Phono Preamp Reviews

One of the pleasures of reviewing—and also using—products from Pass Laboratories is an encounter with Nelson Pass's writing, which can usually be found in the owner's manual and is always competent, insightful, and sometimes funny. How often do you get real pleasure and insight from reading an owner's manual?

Pass Labs has a lot of owner's manuals online. Reading through one, I encountered the following passage; you'll find the same or similar language in other manuals and on the Pass Labs website. I present it not only because I admire it and agree with the philosophy it expresses but also because it captures the spirit of the product under review—the XP-27 phono preamplifier ($12,075 in silver)—at least as I've experienced it during an extended review period. Here it is, quoted at length with some slight adjustments to make it consistent with Stereophile's editorial style:

"For a very long time, there has been faith in the technical community that eventually some objective analysis would reconcile critical listeners' subjective experience with a repeatable laboratory measurement protocol. Perhaps this will ultimately occur, but in the meantime, audiophiles largely reject bench specifications as an indicator of audio quality. This is appropriate; the appreciation of audio is a completely subjective human experience. We should no more let the numbers define audio quality than we would let chemical analysis be the ultimate arbiter of fine wines. Measurements are certainly critical, they can and do provide a measure of insight, but are no substitute for human judgment of that which is pleasant.

"As in art, classic audio components are the results of individual and collective efforts that reflect a coherent underlying goal and philosophy by the major participants. If successful, they make both a subjective and an objective statement of quality, which is meant to elicit appreciation in the final product. It is essential that the circuitry of an audio component reflects a philosophy which addresses the subjective nature of its performance first and foremost.

"Lacking the ability to completely characterize performance in an objective manner, we should take a step back from the resulting waveform and take into account the process by which it has been achieved. The history of what has been done to the music is important and must be considered a part of the result. Everything that has been done to the signal is embedded in that signal, however subtly."

This passage, and the philosophy it espouses, could serve as a how-to manual for aspiring Stereophile reviewers. Objective aspects—technical details and measurements—matter, but they're secondary to the sonic result, and the component should be considered in context: the context of the review system and of the product's development history. I aim to do that in this review.

The essay, which was written by Nelson Pass (I asked), goes on to list three principles that inform Pass Labs designs: Keep circuits simple and reduce the parts count, especially in the signal path; pay attention to and exploit the unique characteristics of the gain device employed; maximize intrinsic linearity, then minimize feedback (the main purpose of which is to increase linearity) because "Experience suggests that feedback ... removes information from the signal."

In doing research for this review, I was surprised to learn that this language goes all the way back to the manual for the Aleph Ono phono preamplifier, which debuted at the tail end of the 20th century. That was, it's safe to say, the first iteration of the phono preamplifier that eventually became the XP-27. So let's start there.

Like many early-ish Pass Labs designs, the Aleph Ono was innovative (footnote 1). It used a combination of JFETs, MOSFETs, and bipolar transistors to produce very high gain at very low noise with no global feedback, while providing great flexibility in cartridge loading. The specifications were excellent for the time, yet that was hardly the point. Nelson wrote in the owner's manual, "it is perhaps remarkable that the final product displays such spectacular objective qualities [footnote 2]. However, we view the specifications as a secondary source of pride in this product. ... The measurements only hint at the listening experience with the Aleph Ono. The sound is lush and smooth, with a wide soundstage, fine detail and layering, and a nice, firm bottom."

Jump forward a decade or so, to the XP-25 phono stage, which was introduced at the 2010 Consumer Electronics Show. Though built in the spirit of the Aleph Ono, the XP-25 was, in my view, the first modern, mainstream Pass Labs phono preamp—modern and mainstream in that it combined high performance with support for two tonearms/cartridges and all adjustments on the front panel (as opposed to rear panel DIP switches). It also employed some negative feedback.

The Pass Labs XP-27 is very similar to the XP-25, inside and out. Both put the power supply in a separate, second chassis.

If you stand back so far that you can't see the writing on the front panel, you won't be able to tell the two preamps apart. The owner's manuals even share much of the same language.

Then why does the XP-27 exist? Because designer Wayne Colburn and the Pass Labs crew kept asking themselves how they could improve the XP-25. Once they had accumulated enough ideas to improve the XP-25, they created the XP-27. In the next section, I'll describe those upgrades in detail.

What it is
The XP-27 is a fully balanced, dual-input, very adjustable phono preamplifier with all its adjustments on the front panel. As already mentioned, it employs separate chassis for the power supply and the signal circuitry. Why two chassis? An important source of noise pollution in low-level circuitry is coupling from noisy AC power. Putting the power supply in a separate chassis keeps all that away from the tiny signal passing out of an MC cartridge and into the phono preamp. A grounded chassis made from metal acts as a cage, keeping electromagnetic radiation from exiting or entering (though it's better at the former than the latter).

