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Pass Labs HPA-1 Headphone Amp Review

Rate this headphone amplifier:

  • 1. Poor (headless panther)

    Votes: 321 90.2%
  • 2. Not terrible (postman panther)

    Votes: 19 5.3%
  • 3. Fine (happy panther)

    Votes: 6 1.7%
  • 4. Great (golfing panther)

    Votes: 10 2.8%

  • Total voters


Staff Member
CFO (Chief Fun Officer)
Feb 13, 2016
Seattle Area
This is a review and detailed measurements of Pass Labs HPA-1 headphone amplifier. It is on kind loan from a member and costs US $3,675.
Pass Labs HPA-1 Headphone Amplifier Review.jpg

The box is quite heavy which is "nice" given how much it costs. The look would be good if it were not for the locking headphone plug. A simple hole would have fit better with the rest of the input switches to the left. Volume control has substantial weight to it which is also appreciated.

Back panel surprised me by lack of balanced XLR input (and output for that matter):
Pass Labs HPA-1 Headphone Amplifier back panel high-end Review.jpg

When I attempted to power on the unit, none of the front panel LEDs lit up. I tried different cables, read the manual, etc. to no avail. So I took the top panel off. There are four hex head screws but the thread is cut poorly into the chassis, making them difficult to turn -- I expected a lot better at this price. Once open, the problem was obvious:

Pass Labs HPA-1 Not Powering On Cable Loose.jpg

As you see, the ribbon cable connector has come out completely. Safe assumption would be that it was barely in and came out during shipping. Or else the ribbon cable puts too much negative pressure on it causing it to pull out. Either way, not good. Anyway, once in there the unit was fully functional. While I was in there, I thought I take a shot of the full design:
Pass Labs HPA-1 Headphone Amplifier inside PCB design.jpg

As you see, it is mostly a discrete class A design. A beefy transformer and lots of heat sinks add to the weight. Here are the specs:
Pass Labs HPA-1 Specifications specs.png

The 8 dB gain is quite low which will impact our power measurements as you see later. Sad to see inadequate specifications such as not indicating the load for frequency response and THD+Noise.

Pass Labs HPA-1 Measurements
Company all but mandates leaving the unit on all the time and at least 1 hour of warm up time. I tested the latter claim but found it without merit as far as noise and distortion is concerned:
Pass Labs HPA-1 Headphone Amplifier warm up measurements.png

As you see, performance is stable more or less when you power it up. Speaking of performance, let's get our dashboard:

Pass Labs HPA-1 Headphone Amplifier dashboard measurements.png

In a world where we have headphone amplifiers with SINAD of 120 dB, this is quite poor performance (although nearly matching spec). Second harmonic distortion dominates as does lots of power supply noise that I could not impact no matter what I did with grounding. The distortion may be part of the "low-feedback" design but what is up with the power supply spikes? That impacts signal to noise ratio:
Pass Labs HPA-1 Headphone Amplifier SNR measurements.png

High noise floor lands the HPA-1 near the bottom of our rankings:

most quiet headphone amplifier review.png

Best to stick with low sensitivity headphones as we are some 30 dB short of where we should be landing.

Even in a simple frequency response measurement, there is something to complain about:

Pass Labs HPA-1 Headphone Amplifier frequency response measurements.png

Company says -1 dB at 100 kHz but I am seeing -1 dB 40 kHz or so. Good headphone amps routinely produce a flat line to 100 kHz here. Not a real audible concern but still, if they are going to spec this, it better do a good job here.

Back to non-linearity, here is our multi-tone test:
Pass Labs HPA-1 Headphone Amplifier Multitone 600 ohm measurements.png

We see a substantial rise in distortion as frequencies increase, indicating that our dashboard numbers are exaggerating the true performance. We see the same in a sweep test:
Pass Labs HPA-1 Headphone Amplifier Frequency vs Distortion measurements.png

Let's see how much power we have into 300 ohm load:
Pass Labs HPA-1 Headphone Amplifier Power into 300 ohm measurements.png

My minimum standard here is 100 milliwatts and HPA-1 misses even that target with 92 milliwatt output. There is no clipping indicating more power could be had. Same is true with 32 ohm load:
Pass Labs HPA-1 Headphone Amplifier Power into 32 ohm measurements.png

Stepping through different load impedances we see that distortion progressively gets worse as impedances go lower (become more difficult):
Pass Labs HPA-1 Headphone Amplifier Power vs load vs distortion measurements.png

The only good news here is the nice volume control which provides near perfect channel balance in my sample:

Pass Labs HPA-1 Headphone Amplifier Channel Balance measurements.png

Pass Labs HPA-1 Headphone Listening Tests
I started my testing with my everyday headphone, the Dan Clark Expanse (I purchased the review sample). This is a difficult to drive headphone and it showed. Even at max volume, there was not a lot of power coming out of HPA-1. What did come out was progressively more distortion especially in high frequencies. The highs were grungy and bass not clear and impactful. I switched back to my RME ADI-2 Pro which is also my everyday driver, and boy, did the beauty of these headphones came out.

