Katz's Corner Episode 24: Pass Labs HPA-1 Headphone Amplifier

Fig. 1: Pass HPA-1 amplifier, front panel

Nelson Pass's Lab has produced an impeccable headphone amplifier with more than enough power, that's quiet, solid, and very clean. Several other reviewers have reflected on this excellent amplifier, but I want to weigh in with my unique sonic perspective and also measurements of its performance.


Fig 2: Pass HPA-1 amplifier, rear panel

The Pass's sculpted front panel is very pretty and makes a distinctive show piece; there's no need to hide this baby. The rest of the case consists of rectangular brushed aluminum panels. Its large, weighted volume control is unique in my experience—rotational feel is more solid than any other rotary control I've ever used. It's also very precise, as I measured nearly perfect channel matching, zero error (level difference) at full gain, 0.06dB error at 3 o'clock and 0.25dB error at 12 o'clock. Jam Somasundram, its designer, has made a very wise decision, to set the voltage gain to the absolute minimum to be compatible with any current headphone and DAC source. It's specified as only 6dB though I measured 7.6dB gain into 20 Ohms at 1kHz. Even with the volume control all the way to the top, there is no noise to be heard with the most sensitive headphones.

He has implemented an unbalanced circuit, with two selectable stereo RCA inputs and a locking 1/4" TRS jack, yet it delivers plenty of power, demonstrating that it is not necessary to have a balanced output to produce superior performance. An unbalanced amp needs a higher voltage power supply than a balanced amp with the same specification—however, the tradeoff is that fully-balanced requires double the components and an expensive, high precision 4-gang volume control. Balanced is strictly a designer's choice, not demonstrably superior (or inferior) to unbalanced—if the designers optimize the characteristics of each design. Ironically, most times Nelson Pass has advocated a fully-symmetrical approach in his products, but not in this headphone amplifier, which has a clean, minimalist design and few active components to pass the signal through.

Yes, balanced inputs may improve hum performance when very long connections are used, but that is not the case in most consumer's homes. Balanced outputs for headphones are absolutely not necessary, and even the use of the term is questionable—I prefer to say "push-pull" because headphones are floating, they do not have a ground reference. The amplifier also has line level outputs, available at the press of the "preamp" switch. The line-level preamplifier comes direct from the headphone circuit with the addition of a premium coupling capacitor. I did not listen to the preamp output but I did test its functionality. Pass's technical support engineer Kent English points out "One of the goals was to enable 'not yet audiophiles', allowing them to hear their own music better than they have heard it before....for the first time. In other words build something that appeals to the music lover first, number chasers second."

Jam, who is a very enthusiastic engineer, gave me some insight on the design: The HPA-1 was designed as a small power amp, requiring an oversized power transformer and a +/-24 Volt DC supply, while most headphone amps suffice with only +/- 15 volts. The low feedback, wide bandwidth amp has J-FET inputs and a direct-coupled MOSFET output stage, fully biased in Class A throughout. But unlike a loudspeaker amp a headphone amp has to meet a huge bunch of conflicting requirements. It has to drive a wide range of loads, from very sensitive headphones with as low as 15 Ohm impedance that require high current, to insensitive headphones with impedance as high as 600 Ohms that require high voltage drive. So after a certain point design choices become subjective, deciding the least compromise in each area to ensure low noise, low distortion, wide bandwidth and good sound.

For example, there is a balancing act between negative feedback and open loop gain as too much feedback results in a shallow midrange. There are no integrated circuits in the signal path. The power supply regulator is discrete, which Jam says sounds better than an integrated circuit. It took a full year to perfect the design, which included choice of components by listening. Eliminating unnecessary components was a conscious part of the design, and with its optimized gain and imperceptible noise, Pass has avoided the need for complex gain-set switches, that can introduce noise or distortion if not set properly.


I made some basic measurements to confirm performance. Voltage gain (unloaded) to either the preamp or the headphone output was 8.25/8.28dB at 1kHz. Loaded by 20 Ohms, the output dropped by 0.62/0.54dB, indicating an output impedance of 1.45 Ohms (left channel) at 1kHz, close to its specification of "less than 2 Ohms". This moderate output impedance indicates that high negative feedback was not a design goal, so to obtain good linearity the designer had to begin with a very linear open-loop circuit.

With a 1kHz tone, 0dBu output into 110 Ohms (5.5mW) produces nearly all 2nd harmonic, at a level of -90/-90dBu, or 0.003% THD. We can see a trace of 3rd and 4th harmonic peeking above the noise floor in the left channel only, but at insignificant level. The rest is noise and hum (See Fig. 3). Keep in mind that second harmonic is the most innocuous and "invisible" harmonic inasmuch as it is exactly one octave above the fundamental. Besides, a level as significant as 0dBu would yield a ridiculously loud 101.72dB SPL in the Audeze LCD-4 headphones (according to manufacturer's specification). The very low second harmonic is at an inaudible 11.72dB SPL! The absence of other harmonics is a testament to the very linear design and promise of good sonics.

