Phono Preamp Reviews

I measured the PS Audio Stellar Phono phono preamplifier using my Audio Precision SYS2722 system (see the January 2008 "As We See It"). For logistical reasons, I tested a different sample (serial number SPH-B-9J0100) from that auditioned by Michael Fremer. To get the lowest measured noise, I floated the signal generator's unbalanced output from ground and ran a separate connection from the analyzer's ground to the grounding post on the preamplifier's rear panel.

PS Audio specifies the voltage gain for the moving-magnet input as Low (44dB), Medium (50dB), or High (56dB), depending on the setting selected with the remote control. Looking at the Stellar's balanced output, I measured gains of 43.9dB, 49.35dB, and 54.8dB, very close to those specified. The gain at the unbalanced output was exactly 6dB lower at each setting. The nominal MC input gain is 60dB, 66dB, or 72dB. With the MC input's input impedance set to 47k ohms, I measured gains of 59.9dB, 65.4dB, and 70.8dB. The gains at the unbalanced output were again 6dB lower. Both the moving-magnet and moving-coil inputs preserved absolute polarity.

My estimates of the PS Audio preamp's output impedance were lower than the specified values, at 100 ohms vs <200 ohms unbalanced, and 300 ohms vs 400 ohms balanced. The differences will be inconsequential given the much higher input impedances offered by the line preamplifiers with which the Stellar will be partnered. The Stellar's moving-magnet input impedance was 47k ohms at 20Hz and 1kHz, dropping slightly to 42.4k ohms at 20kHz. Set to 47k ohms, the moving- coil input's input impedance was 45k ohms at low and middle frequencies and 40.5k ohms at the top of the audioband. Set to 200 ohms and 100 ohms, the measured MC input impedance was identical to the specified values and consistent from 20Hz to 20kHz. With it set to 60 ohms, I measured 67 ohms, this again consistent across the audioband.

The PS Audio Stellar Phono Preamplifier offers superbly accurate RIAA equalization (fig.1), with an inconsequential 0.1dB difference between the channels in the midrange. This graph was taken with the MM input; repeating the measurement with the MC input reduced the upper –3dB frequency from 80kHz to 45kHz (both frequencies the average of the two channels), but the response was still flat to 20kHz (fig.2). Channel separation was good, at 63dB in both directions below 500Hz, increasing to 73dB at 3kHz, though it was a little asymmetrical above that frequency.

Spectral analysis of the PS Audio's low-frequency noise floor with the MM input set to High gain (fig.3) indicated that random noise components were low in level. However, very low-level, power supply–related spuriae at 60Hz, 120Hz, and 240Hz can be seen in this graph. The PS Audio's unweighted, wideband S/N ratio, measured with the input short-circuited to ground and with the preamp again set to MM High gain, was an excellent 80dB with an input signal of 1kHz at 5mV. Restricting the measurement bandwidth to 22Hz–22kHz increased the ratio to 87.6dB, while switching an A-weighting filter into circuit increased it further, to 89.3dB. Switching the MM gain to Medium, then to Low, respectively reduced the unweighted, wideband S/N ratio by 6dB and 12dB.

Spectral analysis with these two lower gains revealed that the decreases in the S/N ratios were due to the supply-related spuriae increasing in level. These spuriae must therefore be introduced after the MM gain stage—as the output level increases as the gain increases, a constant level of noise becomes correspondingly lower when referenced to the output. However, even in the Low gain condition, the MM input is still very quiet in absolute terms. The MC input is also very quiet, and by contrast with the MM input, its S/N ratio didn't change by much when the gain was changed. The unweighted, wideband ratio, referenced to an input of 1kHz at 500µV, measured 58.4dB with Low gain, 59.4dB with High gain, these figures increasing to an excellent 73.1dB and 75.8dB, respectively, when A-weighted.

As you might expect, both overload margins and distortion levels varied according to which input was in use and which gain had been selected. With the MM input set to Low gain, the overload margin, ref. 1kHz at 5mV, was an extraordinary 30dB at 20Hz and 1kHz, dropping to a still good 13.25dB at 20kHz. Surprisingly, the MC margins were 4dB higher, despite the higher gains. The distortion signature was virtually pure second harmonic (fig.4). Increasing the gain by 6dB reduced the overload margins by the same 6dB and while the distortion was still predominantly second harmonic, its level had increased by 9dB (fig.5) and the third harmonic appeared at –90dB (0.003%). With the MM input set to High gain, the overload margins were reduced by another 6dB, which meant there was almost no margin at 20kHz. The distortion also increased (fig.6), with the second harmonic now lying at –56dB (0.2%) and the third at –80dB (0.01%). However, reducing the load impedance to the current-demanding 600 ohms didn't increase the levels of these harmonics. The PS Audio Stellar phono preamplifier must have a beefy output stage.

Fig.7 shows how the PS Audio Stellar's MM input, set to Low gain, handled an equal mix of 19kHz and 20kHz tones, at a peak input level equivalent to 1kHz at 10.5mV. This is 7dB below the 20kHz input level where the harmonic distortion reaches 1%, but while higher-order intermodulation products are low on level, the second- order difference product at 1kHz has reached 1% (–40dB). This graph was taken from the single-ended outputs; as with the harmonic distortion measurements, the behavior was identical from the balanced outputs, apart from the doubled output level.

Overall, the PS Audio Stellar appears to be a well-engineered, versatile device, and is among the quietest phono preamplifiers I have encountered. The way in which its measured distortion and overload margin vary with the gain setting suggests that its owner use the lowest gain setting that will give acceptably loud listening levels with their preferred cartridges, both moving-magnet or moving-coil types.—John Atkinson