Sutherland Engineering Phono Block monoblock phono preamplifier Measurements
Sidebar 3: Measurements
I measured the Sutherland Phono Block with Stereophile's loan sample of the Audio Precision SYS2722 system (see the January 2008 "As We See It" and www.ap.com). For logistical reasons, I was able to measure only one of the two monophonic amplifiers auditioned by Brian Damkroger, serial no.1060. The voltage gain at 1kHz accurately tracked the nominal gain of the daughterboards used: "45" measured 45.0dB, "50" 50.2dB, "55" 54.9dB, and "60" 60.1dB. The preamp preserved absolute polarity (ie, was non-inverting) with all gain settings.
The Sutherland's output impedance was usefully low at 200 ohms across the audioband, and the RIAA equalization was the most accurate I have ever encountered: ±0.03dB to well above the audioband (fig.1). The input impedance was close to the nominal value at low settings: "50 ohms" measured 52 ohms at all audioband frequencies; "100" was 100.5 ohms; "200" was 200.1 ohms; "475" was 473 ohms at 20Hz, 469 ohms at middle and high frequencies; "1000" was 995 ohms at 20Hz, 975 ohms at higher frequencies; "10k" was 9500 ohms at 20Hz, 8000 ohms at 1kHz, and 7800 ohms at 20kHz; and "47k" measured 148k ohms at 20Hz, 41k ohms at 1kHz, and 37k ohms at 20kHz. With its adjustable gain and loading, the Phono Block can be optimized for any phono cartridge out there, other than models like the Haniwa HCTR01 (which Michael Fremer reviewed in November), with its need for a very low load impedance; or models with a pathologically low output level, such as the Ortofon MC3000 from the 1980s.
I was pleased to see that the Phono Block offered a choice of chassis-grounding options, to allow its noise floor to be minimized in the specific system in which it will be used. I got the lowest level of noise by floating the ground of the Audio Precision's single-ended output and using the Sutherland's 50 ohm "soft" chassis ground connection. In that condition, the Phono Block was deathly quiet. With its gain set to 45dB, the wideband, unweighted signal/noise ratio with the input shorted was 80dB ref. 1kHz at 5mV, the typical moving-magnet output level. With 60dB of gain, the ratio ref. 1kHz at 500µV, which is appropriate for a typical moving-coil cartridge, was still 66.5dB, which is excellent. Switching an A-weighting filter into circuit improved these figures to 90 and 73.7dB, respectively.
The Phono Block's overload margin varied with the gain selected and with frequency. At low and middle frequencies, the margin with the gain set to 45dB was a respectable 20.2dB ref. 1kHz at 5mV. This decreased to 15.1dB at 50dB, again ref. 1kHz at 5mV. With the MC gain settings, the lower-frequency margin was 30.4dB at 55dB gain and 25.1dB at 60dB gain, both figures ref. 1kHz at 500µV. The margin shrank at 20kHz, to 12.7dB (45dB gain), 7.6dB (50dB gain), 22.8dB (55dB gain), and 17.6dB (60dB gain). This suggests that the Phono Bock's owner should use the lowest gain setting that gives a satisfactorily loud listening level, for the optimal combination of very low noise and high overload margin.
Below clipping, the Sutherland offers very low distortion, as shown by fig.2, which plots the THD+noise percentage against frequency with the gain set to 45dB. The level was 2V, equivalent to a 1kHz input level of 11.2mV. This graph was taken into the benign 100k ohm load, but even into a demanding 600 ohms (fig.3), the only distortion harmonic visible above the very low noise floor is the subjectively inconsequential second harmonic, at 100dB (0.001%)! Intermodulation distortion was also vanishingly low (fig.4).
The Phono Block offered superb measured performance, as I have come to expect from a Ron Sutherland design.John Atkinson