Part 2: RAAL-requisite SR1a Earfield Headphone Monitor & HSA-1b Headphone & Speaker Amplifier Review

RAAL-requisite SR1a Earfield Headphone Monitor & 

HSA-1b Headphone & Speaker Amplifier Review

Part 2 of 2: Technology Q&A, Optimizations, Competitive Comparisons, and Conclusions

Rajiv Arora

• Part 1: Introduction, Setup, and Initial Impressions (link)

Introduction

If you haven’t already, please first read Part 1 of this review, where I introduce and describe the SR1a and HSA-1b, along with my listening setup. I also describe my initial listening impressions with this combo. 

In this concluding Part 2, we start with a short technology Q&A with Alex Radisavljevic, President, RAAL-requisite Serbia. I’ll then describe various adjustments and tweaks I made to see how much more the sound quality (SQ) of the SR-1a and HSA-1b could be improved. Following that, we get to the section I enjoy most: comparisons with the competition! Finally, I’ll wrap up with my conclusions and overall assessment of the SR1a and HSA-1b.

Technology Q&A with Alex Radisavljevic, President, RAAL-requisite Serbia

The SR1a’s low-impedance drivers and their impact on amp design sparked several questions from me, and Alex obliged by answering in refreshing detail. He has graciously allowed me to share his answers here. More technical-minded readers should find this very interesting.

Rajiv: What is the sensitivity of the SR1a, in terms of dB @ 1mW/1kHz? 

Alex: It is 84dB/mW, but it’s not applicable in a straightforward manner because the driver itself has 18 mOhms (0.018 Ohms). Power handling is ~450mW with a full range signal, reaching 111dB of SPL. In order to develop 450mW across 18mOhms, you need 5Arms. Our cables are ranging from 0.15 to 0.38 Ohms, so roughly, the load presented to the amplifier is from 0.17 to 0.4 Ohms, so you’re not dealing with just the driver impedance, as cabling is the dominant factor. So, if you’re using 5Arms, you are actually demanding 10W of power from the amp, as 5A^2*0.4Ohm=10W, while still developing only 450mW at the driver and burning 9.55W at the cable. So, with a 0.4 Ohms cable, the sensitivity of the cable-driver system is ~72dB/mW. At the moment of conceptualizing the true-ribbon headphones, we only had loudspeaker amplifiers as a source of 5Arms, Jot-R or HSA did not exist, so we have adopted the concept of our Interface Box that can be simplified as a big power resistor of 5.6 Ohms. With the 0.4Ohms cable, it presents a 6 Ohms load to the amp. To get our 5Arms, you hook that up to an amp that is rated at 100W at 8 Ohms. Across 6 Ohms load, it will deliver 5Arms, and at that point, the Interface Box-cable-driver system has 62dB/mW, or 92dB/W. Therefore, we have omitted the traditional dB/mW efficiency rating, as our concept is different and this kind of rating is not practical.

Rajiv: What is the output impedance of the ribbon headphone out of the HSA-1b? 

Alex: It is 1.0 Ohms. In essence, having low impedance amplifier output does not concern SR1-a, as the cable already has 20 times greater resistance than the driver, so a high damping factor amp will do nothing. Therefore, we have designed the driver in such a way that it doesn’t depend on a high damping factor. Actually, dealing with this kind of drivers, and potentially high contact resistance from cable plugs, you may benefit from current-source amps (infinite output impedance), which is what we’re emulating with our amps and Interface Box. Neither is “better,” though, and it is inconsequential for the cable-ribbon driver system, anyhow.

Rajiv: What is the max power delivery into 0.2 ohms of the ribbon headphone out? Either watts, or amps.

Alex: It is 6.5A rms.

Rajiv: What mode does HSA-1b operate in?

Alex: AB

Rajiv: What is the default DIP switch attenuation setting on the HSA-1b, and was this the setting on the review unit you sent me? 

Alex: –6dB and yes.

Rajiv: Please describe the HSA-1b's power supply in a couple of sentences. For example, linear or switch-mode, transformer VA rating, any other attributes that are noteworthy? 

Alex: Linear, 150VA transformer with regulation where needed.

Rajiv: Thanks very much, Alex, for your detailed answers!

My Listening Setup

Just for easy reference, here again is a diagram of my listening setup.

Optimizing the SQ of the SR1a

Based on my mostly positive impressions, I tried various adjustments and tweaks to see how much more the SQ could be improved.

