Review: Pass Labs HPA-1 headphone amplifier


by John Grandberg

Have you ever waited and waited and waited for something to show up? And then when it does, it actually manages to exceed your admittedly over-inflated expectations?

I’m not talking about impatiently sitting by the window watching for the FedEx truck. I mean things that don’t exactly exist yet. An upcoming album by a favorite band. That new movie from the relatively obscure director that you’ve been following for years. It could be a new speaker, a new craft beer, a new book, a new anything, really. My point being – it’s easy to build up expectations to an unrealistic level, which almost inevitably leads to disappointment.

I say “almost” because every once in a while things actually work out. The movie, book, album, craft beer, whatever it is, ends up being so thoroughly satisfying that it actually lives up to your self-imposed hype. This is a rare occurrence indeed.

Thankfully, it just happened with the Pass Labs HPA-1 headphone amplifier.


Most everyone knows Pass Labs and the excellence involved with that name. From the overkill three-box XP-30 preamplifier — merely a mid-level Pass offering — to massive monoblock amps weighing well over a hundred pounds each, Pass knows how to do cost-no-object designs. At the same time they offer an excellent line of more affordable (but certainly not cheap) stereo and integrated amps which don’t cost as much as a new car. Even folks who end up choosing other equipment still tend to have a genuine respect for the Pass brand and their products.

So what happens when Pass applies their expertise to the headphone domain? The result is the HPA-1, a world-beater headphone amp and surprisingly useful (if somewhat feature-limited) preamplifier. Spoiler alert – this is among the best headphone amps I’ve ever encountered.

When you hear the name Pass Labs, it’s impossible not to think of Nelson Pass — Godfather of DIY amp design and all-around audio legend. You’d be correct in thinking of him since Pass started the company from his home back in 1991. These days he focuses more on his First Watt projects, while many of the Pass Labs designs are now handled by Wayne Colburn. Colburn has been with Pass Labs for over 20 years and is responsible for most of the newer products, though after that much time it’s probably difficult to separate his designs from Pass’. Surely they have both influenced each other in so many ways as to become thoroughly entangled.

In a similar but more recent development, Jam Somasundram joined Pass Labs after consulting for Sumiko and Wadia, then spending time as director of engineering for Cary Audio. Jam has known Mr. Pass for decades and has been very heavily influenced by his design concepts. The HPA-1 is his first design for the company, and Jam tells me he spent about a year designing it plus many additional months voicing it to perfection. I’d say that was time well spent. While some people may be a tad disappointed that Pass himself didn’t design this thing directly, I think it’s much like the Wayne Colburn situation mentioned above. Nelson obviously has faith in Somasundram and his skills, and would not let the amp be released if he wasn’t confident in its status as a Pass Labs product.

The HPA-1 sells for $3500 which is on the low-end of the Pass Labs scale, but nonetheless, near the top of the heap in headphone market at large. Yes, I can name a handful of headphone amplifiers with higher prices. That’s akin to pointing out a $150,000 pair of speakers as being more pricey than a set going for $80k. Both are on the extreme upper end of the spectrum, regardless of the difference.

For the money, Pass Labs delivers their usual experience. Which translates to superb build quality and the stunning Pass aesthetic, thanks to Desmond Harrington’s signature enclosure design. The HPA-1 looks like a mini-me version of the XP series line stages — same enclosure design, same volume knob, same buttons, but less width meaning no room for an LCD display. Which is fine as that really isn’t needed in this particular case. The two inputs selectors and single preamp output each get a very small LED showing active status, and it works perfectly without lighting up a dark listening room.

The one thing that I don’t love is the choice of headphone jack — this particular Neutrik locking 1/4″ design is just not attractive in my opinion. Nor is a locking jack particularly necessary on a headphone amp. I notice the same jack in use on a friend’s Gibson Les Paul guitar and it makes a lot more sense in that context, but not so much on a stationary headphone amplifier. Jam says they tried quite a few options and ultimately went with this choice for its robust built quality, which seems fair enough.


Internally, the HPA-1 is described as being a relatively simple design, though that belies the attention to detail taken to achieve superior results. Jam explained his project to me like so:

“The HP-1 was designed more like a small power amplifier than a traditional headphone amplifier. It will drive headphones that have impedance ranging from 15 to 600 ohms (which is a conflicting set of requirements for a good design). To overcome this we had to use relatively high supply rails as well as a high bias.

