The Pass Labs HPA-1 comes with the Pass pedigree and accompanying price, so this is not the headphone amp for most people. But does it do things sonically that might make it right for some music-lovers? Let’s see. 

Background 

Pass Labs is a venerable amplifier manufacturer. Founder and amplifier savant Nelson Pass has long been associated with big, powerful, musical amplifiers with advanced circuitry and, most memorably, a special topology called class-A. But Pass also designs amps that use new or unusual transistor types because, well, he just seems to love designing amplifiers. And having done this for 50 years or more, his designs reflect a deep knowledge of the many tradeoffs in amplifier designs. Robert Harley did an profile a few years ago of Nelson, along with Threshold (his prior amp company) and Pass Labs and First Watt.  

The thing is, the HPA-1 wasn’t designed by Nelson Pass. Nelson focuses on power amps, while Wayne Colburn does Pass preamps. To deal with the unique goals and demands of a headphone amp, Nelson brought in Jam Somasundrum. I tell this story because Pass clearly respects the need to master the details of amplification in the different voltage, current and impedance domains of power amps, preamps and headphone amps. Someone with Nelson’s knowledge base also seems likely to be a good judge of technical talent. 

The Pass Labs HPA-1 was introduced in 2016. In a world with iPhone releases you can set your Apple Watch to, this seems like a technological eternity. But Moore’s Law, which summarizes the progress of digital semiconductors, doesn’t apply nearly so exactly with analog circuits. We are all conditioned to think this way of course, but I encourage you to forget that for the next 5 minutes. If you can. We wanted to revisit the HPA-1 because reviewer Neil Gader praised it in 2017, but headphones have come a long way since then.  

The HPA-1 is expensive at $3675. For those of you who can’t or won’t afford this, I understand that this is an expensive amp, even if it is less than a family vacation or a mid-priced refrigerator. Today’s question is whether the amp does something or some things that might interest those who can afford it. The answer, in my listening is “yes”. Emphatically so, in fact. 

Sound Quality 

Let me lay out the case (pro and con) for the HPA-1: 

The HPA-1 has a bass and midrange transparency and a treble purity that simply sounds more like real music than most headphone amps. I compared the HPA-1 to 5 headphone amps I had available while writing this, using it with more amps and with 8 pairs of review headphones over the past few months, and the transparency and lifelike tonal rendition was equal to or better than all the others. My reference dCS Lina headphone amp was quite similar in sound, but the HPA-1 micro-dynamics are just a hair more lifelike. You can hear this difference on Max Richter’s Exiles, with Jarvi and the Baltic Sea Philharmonic.  

The dCS does have an output advantage with very low sensitivity headphones like the HIFIMan Susvara, but I wouldn’t say the HPA-1 really struggles with these unless you listen at ear-damaging levels. Perhaps there are even less sensitive headphones, and they indeed might overtax the HPA-1. I know this issue of output level has been a point of criticism, but it seems misplaced to me for many users. You have to be the judge, of course. 

With other top-flight headphones like the Meze Empyrean 2 and Modhouse Tungsten and the Focal Utopia and the Dan Clark E3, the HPA-1 rendered bass and midrange with a suaveness and detail level that is the best I’ve heard, which of course doesn’t include all potential competitors. More importantly, transparency + naturalness is a hard combination to achieve. The HPA-1 does it.  

The effect fits in that category we call “tonal density”. What this means is that instrumentals or vocals, which are of course made up of fundamental tones plus harmonics, have the richness of that tonal palette on full display. You hear the fundamentals and overtones properly balanced in both level and timing, and the result a feeling of “rightness” and expression and vividness that makes a difference. Again, this is not just a midrange phenomenon with the Pass, it extends down into the bass and up into the treble. Try the 2008 remaster of Bonnie Raitt’s Streetlights. The performers are spotlit but not with glaring lights.  

Some other amps I tried had a more treble-focused balance, which seemed unrealistic and frankly distracting in comparison. This is less about sheer treble elevation and more about an edge in the treble that moves the listener farther from the absolute sound. The Topping A70 Pro was emblematic of this category. Then I found another set of amps that had more natural treble transients but seemed to do this with a loss of a slight bit of low-level detail. Here I would mention the Icon HP8. The point is that these are good amps that sound fine, but where you can identify qualities where the Pass gets a step closer to a natural sound profile. What impresses with the HPA-1 is that it delivers benefits with few obvious sonic tradeoffs.  I was surprised how clearly these differences stood out with a relatively old, but remastered, recording like Sweet Dreams (Are Made of This) by Eurythmics. The 1980s artifacts were there with all the amps but seemed less annoying with the Pass and yet it delineated detail better as well. 

If there are drawbacks to the HPA-1, besides maximum gain or output levels, they might include: 

• The HPA-1 doesn’t strike me as the ideal amp for correcting tonal imbalances in your headphones. First, the Pass amp sounds quite neutrally balanced. Second, the deviations of most headphones from your desired frequency response are likely to be much larger in magnitude than a well-balanced amp like this can address.

• The HPA-1 is single-ended only. Inputs are RCA jacks and the one output is for a single ¼” phone plug. I assume the internals of the HPA-1 are single ended, so Pass felt the inputs and outputs would best be left as is. But if you have a strong preference for balanced output, this will not be your amplifier. 

• The HPA-1 is medium-size, and at 11” wide by 4.5” high by 13” deep. For some desktops, this may be uncomfortably big. 

The volume control is smooth and easily adjusted in my experience. There is no remote, although the HPA-1 has preamp outputs and thus can be used with a power amp to drive speakers. Somewhat obviously, the application Pass had in mind seems to have been the desktop.  

Summary 

I had the good fortune of using the Pass HPA-1 for several months. This allowed me to try it with many headphones and compare it to many amps that were here sometimes for only a few days. Initially I found myself thinking “this sounds very good” each time I had new gear in the lab. Then when I got down to more careful notetaking, my general sense of the amp quickly snapped into focus.  The Pass HPA-1 delivers a sense of tonal rightness across the frequency spectrum done with very few downsides.