There are also visible hum-related products near the noise floor, at a higher level than I've seen in some other headphone amplifiers. Nevertheless, this hum is inaudible with any headphone I own. The noise floor measured -95.63/-95.77dBu unweighted, 10-20kHz, so it is also inaudible. I measured an acceptable crosstalk figure (110 Ohm load) of -70dB (in either direction) throughout the majority of the spectrum, rising to -55 at 20kHz. An IM test, 19/20kHz at a total of 0dBu into 110 Ohms produced a 1kHz difference tone at a very low-105dBu and supersonic sums circa 40kHz at -75dBu, which represents very good performance (fig 4). The rise in noise floor above 40kHz is due to my Prism interface's noise-shaped converter.

For frequency response, I can measure up to about 80kHz, and within that bandwidth, I measured the Pass's response (unloaded) as ruler flat from 10 Hz (where I began measuring) and slowly rolling off to -2.25/-2.5 at 80kHz at 0dBu output.


Fig. 3: Pass HPA-1 THD at 1kHz and noise


Fig 4: Pass HPA-1 IM distortion

While attempting to measure maximum power output I ran into the first "limitation" of the HPA-1's low gain: I could not drive the amplifier into clipping with the Prism Callia's unbalanced outputs, whose maximum level at full scale is 2V RMS (+8.23dBu), the same maximum output level as a consumer CD player. The reason I put the word "limitation" in quotes is that the Callia's unbalanced outputs at full scale can drive the HPA-1 to a very healthy 15.96/15.99dBu (1.18 watts) into 20 Ohms at 1kHz, yielding 0.1% THD and which would produce a deafening 127.8dB SPL at full scale even with the insensitive Audeze LCD-4. Not an advisable condition, so the Pass's low gain is not a limitation at all. If you insist on driving your eardrums into submission you'll need a DAC or preamp with a higher level output. You could also get 6dB more level with a balanced to unbalanced adapter. Not advised!

Nevertheless, I persisted. I switched to an interface with higher output level, the Prism Lyra, so finally I can confirm that the Pass clips, with 1.07/0.7% THD at 1kHz, +21.28/+21.30dBu (4 watts) into 20 Ohms, 1/2 a watt higher than the specification. For the record, that is equivalent to an even more deafening 133dB SPL (LCD-4) which would certainly damage headphones as well as ears in continuous testing. This is a measure of what the amplifier can deliver on peaks, so if we subtract a nominal 20dB, it would satisfy some mythical listener with mezzo-forte passages of 113dB! Fogedabodit.

The Listening
I warmed up the unit for several days. The sound definitely improves throughout the first day, and I swear even after the seventh day of continuous power it keeps on sounding better. Pass advocates leaving the power on 24/7. I decided to pit the sound of the HPA-1 against the Mjolnir Pure Bipolar as a reference that I'm familiar with. I matched the gain of the Mjolnir (balanced) and the Pass (unbalanced) within 0.1dB at 1kHz into a 110 Ohm load. The Pass volume control was set to full level and the Mjolnir ended up at about 3 o'clock. I built a high quality TRS male to XLR 4 pin female adapter so I could quickly exchange balanced headphones between the two amps.

I used JRiver Media Center on PC to play high resolution files into the Prism Callia DAC via AES/EBU from my Lynx interface. JRiver fed Acourate Convolver via Acourate ASIO's 64-bit connection so that I could insert my custom headphone EQ filters if I wished, but I listened with no EQ (flat) to all my tracks. With the Pass driven by the Callia's unbalanced output, I was able to produce a "loud" presentation of a well-recorded popular music piece at -11dB digital attenuation. Acourate Convolver dithers all its outputs, so there's nothing to fear from its digital attenuation. This leaves up to 11dB available gain, enough to play the softest, widest range classical music piece in my collection. By contrast, the Mjolnir had additional available analog gain, conceivably to play even softer music, but I have never seen any recording that would require it, so I am convinced that the Pass has adequate voltage gain to handle any music you might decide to feed it.

Lindsey Webster "Back To Your Heart"

This is one of my own masters, at 2496. It's a full-bodied, full range smooth jazz recording. With the Pass, the sound is very attractive, punchy and fat. The sound is subtly on the warm side: Lindsey's voice sounds warm and intimate. The cymbals are round, clear, yet extended. The ambience seems reasonably wide; I don't notice any loss of separation. Bass is solid and deep.

Switching to the Mjolnir, the sound is also clean, pure, and extended from bottom to top. The recording's ambience stretches pleasantly, it seems a bit more outside the cans than it was with the Pass. Vocal is perfectly centered within my head. All the clarity is there and it's driving the transients effortlessly. I'm quite surprised that the Mjolnir seems a tiny bit brighter than the Pass on this material since I expected it to sound warmer based on previous experience. Bass is big and beautiful. The snare pops. Cymbals are pure and tight. I don't feel we're missing any high end, even though I'm not using the headphone EQ that I'm accustomed to using with the LCD-4s and the Mjolnir.

