The Unison Research's digital inputs preserved absolute polarity from both output types. With the volume control set to the maximum, the output level with a 1kHz tone at a low –30dBFS was 132.8mV from the Subwoofer output and 3.96V from the speaker output into 8 ohms. The latter is 13dB below the clipping voltage into that load; the S6 Black Edition's DAC offers 17dB higher gain than is strictly necessary. To avoid overloading the amplifier's output stage, I performed all the subsequent digital input testing at the Subwoofer output with the volume control set to –20dB.
The Unison Research S6 Black Edition offers a single reconstruction filter for PCM data. Fig.10 shows the filter's impulse response with 44.1kHz data and indicates that the filter is a linear-phase type with equal amounts of ringing before and after the single high sample. The magenta and red traces in fig.11 show the filter's ultrasonic rolloff with data sampled at 44.1kHz. This is interrupted at 24kHz by a rise in level of the ultrasonic noisefloor, which might be related to the high levels of distortion. The response doesn't reach full stop-band attenuation until twice the sample rate. The aliased image at 25kHz of a 19.1kHz tone at –100dBFS (cyan, blue) is fully suppressed, but the harmonics associated with the 19.1kHz tone are all relatively high in level. The second harmonic is the highest in level, at –44dB (0.6%).
Fig.12 shows the frequency response with data sampled at 44.1kHz, 96kHz, and 192kHz. The audioband response is very similar to that of the analog inputs with all three sample rates, with then a sharp rolloff just below half of each sample rate.
While the level of a dithered 24-bit 1kHz tone at –90dBFS was correct (fig.13), the noisefloor was dominated by the power supply-related spuriae seen in the loudspeaker outputs. This suggests that the Subwoofer output might be taken after the tubed output stage. With undithered data representing a tone at exactly –90.31dBFS, the three DC voltage levels described by the data were obscured by noise (fig.14).
With an equal mix of 19 and 20kHz tones, each at –6dBFS, the second-order intermodulation product at 1kHz and the higher-order products were slightly higher than they had been with the analog inputs, and again the noisefloor is obscured by the supply-related spuriae (fig.15). These spuriae also affected my attempt to examine the S6 Black Edition's rejection of word-clock jitter, obscuring the levels of the odd-order harmonics of the undithered low-frequency, LSB-level squarewave (fig.16).
With its analog inputs Unison Research's S6 Black Edition behaved pretty much as I would expect from a tube amplifier with a single-ended output stage and little or no global negative feedback. While the distortion signature is predominantly the innocuous second harmonic, the high levels of distortion at powers of more than 1W, especially into impedances below 8 ohms, mean that this amplifier must be used with loudspeakers that have both a high sensitivity and a high impedance. Measured via the subwoofer output, the measured performance of the digital inputs was compromised by the presence of the supply-related spuriae.—John Atkinson
