Pass Labs HPA-1 Headphone Amp Review

[respice finem]

Make them unique with features such as EQ, balanced connections, Dirac...

This might be either out of their technical reach, or not matching their "vision".
To compete with companies like RME?

 

[phoenixdogfan]

This kind of design goes all the way back to the '70's audiophile ideal of a great amplifier. It was based on three premises. 1). The amp was a voltage source invariant with load. This means that it's power in watts effectively doubles every time resistance is halved. This was the idea behind a lot of the early Krell and Levinson amps (like the ML-2) in that days. 2). Pure Class A operation at all times and 3.) No negative feedback. There was ititially some merit to the idea that while negative feedback lowered overall THD, it amplified higher level distortion product. It's explained in an article on Audio Woo in RationalWiki:

"Early solid state amplifiers (and even some later ones), while measuring far better than their tube (valve) predecessors, sounded worse. It turned out that inadequate amounts of negative feedback, while measurably reducing the total harmonic distortion (THD), were actually increasing the perceived harmonic distortion, by creating new higher-order distortion components that are more audible. Reducing the THD number does not necessarily reduce perceived distortion, since low-order distortion components are masked more by the ear. Of course, once the engineers realized this, they solved the problem by applying lots more negative feedback to reduce all the harmonics to inaudibility.[24] Nevertheless, an irrational aversion to any form of negative feedback persists to this day"

Complete article makes for fascinating reading:

Audio woo

Audio woo consists of various vague and unsupported claims for devices or methods for getting better sound quality from systems that reproduce recorded music. Such claims are made by manufacturers, hobbyists, and writers in the field.

rationalwiki.org

 

[Lorenzo74]

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.
View attachment 244343
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):
View attachment 244344

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:

View attachment 244345

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:
View attachment 244346

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:
View attachment 244347

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:
View attachment 244348

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

View attachment 244349

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:
View attachment 244351
High noise floor lands the HPA-1 near the bottom of our rankings:

View attachment 244352

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:

View attachment 244353

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:
View attachment 244354

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:
View attachment 244355

Let's see how much power we have into 300 ohm load:
View attachment 244356

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:
View attachment 244357

Stepping through different load impedances we see that distortion progressively gets worse as impedances go lower (become more difficult):
View attachment 244358

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

View attachment 244359

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.

Conclusions
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/

Image that everything tested in stereophile (John Atkinson measurements are still of great value to all of us ) is then sent by default to Seattle for amir’s assessment.

In 12 months this world will be cleaned. No more smoke sellers (cables, heavy speaker, heavy amplifiers,…) and audiofoleries.

This world will be tidy and value will be unleashed for all customers.

@amirm call John A. And Kalman… It is in their interest too right? Or not?
My Best
L.

 

[TonyJZX]

i looked at the price.... $3,600

and the single board inside and thought.... where's all the goddamn money gone???

charitably is there even $360 worth of parts in there???


also these are like how Tom Clancy novels are still coming out.... he passed about a decade ago


HARD PASSS

 

[pseudoid]

@amirm is NOT getting a Christmas card from Pass.

But look at it from poor @amirm's perspective. He is like the Grinch bringing bad news to some member who was kind enough to pass his Pass to him.
Some thank you!

 

[AudioSceptic]

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.
View attachment 244343
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):
View attachment 244344

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:

View attachment 244345

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:
View attachment 244346

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:
View attachment 244347

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:
View attachment 244348

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

View attachment 244349

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:
View attachment 244351
High noise floor lands the HPA-1 near the bottom of our rankings:

View attachment 244352

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:

View attachment 244353

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:
View attachment 244354

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:
View attachment 244355

Let's see how much power we have into 300 ohm load:
View attachment 244356

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:
View attachment 244357

Stepping through different load impedances we see that distortion progressively gets worse as impedances go lower (become more difficult):
View attachment 244358

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

View attachment 244359

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.

Conclusions
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/

I feel really sorry for the owner. I wonder what they thought the result would be? Mind you, even excellent performance would still make this poor value.

 

[Palladium]

You’re quite closeminded for even speaking that first sentence. As for the second, the 90% is a very wide estimate. This amp is almost $4000, I highly doubt most voters have heard it, so what point is the vote besides to regurgitate the opinion in the original review?

Dirt has called, they want fallacious appeal-to-wealth rebuttals older than them back from naked emperors.

 

[pseudoid]

I feel really sorry for the owner. I wonder what they thought the result would be? Mind you, even excellent performance would still make this poor value.

stop it... that is like pouring salt into a wound or thumbing an eye!
I know what it feels like dumping $$ and the subsequent grinch-news delivery!