Switching back to the Pass, I notice the sound level seems to have dropped just a hair. Aha, that's a clue to the sonic difference: I wager that the Pass sounds a hair softer (even though the levels were matched) because it is much cleaner, has far less harmonic distortion than the Mjolnir. This gives the Mjolnir an unfair loudness advantage. I don't think it will ever be fair to compare these two amps at matched 1kHz levels so it's perfectly possible that the sonic differences amount to the harmonic distortion contributing to a perceived loudness difference.

Either way, the sound of the Pass is extremely satisfying, warm, natural and beautiful, just not as bright circa 8-10kHz as the Mjolnir with this particular recording, but we're talking about the tiniest hair, so small that even an expert could not decide which presentation is more "right". There is no "right" when two amplifiers are both this good. And frankly, I could not pass a blind test to determine which amp is playing, as there is more similarity than difference when the two amps are matched at 1kHz.

San Francisco Symphony, West Side Story

For this gorgeous 24/96 recording from Blue Coast Records, I had to raise the digital gain by 6dB—attenuation is now at -5dB. On this recording, which I know to be bright-sounding on its peaks, the Mjolnir is a bit more forgiving as it saturates at higher levels and gets warmer via the additional 2nd and 3rd harmonic, while the Pass remains linear. So when the Jets chorus sings loudly, the vocals sound sweeter and more palatable on the Mjolnir than the Pass. But for the vast majority of this recording, except for the fortissimo passages, the Pass sounds beautiful, warm and pure.

It's critical to note that when I play this recording on my speakers, I EQ the high end down or it would sound bright and a bit harsh. So when playing it flat through the Pass I hear the same issues as on my speakers and amp. It's clear that the Pass amp is subtly on the warm side of accurate, and the Mjolnir on the euphonic side, at least on the loud passages. A bit of program EQ would fix the presentation of this recording on the Pass. Recordings differ and some of them need EQ. Live with that!

Janice Ian, Walking on Sacred Ground

This is from the album "Breaking Silence", transferred from the 16/44 CD. I began with my headphones on the Pass amp. This excellent recording, mastered by Doug Sax, brings goose bumps! Doug's great tube transfer chain really makes this recording sound beautiful, sweet and perfectly reproduced on the Pass. The highs and transients in this recording are still punchy and clear without getting fatiguing. This segues into Janis' "Ride Me Like a Wave", which also does not disappoint. This is a very pleasant listening experience. I do not switch the Mjolnir to compare.

"Do You Remember" by Jill Scott.
16/44 from the album "All or Nothing". This Neo Soul album's bass, in tandem with bass drum, goes down very deep. Her voice sounds warm and full on the Pass. The transients really move, and there's effortless power to spare. This a reference recording for the soul fanatic where the touch of hip-hop feel provides the "neo". I do not switch to the Mjolnir.

"Erienda", by Kenny Rankin
From the album "Because of You", direct from the 16/44 CD that I was the recording engineer for. I recorded this song with minimalist miking, primarily a Blumlein pair, in RCA's Studio A, New York City for Chesky Records with a tube chain and custom A to D Converter. Kenny's voice is just as I remember it, filling the hall so nicely. Highly recommended, if I may say so myself. The tone of all the instruments is just right on the Pass. It's pure pleasure. The ambience of Studio A is reproduced and extended well outside the phones. I do not switch to the Mjolnir.

"Speechless", by Laurie Anderson
This is from the CD "Bright Red". Recorded by the great Kevin Killen, mastered by Bob Ludwig, this a mesmerizing and perfect reference for audiophile fans of rock and performance art. Pick it up right away if you are a Laurie Anderson fan. Pick it up even if you don't know who Laurie Anderson is. The drums are among the best rock drums on any recording ever made, ever! The dynamics are impressive and the Pass does it perfect justice, extending the ambience well beyond the sides of my head. Bass drum goes down to the center of the earth, or at least my head. Mesmerizing. Did I already tell you that? I do not switch to the Mjolnir.

FB Pocket Orchestra, "I Lost My Girl from Memphis"
This swing combo is from my 2496 master of the album "Guerilla Jazz". The Pass reproduces the instrumental and percussion transients particularly well, making me rock involuntarily with the rhythm.

My impression is that the Pass is an impressive, impacting and musical headphone amplifier with absolutely no defects. It's musical, slightly on the warm side, yet extremely true to the source.

Matt and David Take a Listen
Let's check the perspective of a couple of young audio ears: my two assistants, Matt Davis and David Corson, with different tastes in music, so let's see how their favorites play on this amp. (I did suggest some of the tracks). Initially they listened without any headphone EQ. Again, amplifier levels matched within 0.1dB at 1kHz into a 110 Ohm load.

Vienna Philharmonic, Mahler Third, 2488 from HD Tracks
Matt - On this piece I can't hear any appreciable difference between the Mjolnir and the Pass.

David - The Pass is very similar in tone to the Mjolnir. Though it's really close, the Mjolnir has more impact and microdynamics than the Pass. But the tonality is too close to call.

