Phono Preamp Reviews

I measured the Sutherland Duo phono preamplifier using my Audio Precision SYS2722 system (see www.ap.com, and the January 2008 "As We See It"). As always with phono stages, I experimented with the grounding between the Sutherland and the Audio Precision system to get the lowest level of noise, though the Duo's twin enclosures made this more complicated than usual. I ended up floating the Audio Precision's outputs from ground and connecting both Duos' grounding posts to the analyzer ground.

The Duo preserved absolute polarity, and the output impedance was a low 199 ohms at all audioband frequencies. The measured gain was close to specification, ranging from 40.6dB when set to "40," to 46.25dB ("46"), 52.3dB ("52"), 57.7dB ("58"), and 63.67dB ("64"). So was the input impedance at 1kHz, which ranged from 100.6 ohms when set to "100 ohms," to 201 ohms ("200"), 474 ohms ("475"), 1003 ohms ("1k"), and 42.25k ohms ("47k"). The input impedance was the same at 20kHz as it was at 1kHz, except when it was set to 47k, when it dropped to 38k ohms; it rose very slightly at 20Hz.

The Duo's response with RIAA equalization (fig.1) was one of the flattest I've measured, falling within limits of ±0.02dB. The two enclosures meant that the Duo's channel separation was so low as to be unmeasurable. Though with the optimal grounding I settled on there were still some low-level spuriae at 60Hz and its odd-order harmonics (fig.2)—odd, given the Duo's dual-mono switched-mode power supplies—this was still a very quiet phono preamplifier. Set to the lowest gain, appropriate for moving-magnet cartridges, the wideband, unweighted signal/noise ratio (ref. 1kHz at 5mV and measured with the input shorted) was 70.1dB (average of both channels). This improved to 73.7dB when the measurement bandwidth was restricted to the audioband, and to 85.2dB when A-weighted. Set to the highest gain, appropriate for low-output moving-coil cartridges, the respective ratios were 65dB, 67.9dB, and 73.5dBA, this time ref. 1kHz at 500µV.

The overload margins were related to the gain setting. At 40dB gain, the margin was 25.1dB at 20Hz and 1kHz, dropping to 17dB at 20kHz, all ref. the MM output level of 1kHz at 5mV. Each time the gain was increased by 6dB, the overload margins dropped by the same 6dB. When the Duo was set to 64dB, the highest gain for MC cartridges, the margins were a healthy 22dB at 20Hz and 1kHz, and still 13.9dB at 20kHz.

The Duo offers very low levels of harmonic distortion. Even into the punishing 600 ohm load, with the gain set to 40dB and the output level set to 2V, the second and third harmonics were almost nonexistent at close to –120dB (0.0001%, fig.3). Intermodulation distortion at the same 2V output level (fig.4) was also extremely low.

Its measured performance reveals the Sutherland Duo to be a superbly quiet, superbly linear, superbly accurate phono preamplifier.—John Atkinson