The Pass Labs HPA-1c (photo by Juan C. Ayllon)
If you don't already own one, you've probably seen them on the equipment racks of high profile audio reviewers: rugged and utilitarian, a milled metal block with an oversized volume knob that sounds sumptuous. For the past two years, I've been enjoying the lush delivery of the widely admired Pass Labs HPA-1 Headphone Amplifier designed by Jam Somasundram, who'd left his position of Director of Engineering at Cary Audio Design in 2013 to join Pass Labs, and has been serving as president of Quadratic Audio since January 2021 (you can read my review of it HERE). Originally released in 2016, it's a powerful standout headphone amp that can double as an excellent preamplifier.
"Boasting the iconic and minimalistic machined aluminum chassis typical of Pass Labs," I'd written, "it delivers a deeply satisfying, vivid, and robust aural experience, whether it's to a roomful of guests or an audience of one tethered with a headset."
Then this past June, I received an email from Anastasia Protopappas, president at Pass Labs, inviting me to review their new update (you can read my interview with her HERE).
"We have recently released a new and improved version of the HPA-1, now called the HPA-1c," she informed. "Wayne Colburn has redesigned the board to lower the noise floor and improve the sound quality. Since you have reviewed the old version, I thought I would offer you a new HPA-1c to review."
Nelson Pass (photo by David Robinson)
A Few Words on Pass Labs
A Physics graduate from UC-Davis in 1974, the venerable Nelson Pass designed crossovers and speaker enclosures for ESS Speakers while a student and, after graduation, co-founded Threshold Audio with industrial designer Rene Besne, where he gained renown for their Threshold Stasis amplifier.
Nelson also gained wide acclaim for his Adcom GFA 555 amplifier design.
While at Threshold, he met and hired Joe Sammut, who eventually rose to the position of business partner; he would move on to the presidency at Krell and, eventually, Pass Labs.
Nelson also hired Wayne Colburn, a future partner at Pass Labs, as the "Wizard's assistant" at Threshold in 1989.
Official Pass Labs operations commenced in 1991 at Nelson Pass's home (he and Mike Burley created a machine shop in Nelson's garage with three CNC machines they'd built to produce the amps), where he gained notoriety with his single-ended Aleph. The first version, the Aleph 0, was a monoblock rated 70-watts into 8 ohms and, according to passlabs.com, "was unique in using three gain stages of power Mosfets, including a push-pull output stage biased by a big constant current source. The amplifier operated as single-ended Class A up to its 70-watt rating, and in push pull Class A at higher wattage."
Wayne Colburn (image from "Burning Amp 2017 - Wayne Colburn." Linear Integrated Systems YouTube channel)
In 1994, Wayne Colburn rejoined Nelson. At Pass Labs, he improved the Aleph P line stage, giving it a new active volume control circuit and remote control, created a new phono stage and, in the process, Nelson Pass received two patents, with one of them the design of the Super Symmetric circuit that's still used today. He also created their D1 DAC that's still highly regarded nowadays, but currently focuses primarily on preamps.
In 1995, Joe Sammut left Krell to become the head of operations at Pass Labs and in 1996, designer Desmond Harrington, also from Krell, followed suit. Sammut would go on to become President of Pass Labs, allowing Nelson to focus all his time on product development. (You can read more about the Pass Labs story HERE.)
Then after Mr. Sammut passed away, his wife, Barbara Sammut (a longtime Pass Labs employee) returned to work on book keeping and payroll at PL and soon after, her daughter (and Joe's stepdaughter), Anastasia Protopappas, joined their ranks. A decorated MBA graduate of U.C. Davis, she spent 20 years in corporate finance, with 15 years at Hewlett Packard focusing on research and development, acquisitions, and became senior financial analyst; from there, she moved onto four more years at Oracle, serving as senior financial manager.
Anastasia Protopappas, President, Pass Labs (photo by Juan C. Ayllon)
"My mother had come back to work here after my stepdad, Joe, had passed away, and she was sort of lamenting that she missed being retired, and getting to do the things she wanted to do," she said to me in an interview. "And I wasn't loving my job at the time, and so it's like, 'Well, I could do your job,' and she was doing bookkeeping and payroll and all of that kind of stuff. And so I came here in 2019, and just kept picking up whatever they could throw at me—basically anything that wasn't engineering and technical! (she laughs)." And then...(in January 2022) they made me president." (You can read more from that interview HERE.)
