Solid State Preamp Reviews

I measured the Pass Laboratories XP-22's performance with my Audio Precision SYS2722 system (see the January 2008 "As We See It"). The maximum gain was the specified 9.5dB for the balanced inputs to the balanced outputs, but 6.07dB from the unbalanced input to the unbalanced output, the latter result 3dB higher than the specified gain. The preamplifier preserved absolute polarity (ie, was non-inverting) with both balanced and unbalanced inputs and outputs. (Its XLR jacks are wired with pin 2 hot, the AES convention.)

The XP-22's unbalanced input impedance was close to specification at 20Hz and 1kHz, at 20.5k ohms, but a little lower at 20kHz, at 17.5k ohms. The balanced input impedance was twice the unbalanced, as expected. The unbalanced output impedance was 51 ohms at 20Hz, and 44 ohms at 1kHz and 20kHz. The balanced impedance was 44 ohms at 1kHz and 20kHz, and 65 ohms at 20Hz.

The preamplifier's balanced frequency response into high impedances was flat from 100Hz to 30kHz (fig.1, blue and red traces), though the low frequencies rolled off slightly into 600 ohms, reaching –1.5dB at 20Hz (cyan, magenta). Fig.1 was taken with the XP-22's volume control at its maximum setting of "99." At lower settings the ultrasonic output rolled off slightly faster, reaching –2dB at 90kHz with the control set to unity gain ("90"). Channel separation was superb, at >110dB in both directions below 4kHz, and decreasing only slightly, to 96dB, at 25kHz (not shown).

From balanced inputs to balanced output, the Pass Labs preamp offered extremely low noise, with virtually no power-supply–related spuriae in its output, even with the preamplifier sitting atop its separate power supply (fig.2). From unbalanced inputs to unbalanced output, however, some spuriae crept into the spectrum at both the even- and odd-order harmonics of the 60Hz power-line frequency, even with the power supply sitting on the floor at the full length of the umbilical cord (fig.3). Even so, these spuriae were very low in level. The wideband, unweighted signal/noise ratio, measured with the unbalanced input shorted to ground but the volume control set to its maximum, was still 83.8dB ref. 1V output (average of both channels, which were very similar). Restricting the measurement bandwidth to the audioband increased the S/N to an excellent 85.35dB, while switching an A-weighting filter into circuit further improved this ratio, to 99dB.

Fig.4 plots the percentage of THD+noise in the XP-22's balanced output into 100k ohms. The THD+N rises below 5V output due to the fixed level of noise becoming an increasing percentage of the signal level. The XP-22's balanced output doesn't clip (ie, when the THD+N reaches 1%) until 34V, which is almost an order of magnitude above the maximum level the preamplifier will be asked to deliver in typical use. As expected, the unbalanced output clipped at half this output level.

To be sure that the reading was not dominated by noise, I measured how the XP-22's distortion changed with frequency at a very high level, 6V. The THD+N percentage was extremely low throughout the audioband into both 100k ohm and 600 ohm loads (fig.5), though with the usual rise in the top audio octaves. I looked at the spectrum of the distortion at a similarly high output level because, at 1V, no distortion harmonics were visible above the noise floor. The result is shown in fig.6; while the second and third harmonics can be seen, these are still at astonishingly low levels: below –100dB. Tested for intermodulation distortion with an equal mix of 19 and 20kHz tones at a more typical level of 2V, the second-order difference product at 1kHz was absent, and the higher-order products all lay at or below –97dB (0.0014%) (fig.7).

Like the other Pass Laboratories preamplifiers we've reviewed, the XP-22 offered superb measured performance.—John Atkinson