Sidebar 3: Measurements
I measured the Pass Laboratories XP-32's performance with my Audio Precision SYS2722 system (see the January 2008 "As We See It"), repeating some tests with the magazine's more-recent APx555 system. The maximum gain was a little higher than the specified 9.6dB for the balanced and unbalanced inputs to the balanced outputs, at 10.2dB. The gain from the unbalanced input to the unbalanced output was 6.5dB. The preamplifier preserved absolute polarity (ie, was noninverting) with both balanced and unbalanced inputs and outputs. (Its XLR jacks are wired with pin 2 hot, the AES convention.)
The XP-32's unbalanced input impedance was close to specification at 20Hz and 1kHz, at 20.3k ohms, but slightly lower at 20kHz, at 17.6k ohms. The balanced input impedance was to specification and twice the unbalanced, as expected. The unbalanced output impedance was a low 38 ohms at 20Hz, and 32 ohms at 1kHz and 20kHz. The balanced output impedance was twice the unbalanced, again as expected, as there are two single-ended stages in series for this output.
The preamplifier's frequency response into high impedances was flat from 10Hz to 20kHz (fig.1, blue and red traces) in both balanced and unbalanced modes, with the ultrasonic rolloff reaching –1dB just above 100kHz. While the XP-30's low frequencies rolled off into 600 ohms, reaching –6dB at 32Hz (footnote 1), the XP-32's response into this demanding load was flat down to 20Hz (fig.1, cyan, magenta traces). Fig.1 was taken with the XP-32's volume control at its maximum setting of "199." Both the frequency response and the superb channel matching were preserved at lower settings of the control. The volume control operated in accurate 0.5dB steps for almost its entire range of operation.
Figs.3 and 4 respectively plot the percentage of THD+noise in the XP-32's balanced output against the output voltage into 100k ohms and 600 ohms. We usually specify a preamplifier's clipping voltage as being when the THD+N reaches 1%. However, as you can see from this graph, the output voltage stops rising when the THD+N reaches 0.2%. The XP-32's balanced output level at this THD percentage was just below 30V, even into 600 ohms, which is almost an order of magnitude above the maximum level the preamplifier will be asked to deliver in typical use.
Footnote 1: See fig.1 here.
Fig.1 Pass Labs XP-32, balanced frequency response with volume control set to "199" at 1V into: 100k ohms (left channel blue, right red), 600 ohms (left cyan, right magenta) (0.5dB/vertical div.).
As you'd expect from the use of separate chassis for each channel, the Pass Labs preamp's crosstalk was superbly low, at –130dB in both directions below 1kHz, and decreasing only slightly, to –120dB at the top of the audioband (not shown). From balanced inputs to balanced output, the XP-32 offered extremely low noise, with no power supply–related spuriae in its output even with the two preamplifier chassis sitting on top of the separate power supply/control unit (fig.2). The wideband, unweighted signal/noise ratio, measured with the unbalanced input shorted to ground but the volume control set to its maximum, was a high 85dB 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 94.5dB, while switching an A-weighting filter into circuit further improved this ratio, to 97.2dB.
Fig.2 Pass Labs XP-32, balanced spectrum of 1kHz sinewave, DC–1kHz, at 1V into 100k ohms (left channel blue, right red; linear frequency scale).
Fig.3 Pass Labs XP-32, balanced distortion (%) vs 1kHz output voltage into 100k ohms.
Fig.4 Pass Labs XP-32, balanced distortion (%) vs 1kHz output voltage into 600 ohms.
The THD+N was extremely low at output levels of a few volts, so to be sure that the reading was not dominated by noise, I measured how the XP-32's distortion changed with frequency at 4V. The THD+N percentage was extremely low throughout the audioband into both 100k ohm and 600 ohm loads (fig.5), though with a slight increase 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; the only harmonic that can be seen is the third harmonic at –114dB (0.0002%). This harmonic rose by only a couple of dB into the current-hungry 600 ohm load. Tested for intermodulation distortion with an equal mix of 19 and 20kHz tones at the same peak voltage level, the second-order difference product at 1kHz lay below –130dB, and the higher-order products were all below –110dB (0.0003%) (fig.7).
Fig.5 Pass Labs XP-32, balanced distortion (%) vs frequency at 4V into: 100k ohms (left channel blue, right red), 600 ohms (left cyan, right magenta).
Fig.6 Pass Labs XP-32, balanced spectrum of 1kHz sinewave, DC–1kHz, at 4V into 100k ohms (left channel blue, right red; linear frequency scale).
Fig.7 Pass Labs XP-32, balanced HF intermodulation spectrum, DC–30kHz, 19+20kHz at 4V into 100k ohms (left channel blue, right red; linear frequency scale).
The XP-32 offered superb measured performance, better than that of the XP-30 and the more recent, two-chassis XP-22.—John Atkinson
Footnote 1: See fig.1 here.