The amplifier uses a complementary topology with a cascoded J-Fet input stage and a Mosfet output stage. The design is fully discreet and uses very small amount of feedback and has a wide bandwidth.

The output stage is biased into Class A for more linear operation and improved distortion characteristics.

The transformer has been custom designed for the HPA-1. It is a toroid that is rated at over three times of what is required for the circuit and has a Faraday shield built in as well as magnetic shielding around the circumference of the unit, to reduce noise. We had to go through several prototypes before we came up with one that met our requirements. Contrary to popular belief, transformers can make a huge difference.

The importance of the power supply cannot be ignored in a good design and to this end, the regulator is composed of discreet components and is a very low noise design with over 40,000 uF of capacitance with local decoupling for each channel.

Switching and mute functions are controlled by a custom programmed micro-controller.

We have left out any unnecessary features that we deemed would compromise the sonic performance of the unit; less can sometimes be more.”

Jam then went on to explain some operational aspects of the device:

“The unit is designed to stay on at all times. The power switch is located in the rear.

When the power is turned on the unit goes into a mute state for twenty seconds. This is done to allow the circuitry to stabilize. During this period, no functions are available, and the power led will flash. When the unit goes out of mute into operate mode the power led will stop flashing and stay on.

The amp has two inputs that can be selected from the front panel.

The unit also has a pre-amp function, which is selected from the button on the front panel. When the pre-amp function is engaged, the headphone output is disconnected — only one output (either headphone or pre-amp) is available at any one time. This is done as a safety feature.

The unit gets to optimum performance about one hour after turn on. In the event of a power loss, the unit shuts off, and upon restoration of power the unit will go through the mute cycle and return to the state it was in prior to the loss of power. It will remember the last input and function and will return to that setting. This is also true when the unit is switched off with the power switch.

Finally I suggest you try the unit as a pre-amp, we believe it will hold its own against products which cost much more.”

I pressed Jam on some specifics regarding his design philosophy and how he arrived at this particular configuration. My questions, along with his answers, are presented below:

Q: Is there any area about the HPA-1 which you feel could have been improved if cost was no object? It’s already quite expensive as headphone amps go but is there anything that was “held back” for any reason?

A: If cost were no object I suppose we could improve the power supply further, or use a dual mono version, all of which I have tried with minimum improvement. Sometimes added complexity can work against you. If you look at the price/performance curve, I think we are in a pretty good spot.

Q: Similar to question 1, is there any design aspect or parts implementation that would have been used if there wasn’t any regard for realistic manufacturing processes etc? I know the DIY community does some crazy stuff that wouldn’t be tenable for a company to build even in modest numbers.

A: I actually tried some crazy stuff (ideas) in the design but only used them if they worked. Parts were chosen for the best performance, and we found that some audiophile (boutique) rated components actually hurt the sound. We tested the amplifier with a wide variety of headphones to check that we were not missing anything. Also, reliability plays a key role in a product like ours, and you do not want to skirt too close to the edge.

Q: Are there any similarities to any of the existing Pass models? I’m not referring to general similarities like low feedback but rather specifics…. was anything borrowed or inspired heavily from any portion of another Pass product? I guess I’m trying to clarify if this is a complete “ground up” design.

A: The design is quite different from existing Pass models, primarily because of the requirements of a headphone amplifier as I mentioned in my other e-mail. This is a totally ground up design, we might use similar parts but the topology is completely different.

Q: I like your comment on voicing. Can you explain in your words the difference between “design” and “voicing”?

A: I would say design is the basic topology/circuit used in the product that works and meets your basic objectives. Voicing is the tricky part which includes for example your choice of components, amount of feedback, and biasing, among others. In my opinion voicing can make the difference between a good or acceptable product and a great one. On the other hand, voicing is not going to help a bad design.

Q: Also related to the above question: as you know, some of the more objectivist designers frown on the term voicing… I suspect you understand their point of view but simply disagree, so where do you think they go wrong?

