I examined the behavior of the WiiM's D/A conversion circuitry with Ethernet data sourced from Roon and with the optical input. The latter locked to S/PDIF data with sample rates up to 192kHz. The Amp's digital inputs preserved absolute polarity and with the volume control set to the maximum, a 1kHz digital signal at –20dBFS resulted in an output level of 2.283V from loudspeaker outputs, which is exactly 20dB below the amplifier's clipping voltage into 8 ohms. It is rare to find a gain architecture as well-managed as this.
The Amp's reconstruction filter is a conventional linear-phase type, with time-symmetrical ringing on either side of the single full-scale sample (fig.13). With 44.1kHz-sampled white noise, the Amp's response rolled off sharply above 20kHz (fig.14, red and magenta traces), reaching full stop-band suppression at 24kHz. No aliased images are visible with a full-scale tone at 19.1kHz (blue and cyan traces) and the third harmonic is the highest in level, at –66dB (0.05%). Some low-level sidebands are visible around the 19.1kHz tone and its harmonics, these of unknown origin, but possibly due to jitter (see later).
The WiiM's frequency response with data sampled at 44.1, 96, and 192kHz (fig.15) followed the same basic shape before dropping off sharply just below half of each the sample rate. Channel separation (not shown) was significantly better than it had been from the analog input, at >110dB at low frequencies and still 80dB at the top of the audioband.
Fig.16 shows the spectrum of the WiiM's output as it decoded dithered data representing a 1kHz tone at –90dBFS with 16-bit and 24-bit data. The increase in bit depth lowers the noisefloor level by 10dB, which suggests that the Amp's DAC offers between 17 and 18 bits of resolution. With undithered 16-bit data representing a tone at exactly –90.31dBFS, the three DC voltage levels described by the data were obscured by noise (fig.17).
Intermodulation distortion with an equal mix of 19kHz and 20kHz tones sampled at 44.1kHz and peaking at 0dBFS was very low, though there was an odd rise in the noisefloor on either side of the tones with Ethernet data (fig.18) that was not present with optical data.
The Amp offered good rejection of word-clock jitter, with both optical and Ethernet data. Fig.19 was taken with 16-bit optical J-Test data. All the odd-order harmonics of the LSB-level, low-frequency squarewave lay at the correct levels, shown by the sloping green line, and there is no broadening of the spectral spike at one-quarter the sample rate. However, a pair of low-level sidebands was present at ±328Hz. These were also present with 24-bit J-Test data (fig.20).
Overall, the WiiM Amp's measured performance is excellent with both analog and digital data, which I found astonishing considering it costs just $299!—John Atkinson
