Simaudio Moon i-1 integrated amplifier Measurements

Sidebar 3: Measurements

I primarily used Stereophile's loaner sample of the top-of-the-line Audio Precision SYS2722 system (see www.stereophile.com/asweseeit/108awsi and www.ap.com) to examine the Simaudio's measured performance.

Before performing tests on an amplifier, I thermally stress it by running it for 60 minutes at one-third its specified power into 8 ohms, which is the worst case for an amplifier with a class-B or -AB output stage. Despite its lack of external or obvious heatsinking, the Simaudio Moon i-1 survived this preconditioning period without incident, though the distortion rose from 0.015% with the amplifier stone cold to 0.028% with it baking hot. (Actually, only the bottom panel got that hot; the top panel was merely warm.)

The maximum voltage gain from the speaker terminals (into 8 ohms) was 35.75dB, and a sensibly low 5.7dB from the preamplifier output jacks. The i-1 was non-inverting from both sets of outputs; ie, the amp preserved absolute polarity. The input impedance was a moderately low 11.3k ohms at bass and midrange frequencies, dropping to 7k ohms at the top of the audioband. The source impedance from the preamp output jacks was 100 ohms across the band; from the speaker terminals, it was 0.075 ohm at low and middle frequencies, rising inconsequentially to 0.1 ohm at 20kHz.

As a result of this low output impedance, the modification of the amplifier's frequency response due to the Ohm's Law interaction between the impedance and that of the loudspeaker (fig.1, green trace) was minimal. This graph also shows that the i-1 has a very wide small-signal bandwidth, at least into higher impedances, the response into 8 ohms (blue and red traces) not dropping by 1dB until 100kHz, which results in a very sharply defined 10kHz squarewave (fig.2). However, the response into 2 ohms (fig.1, black trace), was –3dB at the same frequency. These responses, incidentally, were taken with the volume control set to its maximum; the bandwidth didn't change at lower settings of the volume control, which is good design.

Fig.1 Simaudio Moon i-1, frequency response at 2.83V into: simulated loudspeaker load (green), 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), 2 ohms (black). (1dB/vertical div.)

Fig.2 Simaudio Moon i-1, small-signal 10kHz squarewave into 8 ohms.

Channel separation was good rather than great, at approximately 80dB at 1kHz in both directions. The L–R figure remained the same at higher and lower frequencies, but the R–L figure dropped to a still-sufficient 60dB at 20kHz. The signal/noise ratio, with the volume control at its maximum and the input short-circuited, was an okay 60.4dB (wideband, unweighted, ref. 1W into 8 ohms), which increased to 85.7dB when the measurement bandwidth was reduced to the audioband.

Specified at 50Wpc into 8 ohms (17dBW), the Simaudio doesn't actually clip (1% THD+noise) until 62W with both channels driven (17.9dBW), shown graphically by fig.3, which plots THD+N against output power. This graph also reveals that the i-1 exactly meets is 100Wpc into 4 ohms specification (17dBW). (I didn't plot the THD vs power into 2 ohms, because the amplifier isn't specified into that low an impedance and because I already suspected that the amplifier would misbehave into that load.) The traces in fig.3 slope down with increasing power below a few watts, indicating that the measurement is dominated by noise rather than actual distortion in this region. There is a very slight rise in THD with the amplifier delivering a few tens of watts, but the transition to actual waveform clipping is very sharply defined.

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

Fig.4 plots the i-1's THD+N percentage against frequency at a level (10V) where the distortion components at 1kHz are starting to rise above the noise floor. The distortion into 8 and 4 ohms (blue, red, cyan, and magenta traces) rises a little in the upper octaves, but not to high levels. However, the amplifier is not happy driving 2 ohms (green trace), suffering from a catastrophic rise in distortion at this output voltage. At only slightly higher output levels into 2 ohms, the amplifier went into protection and shut itself down, fortunately without suffering any damage.

Fig.4 Simaudio Moon i-1, THD+N (%) vs frequency at 10V into: 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), 2 ohms (left green).

The distortion with higher load impedances is predominantly the subjectively innocuous third harmonic (fig.5), though higher-order harmonics appear as the amplifier is asked to simultaneously deliver high voltages and high currents (fig.6). Even so, the harmonics are all lower than the level of the power-supply component at 120Hz, which lies at –80dB (0.01%). The only real clue that this is an affordably priced amplifier is the appearance of sidebands at the power-supply–related frequencies of ±120Hz around all the harmonic components at powers close to clipping, both with single tones (fig.7) and with an equal mix of high-frequency tones (fig.8). I suspect that it is this lack of power-supply grunt that leads to BJR finding the i-1's mid-bass to sound a touch warm. Actual intermodulation products with the latter are all below –74dB (0.02%), however.

Fig.5 Simaudio Moon i-1, 1kHz waveform at 10W into 4 ohms (top), 0.028% THD+N; distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.6 Simaudio Moon i-1, spectrum of 50Hz sinewave, DC–1kHz, at 60W into 4 ohms (linear frequency scale; left channel blue, right red).

Fig.7 Simaudio Moon i-1, spectrum of 1kHz sinewave, DC–10kHz, at 60W into 4 ohms (linear frequency scale; left channel blue, right red).

Fig.8 Simaudio Moon i-1, HF intermodulation spectrum, DC–24kHz, 19+20kHz at 40W peak into 8 ohms (linear frequency scale; left channel blue, right red).

You can't expect bombproof behavior from an amplifier costing just $1500; the trick the designer needs to pull off is to make sure that the amplifier's performance in the middle of the envelope is not compromised, while allowing it to misbehave at the extremes. Simaudio's Moon I-1 integrated amplifier gracefully manages that trick; the real mystery is how it can be possible for a Canadian manufacturer to make this product at all in North America while keeping the price competitive.—John Atkinson

COMPANY INFO
Simaudio Ltd.
2002 Ridge Road
Champlain, NY 12919
(877) 980-2400
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