Effect of mechanical adjustments

Due to its unique design, these headphones are essentially a headband in terms of contact with your head. SInce they are not circumaural, they are not constrained to enclose, or even rest on the ear. The transducers are on flaps that swivel on a hinge. So, the flaps can either be opened up at a wide angle, or closed in to be almost parallel to the ears, up to a point beyond which they start making contact. Additionally, the entire band can be moved forward or back to change the position of the ribbon relative to the ear.

The net of all these adjustments is that it is possible to tailor the soundstage and frequency response just by these mechanical adjustments. Pushing the band a bit forward and opening the flaps wide pushes the sound stage forward, although it’s still nothing like a speaker setup.The soundstage does widen, but I felt it reduced the depth somewhat. The bigger impact is that it weakens the bass even further, so there is a price to be paid for this choice.

Rotating the flaps inwards towards the ears deepens and strengthens the bass in a very welcome way, albeit also restoring a more typical headphone image around the head. Ultimately, I preferred this position for the bass, and the traditional headphone soundstage didn’t bother me, as I’m a headphone guy anyway. If you’re coming from a speaker setup, and harbored hopes that this would give you a speaker-like sound stage, that is unfortunately not the case.

Effect of cables

I next focused my attention on alternative cables.  I’d gotten excellent results so far with the stock cable, but I was fortunate to have on hand both RAAL-requisite’s own SR728 Ribbon Silver cable, along with my long standing Transparent Ultra Headphone Cable system, so I explored the effect these upgraded cables made.

RAAL-requisite SR728 Ribbon Silver Cable

I admit I was puzzled at RAAL-requisite’s choice of silver for the SR728 (MSRP $1050-$1500) Ribbon Silver Cable’s metallurgy, as transparency and clarity, which is what I associate with silver for analog cables, are something the SR1a already has in spades. Would this cable push these attributes over the top and cause the SR1a to sound bright and analytical?

Thankfully, it does not! In fact, this cable manages to extract even more transparency from the SR1a than the stock cables, without adding any brightness or sheen. Tonality remains neutral, but somehow this cable takes you even deeper into the music. In the last movement of the Choral Fantasy (Beethoven: Symphony No. 9 & Choral Fantasy, Pablo Heras-Casado, Freiburger Barockorchester, Kristian Bezuidenhout, harmonia mundi, 24/96), this becomes especially evident in the crescendos, where the violins stay coherently audible below the full chorus.

Transparent Ultra Headphone Cables

The Transparent Ultra headphone cable has been my reference headphone cable for almost 2 years now, so I was very curious to find out if it could meaningfully improve the SR1a/HSA-1b combo. 

The Transparent Ultra cable system (MSRP: $1800+, depending on connectors needed) comprises up to three elements - first, the 3m cable itself, terminated on the headphone end by one section of a 2-piece coupler (see picture above), and on the amp end by a 4-pin male XLR connector. Each headphone is equipped with a short headphone “lead,” which has headphone-specific terminators on one end, and the other section of the coupler on the other. For amplifiers that require a different connector than a 4-pin male XLR, an amp-specific adapter is the third element in the chain. Perfect for the audiophile with multiple headphones! For every new headphone you add to your system, you need to add only another headphone lead (MSRP: $395-$795), and possibly another amp adapter (MSRP: $100-$125).

For the SR1a, Transparent were able to supply me with the requisite (grin) headphone lead, along with a 4-pin female XLR amplifier adapter for the HSA-1b. I was particularly concerned whether the SR1a's extraordinarily low impedance would present a challenge. Transparent assured me it would not, and I needn’t have worried.

Effect of the Transparent Ultra cable

Once again, this cable delivered the sound quality improvements it has on all my other headphones, and showed why it’s remained my reference this long. The SR1a’s low impedance presented no problems. True to its name, transparency is the key attribute of this cable, and it delivers more of it than either the stock or silver cables. It does this while retaining a neutral tonality, another attribute it shares with the silver upgrade cable. 

Where it pulls ahead is in density and texture. On Joaquín Rodrigo’s Concierto Andaluz (Rodrigo, The Romeros, Mercury Living Presence, DSD64), the Transparent Ultra cable makes instruments more fleshed out, with more meat on the bone, and greater texture and realism. On the SR1a/HSA-1b combo, this full-bodied presentation does wonders in alleviating the slightly thin bass, although it does not boost the bass frequencies per se.

