was changed or the input was switched to "A1." The shape of a 10kHz squarewave (fig.2) was essentially perfect, other than the slight slowing of its leading edges by the ultrasonic rolloff.

Fig.1 Simaudio Moon i-5, volume control at unity gain, frequency response at (from top to bottom at 2kHz): 2.83V into dummy loudspeaker load, 1W into 8 ohms, 2W into 4 ohms, 4W into 2 ohms (0.5dB/vertical div., right channel dashed).

Fig.2 Simaudio Moon i-5, small-signal 10kHz squarewave into 8 ohms.

Due to the usual capacitive coupling, channel separation (fig.3) was only adequate at high frequencies: 50dB at 20kHz. However, it did improve to better than 80dB in the midband. The A-weighted signal/noise ratio (ref. 1W into 8 ohms) was a good 81.4dBA, this worsening to a still good 72.4dB when the A-weighting filter was removed and the measurement bandwidth widened to <10Hz to >500kHz.

Fig.3 Simaudio Moon i-5, channel separation (L-R dashed, 10dB/vertical div.).

Small-signal distortion was very low in the midband, but did rise out of the noise floor at high and low frequencies (fig.4). It also rose as the load impedance dropped, reaching above 0.1% in the very low bass and top audio octave. However, as can be seen from the waveform of the distortion residual (fig.5), it is heavily second-harmonic, which, all things being equal, will tend to be subjectively benign. At higher levels, the third harmonic joins the second, though each remains at the -60dB level (0.1%), even into 4 ohms (fig.6). Intermodulation distortion (fig.7) was moderate in level; the 1kHz difference component produced from an equal mix of 19kHz and 20kHz tones driven a couple of dB below clipping into 4 ohms remained at -66dB (0.05%).

Fig.4 Simaudio Moon i-5, THD+N (%) vs frequency at 2.83V into (from bottom to top at 1kHz): simulated loudspeaker load, 8 ohms, 4 ohms, 2 ohms.

Fig.5 Simaudio Moon i-5, 1kHz waveform at 14.5W into 8 ohms (top), distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.6 Simaudio Moon i-5, spectrum of 50Hz sinewave, DC-1kHz, at 110W into 4 ohms (linear frequency scale).

Fig.7 Simaudio Moon i-5, HF intermodulation spectrum, DC-24kHz, 19+20kHz at 66W into 4 ohms (linear frequency scale).

With both channels driven by continuous tones, the i-5 exceeded its specified power delivery into both 8 and 4 ohms, raising 82W (19.1dBW) and 150W (18.75dBW), respectively, at our usual 1% THD+N clipping point (fig.8). With one channel driven, these figures increased to 90W and 165W, with 249W (17.9dBW) available into 2 ohms (at which point the rear-panel fuse blew).

Fig.8 Simaudio Moon i-5, distortion (%) vs continuous output power into (from bottom to top): 8 ohms, 4 ohms, 2 ohms.

Using the Miller Amplifier Profiler to look at the Simaudio's power delivery with a low-duty-cycle 1kHz toneburst, which is similar to a transient-rich musical signal, the i-5 raised 46W into 16 ohms (fig.9, red trace), 90W into 8 ohms (black), 172W into 4 ohms (blue), 290W into 2 ohms (green), and even 201.5W into 1 ohm (magenta). But, as is also apparent in fig.9, the amplifier produces significant levels of distortion into these very low impedances, even at power levels well below actual clipping. The i-5 is best suited to speakers that don't drop below 4 ohms.

Fig.9 Simaudio Moon i-5, distortion (%) vs 1kHz burst output power into 16 ohms (red), 8 ohms (black), 4 ohms (blue), 2 ohms (green), 1 ohm (magenta).

All in all, and considering the modest size of the Simaudio Moon i-5, this is good measured performance.—John Atkinson

21 Lawrence Paquette Drive
Champlain, NY 12919
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