Sonic Frontiers Phono One phono preamplifier Measurements
Except as otherwise noted, the Sonic Frontiers Phono One was measured as received: set for a 100 ohm input impedance.
The Phono One's output impedance measured 185 ohms, its input impedance 95 ohms. The DC offset at the Phono One's outputs was a reasonable 12.5mV in the left channel, 10mV in the right. The preamp is noninverting. The voltage gain at 1kHz in the 100 ohm input impedance setting measured 59.6dB—more than sufficient for all but the lowest-output moving-coil cartridges. S/N measured an excellent 70.2dB from 10Hz to 500kHz, 85.5dB from 22Hz to 22kHz, and 90.7dB A-weighted.
Except for some small irregularities likely due to noise in the low frequencies, the Phono One's frequency response (fig.1) was exceptionally flat. The channel separation measurements (fig.2) were taken at an input of 5mV (1kHz). While the two channels differ significantly, even the poorer of the two figures (L–R crosstalk) is low enough to be audibly irrelevant, given the far worse crosstalk of even the best phono cartridges.
Fig.1 Sonic Frontiers Phono One, RIAA error (0.5dB/vertical div., right channel dashed).
Fig.2 Sonic Frontiers Phono One, Crosstalk (from top to bottom): L–R, R–L (10dB/vertical div.).
The THD+noise vs frequency measurement is shown in fig.3. Note that this result is shown for two input levels. The curve at 5mV (20dB above the standard MC output level of 500µV at 1kHz) indicates a uniformly high level of distortion across the band, but the reading is probably dominated by noise. Increasing the input level to 39mV at 1kHz clips the circuitry above 7kHz, but reveals the true THD level below that frequency.
Fig.3 Sonic Frontiers Phono One, THD+noise (%) ;wvs$w frequency (from top to bottom at 1kHz): at 5mV at 1kHz; at 39mV at 1kHz.
Fig.4 shows the THD+noise vs output in volts at 1kHz. Note that the THD plateaus above 20V output, though it goes marginally lower at nearly 50V out. This corresponds to an input of 39mV—which is where the figure used for one of the curves in fig.3 comes from. (This high input level was not used for the crosstalk measurements because it was found to make little difference below about 12kHz.) Below this input level, noise causes the distortion reading to be higher; above it, the actual distortion of the preamp itself begins to increase.
Fig.4 Sonic Frontiers Phono One, distortion (%) ;wvs$w output voltage into 100k ohms.
The Phono One's overload margin was excellent at 1kHz and 20kHz for a low-level moving-coil preamplifier. A THD+noise reading of 1% was reached at inputs of 72mV at 1kHz (43.2dB overload margin ref. 500µV) and 215mV at 20kHz (32.7dB overload margin). Because of the relative levels of noise and distortion, it was not possible to obtain a THD+noise level as low as 1% at 20Hz. The minimum THD+noise at this frequency is 2.37% at an output of 1.86V and an input of approximately 0.25mV—a fair overload margin of 14dB.
There was nothing particularly surprising in the Phono One's measurements. This is a respectable set of results for a moving-coil phono stage.—Thomas J. Norton