Lindsey Webster, "Next to Me", 2496 Master
Matt - Pass: Dynamic presentation far surpasses the Mjolnir's. In the Mjolnir, the kick drum is more restrained. The vocal doesn't jump forward as much on her peaks. I'm not hearing the 3D quality: This may be because the Mjolnir's slightly warmer than the Pass as a lot of ambience is contained in the high end.

David - The Mjolnir seems to have a boost in the lows below about 100 and perhaps a bit more presence and microdynamics throughout the whole range.

FB Pocket Orchestra "I Lost My Girl from Memphis", 2496 Master
Matt - The Mjolnir has a harshness and a veiled quality compared to the Pass. The Pass is not brighter. The 6k sizzle in the tambourine favors the Pass over the Mjolnir. The Pass seems sweeter in the highs.

David - The Pass seemed warmer on this one, but did not have as much bottom. Makes me think the Mjolnir has a bit of a smile curve. [BK note: I think the apparent multi-personality of the Mjolnir is because of the sonic unpredictability of 2nd and 3rd harmonic distortion. Sometimes it can sound warmer, sometimes a bit brighter, sometimes more dynamic, sometimes less, depending on the content and the listener's orientation.]

Deepest Crystal Black, "And Where Now", hard rock, by IceCocoon, 2496 Master
Matt - The Mjolnir: This is a thick, closed in mix. The sound is flabby. In contrast, the Pass is leaner, translates better. The Mjolnir is muddy, the Pass is clearer. They both express the deep sub range approximately well, but the sonic differences in the low mids can prejudice the impression of the deep bass.

David - The Mjolnir seems to bring out the bass more as well as the vocals seem clearer, more presence.

Miles Davis "So What" from Kind of Blue, DSD upsampled in JRiver to 176.4
Matt - The distinctions are very subtle in this particular track. The two amps are very close. These two amps are more matched than not.

David - Seems similar to my previous reactions.

Ray Charles: "Here We Go Again", 2496 from "Genius Loves Company" Anniversary Edition
Matt Pass: Does not feel brighter. All signs would point to that since it has a clearer sonic signature. But it feels natural, kind of makes the Mjolnir feel like a murky amp. The Pass has more dimension.

David - Similar to my previous reactions.

Fahrenheit by Telefon Tel Aviv on Tidal – electronic music, aka "IDM" (intelligent dance music)
Matt - Even on something this extreme the Pass has a depth of field that I miss in the Mjolnir. But if I heard the Mjolnir on its own it would stand out well. This is not a glaring issue, a matter of (my) taste.

David - It feels like the Pass is a bit warmer here. But the bass is apparently emphasized in the Mjolnir.

Bruno Mars, 2496 MQA on Tidal "That's What I Like" from "24k Magic"
Matt - This is a nice, well-balanced master by Tom Coyne, a warm but incredibly punchy track, a great reference. My idea of a perfect EQ balance. There is an additional warmth in the Mjolnir that I don't hear in my studio. The Pass is a more present amplifier, and I hear the sound as I do on my system. This comes down to a matter of taste, which sound you prefer and why.

[Bob: Now I throw a monkey wrench in the works, my LCD-4 EQ] With the EQ, now this song feels thin with the Pass and the Mjolnir sounds more like the Pass did without EQ! It could be that the Pass is imparting a subjective Air band boost. The Mjolnir is more "tubey". The triode-like saturation in the Mjolnir can be desirable.

I really like the cleanliness of the Pass. The EQ helps the Mjolnir but I still can't shake the feeling that there's an alteration going on in the midrange. I can see some people seeking that character but in this range I seek neutrality. Most people would prefer the Mjolnir when Bob's headphone EQ is applied.

David - Similar reactions, except now the Pass seems warmer from 60 through 500. But the really thumpy bass is greater on the Mjolnir.

Matt enthusiastically preferred the Pass to the Mjolnir. Matt and David sometimes differed in their reactions, a matter of taste, but keep in mind the Mjolnir was designed to emulate a classic triode circuit, which saturates a bit as things get loud. Single tone tests on the Mjolnir show predominance of 2nd and 3rd harmonic. However, harmonic distortion is a funny beast, sometimes it can manifest itself as presence, sometimes as warmth, and often as fat bass, very dependent on the material, so I'm not surprised at the sometimes apparently contradictory reactions depending on the musical source. Distortion is compression, it can subjectively soften dynamics, but when it's enhancing a range, that range can seem louder and counter intuitively dynamics can appear to increase.

There is enough evidence here that the Pass is the more robust amp and technically the more dynamic of the two amps. We also present some interesting evidence that if you adjust EQ for a headphone, be sure to pick the amplifier you are going to use with this EQ. In fact, when EQ was applied, suddenly one amplifier seemed to take on some of the personality of the other. Harmonic distortion can sound a lot like EQ.

In summary, if you like your sound tight, impacting, clear, accurate and transparent with a nice taste of warmth, I would highly recommend the Pass HPA-1. If you like an amplifier that's a bit more forgiving, especially with harsh sources, I would lean towards recommending the Mjolnir. I would not kick either amplifier out of bed!