Since Protopappas' rise to the helm of PL, the following products have been released:
- First Watt (SIT-4 and SIT-5): Limited-production amplifiers using Static Induction Transistors (SITs) created by Nelson Pass. First Watt is a separate company started by Nelson Pass to explore unique amplifier designs.
- XS Pre 2 (2025): The new flagship, three-chassis preamplifier features separate mono amplification sections and a dedicated power supply for each channel and a digital section. The design is claimed to be Wayne Colburn's best work in 35 years.
- HPA-1c (2025): An updated version of the award-winning HPA-1. The "c" version features an upgraded power supply and improved grounding, leading to lower noise and greater clarity, especially with high-sensitivity headphones. (Google A.I. search, September 17, 2025)
A Peak Under the Hood
The interior of the Pass Labs HPA-1c Headphone amplifier, above, and the original HPA-1 below.
Save for the "C" suffix on the milled name upfront (ostensibly for Colburn, the longtime partner and preamplifier designer at Pass Labs), the two look virtually identical to one another. And removing their tops, I found the two looked remarkably the same, save for a few minor differences, with the primary difference being—an industry friend suggested—that they may have used a different regulator for a lower noise floor versus the more intimate-sounding regulator in the original.
Photo courtesy of Pass Labs
Like its forerunner, the Pass Labs HPA-1c is compact at 11" W x 13.5" D x 4" H, upfront its brushed aluminum faceplate boasts, left to right: its moniker, "HPA-1c" on the bottom left corner; three small, shiny round metal selector buttons: Input 1, Input 2, and Preamp (tiny blue power and selection indicator lights reside above the buttons); a high-quality "locking" 1/4-inch headphone jack on with the engraved Pass logo above; and a large, steely volume knob, protruding from a black circular backdrop, connected to an ALPS Potentiometer or circuit.Visually, the knob echoes the bright blue A/B meters on Pass Labs amps with their black plastic surrounds accessorizing their silvery uniform with a deep groove running across the middle of their machined aluminum chassis that rounds out their iconic look.
Photo courtesy of Pass Labs
At the rear, from left to right, it features an IEC plug input with a fuse box and an On/Off power switch above; a pair of switchable, single-ended analog RCA preamp line level output jacks; and two pairs of single-ended RCA inputs.
A pair of Dan Clark Audio Stealth Headphones rests atop the HPA-1c, at left, and the original HPA-1, at right, while a Richard Lee Audio passive preamplifier serves as a toggling device between the two.
Specifications
- Gain (dB): 8
- Frequency Response: 10Hz – 100kHz (-1 dB)
- Output Power into 20 ohms: 3500 mW
- THD + Noise: < 0.005 at 1V
- Output Power into 300 ohms: 200 mW
- Input Impedance: 50k Ohms
- Output Impedance: < 2 ohms
- Power Consumption (watts): 23
- Unit Dimension (wdh in inches): 11 x 13.5 x 4
- Unit Weight (LBS): 14
Listening Tests with Over-the Ear Headphones
Donning a pair of Dan Clark Audio Stealth planar headphones ($3999), rated a low sensitivity of 86 to 87 dB per milliwatt, with their Straight Wire OEM cables connected to a 10' length of Mogami Gold headphone extension cable I compared the HPA-1c and HPA-1 headphone amps. With the first song, I use the same power cord and RCA cables, alternatively plugging in the HPA-1c and HPA-1, listening, and logging my observations.
First, Patricia Barber's smoky and melancholic vocals, somber piano, Adam Cruz's punchy drum kit, and Mark Johnson's resonant double bass have an uncanny presence in this excellent recording of "Bye Bye Blackbird" (Patricia Barber, Nightclub. 192kHz, 24-bit recording of a Reel to Reel Master Tape. Premonition Records, 2000, UPC: PREMRR90763). After half an hour of back and forth comparisons between the HPA-1 and HPA-1c, I recognize the futility of distinguishing between the two; had I been doing this blind, I would not be able to tell them apart. Sensing the delays with plugging, powering up, listening, unplugging, switching the cables and IEC plug, playing and observing again might create over reliance on auditory memory, I streamline matters.
To reduce the lag time, I alternatively plug the Stealth headphones into the HPA-1c and HPA-1 headphone amps (set at unity) that are linked to a switchable Richard Lee Audio passive preamplifier (again, set at unity) which, in turn, is connected to the Pass Labs XP-12 preamplifier.
