The HPA-1 is a solid-looking box, measuring 4.5x11x14 inches and weighing 14 pounds. It's brushed aluminum and it's got a simplicity to it, as if there's absolutely nothing there that doesn't need to be. The front panel features:

• a single, locking Neutrik 1/4" headphone jack;
• a large volume control knob; and
• three buttons to select inputs or to engage the preamp output.

The three buttons select the chosen input and, using high-quality relays, switch between the rear-panel preamp-output jacks and the front-panel headphone jack.

On the rear panel, you've got:

• two pairs of single-ended analog inputs via RCA jacks (to connect to a CD player, turntable, etc.);
• a set of switchable "preamp" line-level output jacks; and
• a fuse and power switch.

Note that the unit is designed to stay on at all times. When the power is turned on the unit goes into a mute state for 20 seconds, this is to allow the circuitry to stabilize. During this period no functions are available. During the mute period the power LED will flash. When the unit goes out of mute into operate mode, the power LED will stop flashing and stay on.

I have no complaints about the design: sturdy, compact, and attractive. My only comment pertains to the headphone jack: it doesn't seem to "go" with the design. But since I've brought it up, here what Pass Labs says about it:

"The Neutrik locking headphone connector has become the de-facto standard in the broadcast world because of its rugged reliability. But beyond that it has special attributes, that make it ideal for our purposes and your listening pleasure. First, the Neutrik locking connector provides an isolated signal ground connection for greater signal purity. The high contact pressure and positive locking mechanism of this piece work in conjunction to maintain the best possible signal integrity between the headphone amp circuit and the headphones. The silver contacts are rated to reliably carry 10A of current without overheating and are largely self cleaning. If there is a better sounding and more trouble-free 1/4” audio jack, we’ve yet to experience it!"

As far as the tactile experience, it's a win in my book. I love the large volume knob with a convex center. I also like the three push buttons on the front panel. Do you remember button candy ... the little dots you would peel off of strips of paper? They remind me of those. Plus, there's an oddly satisfying three-syllable audible click when you push one.

Pass Labs conceptually designed the HPA-1 as a real Class-A power amplifier, not as an accessory offering only incremental performance gains. The HPA-1 spares nothing in the circuit design and spares no necessary expense in execution. The foundation of the HPA-1's engineering is a custom, low-noise shielded toroidal power transformer feeding a discreet low noise regulated power supply for the audio circuits. As Nelson Pass has been known to say, the importance of the power supply is often overlooked and plays a large part in the overall performance of the amplifier.

The HPA-1's amplifier circuits are low-feedback, wide-bandwidth discreet designs employing J-Fet input stages and Class A-biased direct-coupled MOSFET output stages.

The Pass Labs HPA-1 front and rear panels.

Testing setup: I tested the HPA-1 using the DAC inside of the Matrix Audio Element X (with the Element X's preamp/headphone amp section turned off). Later, for a comparison, I tested the Element X with the headphone amp function turned on. My impressions here are based off of the AEON 2 open-back headphones from Dan Clark Audio.

It's easy to hear right away that the HPA-1 has an almost tube-like sound, with a warm-ish coloration. I'd classify it as warm side of neutral. The sound is very musical, with a generous soundstage and plenty of detail. Now, they say the midrange is where the magic is, and this is as true as ever with the HPA-1. Vocals and acoustic instruments, in particular, have great presence that is natural and tonally accurate. Bass is tight and nicely defined, with a nice amount of body, while highs are sweet and pretty yet wonderfully smooth.

"Skating Away on the Thin Ice of a New Day" by Jethro Tull is one of my go-to test tracks. Here are a few of the things I look for: clarity in the opening sequence where Ian Anderson is making tea; separation between dominant and secondary instruments, especially when the music gets frenetic; highs that aren't too bright (flute, bells, etc.). What I find with the HPA-1 is that the opening sequence as well as vocals sound very close - so close that I can actually feel the clink of the tea spoon against the cup. Bass sounds nicely defined, as do the various instruments. I can easily track them when the music gets busy. The highs sound very sweet but not unpleasant. I noticed that the guitar blasts sound less harsh than usual -- they can be quite jarring on a more analytical system.

On "Architect" by Trey Anastasio, I enjoy the full and ethereal sound with a nice sense of spaciousness. Instrumentals in the song are meant to mimic construction sounds, and I love how they sound crisp and defined but not to the point of drowning out vocals; in fact, vocals are quite front and center here, with the quality of Anastasio's voice matching the reflective tone of the song.

Feeling nostalgic during my first testing session, I called up a song from my high school days: "The Low Spark of High Heeled Boys" from Traffic.

Albums:

• War Child - Jethro Tull
• Traveler - Trey Anastasio
• Low Spark of High Heeled Boys - Traffic
• Fragile - Yes
• Lover - Taylor Swift

If you order the HPA-1 from Moon Audio before Dec. 31, 2021, we will include a free 4-foot Blue Dragon power cable - a $200 value. This neutral-sounding cable features UP-OCC copper and is double shielded to ward off all outside interference. Shielding is paramount in power cables, not only to protect the cable but to eliminate the power lines from corrupting your line-level interconnects in close proximity. The cable can be covered with a layer of Techflex braiding for added wear protection and good looks. Contact us for this option.