Digital Processor Reviews

I measured the Simaudio Moon Neo 230HAD with my Audio Precision SYS2722 system (see the January 2008 "As We See It"). As well as the Audio Precision's analog and digital outputs, I used WAV and AIFF test-tone files sourced via USB from my MacBook Pro running on battery power with Pure Music 3.0. Apple's USB Prober utility identified the Neo 230HAD as "MOON USB DSD HD Audio" from "Simaudio Ltd.," and its serial number as "413-001." The Neo 230HAD's USB port operated in the optimal isochronous asynchronous mode, and Apple's AudioMIDI utility revealed that it accepted 32-bit integer data via USB sampled at all rates from 32 to 384kHz.

Looking first at the behavior of the Neo 230HAD's analog inputs and outputs, the maximum gain at 1kHz was 11.9dB from the front-panel ¼" headphone jack and 5.9dB from the variable rear-panel RCA jacks. Measured at the fixed RCA jacks, the gain was 5.9dB. All three sets of outputs preserved absolute polarity (ie, were non-inverting) with both analog and digital input signals. The input impedance was the specified 22k ohms at low and middle frequencies, but dropped to 9.7k ohms at the top of the audioband. The output impedance was a low 22.5 ohms from the variable RCA jacks, 50 ohms from the fixed RCA jacks, and an appropriately low 0.7 ohm from the ¼" headphone jack, all figures constant across the audioband.

The analog input/output frequency response was flat within the audioband, with the ultrasonic –3dB point above 200kHz (fig.1). This graph was taken from the variable outputs with the volume control set to its maximum; note the excellent channel matching—neither this nor the response changed at different volume-control settings, from the other outputs, or with lower load impedances. Channel separation (not shown) was >100dB below 2kHz, and was still 80dB at 20kHz. Again, this behavior was consistent from all three sets of outputs.

The Neo 230HAD offered a high maximum output level, clipping at just over 10V into 100k ohms (variable and fixed outputs), and into 300 ohms from the headphone output (fig.2). Reducing the load to 30 ohms reduced the headphone clipping output to 6.5V (fig.3), but this will still be enough to drive low-impedance headphones to ear-shattering levels. The distortion signature from the variable and fixed outputs was the second harmonic (fig.4), but at –114dB ref. 2V (0.0002%) this will be inconsequential, to say the least. The third harmonic was the highest in level from the headphone output (fig.5), but this is still extremely low in level. Intermodulation distortion (fig.6) was also vanishingly low in level.

Turning to the behavior of the Neo 230HAD with digital input signals, the maximum output levels at 1kHz in DAC mode were 8.1V (headphone output) and 4.05V (variable and fixed RCA outputs). The impulse response with 44.1kHz data was typical of a DAC using a conventional finite impulse-response reconstruction filter (fig.7), with the symmetrical ringing to either side of the pulse mapping the filter's coefficient values. This filter has a very steep rolloff above the audioband (fig.8, magenta and red traces), but, peculiarly, the aliased image at 25kHz of a full-scale 19.1kHz tone was reduced in level by only 85dB (cyan, blue, footnote 1). Harmonic distortion associated with that tone was low, however, with the third harmonic highest in level at –86dB (0.005%). Fig.9 is a more conventional means of displaying frequency response, this time with data sampled at 44.1, 96, 192, and 384kHz. Again note the superb channel matching, and that, at the three lower sample rates, the smooth rolloff above the audioband is broken by a sharp drop in output just below each Nyquist frequency (ie, half the sample rate).

The Neo 230HAD's DAC offers high resolution. With data representing a dithered 1kHz tone at –90dBFS, increasing the bit depth from 16 to 24 drops the noise floor by almost 20dB (fig.10), which suggests resolution of greater than 19 bits. This graph was taken with S/PDIF data; the resolution was equally excellent with USB data (fig.11). An undithered tone at exactly –90.31dBFS was reproduced with a superbly symmetrical waveform, with the three DC voltage levels well defined (fig.12). With undithered 24-bit data at this low level, the Moon output a well-formed sinewave (fig.13).

Both harmonic and intermodulation distortions were very low in level with digital data, but with the latter (fig.14), I was puzzled to see what appeared to be some modulation of the noise floor around 8–9kHz and 17–18kHz. Though the 230HAD's rejection of word-clock jitter was excellent for both the S/PDIF and USB inputs, this modulation can be seen with both 16-bit (fig.15) and 24-bit (fig.16) J-Test data. A mystery.

Overall, the Moon Neo 230HAD's digital input behaved very similarly to the digital-input module in Simaudio's Moon Neo 340i integrated amplifier, which Herb Reichert reviewed in 2016, but its noise floor is not compromised by the digital circuitry being close to circuits carrying very high currents. As a headphone amplifier, the 230HAD offers superbly low distortion, very high dynamic range, and excellent channel matching.

At $1499, the Moon Neo 230HAD is a little less expensive than Ayre Acoustics' Codex, which retails for $1795 and which Jon Iverson reviewed in June 2016. Unlike the Ayre, the Moon has analog inputs but no balanced headphone output. Both handle DSD data, and PCM data sampled at up to 384kHz; the Neo 230HAD has a conventional digital filter, whereas the Ayre offers a very short minimum-phase filter with a much slower rolloff above the audioband. These two headphone amplifiers very much typify the two current approaches to high-performance DAC design, therefore.—John Atkinson