Second, the crowd's cheers are palpable, the acoustic guitar's attack and Church's raspy vocals are crisp and clearly defined, and there's loads of organic and spatial detail in Eric Church's "Mistress Names Music Red Rocks Medley (Live)" (Eric Church, Mr. Misunderstood On the Rocks: Live & (Mostly) Unplugged. Qobuz FLAC 44.1kHz, 16-bit download. EMI Records Nashville, November 4, 2015, B0025839-02). Flipping back and forth after a minute has elapsed, then in between vocal phrases and instrumental runs, I continue having great difficulty differentiating the HPA-1 from the HPA-1c. Like their covers and internals, they are indistinguishable.
And third, the cheers, the meaty sound of hands clapping, John Ferraro's snare and Hi-Hat vamp in 4/4 time, the snap of Matt Bissonette's bass guitar, the pad of Jim Cox's keys, and Tom Poole's trumpet overlayed with Tom Scott's flute all sound vivid, clearly articulated and spectacular, while the warmth and comfort of Boz Scaggs' vocals—augmented by Barbar Wilson's background vocals—soothe and enchant in "Lowdown" (Boz Scaggs, Boz Scaggs. Greatest Hits Live. Qobuz FLAC 44.1kHz, 16-bit. ) It's lush, detailed, and lifelike, but once again, toggling back and forth, I am not hearing a difference!
Swallowing hard, I pick up the phone and call Pass Labs. Their president, Anastasia Protopappas, answers the phone, hears me out, and passes me onto Kent English, their sales person.
"You were listening with over the ear headphones?" he clarifies in his calming, professorial tone. I say, yes. "Ohhhh. I couldn't tell the difference with Focal over-the-ear headphones, either," he replies, and mentions another brand of over-the-ear headphones that yielded the same results.
He then explains that people who listened to music using in-ear monitors with seven-way systems and 120 decibel per milliwatt sensitivity (versus the 90 to 105 dB per milliwatts of over-the-ear headphones) were having noise issues with their HPA-1. Hence, to fix the problem, Pass Labs focused primarily on the power chain. Enlisting power filtration and applying a noise analog signal out of phase with the DC line resulted in a noise reduction of half the noise. However, the catch is that in order to actually hear the lowered sound floor, I will have to use headphones with greater sensitivity.
He hands me back to Anastasia, who promises to email their contact at Moon Audio to see if they can lend me a pair of their in-ear monitors for the review.
Weeks later, a pair of 64 Audio Fourte In-Ear Monitors ($3599) arrived.
At 114 dB/milliwatt at 1kHz and 10 Ohms impedance, they were definitely more efficient than the Dan Clark Audio Stealth planar headphones (at roughly 86-87 dB/mW and 22-23 Ohms)! I moderately lowered the volume setting on the HPA-1 from what I used for the DC planars (which are harder to drive), I listened to a cut and focused on clarity, balance, and tonal characteristics but soon realized I was playing them way too loud. My ears were ringing! Restarting with both the HPA-1 and HPA-1c set at unity, I gradually raised the volume on the Pass Labs XP-12 preamplifier's single stage attenuator from level 50 up to 60.
Turning to an old standby, "Water: Giver of Life," I hear the attack, leading and trailing edges of Pat Metheny's plucked guitar strings tactilely detailed, rich, and resonant as their notes decay, while Dave Liebman's succulent soprano saxophone wails, plaintive and plush. Bill Hart's brushed snare and cymbals, and Cecil McBee's deep and dour double bass sound sublime (Dave Liebman, The Elements: Water. Qobuz FLAC 44.1kHz, 16-bit. Arkadia Jazz, Jul 2022). But here's the rub: switching back and forth between the HPA-1 and HPA-1c, I strain to hear a difference.
With Boz Scaggs cut, toggling back and forth between sections, and then mid-phrase, I noted perhaps a very, very subtle, slightly lower noise floor, a little crisper edge to the sounds of clapping hands, the crowd's cheers, and the cymbals, Hi Hat, and snare. As a side note, I noticed that with these 64 Audio Fourtes, details seemed a bit more highlighted than I'm used to with the over-the-ear headphones, as in the guitar comps in "Lowdown."
In Eric Church's "Mistress Names Music Red Rocks Medley (Live)", the acoustic guitar's attack was hard-edged and detailed, and Church's gritty baritone vocals, tinged with a little reverb, commanded my attention. Switching over to the HPA-1c, I thought for a moment that it sounded a hint warmer, and in another pass a slight decrease in ambience noise, but subsequent switching back and forth reinforced the overwhelming impression that they're extremely similar and difficult to tell apart.