Inside the power supply chassis, that 120V AC is rectified, regulated, and down-converted so that all that emerges and passes to the other chassis (via nine silver-coated conductors inside a very nice, well-shielded umbilical cable) is precisely regulated constant voltage. Because it has no time-varying component, it cannot pollute that tiny input signal in the main chassis.

Like its predecessor, the XP-27 doesn't distinguish between MM and MC cartridges; just plug the tonearm cable in to one of the two inputs and select the appropriate gain and loading parameters. All choices are made from the front panel. There are three gain settings: 53dB, 66dB, and 76dB. (That's for balanced output; unbalanced gain is 6dB lower.) Load resistance can be varied from 30 ohms to 1k ohm in eight steps for MC plus 47k ohms for MM. Load capacitance varies from 100–750pF in six steps, appropriate for loading an MM cartridge, which is typical; one rarely sees capacitive loading for MC cartridges, which requires much larger capacitance values.

All these changes can be made on the fly from the front panel—though not from your listening seat, since there's no remote control. This pleases me: I'm happy to get up from my chair, and I don't want another remote control to deal with.

One thing you'll notice when changing settings on the fly: The music is muted for a few seconds. Why? Wayne Colburn said, "The front panel knob settings are controlled by a micro and sent over a serial bus to a serial to parallel expander." The circuitry for that is inside the main chassis. "The expander goes into sleep mode after the settings are changed." Whenever the control circuit is active, then, the music is muted. Noise from the controller cannot affect the music.

Is that noise even audible to start with? Colburn again: "I don't know that this really affects the audio, but it does help lower EMI for agency approvals." No BS.

There is also a mute button on the front panel, which does what you'd expect. Mute is automatically activated when gain or loading is changed, but it is also user-selectable. You can mute the sound when dropping the needle, for example, to avoid the unpleasant sound that results when the stylus misses the groove at the start of an album, or simply if you prefer silent queuing.

In addition to the source selector and Mute buttons, there's one more small button on the front panel, for a defeatable rumble filter. I never used it.

The first gain stage in the XP-27 employs lower-noise resistors—lower than in the XP-25—and all film and electrolytic capacitors have been upgraded. The same transistors are used, from Toshiba. The second gain stage "has similar topology but is now DC coupled [to the first stage] and has some upgraded parts and improved circuit layout." That's Colburn again.

A point of emphasis for Pass Labs phono stages is accuracy of RIAA correction. One of the biggest design changes in the XP-27 affects how gain is adjusted and how that adjustment interacts with the RIAA circuit. "On the XP-25," Colburn said, "I varied the feedback to set the gain." Varying feedback, though, can affect the sound, with the result that the preamp could sound different at high gain than at low gain. Nelson Pass: "The level-control circuit is now a tapped resistor bank between the low-frequency RIAA EQ circuit and the gain stage, which follows." The gain setting should now have no impact on the RIAA circuit.

A good bit of work has been put into the power supply, which now has two power transformers, one for the digital control circuitry, the other for the analog parts. These transformers are toroidal, epoxy-sealed under vacuum; this will lengthen their life and stabilize them to reduce (probably eliminate) mechanical noise. They're also double shielded, with mu-metal, a nickel-iron alloy that's especially good at keeping magnetic fields in or out. Here, this should reduce any coupling between the AC and the surrounding circuitry.

"The power filtering is better," Colburn said, "with added capacitance and preregulation. Improvements were made for lower noise and safety to meet new worldwide regulations. The cable between the supply and amplifier is also new—heavier and shielded, with nice connectors made by Japan Aerospace.

"The rectifiers are upgraded," Colburn said, "and have suppression to stop ringing and to lower EMI." Nelson was slightly more specific, noting the addition of "separate snubbed rectifier bridges for each polarity." Nelson also mentioned CRC filters: "All this to lower the noise," both over the air RF and on the AC line.

I noticed that, despite all these changes, the noise specification is unchanged—the same as for the XP-25. I asked Colburn about this. "The noise differences don't show up in the specs because they are out of the Audio Precision analyzer's measurement range," he responded. "I measure on a stand-alone spectrum analyzer. Having had my hearing tested, it also falls out of my ear's range. Why it seems quieter, I don't know."

"The XP-27 doesn't represent much in the way of fundamental changes from its predecessor," Nelson Pass said, "just a list of incremental improvements."

Colburn noted "lower noise and better bass performance. Noise is such a large portion of the THD+N in phono stages due to the low signal levels that lowering noise really helps out in small details. The XP-25 was quite good, but these improvements add up to make the new one even better."

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Footnote 1: Wayne Colburn—not Nelson Pass—has been designing Pass Labs preamplifiers for decades. The Aleph Ono and all the other phono preamplifiers mentioned in this review are Wayne Colburn's work.

Footnote 2: Upon measuring the Aleph Ono, Tom Norton, who at the time was Stereophile's technical editor, wrote, "There were no surprises in the Aleph Ono's excellent test-bench results." See Michael Fremer's review.