 

[amirm]

I feel really sorry for the owner.

The owner is downsizing his audio system and sent this to be tested before he sells it. So it is not an issue for him although you could say it is fortuitous that it is not a loss anyway.

 

[Fezzo]

Puzzling variance with other objective and subjective review data.

 

[Palladium]

i looked at the price.... $3,600

and the single board inside and thought.... where's all the goddamn money gone???

charitably is there even $360 worth of parts in there???


also these are like how Tom Clancy novels are still coming out.... he passed about a decade ago


HARD PASSS

The fact you get more HPA performance from a single op-amp in the Topping DX3 Pro+ that is mostly an afterthought feature of a SOTA USB/SPDIF/Coax DAC and Bluetooth receiver makes it even more puzzling.

 

[Keened]

The owner is downsizing his audio system and sent this to be tested before he sells it. So it is not an issue for him although you could say it is fortuitous that it is not a loss anyway.

But now he has to sell it knowing it's junk; a heavy price to pay on his conscious.

 

[Somafunk]

Designed for rugby ears perhaps?

 

[al2002]

This kind of design goes all the way back to the '70's audiophile ideal of a great amplifier. It was based on three premises. 1). The amp was a voltage source invariant with load. This means that it's power in watts effectively doubles every time resistance is halved. This was the idea behind a lot of the early Krell and Levinson amps (like the ML-2) in that days. 2). Pure Class A operation at all times and 3.) No negative feedback. There was ititially some merit to the idea that while negative feedback lowered overall THD, it amplified higher level distortion product. It's explained in an article on Audio Woo in RationalWiki:

"Early solid state amplifiers (and even some later ones), while measuring far better than their tube (valve) predecessors, sounded worse. It turned out that inadequate amounts of negative feedback, while measurably reducing the total harmonic distortion (THD), were actually increasing the perceived harmonic distortion, by creating new higher-order distortion components that are more audible. Reducing the THD number does not necessarily reduce perceived distortion, since low-order distortion components are masked more by the ear. Of course, once the engineers realized this, they solved the problem by applying lots more negative feedback to reduce all the harmonics to inaudibility.[24] Nevertheless, an irrational aversion to any form of negative feedback persists to this day"

Complete article makes for fascinating reading:

Audio woo

Audio woo consists of various vague and unsupported claims for devices or methods for getting better sound quality from systems that reproduce recorded music. Such claims are made by manufacturers, hobbyists, and writers in the field.

rationalwiki.org

The ‘Feedback is Evil’ fantasy was thoroughly debunked by Cherry and Cordell. Their articles in the JAES are worth reading. In a series of articles in Wireless World, reprinted by Linear Audio, Baxandall showed that, in fact, high amounts of feedback also reduced higher order harmonics, again these articles are worth reading.

Nelson Pass is one of the good guys in audio - he freely shares many of his recent designs over on DIYaudio.com - and knows perfectly well what he is doiing. He can design low distortion amps with the best of them - see tests of his old Threshold amps - but chooses not to do so. This amp is intentionally designed with low/no feedback. Nelson Pass has stated that he prefers the way low/no feedback amps sound. Clearly many audiophiles agree with his philosophy for the company is, as far as one can tell, quite successful.

 

[mathman]

Well, one might argue that it somehow didn't pass QC before it was shipped out, but the label clearly says "PASS" ; so it's indeed just a overpriced piece of junk...

 

[restorer-john]

Great review.

I think for a headphone amplifier, one shouldn't be running out of power or hitting audible distortion limits- that kind of defeats its reason for existence. So it strikes out there.

The unit is nicely made, should last a long time and is eminently repairable down the track. But the performance and the price are not correlated- not even close. The price is obscene and surely NP must chuckle to himself every time an order rolls in. But that is the nature of some audiophiles I guess, they are buying his name and paying a lot for it.

 

[pseudoid]

stop it... that is like pouring salt into a wound or thumbing an eye!
I know what it feels like dumping $$ and the subsequent grinch-news delivery!

But now he has to sell it knowing it's junk; a heavy price to pay on his conscious.

I am keeping my doorstop (that @amirm tested) for the same reason.

 

[okok]

please show some respect folks, 1 out of 100 products there might be some bad design overall, can't hurt the reputation

 

[Ken1951]

please show some respect folks

Why? High price and lousy implementation. Nelson Pass designed my GFA-555, but saying this headphone amp is lacking shows no disrespect to my Adcom or any other electronics he's responsible for that were designed well.

 

[Sonny1]

Dang! I’ll PASS on this one…