Where's the purported background noise that in-ear monitor users purportedly complained about with the original HPA-1? If it's there, I'm not hearing it.
I reached out to Wayne Colburn, advising him of my findings, and inquiring if the new changes implemented in the HPA-1C are to target the measured distortion and "noise" that Amir Majidimehr, an electrical engineer and former executive in digital media whose popular site, Audio Science Review (ASR) critiques audio equipment solely based on measurements—an approach which Schiit Audio partner and designer, Mike Moffat, said, "boil down to mosquito farts in the context of jet engines" in my 2021 interview with him (HERE).
"I know that for some audio aficionados, measurements are king," I write. "As I recall, (Schiit Audio partner) Jason Stoddard mentioned a DAC in a YouTube video that measured better in an apparent nod to their demands. Is that the case with the HPA-1c? If I could have some clarification from Wayne Colburn on the noise the HPA-1c targets, as well as what methods he employs to ameliorate it, that would be helpful."
Photo by Juan C. Ayllon
Wayne Speaks
"The changes to the headphone amp are mostly due to the discontinued opa 2604 servo amplifier," Colburn responded via email. "While we have a stash of them when they go away we wanted to be ready for it. These had the ability to run right off the main supply of +- 24 volts. Most op amps have +- 18 volt limits. I added regulators for this circuit to use a lower Voltage part.
"Amir's comments had me take a look at the measured performance of the amplifier. I thought it measured well like John Atkinson referenced in his review. I did see two peaks in the FFT that could maybe be improved. These were power supply harmonics at 120Hz and 180Hz. While low and probably below audibility I wanted to improve them. On some units it was worse than others.
I therefore added another stage of resistor and capacitor filtering and Voltage regulation with active noise cancellation. I also installed improved RFI filtering mostly for agency approval. This all lowered these above-mentioned two peaks by about 9 dB. The overall broad band 20 - 20kHz noise went down a couple of dB. Not enough to make Amir happy but we don't sell to his readers most likely anyhow. While we have plenty of test equipment, it is not the final arbiter of our sound.
"When listening to three versions of the HPA the new version came out on top. On planars the difference wasn't audible. Mike Jaynes used an older pair of Grado pros made for Germany to hear a slight difference. The amplifier circuit had one minor capacitor change of a few picofarads. Any change to the basic circuit sounded worse. One version measured very well but hurt people's ears. It was really awful."
Photo courtesy of Pass Labs
Final Thoughts
When it comes to consumer audio playback, the consensus among high-end manufacturers is that measurement performance is a good starting point, but not the endgame, and as Wayne Colburn said, it's certainly not the sole arbiter of Pass Labs designs—and with good reason.
Back in 2021, I reviewed the Ayre QB-9 Twenty DAC (HERE) and noted the differences between two selectable digital filters: one for "Listen" (its default setting) that provided greater accuracy in the 'time domain," according to its user manual, and the second, "Measure," which is optimized for greater frequency response accuracy. Based on Amir's criteria, the option should have been superior; however, experienced firsthand, it was unnaturally hard-edged, fatiguing, and irritating for extended listening, while the "Listen" algorithm setting was more natural sounding, enjoyable, and impressive with quality source material.
As another industry designer said, given that vacuum tubes measure far worse than solid-state, why do they remain so popular in audiophile circles? Or, as Jam Somasundram, the designer of the original HPA-1 Headphone Amplifier, said in my review of it, "My whole take on it is if distortion is the sole criterion on how something sounds, you might as well own a Yamaha receiver, because that gives you pretty good distortion numbers, but it doesn't sound very good." (You can read that review HERE.)
In contrast, with the Pass Labs HPA-1c, you get the best of both worlds. Not only does the Pass Labs HPA-1c sound every bit as good as its predecessor, the original HPA-1, but it does so with lower distortion numbers. And although these improvements were inaudible to me, for some, the better measurements may be a compelling consideration. Here's my take: If you already have a Pass Labs HPA-1 that's in good working order, I'd hold onto it. However, if you don't, if you have one that's growing long in the tooth and needs a replacement, or you crave legendary sonic delivery with improved measurements, then look no further!
Bottom line, the Pass Labs HPA-1c is a formidable headphone amplifier worthy of your consideration and, as such, I recommend it.
HPA-1c Headphone Amplifier
Retail: $4045 in silver and $4545 in black
Pass Labs
