Solid State Power Amp Reviews

The Moon Evolution W-8 is surprisingly heavy for its size, suggesting a goodly amount of essential iron in its power transformer. Stone cold, the amplifier's THD at one-third power into 8 ohms was low at 0.017% but featured some crossover distortion. After 60 minutes at this power level, the THD had dropped to 0.004% and was almost pure third-harmonic in nature. The heatsinks were too hot to touch by this time. The W-8's voltage gain into 8 ohms was almost 4dB higher than average, at 30.9dB, this the same through both the unbalanced and balanced inputs. It preserved absolute polarity through both inputs, and its input impedance was a usefully high 50k ohms unbalanced and 96k ohms balanced over most of the audioband, dropping slightly at 20kHz to 40k ohms unbalanced and 86k ohms balanced.

The Moon W-8's output impedance was a very low 0.065 ohm across the audioband, rising inconsequentially to 0.08 ohm at 20kHz. (Both figures include the series resistance of 6' of speaker cable.) As a result of this low source impedance, the variation of frequency response into our standard simulated speaker load was negligible (fig.1, top trace at 2kHz). This graph also reveals a sensible degree of curtailing of the amplifier's ultrasonic response, though the –3dB point was different for the two channels: 85kHz left and 74kHz right. (This graph was taken using the balanced inputs; the unbalanced response was identical.) Even with this ultrasonic rolloff, the W-8's reproduction of a 10kHz squarewave was excellent (fig.2).

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

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

Channel separation (not shown) was better than 100dB below 1kHz, while the background noise level was also very low. Referenced to 1W into 8 ohms with the input shorted, the A-weighted signal/noise ratio was 96.5dB, and decreased to a still good 87.4dB with a wideband, unweighted measurement. Given the high output power, the W-8 offers a very wide dynamic range, joining current designs that comfortably exceed 16-bit CD in this respect.

Fig.3 shows how the percentage of THD+noise in the amplifier's output varies with output power into 8, 4, and 2 ohms. The broad inflection point in the 8 ohm trace between 3W and 5W suggests that this is where the THD starts to rise above the noise floor. The W-8 is certainly a very linear design. The maximum output powers (at 1% THD) were 310W into 8 ohms (0.9dB above the 24dB specification), 590W into 4 ohms (24.7dBW), and 990W into 2 ohms (23.9dB). The 0.1dB shortfall in the 2 ohm power output is explained by the fact that my AC supply voltage had dropped to 119V at this point. While I was performing these measurements, keeping the amplifier operating the highest power levels into 4 ohms and 2 ohms for more than a second or so caused the 10A rear-panel fuse to blow. Replacing the fuse restored normal operation.

Fig.3 Simaudio Moon W-8, distortion (%) vs 1kHz continuous output power into (from bottom to top at 1W): 8, 4, 2 ohms.

At small signal levels, the THD+N percentage was always low, but did rise into low impedances and toward the top of the audioband (fig.4). As mentioned earlier, with the amplifier warmed up, the distortion content was almost pure third-harmonic (fig.5), though some second harmonic appeared at high powers into lower impedances (fig.6). Intermodulation was very low (–92.5dB) with respect to the low-frequency difference product (fig.7), but higher-order products reached a still-low –76dB (0.015%).

Fig.4 Simaudio Moon W-8, THD+N (%) vs frequency at 3.5V into (from bottom to top): 8, 4, 2 ohms (right channel dashed).

Fig.5 Simaudio Moon W-8, 1kHz waveform at 75W into 8 ohms (top), 0.016% THD+N; distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.6 Simaudio Moon W-8, spectrum of 50Hz sinewave, DC–1kHz, at 205W into 4 ohms (linear frequency scale).

Fig.7 Simaudio Moon W-8, HF intermodulation spectrum, DC–24kHz, 19+20kHz at 162W peak into 8 ohms (linear frequency scale).

Simaudio's Moon Evolution W-8 joins that select company of modern amplifier designs that offer high power combined with a very low noise floor and excellent linearity.—John Atkinson