Sidebar 3: Measurements
I performed a full set of measurements on the MOON 371 with my Audio Precision SYS2722 system. As is typical with modern products, when I connected its Ethernet port to my router, the MOON suggested that I update its firmware. I did so; after the update it was running firmware V.1.02 with the MiND module running V.2.07.
Before I started the measurements, I preconditioned the amplifier by operating it at 1/8 the specified power into 8 ohms for 30 minutes. At the end of that time, the temperature of the heatsinks on the amplifier's sides was 137.8°F/58.6°C. The temperature of the top panel was lower, at 94°F/37.4°C; still, this amplifier needs plenty of ventilation.
Looking first at the MOON 371's balanced and single-ended line-level inputs, the amplifier preserved absolute polarity, ie, was noninverting, at the loudspeaker, single-ended "Analog" RCA, and headphone outputs. The unbalanced line input impedance was close to the specified 9.5k ohms, at 8k ohms at 20Hz and 1kHz, dropping inconsequentially to 7.5k ohms at 20kHz. The balanced input impedance was 19.2k ohms across the audioband. The volume control operates in accurate 0.5dB steps; with the volume control set to the maximum of "80," the voltage gain at 1kHz with both types of inputs was 37.5dB from the loudspeaker output into 8 ohms and from the headphone output into 100k ohms. The gain from the RCA output was 6dB, again with both input types.
I performed a full set of measurements on the MOON 371 with my Audio Precision SYS2722 system. As is typical with modern products, when I connected its Ethernet port to my router, the MOON suggested that I update its firmware. I did so; after the update it was running firmware V.1.02 with the MiND module running V.2.07.
Before I started the measurements, I preconditioned the amplifier by operating it at 1/8 the specified power into 8 ohms for 30 minutes. At the end of that time, the temperature of the heatsinks on the amplifier's sides was 137.8°F/58.6°C. The temperature of the top panel was lower, at 94°F/37.4°C; still, this amplifier needs plenty of ventilation.
Looking first at the MOON 371's balanced and single-ended line-level inputs, the amplifier preserved absolute polarity, ie, was noninverting, at the loudspeaker, single-ended "Analog" RCA, and headphone outputs. The unbalanced line input impedance was close to the specified 9.5k ohms, at 8k ohms at 20Hz and 1kHz, dropping inconsequentially to 7.5k ohms at 20kHz. The balanced input impedance was 19.2k ohms across the audioband. The volume control operates in accurate 0.5dB steps; with the volume control set to the maximum of "80," the voltage gain at 1kHz with both types of inputs was 37.5dB from the loudspeaker output into 8 ohms and from the headphone output into 100k ohms. The gain from the RCA output was 6dB, again with both input types.
Fig.1 MOON 371, line input, frequency response at 2.83V into 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta) (1dB/vertical div.).
Fig.2 MOON 371, line input, small-signal, 10kHz squarewave into 8 ohms.
The RCA output impedance was a usefully low 50 ohms from 20Hz to 20kHz; the headphone output impedance was relatively high, at 327 ohms, again from 20Hz to 20kHz. The loudspeaker output impedance was very low, at 0.02 ohm at 20Hz and 1kHz, rising slightly to 0.05 ohm at 20kHz. The modulation of the MOON 371's frequency response due to the Ohm's law interaction between this impedance and the impedance of our standard simulated loudspeaker was therefore negligible (not shown). The amplifier's response into resistive loads (blue, red, cyan, and magenta traces) was flat in the audioband and –2dB at 100kHz. Both the very close channel balance and the overall response were preserved at lower settings of the volume control, with the balanced and unbalanced inputs. There was no overshoot or ringing on the MOON 371's reproduction of a 10kHz squarewave into 8 ohms (fig.2).
Fig.3 MOON 371, line input, spectrum of 1kHz sinewave, DC–1kHz, at 1Wpc into 8 ohms with volume control set to the maximum (left channel blue, right red) and to –6dB (left green, right gray) (linear frequency scale).
Channel separation was excellent, at>110dB in both directions below 3kHz and still 100dB at the top of the audioband. The wideband, unweighted signal/noise ratio at the loudspeaker outputs, taken with the single-ended line inputs shorted to ground and the volume control set to its maximum, was a good 70dB (average of both channels), ref. 2.83V, which is equivalent to 1W into 8 ohms. This ratio improved to 83.5dB when the measurement bandwidth was restricted to the audioband and to 86.2dB when A-weighted. The spectrum of the MOON 371's low-frequency noisefloor at 1Wpc into 8 ohms, with the volume control set to its maximum, is shown by the red and blue traces in fig.3. There are no supply-related spuriae, and the levels of the random noise components dropped by around 6dB when I set the volume control to "68," equivalent to –6dB, and increased the level of the input signal so that the output was the same 1Wpc into 8 ohms (green, gray traces).
Fig.4 MOON 371, line input, THD+N (%) vs 1kHz continuous output power into 8 ohms.
Fig.5 MOON 371, line input, THD+N (%) vs 1kHz continuous output power into 4 ohms.
MOON specifies the 371's maximum power as 100W into 8 ohms and 200W into 4 ohms, both equivalent to 20dBW. Figs.4 and 5 respectively plot how the THD+noise percentage in the MOON 371's output varies with power into 8 ohms and 4 ohms with both channels driven. Stereophile defines clipping as when the THD+N reaches 1%. The MOON 371 clipped at 135Wpc into 8 ohms (21.3dBW) and at 265Wpc into 4 ohms (21.2dBW). The FTC's updated "Amplifier Rule" states that maximum power should also be assessed at frequencies other than 1kHz. I therefore repeated the power test with a 20kHz signal. The amplifier clipped at the same 135Wpc into 8 ohms at this frequency as it had at 1kHz.
Fig.6 MOON 371, line input, THD+N (%) vs frequency at 20V into 8 ohms (left channel blue, right red) and 4 ohms (left green, right gray).
The downward slope of the traces in figs.4 and 5 indicates that distortion lies beneath the noisefloor below waveform clipping. Fig.6 shows how the MOON 371's THD+N percentage changed with frequency at 20V, which is equivalent to 50W into 8 ohms (blue, red traces) and 100W into 4 ohms (green, gray traces). The distortion+noise into both loads is very low, though slightly higher in the right channel into 4 ohms (gray trace), with a small rise in the top audio octaves in both channels into both loads.
Fig.7 MOON 371, line input, spectrum of 50Hz sinewave, DC–1kHz, at 50Wpc into 8 ohms (left channel blue, right red, linear frequency scale).
Fig.8 MOON 371, line input, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 100Wpc peak into 4 ohms (left channel blue, right red, linear frequency scale).
The distortion signature was primarily the second harmonic; even so, at less than –110dB (0.0003%) at 50W into 8 ohms (fig.7), it is negligible. Even at the same peak voltage into 4 ohms, intermodulation distortion with an equal mix of 19kHz and 20kHz tones was extremely low in level (fig.8). The difference tone at 1kHz lay at –114dB (0.0002%), and the higher-order products all lay at or below –100dB (0.001%).—John Atkinson