Switching to high impedance Sennheiser HD-650, to my surprise, made it worse. Yes, there was enough volume now but distortion would set in early and keep getting worse. At max volume, the sound was miserable. Again, I switched to RME ADI-2 Pro and the improvement in all aspects from clarity to bass impact and clean highs was remarkably clear.

I heard nothing euphonic. At best the sound was OK (at low volume) but quickly degraded as you turned up the volume.

I can just hear the conversation in high-end audio companies: "everyone is listening to headphones; we need a headphone amp!" That is well enough but do you not research the market some to learn the feature set you need to have and performance to go with it? I mean where is the gain switch? Why no balanced out or balanced in? The feature set here is primitive at almost any price north of $100. Then there is the distortion which completely ruined the subjective performance for me. Folks must not be critical listeners or use this box at extremely low volumes in which case, there is no distortion to presumably sweeten the sound.

I can't express enough how the HPA-1 ruined the sound of the two headphones I tested with it. Had this been my only experience, I would have thought neither headphone is any good! This mirage of more distortion is good for you needs to go go away and commitment to fidelity restored. Or else show me one controlled blind test that shows there is real benefit here. Folks need to stop buying into stories that worse fidelity makes things sound better. It doesn't.

To be clear, I don't mind the price at all if it delivered performance in such a substantial enclosure and nice volume control. Just don't charge me and well underdeliver.

Anyway, I can't recommend the Pass Labs HAP-1. They need to get away from telling stories and wasting design and manufacturing skills this way.

As always, questions, comments, recommendations, etc. are welcome.

Any donations are much appreciated using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/
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Thanks, @amirm . As usual from these Hi-End companies, they only care for nonsense marketing buzzwords.
Seriously... how could you say to your customers to wait for one hour before the unit starts to perform optimal? o_O

I switched back to my RME ADI-1 Pro which is also my everyday
Do you have some unreleased RME that we aren't aware of? ;)
This Pass did not a pass Amir's scrutiny.

That said. Pass aficionados expect harmonics and higher 2nd than 3rd and no higher harmonics (tube like).
Pass delivered on that and with only slight hints of that distortion.
Connectivity, price and a lack of quality control is not becoming of a 'brand name', certainly not at this price level. You buy the name and designers fame...
At least there is an RK27 volpot and mains filter and proper chassis grounding.
8dB gain means that to reach clipping level you need a (SE) DAC output that reaches 3.2V (in order to clip the amp at 8V out)
On a 2V DAC (which most are) one can only reach 5V (as seen in Amirs measurements)

Would pass on this Pass.
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What were they thinking when designing this? (The $$$ no doubt).

I have a question for the member that owns this if they are prepared to reveal themselves- what process did you go through to arrive at choosing this? I ask not to be unkind or mocking, but to truly try and understand how this becomes an option for someone.
So fancy case apart, what's the parts cost in this thing? So much high end gear (except power amps with huge expensive transformers and supply caps) seems to be $/£100 parts inside an expensive box, the whole costing thousands :(

As usual from these Hi-End companies, they only care for nonsense marketing buzzwords.
Seriously... how could you say to your customers to wait for one hour before the unit starts to perform optimal? o_O

That's nothing compared to this:

As I always do, I first inserted the Naim DAC into my usual chain. Though dispatched preconditioned, 24 to 48 hours of warm-up noticeably relaxed the sound.

I have tried it many times with all my Naim gears. Naim says 48 hours of warm up is required to get them sound their best and they are absolutely correct. The sound is so much more alive after about 2 days of warm up (just switched on), after that experience when you try listen to it cold you can immediately recognize the loss of life and excitement in sound. It is very obvious.

Too bad if you arrive at a Hifi dealer on a Monday morning, and want to listen to a DAC or amp.

- Sorry, but you can't listen to a cold Naim. Please come back in two days.
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wow, looks like good performance match offered by the shittiness of Pass ACA power amp
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