The Transparent Ultra cable finally allowed me to start appreciating the SR1a’s outstanding qualities, without being distracted by its bass-lightness. I highly recommend it with the SR1a.

Yes, this is an expensive cable, and yes, it delivers extraordinary sound quality. Such is the nature of high-end audio. If you, like me, have 2 or more headphones you regularly use, I would argue this cable system actually becomes quite an irresistible buy, when compared to buying upgraded cables for each headphone.

Effect of EQ

During the course of my evaluation of the SR1a, I received an email from Chris, excitedly saying: “you have to hear this.” The “this” he was referring to were convolution filters that Mitch Barnett @mitchco had created after doing measurements on the SR1a. These are now described in Chris’s article. Readers interested in these filters should contact Mitch directly at this link.

While my highest quality sources for listening are PGGB-remastered source files, the filters Mitch provided were designed for Roon and HQPlayer only. So I did my initial evaluation using HQPlayer doing real-time PCM upsampling with the sinc-M filter and LNS-15 noise shaper.

I started with the neutral EQ filter first, and was immediately blown away by the improvement it made over no EQ! Gone was the slight tendency to brightness I felt was part of the SR1a tonal profile. And the bass lightness was a thing of the past. Don’t get me wrong: the SR1a is never going to be considered a bass monster, but at least now, it didn’t sound lacking in that department. 

While I could easily live with the neutral EQ filter, I found the “neutral plus bass extension” EQ filter to be more satisfying. This boosts the sub-100Hz region, and added a bit more of the low bass I was missing. Having settled on this EQ filter, I was delighted to learn that @Zaphod Beeblebrox could adapt this EQ file to a form that PGGB would accept, and I remastered my evaluation tracks to get the benefits of both PGGB and the bass extension EQ filter.

Listening Impressions with EQ

Now we’re talking! If the Transparent cable had taken me a long way toward focusing on the SR1a’s amazing strengths, rather than its shortcomings in the bass, this EQ filter took me all the way. Listening to the SR1a/HSA-1b combo with cabling and EQ in place, I finally began to appreciate just how special these headphones really are.

Comparisons of SR1a with other headphones

With the outstanding SQ resulting from the combination of adjustments, upgraded cabling, and EQ, it was now time to pit the SR1a/HSA-1b against the competition, i.e. my reference headphones. Obviously, I did not have an exhaustive collection on hand, but there is a reason the Meze Empyrean and the Abyss AB-1266 Phi CC have a permanent spot on my headphone stand. To my ears, they represent some of the best headphones out there. Obviously, this is a matter of personal preference. I’ve listened to other top headphones, like the HiFiMan Susvara and the Focal Utopia, but they didn’t really speak to me. Nor am I enamored with any of the current crop of electrostatics. I do plan to take a closer look at the new Audeze CRBN - but that’s the topic of another review.

Meze Empyrean

The Empyrean gets the most use in my system, and it’s fair to say it’s my default headphone of choice. I use them exclusively with the leather earpads, preferring them by a wide margin to the Alcantara suede. Due to its high sensitivity (100dB, 1mW/1kHz) and moderate impedance (32Ω), the DAVE can effortlessly drive them directly. So this is how I listened to them, driven from the 6.3m single-ended output of the DAVE, using the same Transparent Ultra headphone cable. In contrast, the SR1a was driven by the HSA-1b fed by the DAVE’s XLR outputs.

I discussed the evolution of my listening setup in an earlier section. While the resulting sound quality improvements have been in every department, including dynamics, tonality, and freedom from fatigue, the biggest improvements have come in the areas of transparency and transient response. These are attributes I value highly, and are strengths of the Extreme, the DAVE, the DC3 PSU, and PGGB upsampling. 

I mention the above as a prelude to describing the Empyrean’s character in my system. The transparency and transient response improvements upstream synergize really well with the Empyrean. All the micro-details are there, leading edges are crisp, and they are accompanied of course by the other strengths of the Empyrean: outstanding bass, and a relaxed and smooth tonal character. The reason I love these headphones so much is I can listen to them for hours on end on every genre I throw at it.

In comparison, the SR1a through the HSA-1b were quite a contrast! Where the Empyrean lead with their smooth and relaxed character, before revealing their resolution and transparency, the SR1a give almost the opposite impression. Even with the cabling and EQ, the SR1a first impression is still one of speed, transparency, and resolution, only after which you notice its neutral tonality. Is one of these better than the other? Only as a matter of subjective preference!