Pass Laboratories, Inc.
(530) 878-5350

Vic's picture

To rabid Rob Watts fanboys, there is nothing better in the entire universe. Period. Anyone who thinks there might be is simply listening wrong. They should probably watch more Rob Watts videos or something.

I say this lovingly as a Chord DAVE owner who enjoys it well enough, but can admit that other devices have their own merit.

tony's picture

I've never heard of anyone "not" liking Pass stuff. ( period ! )

I tried to carry the Threshold Line in 1985 but they already had someone...

Since the Spring of 2015 when the Pass designer was presenting this Amp's Prototype at RMAF, people have been praising !

It seems a "given" that any Pass Design will be "Solid" A+ Recommended Componant. Isn't it?

So, I suppose that you agree with everyone else.

I checked eBay for "used" Pass gear. hmm, pretty thin offerings considering N.Pass has been making great stuff 4-EVA !

Why review this Amp?

Why no story about it being a Pre-Amp?


that it doesn't have balanced Outputs. Hmm again.

I see a much bigger Picture of this Amp. I see the vast majority of A+ Rated Electronics achieving superior performance levels without the negative drag from having to do Phono levels of step-up and amplification. Phew!

A level ( 2 Volt ) playing field and an Abundance of Repeatable Signal Sourcing from RedBook and higher.

My take is that today's Electronic Designer works to a budget and hits performance levels with consistency.

Beautiful Chassis Packaging is the big additional Cost, strip the pricy boxes and we have Schiity Gear.

So, maybe an Audiophile level review of the New Schiit LYR 3 is in order.

For anything from Bob Katz, I'm all Ears ( and eyes )!

Thanks for reporting !!!

Tony in Michigan

ps. I'm still collecting Bombay Dub Orchestra and luv'n it.

zobel's picture

We all seem to entirely agree on every aspect of audio involving reproducing music at home on our stereo systems. No one here has ever felt that they have spent too much for any audio component, or that they might have been duped into purchase of expensive snake oil.

Anything anyone writes here is truth, and should be respected, no, treasured, ...as scripture, and to be taken as irrefutable, and not to be challenged, beyond discussion, as sacred gems of knowledge.

Suggestions of what might, or might not be included, will be taken as heresy. Opinions to the contrary to anything stated by the audio gods will be considered blasphemy. This is why we all kneel, worship, and praise, the experts who have lowered themselves to inform us of the true nature of reality. That is also why we are always in complete agreement on any viewpoint expressed herein on any topic in audio. Soon we will all agree on what is the best music.

Vic's picture

Bob listened to the device at length, wrote up his findings, and presented them for the benefit (hopefully) of the readership. Obviously his methods and conclusions won't be universal, and that's okay. That's pretty much how this works.... you read it, take it as a point of information among others, and move on. It's up to each reader to decide if they trust the writer, agree with his methods, like the same kind of music, and so on.

Does anyone really benefit from these wacky comments demanding ABX testing, complaining about pricing, shilling Rob Watts, or calling out Stereophile (for whatever reason...) as a flawed endeavor? "Hey Bob, this review was garbage because you didn't listen to a single bit of 18th century Flemish harpsichord music." "Hey Bob, I feel this thing is overpriced, because another magazine sometimes recommends overpriced gear." "Hey Bob, all amps and DACs sound the same, you are fooling yourself". It's the equivalent of that car you see rolling down the street with a bunch of militant bumper stickers on it... why does anyone think we want to know their unique political views in that context?

zobel's picture

You make assumptions. About subjectivity... if everyone was on your bandwagon, vic, and did as you wished, there would be no progress in audio reporting. This is a discussion, and if you have nothing to add to it, why post here at all?

Of course different amps and dacs sound different. That is not something anyone with decent hearing can deny. They don't sound nearly as different headphones do though. That is why it is much harder to find objective parameters to categorize and describe the sound differences, that become increasingly small, when amps and dacs are of decent design.

Tyll has always maintained a link to objectivity, tying his perceptions whenever possible to his measurements. He made an effort to measure amps, and apply those tests to what was heard from those amps, however, he made no real progress there. There is a reason for that. Trying to nail those very small differences down requires a very disciplined approach in testing, as any competent designer knows.

The efforts Tyll makes to relate his subjective findings not only to the cans on hand, but in respect to the competing headphones is greatly appreciated. I always want at least this in reviews. Have you read about his listening regime, that he keeps as a constant, in order to take subjective and emotional aspects out of the picture as much as possible? That is all I ask of any reviewer, to make a fair attempt at objectivity, and relevance, not just in comparison with other products, but also addressing compatibility with other components in the listening chain.

I have never suggested an ABX test to be part of any review, and realize the limitations of that test. ABX test... I think it is very useful and qualifies as a good yardstick, however, not usually a necessary one. I think Bob writes very well, is extremely knowledgeable, and have enjoyed many of his articles. I have learned a lot from him. This particular article is not anything of use to me, and I think it has very limited value to anyone, outside of entertainment. You are entitled to your little opinion as well vic. Stop trying to 'correct' me. Your lame analogies are a slap in the face, and I find your attitude in offering them to be very rude. Please stick to the subject being discussed. I'm not the subject. I could go on a rant here about you, but that too, would be a waste of time.

