Sidebar 4: More Measurements
Turning to the MOON 371's phono input, this can be set with the menu on the front-panel display to moving coil (MC) or moving magnet (MM). To measure the behavior of this input, I connected the ground terminal on the amplifier's rear panel to the analyzer's chassis ground. Both phono input modes preserved absolute polarity at all three output types. The MM input impedance is specified as 47k ohms; I measured 43k ohms at 20Hz and 1kHz, and 34.4k ohms at 20kHz. The MC input impedance is specified as 1000 ohms. I measured 980 ohms from 20Hz to 20kHz.
The specified gain for the phono input modes is 40dB in MM mode and 60dB in MC mode. With the volume control set to the maximum, the gain in MM mode was 45.7dB from the RCA output and 77.8dB at the loudspeaker and headphone outputs, which confirms that the power amplifier stage adds around 32.1dB of gain, close to the specified 31dB. The gain in MC mode with the volume control set to the maximum was 20dB greater than it was in MM mode.
I examined the phono input's performance at the headphone output, which mutes the other two pairs of outputs (footnote 1). To avoid clipping the headphone output, I performed these measurements with the volume control set to "68" (–6dB), "56" (–12dB), or "40" (–20dB) depending on which test I was performing.
With the volume control set to the maximum, the output level with a 1kHz tone at –30dBFS was 134.3mV from the RCA output and 5.15V from the headphone output into 100k ohms and from the loudspeaker output into 8 ohms; the last is 16dB below the clipping voltage of 32.9V. Except where noted, I examined the digital inputs' behavior at the headphone output with the volume control set to "44" (–18dB) to avoid overloading the output stage.
Footnote 1: In his Manufacturer's Comment, Dominique Poupart, MOON by Simaudio's Product Director, noted that the phono stage measurements were taken from the headphone output rather than the preamplifier line-level output. As the headphone output is driven by a padded-down signal derived from the main internal power amplifier, this operating condition can account for the slightly higher noise and distortion figures observed, since the signal follows a non-typical measurement path rather than the conventional low-level phono line-level output used for direct comparison with other designs. Footnote 2: Unfortunately, when I tried to repeat the overload margin and S/N ratio testing in MC mode, the amplifier turned itself off and wouldn't turn on again. (The fuse on the rear pane; was intact.) It is possible, therefore, that something had failed while I was performing these tests.
Fig.9 MOON 371, MM phono input, response with RIAA correction (left channel blue, right red) (0.5dB/vertical div.).
The MOON 371's RIAA equalization, measured in MM mode (fig.9), was accurate, with very close channel matching. The MOON 371 phono stage's unweighted, wideband S/N ratio in MM mode, measured at the headphone output with the input shorted to ground and the volume set to "68," was a good 67.5dB in both channels ref. 1kHz at 5mV. Restricting the measurement bandwidth to 22Hz–22kHz increased the ratio by 5dB, while the A-weighted ratio was a very good 84.5dB. The S/N ratios in MC mode, ref. 1kHz at 500µV, were around 20dB lower due to the presence of very low-frequency noise (see later).
Fig.10 MOON 371, MM phono input, spectrum of 1kHz sinewave, DC–10kHz, at 11mV input (left channel blue, right red, linear frequency scale).
Fig.11 MOON 371, MM phono input, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 100mV input (left channel blue, right red, linear frequency scale).
I measured the MOON 371 phono input's overload margins with the volume control set to "40" to make sure I was looking at true input overload rather than output stage clipping. The margins in MM mode were a very good 18.9dB at 20Hz, 18.1dB at 1kHz, and 17.6dB at 20kHz, all ref. 1kHz at 5mV. The margins in MC mode, ref. 500µV at 1kHz, were even larger, at 29.8dB from 20Hz to 20kHz. Harmonic distortion was very low, even at 12dB below the MM input overload voltage (fig.10), as was intermodulation distortion (fig.11). The difference product at 1kHz with an equal mix of 19kHz and 20kHz tones at a peak level of 100mV, with the volume control set to "56" in MM mode, lay at just –94dB (0.002%).
I tested the 371's digital conversion via the optical and coaxial S/PDIF inputs and via the Ethernet input using network data played with Roon and the MiND app. The S/PDIF inputs all locked to data with a sample rate of 192kHz. The digital inputs preserved absolute polarity at the speaker, RCA, and headphone outputs.