Abyss AB-1266 Phi CC

While I do not have the latest TC upgrade, the CC’s I have are another useful comparison to the SR1a. Due to the low sensitivity (88 dB, 1mW/1kHz) of the Abyss, they sound a bit anemic driven directly from the DAVE. In my system, I drove them both from my reference Cavalli Liquid Gold amp, as well as the HSA-1B’s high-current ribbon output. I’ll talk about amp differences in a later section.

As I expected, compared to the other headphones, the Abyss fell in the middle of the two extremes represented by the Empyrean and the SR1a. Its presentation was not as forward as the SR1a, although it retained the transparency and resolution. While the SR1a still had the edge on sheer speed, the Abyss was not far behind, and had the advantage of a more balanced tonality. On bass, there was really no contest, as the Abyss went down to subterranean depths that the SR1a could not match.

How did all these differences manifest in actual music?

Comparative Listening Impressions

A fellow forum member, who owns an R2R (reel-to-reel) tape recording of Traffic’s legendary album, Low Spark of High Heeled Boys, shared a DSD128 digital capture with me. I further processed these files with PGGB from DSD128 to 24/705.6. Wow, this version just blew away any of the extant digital releases, including the 2002 remaster (which I own), and gave me an inkling of why so many classic and progressive rock fans love their analog rigs. I’m old enough to remember hearing this album on vinyl, and this R2R capture reminded me of the tonal richness and satisfying bass that modern digital rock masterings just seem to lack. I will say that I don’t consider this a problem with all digital recordings or remsasters, but it seems to afflict pop/rock recordings disproportionately.

The SR1a pulled the percussive elements like the keyboards, the drums, and cymbals out front and center, and the rich texture of these instruments was presented in their full glory. On the Empyrean, the presentation was more laid back, but the sound had a meatiness to it, which made you go “oh yeah!” This is not to say tha the SR1a lacked in body and bass heft, nor was it to say that the Empyrean was lacking detail and resolution. It just reflects the character of these two outstanding products. The Abyss fell in the middle, where these instruments were not so prominent, and felt like the most accurate of all three. The bass was just divine on the Abyss!

On the first movement of Carl Maria von Weber’s Piano Concerto No. 1 (Complete Works for Piano and Orchestra, BIS 24/96), the SR1a threw a massive sound stage, with excellent separation and air. The piano, in particular, was just captivating, with crystal clear hammer strokes, and a beautiful tone. The Empyrean and Abyss were not far behind, while excelling in other areas. The Abyss really shone in dynamics, especially rendering the crescendos with gut-punching physicality. The Empyrean seemed to be the most organic, and allowed me to sink into the sheer beauty of the piece. Listening with these 3 headphones, I was reminded just how fortunate I was, and why reviewing is such a fun gig!

Is one “better” than the other? Sure, most people, given the chance to compare head to head, would prefer one over the other, but to declare winners and losers would just be silly. Over the course of the review, I had several friends and family members listen to these three, and as I expected, there was no consensus on favorites. 

Listening impressions of HSA-1b driving other headphones

While I now had a good handle of the SR1a’s sonic character, what about the HSA-1b amp? How could I judge its quality relative to other amps? Unfortunately, I was out of luck when it came to driving the SR1a. Other than the HSA-1b direct-drive amp, the other options are the Schiit Jotunheim-R, which I did not have, or to use the interface box with sufficiently powerful conventional (speaker) power amps. Being a headphone guy, I didn’t have a stash of high-end power amps handy.

However, the HSA-1b can drive other headphones, so I could use this capability to compare against my existing headphone amp references: the Cavalli Liquid Gold amp, or direct output from the DAVE.

Before getting into comparisons, I first determined the HSA-1b configuration that provided the best SQ. First up: single-ended or balanced inputs? I know the DAVE’s SE outputs are slightly more transparent than the XLRs, but much depends on the amp’s capabilities. It only took a few minutes of experimentation to realize the HSA-1b had much better SQ through its balanced XLR inputs, so I used them for the duration of the review.

The second variable was the choice of headphone outputs. Was there a difference in SQ using the Conventional HP output vs. the Ribbon output? The latter has a switch that applies “baffle-step compensation” when set at the SR1 position. I set this switch to the HP setting and used the Empyrean headphones to compare between the conventional HP and ribbon outputs. Due to the high sensitivity of the Empyrean, the HSA-1b barely broke a sweat, even with conventional HP output. The ribbon output was louder, as expected, so I used the attenuator to compensate and match levels.