Vic's picture

In my view, Tyll and the other contributors are pretty well established. You know what they have to offer and what they don't. It should not be a surprise by now. None of them seem interested in replacing the late Peter Aczel with his Audio Critic approach, yet you and XNOR won't be happy with anything less.

I find it incredibly rude of you to keep commenting on how little this article has to offer. Obviously Bob did a lot of work and is presenting his conclusion in a way that he feels is meaningful. If you don't care for it, that's totally fine, but why do the rest of us need to be reminded of that after each article you aren't thrilled with? Nobody comes here wondering if a particular review has earned the Zobel seal of approval.

I strongly dislike Jonathan Valin over at TAS but I don't feel compelled to go post on each of his reviews to tell him he is worthless. Instead, I just steer clear of his work. Problem solved. Saves me a lot of hassle, and saves other readers from having to see my grouchy opinion. It's win-win. And I don't at all feel I'm somehow missing an important opportunity to "improve audio journalism" as that's simply not my job.

Jazz Casual's picture

so I suggest that you give it a rest zobel. You could use the time out to hone your sarcasm.

zobel's picture

Sit on it Jazz Casual.

Jazz Casual's picture

Just look at your reply.

castleofargh's picture

@Bob Katz. did your opinion on agreeable distortions and less negative feedback lead you to Pass gears or did Pass push you toward your actual opinion on NFB and consequences? ^_^

I'll leave you alone about blind testing. I have strong feelings on the matter of what defines a listening test, very similar to xnor's, but there was no rule saying that you couldn't come here and share a subjective experience with us. Tyll does just that all the time and we don't crucify him on every single article.
so I think you deserve the same tolerance and I'll bring my Spanish inquisition toolkit another time when test method is the main topic >:)

Bob Katz's picture

Someone who is fair-minded and asks good questions! I'm still formulating my opinion on NFB and in fact my next episode, upcoming Episode 25 deals with the issue of NFB EXTENSIVELY and makes some strong experimental discoveries (sighted, though, sighted). Anyway, I've had the opinion that less NFB is often a good thing for a long time. But generalizations are just that, and unless I get into issues of open-loop bandwidth, open loop gain and harmonic distribution of the NFB'ed device, then they are just generalizations. That's why I wrote Episode 25, which I hope you will find to be a groundbreaking investigation and conclusion about these things we've only buzz worded for a number of years. So please stay tuned for the fireworks.

As for blind testing, I'm VERY close friends with two of the world's psychoacoustic experts on blind testing. After much discussion and some rigid, expensive, time-consuming blind tests which I did myself, the bottom line I have to say is: if you cannot blind test correctly, with BS.1116 protocol and proper training on how to use BS.1116 protocol, you're better off sighted. Because your results are likely to be null, and just support the conclusion that there is no sonic difference. Some of the most silly audible things have been shown statistically as likely inaudible. It's it a bit silly to advocate a test method which historically has shown that statistically, almost no subtle difference is audible? So we're in a dilemma: sighted is unreliable, blind is very difficult to do right, so time-consuming to do right.... And I'm ok with that dilemma and I work sighted most of the time.... that's the only practical way as there are so many questions, and so little time. Remember also that I am a professional mastering engineer, I make subjective judgments and preference decisions many times a day, sighted.... rarely do I perform them blind. And people pay me money to do that! What a wonderful thing! And people like my work, too. Another wonderful thing. Is my work consistent? Is it reliable? Is it sometimes flawed? Answer: As much as humanly possible. Imagine doing a blind test on that +0.1 dB at 20 kHz EQ that I used yesterday :-).

I don't have the time to respond in more detail. It's not an argument anyone can win, on either side. The science has the leg up on not being fallible, if the statistics show high enough percent correct. No question. The sighted tester has nothing to go on except his or her credibility, reputation, and integrity, and relative infallibility, or, if you prefer, entertainment value!

So, thanks for postponing the dialogue about blind testing for another moment. Please make sure you've studied BS.1116 before discussing, though.

money4me247's picture

I don't want to further a pointless argument, but I think it would be an enlightening experience to shift perspective. So audio is a hobby and how we approach the hobby varies greatly. Whether someone wants to put more weight into personal sighted impressions or want to hear about blinded comparison testing is really a consumer choice, no big deal really.

Things may be a bit interesting if the principles being discussed about is moved out of the field of audio and into something else, like the field of medicine. So medicine is considered to be very strictly adherent to scientifically rigorous protocols right? You would imagine everything the doctor recommends to you definitely has some really strong evidence and testing behind it. When a life-altering drug/treatment being is offered to you or a loved one, do you ever stop and wonder how strong is the evidence backing it is? Is it backed by a double-blinded placebo-controlled clinical trial (aka Level I evidence)? Or maybe it is simply the subjective consensus opinion of experts in the field (Level III evidence)? Or maybe its a recommendation without any prior testing or published literature supporting it?