Fig.12 MOON 371, digital inputs, impulse response (one sample at 0dBFS, 44.1kHz sampling, 4ms time window).
Fig.13 MOON 371, digital inputs, wideband spectrum of white noise at –4dBFS (left channel red, right magenta) and 19.1kHz tone at 0dBFS (left blue, right cyan) with data sampled at 44.1kHz (20dB/vertical div.).
Fig.14 MOON 371, digital inputs, frequency response at –12dBFS with data sampled at: 44.1kHz (left channel green, right gray), 96kHz (left cyan, right magenta), and 192kHz (left blue, right red) (1dB/vertical div.).
The MOON 371's impulse response with PCM data sourced from Roon (fig.12) revealed that the review sample's reconstruction filter is a long minimum-phase type, with all the ringing following the single sample at 0dBFS. The magenta and red traces in fig.13 show the ultrasonic rolloff of the MOON 371's digital inputs with white noise data sampled at 44.1kHz. The traces reach full stop-band attenuation at 24kHz, just above half the sample rate, which is indicated by the vertical green line. The aliased image at 25kHz of a 19.1kHz tone at 0dBFS (cyan, blue) is suppressed by 100dB, and the harmonics associated with the 19.1kHz tone all lay at or below –90dB (0.003%). The digital frequency response with data sampled at 44.1kHz, 96kHz, and 192kHz (fig.14) was flat in the audioband with a sharp rolloff just below half of each sample rate.
Fig.15 MOON 371, digital inputs, spectrum with noise and spuriae of dithered 1kHz tone at –90dBFS with: 16-bit data (left channel green, right gray), 24-bit data (left blue, right red) (20dB/vertical div.).
Fig.16 MOON 371, digital inputs, waveform of undithered 1kHz sinewave at –90.31dBFS, 16-bit data (left channel blue, right red).
Fig.17 MOON 371, waveform of undithered 1kHz sinewave at –90.31dBFS, 24-bit data (left channel blue, right red).
An increase in bit depth from 16 to 24, with dithered data representing a 1kHz tone at –90dBFS and the volume control set to "68" (–6dB), dropped the MOON 371's noisefloor by 18dB (fig.15), which implies a measured resolution of 19 bits. With undithered data representing a tone at exactly –90.31dBFS, the waveform was symmetrical, with negligible DC offset, and the three DC voltage levels described by the data were clearly defined (fig.16). With undithered 24-bit data (fig.17), the MOON 371 output a relatively clean sinewave.
Fig.18 MOON 371, digital inputs, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 0dBFS peak (left channel blue, right red, linear frequency scale).
Intermodulation distortion with 24-bit data representing an equal mix of 19 and 20kHz tones, each at –6dBFS, was low in level, the difference product at 1kHz lying at –114dB (0.0002%; fig.18). Aliased images of the two tones were present, but their levels were below –100dB.
Fig.19 MOON 371, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 16-bit TosLink S/PDIF data (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.
Fig.20 MOON 371, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 24-bit TosLink S/PDIF data (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.
When the MOON 371 was fed 16-bit optical or network J-Test data, the odd-order harmonics of the undithered low-frequency, LSB-level squarewave all lay at the correct levels (fig.19). (As J-Test data peaks at –3dBFS, the volume control was set to "44" for this test, which is why the random noisefloor is higher in this graph than it is in fig.14.) With 24-bit J-Test data (fig.20), no jitter sidebands were present, and the central spectral spike that represents the high-level tone at one-quarter the sample rate was appropriately narrow.
As a conventional integrated, the MOON 371's measured performance with its analog line-level inputs and the digital inputs was very good. The phono input also did well on the test bench in both in moving magnet and moving coil modes, though low-frequency noise in MC mode was higher in level than I expected (footnote 2).—John Atkinson
Footnote 1: In his Manufacturer's Comment, Dominique Poupart, MOON by Simaudio's Product Director, noted that the phono stage measurements were taken from the headphone output rather than the preamplifier line-level output. As the headphone output is driven by a padded-down signal derived from the main internal power amplifier, this operating condition can account for the slightly higher noise and distortion figures observed, since the signal follows a non-typical measurement path rather than the conventional low-level phono line-level output used for direct comparison with other designs. Footnote 2: Unfortunately, when I tried to repeat the overload margin and S/N ratio testing in MC mode, the amplifier turned itself off and wouldn't turn on again. (The fuse on the rear pane; was intact.) It is possible, therefore, that something had failed while I was performing these tests.