A: A long time ago I thought that specifications were everything, but quickly learned that there are some things in audio that can’t be easily quantified, or qualified for that matter. Specifications should be used as a guide, and if higher distortion means a better sound then you have to make a choice…. Of course, some sort of balance has to be struck.

For example, if distortion was the main criteria for the quality of an amplifier, no one would own tube amplifiers. Things like Yamaha receivers would rule the day. In my opinion the technical aspect of a design is only part of the equation, and the rest is an art form. I am not stuck in any design camp, which I find can be counterproductive. For example, the no-feedback idea — you have to use elements that work for your design, otherwise you can paint yourself into a corner with the end result being a handicapped product.

We at Pass Labs listen extensively to everything we manufacture and the process is quite lengthy. Our main ideal is that no product should be released before it’s time.”



Armed with this knowledge of the thought process behind the HPA-1, I threw it in with some of my favorite supporting gear and got to listening. System basics include an oldie-but-goodie Zoethecus rack with their heavy-duty “Z Slab” composite shelving, an EquiTech balanced power conditioner, and cabling from the Cabledyne Silver Reference series for everything except USB, which instead was the active B.M.C. PureUSB1 cable. Source was primarily a Clones Audio HOST running Roon, playing files from the onboard SSD or alternately streamed from my NAS in the other room. I also swapped in my trusty Aurender X100L and my Exemplar-modified Oppo BDP-93 universal player for an occasional change of pace. A Resonessence Labs Veritas decoded those ones and zeros, feeding them to the HPA-1 via single-ended interconnects.


I started things off with the venerable Sennheiser HD650 to get a general baseline. I’ve probably logged more hours with the HD650 than any other headphone in my collection, just owing to the fact that it’s been around for so long. I pretty much know it like the back of my hand. This is a headphone that scales shockingly well with improved source and amplification. Driven by the Pass, this former flagship opened up beyond anything I imagined it to be capable of — image specificity was more distinct, soundstage wider, and realism more palpable than I’ve ever heard from this model. If I didn’t own an HD800 I might call this the most “out of head” experience I’ve had with a Sennheiser. But of course when I switched to the HD800 and later the new HD800S, I heard how much better it can get in terms of technical details. We’re talking lightning fast transients with exceptional clarity — attributes I normally think of when describing an electrostatic system. In fact I do hear distinct similarities to my Stax rig, which is quite an accomplishment for a dynamic headphone setup. I’m told Jam Somasundram has a particular affinity for stats including Sound Lab panels and Stax earspeakers, so this voicing makes perfect sense.

Note that the presentation here with all three Sennheiser models was not at all on the lean side. Some people think of the “electrostatic sound” as being somewhat bright, forward, and lacking in weight — which can indeed be the case with certain Stax models. That was a concern of mine regarding the HD800 in particular, which is tuned to sound less impactful than its brethren. The HPA-1 dispelled all worries I might have had — this thing allows the HD800 to dig deep, with beautiful low-end weight and texture. It’s still not a bass canon, but just about as well-balanced as I’ve heard it.


I ran through my collection of Reference Recordings HRx releases (their term for uncompressed 24-bit/176.4kHz PCM) and marveled at the sheer spectacle of it all. I’ve got plenty of hi-res material that sounds good or even great, but this stuff shows how amazing it can get when every step of the recording and mastering process is done right. Recorded music doesn’t get much better than this, and the HPA-1 drives the Sennheiser trio exceedingly well — the resulting sound is something I could enjoy day after day, limited only by the somewhat small HRx catalog (I own all of them). These same releases in standard 16-bit/44.1kHz format sound mighty nice as well, but the move to high-res is pretty clear as heard through the HPA-1.