And the scary thing is that the answer can range from any of those choices... even when your life is on the line.

So take a breath and relax, no big deal if some audio impressions are sighted and may be prone to some inaccuracies or bias.

Next time you are in the hospital with a possibly life-threatening condition, the doctors will not be conducting any blinded experiments. They will be making a sighted and biased judgement call based on their personal prior experiences. Maybe there was a prior blinded clinical trial that happens to matches your situation showing good evidence on the treatment options and outcomes. Then again, maybe not. On average it costs around $1.3 billion to run a drug through the FDA's phase I/II/III trials.

Regardless of the specific field or specific protocol, anyone who had the pleasure of going through the tedium of creating an experiment that is double-blinded, minimizes confounding variables, has enough statistical power to produce a worthwhile result, and survives critical peer appraisal prior to publication knows that it is a hell of a lot of time, money, and work for sometimes extremely disappointing results. Reading literature on those studies is also a huge pain in the ass. The majority of studies end up being inconclusive at the end of the day and recommending further studies. Only after enough studies are published on a subject, does actual clinical practice begin to shift, and even then it is usually a very slow shift with behind-the-scenes politics and special interests also hindering things.

Rigorous standardized testing is hard and inconvenient. And doesn't always lead to fruitful results. And many times people just don't bother regardless of the stakes. This is life, not only in audio, but in every other field.

So sometimes when an overly inquisitive patient asks too many questions and isn't cooperating with the treatment plan, you just quote some random medical jargon of semi-related information from latest article of the NEJM and say confidently, trust me I'm a doctor.

xnor's picture

"if you cannot blind test correctly, with BS.1116 protocol and proper training on how to use BS.1116 protocol, you're better off sighted."

I'm sorry, but if you had actually understood BS.1116 you wouldn't say something so absurd.
The "careful experimental design and planning" in BS.1116 is so that you don't end up with useless results that approach the uselessness of sighted tests.

So if you don't do a careful comparison, you're screwed anyway .. which is why people hear all kinds of (contradictory) things.
Sighted just makes this situation worse.

Also, you're again using the same bad excuse.
Nobody here is asking for a scientific study, so stop using the excuse that you don't have the time to do one.

How do you not understand that a blind comparison is necessary, especially for yourself, to produce the evidence that what you hear is not biased?


In the other post you essentially said that you cannot hear differences reliably.
Then when do you hear them and how do you know?

How do you distinguish "critical listeners" asserting that they hear a difference although they don't and a person actually hearing a difference?

What is the difference between a "critical listener" asserting that some devices improves the sound and a person asserting that globules (sugar beads) cure diseases?

PS: The consistency, reputation, honesty .. of the person is irrelevant.
I hope you do understand why that is.

wbh's picture

"...I hope you do understand why that is."
Yes, we do ... and...uh...it's BS x.1116

xnor's picture

"Yes, we do ... and...uh...it's BS x.1116"

1) With "we" you refer to commentators that don't know what they're talking about and just decrease SNR?

2) No, it is not "BS x.1116"
We're talking about the Recommendation BS.1116 by ITU-R.

3) Give me the reason then, please. I'm pretty sure you don't know.

wbh's picture

I believe you are mathematically challenged, XNOR. (Surprising, given your pseudo-intellectual "xnor" username).
The "x" is an OPERATOR.
BS times 1116 = bullshit x 1116.
Which is precisely what the ITU is: a worthless, bureaucratic organization along with its hubristic parent, the United effing Nations.

xnor's picture

Bob Katz respects BS.1116, so you are calling Bob Katz a bullshitter?!

The only worthless thing here is your utterly simpleminded trolling.

PS: "x" is not an operator, it's a letter. This is a multiplication sign: ×. That's not an "x", in case you are visually challenged.

Now please go back to the sandbox playing with the other kids.

MRC01's picture

Damn, this is a tough crowd. It's great to get opinions from a knowledgeable professional who is also an audiophile with decades of experience with a variety of equipment and has learned enough to be fair without any axe to grind for or against particular technologies. Sharing opinions from others was even better.

Well designed and built amps driving the kind of loads they were meant to drive will always sound similar even with revealing transducers. That's just a fact of life. But just because differences will be subtle doesn't mean they sound the same. And while a DBT is necessary to *prove* the audibility of sonic differences, the converse is not necessarily true: just because differences do exist doesn't imply a DBT will always detect them. If you believe you heard a difference but can't pick it up in a DBT, they might still exist you just can't be sure. But that doesn't mean you can't share your experience, especially in the transparent way that Bob does.

Rthomas's picture

Nothing brings out the people like a Bob Katz article on Innerfidelity!!

Keep 'em coming Bob.

Please try to review the new $2995 Benchmark HPA4.




Bob Katz's picture

Thanks for the request. I anticipated you, Thomas, by 24 hours, put in a request for an advance review model from Benchmark when they're ready.

Rthomas's picture

Hi Bob,

Brilliant! I look forward to your comparison.

