Pass Laboratories XP-32 line preamplifier

When I studied physics at university too many years ago to admit, I learned about Occam’s razor. Many, many more years ago than that, Franciscan friar William of Occam stated that a hypothesis should provide the simplest possible explanation for a phenomenon.

I apply Occam’s criterion to my audio system: It should be no more complicated than it need be. The D/A processors I have written about over the past few years—dCS Rossini and Vivaldi, MBL Noble Line N31, PS Audio DirectStream, Weiss DAC502—all have high-precision digital volume controls. I therefore haven’t had a conventional preamplifier in my system for a long time, instead connecting the processors’ balanced outputs directly to the power amplifiers. Even for LP playback—don’t tell Michael Fremer!—I connect the balanced, RIAA-equalized output of my Channel D Seta L phono preamplifier to an Ayre QA-9 A/D converter and send the 24/192 PCM data to the digital processor!

However, when I reauditioned the MBL N31 for my follow-up review in the December 2020 issue, I had a disturbing experience when I set the N31’s volume control to its maximum and inserted MBL’s N11 line preamplifier between the DAC and the power amplifiers to control volume. As I wrote in my follow-up, “I was not expecting an increased sense of palpability to the acoustic objects within that soundstage, particularly vocals, but that is what I heard. … [B]ut how can inserting an active device with vanishingly low levels of noise and distortion result in improved sound quality?”

I must have been having a senior moment when I penned those words, because I had forgotten that in my December 2014 review of Ayre Acoustics’ KX-R Twenty line preamplifier, I had described the same experience with the original KX-R: “To my astonishment, the sound of my system with a [Logitech] Transporter D/A processor feeding the preamplifier was better than when the DAC fed the power amplifier directly.”

As I had previously found that the KX-R’s sound quality was equaled by Pass Laboratories’ XP-30 three-chassis preamplifier, when I learned that the XP-30 had been replaced by the XP-32 ($17,500), I asked for a review sample.

The ’32
The new preamplifier, like the XP-30, was designed by Wayne Colburn, who has been responsible for Pass Labs preamplifiers since the mid-1990s. It looks identical to the XP-30, with each channel’s audio circuitry in a separate chassis and the power supply and control circuitry in a third chassis. This last chassis has a rectangular, alphanumeric, blue-fluorescent display in its center, a large volume-control knob to its right, and five control buttons to its left. These controls are duplicated on the aluminum remote control, which also has buttons for channel balance and “Pass Thru.” The latter sets the gain to 0dB (“179” on the display) and locks the other controls so that the XP-32 can be used in a home-theater system, where volume is adjusted elsewhere.

Keeping the “dirty” control circuits and power supply separated from the actual preamplifier channels minimizes the possibility of RF and other spuriae from contaminating the analog signals. (Pass Labs’ XP-22 preamplifier, which Jim Austin favorably reviewed in June 2019, is similar other than having both channels’ audio circuitry in one chassis.) Umbilical cables connect each audio chassis to the control chassis, terminated in locking, military-spec, multipin circular connectors rather than the XP-30’s rectangular DIN-25 connectors. These must be connected prior to powering up the XP-32. (The power supply has to “see” the audio channel before it will send power to the audio chassis.)

Each audio chassis has five numbered inputs, a dedicated home-theater input, and a tape-loop input, all on both XLR and RCA jacks. (When the XLR is used, a jumper connecting the jack’s pin 3 to pin 1 must be removed.) The ground of each RCA input is in parallel with pin 1 of the XLR, and the RCA input feeds a summing junction that, according to the manual, “[maximizes] the patented supersymmetry (X circuit) and preserves the balanced character of the XP-32 from input to output when fed from a single-ended source.” There are three outputs, again on both XLRs and RCAs: one main output that follows the volume control setting; a second output whose level can also be adjusted with the channel’s front-panel gain control to allow use with power amplifiers of differing sensitivities in multichannel applications; and a unity-gain tape-loop output.

XP-32 vs XP-30
Given that the XP-32 appears identical to its predecessor, other than having a volume control that operates in 0.5dB steps rather the earlier one’s 1dB steps, I asked Wayne Colburn what the internal changes were.

“The volume control is single-stage as used in the Xs preamplifier and has more range, with 0.5dB steps; it is quieter and more accurate. While the XP-30 had a 1µF polypropylene input cap, the [XP-32’s] inputs are DC-coupled and a servo is used after trimming. The XP-32 has a new input circuit that is based on the Xs: it is lower in noise and distortion with greater drive capability. Noise is the most prominent part of THD+N at low levels. By lowering noise we get better resolution and dynamics….The basic circuit topology is the same, but the plug-in gain modules are unique to the ’32 and use some new cascode transistors and higher bias. The transistors used are [matched pairs of] Toshiba FETs on the input and MOSFETs on the output. We have a nice stash of these purchased before they were discontinued. For the basic signal path, these are tough to beat and I consider them the standard, as do other audio engineers.

“The output stage runs a higher class-A bias with auto bias and lower output impedance. After all, why would anybody make a pre that wasn’t class-A? The XP-32 can drive long cable runs and low input impedances easily. A servo controls DC offset after it is set by hand and balance-adjusted. The output is still capacitor-coupled even with the servo but can be bypassed with internal jumpers. It is really an interface to other products since I have no guarantee of what it will connect to. While caps still have a sonic signature, they have gotten really good.”

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Pass Laboratories Inc.

13395 New Airport Rd., Suite G

Auburn, CA 95602

(530) 878-5350

passlabs.com

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Specifications
Associated Equipment
Measurements
Jim Austin June 2022

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