I then switched out the Senns to see how well the Pass amp pairs with planar magnetic headphones from HiFiMan and Audeze. These tend to be more difficult to drive, making good use of modern amps with their high-current outputs. The HPA-1 can deliver a maximum of 3.5 Watts into a 20 ohm load — most planar models fall in the 32-50 ohm range, so they should still see a considerable amount of current (the latest Audeze models, which go as high as 200 ohms, being the notable exception — see the LCD-4, for example).

sparkThe results were again highly impressive. Using some of my favorite music of 2016 showed me just how much emotion the Pass was capable of unleashing. Yes, it was technically brilliant on Hiromi’s complex Spark, and exceedingly accurate with the strings of the Cypress String Quartet release. But more than that it conveyed a sense of stunning “believability” which kept me listening for hours on end. Huge soundscapes, rich tone colors, and beautiful transients were the order of the day. Lesser recordings, such as the latest Animals As Leaders album, were not the least bit unlistenable, thanks to the very well controlled upper-midrange and high frequency presentation. While the Pass amp won’t hide any grunge existing there, neither will it accentuate such things as many other “reference” sounding amps are prone to doing. And that’s one of several reasons why I enjoy it so much.

Cypress String Quartet, Brahms String Sextets

When you think “highly neutral, extremely technical, solid-state sound” your mind tends to fill in some additional, and perhaps less flattering, descriptive words. I’m thinking “sterile”, “analytical”, “clinical”, and maybe even “lifeless”. These do not sound like desirable attributes! I’d say in many of us they are brought to mind due to previous exposure with supposed “very neutral” solid-state gear that we ended up not liking very much. For me, and in terms of sources, it was the venerable Benchmark DAC 1, the highly popular Mytek Stereo192-DSD, and to a lesser extent the Esoteric D-07x (which I still own) that I just couldn’t enjoy in my system. On the amplification side I recall struggling with various amps from Krell, Bel Canto, Rowland, Bryston, and Chord over the years (and no, I’m not against class D amps, I currently use and enjoy models from Merrill Audio as well as NuPrime). Obviously, system synergy is key and direct comparisons are impossible in this case; I’m simply pointing out the fact that the Pass Labs HPA-1 is easy to enjoy, that the superb clarity it brings forth is not at all a negative attribute.



The HPA-1 is a spectacular amp, and one that I very much enjoy. But for $3,500, it certainly should be amazing. Perhaps more important is how it compares to other top-level competitors in the field.

First up, the Simaudio Moon 430HA. This amp, in basic configuration without the excellent/optional DAC add-on, is priced identically to the Pass. It’s an amp that InnerFidelity editor Tyll Hertsens absolutely adores, and I can see why. Effortless power, smooth sound, and tons of functionality make it easy to integrate into any system.

Obviously the Pass can’t compete on features. It doesn’t have balanced connectivity. It lacks a remote. It’s not as powerful. And while it can be used as a very high quality preamp, it only has two inputs, making it somewhat limited in all but the most simplistic systems.

The 430HA has a smoother presentation, which I feel makes the rather “immediate” sounding headphone experience very palatable for fans of traditional speaker-based audio. But from swelling Stravinsky pieces to the chest thumping snare on Tesseract’s Polaris (which brings to mind the sound of fist-meeting-face from many a vintage action film), the HPA-1 takes it up several notches in sheer palpability. A notable exception came courtesy of the nigh-impossible to drive HiFiMAN HE-6. In this instance, the powerhouse Simaudio takes a clear lead, as the Pass runs out of oomph and can’t do those headphones justice. While both amps have similarly excellent low-end performance, and both are capable of densely-layered soundstage reproduction, it’s the sense of urgency and snap which I favor on the Pass, and that wins the day for me.

Perhaps a more apples-to-apples in terms of sonic style is the Questyle CMA800R stack, with the dual CMA800R amps run in monoblock mode ($4K total). While it’s kind of a hassle that the setup demands external volume control via DAC or preamp, the listener is rewarded with an uncommon clarity that very few headphone amplifiers can match. The Pass Labs HPA-1, along with perhaps the HeadAmp GS-X mkII, are the only competitors I’ve heard that reach a similarly lofty height.

Speaking of lofty highs — the Questyle stack has a more prominent emphasis on the upper midrange and treble areas, making it initially come across as more sparkly and open than the Pass. Careful back and forth listening shows the Pass to be equally revealing, while doing so in what most people (myself included) would consider a more natural way. The information is there if you want it, without demanding your attention as the Questyle stack can sometimes do. For short-term sessions I felt like the dual-mono stack was perhaps more exciting yet after a continuous album or two (or more if I can find the time) the Pass is more palatable and fatigue-free.