One request for when you review these super expensive ''TOTL'' components is to have a sensibly priced challenger to see just what you get extra when you pay a lot more (if anything)

For example if you have an RME-ADI2 DAC ($999) on hand when reviewing the Pass HPA1 or Benchmark HPA4 I would be very interested to see if you can hear anywhere close to $2K worth of extra performance.

Amir at Audioscience review has the RME and measured it with excellent results.


Thanks again


MRC01's picture

The Oppo HA-1 is extremely well designed and built with excellent specs. We can still read the Innerfidelity review from a few years ago when it first came out. But it was never directly compared to these other amps that cost several times its price. Doing so might help answer the question: what is the least expensive headphone amp that can drive just about any headphone and is indistinguishable from SOTA reference quality?

Bob Katz's picture

I tried to use my Oppo HA-2 for a review a while back (in fact I reported it) and discovered it will not charge while it's in use! That pretty much limits the use to portable applications. i was always interested in the HA-1.... And it would make a good shootout against headphone amps costing 2 to 3 times as much. Especially nice at the ~$1000 price of the HA-2 is the balanced output, very rare at that price point. But with Oppo now discontinuing all their audio products I doubt I can get a review loaner from the company....

John Grandberg's picture

I just wanted to chime in and say the Pass HPA-1 is one of my two favorite solid-state headphone amps of all time (a title it shares with the Violectric V281, for different reasons). The HPA-1 seems strangely overlooked among headphone enthusiasts, and I'm not quite sure what to make of that. Perhaps it is too expensive for many yet "not expensive enough" for the rest? Who knows, the market is weird. All I know is the seemingly popular Simaudio 430HA has absolutely nothing on the Pass in my humble opinion.

Anyway, I enjoyed Bob's write-up and hope others did as well.

zobel's picture

Depending on who you talk to....other amps are better than those you like,which I'm sure is no surprise to you. I value your opinion, and have read some great recommendations from you,but that was on your picks on affordable headphones, such as the AKG K7XX, which I think we agree is a great value, and a good headphone for people who haven't bit into the land of diminishing returns.

What makes me really wonder though, is why not be at least a bit more objective in reviewing expensive gear by making blind comparisons between amps, and between expensive amps to those of lesser cost? What does that hurt? I sure can see what it would help.

As an example, from Tyll's amp/headphone shootout, he preferred amps over your pick, his favorite being "A Neutral Reference Headphone Amp: Simaudio MOON Neo 430HA"

Read more at https://www.innerfidelity.com/content/neutral-reference-headphone-amp-si...

We need a better yardstick, or the reviews are not too helpful.

John Grandberg's picture

I hear you on the affordable aspect. I try to cover less expensive amps whenever I find a good one worth recommending. So far I've placed the Arcam rHead ($599) on the Wall of Fame, and would have done the same with the Massdrop Cavalli CTH ($249) if it wasn't a limited run (which is just how Massdrop works). I am also extremely impressed with the Rupert Neve RNHP ($499) though I haven't actually written it up yet.

Any of those amps plus a K7XX and/or HD6XX, HE400i, etc, would make a really enjoyable combo. Not summit-fi performance, but the price/performance ratio would surely be more favorable.

As for comparisons, I will absolutely point out whenever a good-but-expensive amp is legitimately challenged by a more affordable option. That makes total sense to me.

Tyll and I don't agree on the Simaudio. I had one for a long time and just never got into it as a headphone amp. Not worth the price imho. I ended up using it more as a preamp in my speaker rig, where it did a wonderful job. So yes, we all hear and experience gear differently.

zobel's picture

I heeded your advice and got the Cavalli Massdrop CTH. Again... your review was 100% right-on. I have a Schiit Jotunheim as well, and it has a ton going for it as well, especially in balanced mode. I wonder if there are other amps that run balanced, that are any better than the Jot, and of those, are there any that include a great DAC?

I like my Marantz HD-DAC1 more for it's DAC than it's headphone amp, though that is excellent too. I'm hearing more of what my CDs have now with my Focal Clear, and now notice and appreciate a better amp and DAC more than before.

So far the best sound I have is through a Marantz HDCD-1... toslink to the Marantz HD-DAC1, SE to the Jotunheim, running push-pull (balanced) out to the Focal Clear and Mr.Speakers AEON Flow Closed. That rig really also makes my Sennheiser HD600, and the AKG K7XX, and AKG 553 Pro closed sound more open and detailed and tight and deep in bass, smooth and more airy on top. The Cavalli CTH is wonderful too, with any headphone, straight out of the Marantx HDCD-1. I think I'm done now with my headphone quest, and thank you for all your advice John. I feel lucky to have found what I consider high value and great sound with your help.

John Grandberg's picture
Sounds like you've got quite the nice setup there. Enjoy!
Sean_S's picture

@John Grandberg: Would you mind elaborating on the different reasons you prefer each one?


John Grandberg's picture
Didn't see this until now. The differences are subtle but I find the Pass amp more incisive, detailed, and pure, while the Violectric is a bit more visceral and dynamic. That's an oversimplification but it will do the job.