If we use a Sennheiser HD800 or especially an HD800S, I feel the Questyle remains a very worthy challenger for the HPA-1 — it’s always been an ideal match with the top Sennheiser models. Switching to more difficult to drive planar headphones sees the Pass taking a lead. It seems to capture the midrange flow of the Audeze LCD-3 in a more liquid fashion, and makes the HiFiMAN HE1000 seem like a more balanced headphone overall. The Questyle is quite good in both cases, yes the Pass is noticeably superior upon careful comparison.

In the end, both of these setups offer extreme insight into the recording — the Pass does so at a lower price, and with less system complexity, not to mention the slight advantage in tonality when it comes to lengthy listening sessions. I’d say that makes it the winner.


Last but not least, my reference amp, the Violectric V281 ($2879 with the upgraded relay-based volume solution). Choosing between these two amps is incredibly difficult for me. I love the V281 for its tasteful warmth, punchy dynamics, and emotive midrange. It does highs in an ever-so-slightly smooth way, taking just enough of an edge off without dulling the presentation. That makes it both more musically engaging and less transparent than the Pass amp — which is something that may or may not be a good thing depending on your priorities.

Listening to Jimi Hendrix Cry of Love versus the re-released versions of those same tracks compiled on First Rays of the New Rising Sun, it’s easy to spot the differences with the Pass amp. There’s improved clarity in Jimi’s vocals, while many poor overdubs are removed to reveal more musical content. The Violectric presents this as a sort of “hmmm, yes I suppose it is better”, while the Pass is more “Wow, that’s a huge difference!”. If subtlety and musical illumination is your thing, the Pass really delivers.

That said, the V281 sometimes gets things right to a larger extent. On Yellowcard’s “How I Go” — which is unrelated to but very clearly inspired by the Tim Burton film Big Fish — the Pass amp plays it more light and ethereal, while the Violectric is meatier and more expressive. The female guest vocalist absolutely steals the show on his song (I later discovered her to be Natalie Maines of Dixie Chicks fame) and it seems the Violectic does a better job of conveying the full emotional weight of her voice. The Pass is already excellent here, and I wouldn’t know I was missing anything unless I heard the two amps back to back.

I could go back and forth citing different examples where each amp has an edge over the other. It comes down to individual tracks and of course headphone pairings. I could happily live with either amp or, better yet, live with both where they compliment one another. Those of you who know how much I love the Violectric V281 will understand how big of a compliment this truly is.


Editor's Choice: BMC UltraDACI haven’t even touched on the Pass HPA-1 as preamp in a simplistic setup. Without going into too much detail and making an already long review that much longer, I’ll simply say that the HPA-1 does just as well driving amplifiers as it does driving headphones. It’s transparent, neutral, and most importantly natural sounding, making it a highly desirable device in either scenario. Dual inputs and no remote make it a non-starter for some folks, but if you can work within those limits this might be your best bet into Pass-caliber sonics at a previously unheard of price.

As far as headphones go, the HPA-1 is a brilliant amp which deserves every bit of praise I’ve thus far heaped on it. As a reviewer’s tool, it has become indispensable for getting down to the nitty-gritty in terms of source differences. Typically, when that’s the situation, it means the device in question might not be ideal for day-to-day listening. I’m mainly referencing the Sennheiser HD800 but plenty of other examples exist. Somehow, the HPA-1 manages to transcend that limitation — I could listen to this thing all day everyday without issue.

For being one of the most revealing, true-to-source headphone amps on the planet, the Pass HPA-1 deserves accolades. For doing so while maintaining a sense of musicality and soul, it deserves an Editor’s Choice Award, as well as very serious consideration from anyone in the market for a top-level headphone amp.


  1. Thanks for the comment. I can totally understand – this is an expensive, end-game amplifier, and you can certainly find satisfying gear that comes with a more affordable price. I loved the Questyle CMA800i – have not heard the 600i but I imagine it does a good job as well. They also have a CMA400i coming soon which should come in at less than $1K. I’ll try to check that out when it’s available.

  2. Hi thanks for the great review. A bit more than I want to outlay without a DAC built in. You spoke well of the Questyle 800 stack. I was thinking of their 600i single box with inbuilt dac – any suggestions on how this might perform compared to the Pass or others?

    Many thanks Vincent

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