Here again, it didn’t take protracted listening to determine a winner: it was the Ribbon output. Compared to the conventional HP output, the ribbon output had a more expansive soundstage, sounded more dynamic and open, and had much crisper transient response. Clearly, this wasn’t a power issue, but more likely a consequence of the much lower output impedance of the ribbon output. 

Comparison of HSA-1b with other amps

Given the above findings, I used the XLR inputs and Ribbon output of the HSA-1b for these comparisons. Luckily, the Cavalli amp exhibits no discernible SQ difference when fed from either the SE or the balanced inputs - even though the internal circuit topology is all balanced - so I used the SE output of the DAVE to simultaneously drive the Cavalli. This allowed quick swaps between all 3 amps.

Continuing to Future Times/Rejoice from Tormato (Yes, HDtracks, 24/192), a similar trend emerged. On the Empyrean, the Cavalli rendered the military snare drum with more weight and excitement than the HSA-1b. On the Abyss, the tables turned somewhat. Now the HSA-1b had the better dynamics and bass. The Cavalli still led slightly on transient response. On this track, the HSA-1b pulled ahead slightly on the Abyss, while lagging on the Empyrean. This pattern held true across many more tracks that I tried.

Overall, I deemed the HSA-1b to be of similar sound quality as the Cavalli Liquid Gold, which should not be surprising given their very similar price points - the Cavalli retailed for $4000 when it was new. Let’s face it - very few people will buy the HSA-1b without the SR1a, exclusively to drive other headphones. The HSA-1b is an outstanding amp for the SR1a. The fact that it’s SQ with conventional headphones is also up there with a respected amp like the Cavalli is just an additional bonus!

Summary/Final Thoughts

In my opinion, the RAAL-requisite SR1a joins the pantheon of top of the line headphones that represent the state of the art in headphone reproduction. Its innovative use of ribbon transducers brings an exciting new option to enthusiasts, especially those who strongly value speed, resolution, and transient accuracy. While the tonal signature is a bit bass-light by default, this can be mitigated by a judicious amount of system tuning using alternate cables and EQ, thanks to @mitchco’s pre-built filters.

The RAAL-requisite HSA-1b headphone & speaker amp is the perfect yang to the SR1a’s yin! Together, they performed breathtakingly in my system, going toe to toe with my existing set of reference headphone/amp combos. Furthermore,  I found the HSA-1b to be an outstanding amp with both my Abyss and Empyrean headphones, equalling my reference Cavalli Liquid Gold amp. 

I have long maintained that in the world of TOTL headphones, no one headphone can meet a demanding audiophile’s needs. This makes the HSA-1b a very attractive option for those who own the SR-1a, along with one or more other TOTL headphones. No matter which headphones you choose to listen to, the HSA-1b will make them sing. Well done, RAAL-requisite!

Link to Part 1

Product Information:

• SR1a True Ribbon Earfield Monitor

• HSA-1b Headphone & Speaker Amplifier

• Price (MSRP):

  • SR1a, Case:                                   $3200

  • SR1a, Interface, Case:                 $3500

  • SR1a, HAS-1b, Case:                    $8000

  • SR1a, HSA-1b, Interface, Case:  $8500

• Ribbon 728 Silver Cable:                           $1050-$1500 

• Transparent Ultra Headphone Cable:      $1800+ 

• Accurate Sound SR1a EQ filterset:          $250

Associated Equipment

 Primary System

Music Computer:          Taiko Audio SGM Extreme Music Server, Taiko USB upgrade

Headphone Amplifier:  Cavalli Liquid Gold

Headphones:                 Meze Empyrean, Abyss AB-1266 CC

DAC:                               Chord DAVE

USB to dual-SPDIF:       Audiowise SRC-DX bridge

Ethernet Switches:        SOtM sNH-10G, Uptone EtherREGEN, 

                                        Buffalo BS-GS2016 (modded for LPS)

Power supplies:             Paul Hynes SR7MR3DRXL (dual regulation, 3-rail)  for switches 

                                        Sean Jacobs DC-3 for DAVE

Power Details:                Dedicated 30A 6 AWG AC circuit, 

                                        Sound Application TT-7 Reference Power Conditioner

Power Cables:                Sablon King (wall to TT-7), Sablon Prince (Extreme),

                                        Cardas Clear Beyond (DC-3, SR-7), 

                                        Cardas Clear for all other components

USB cables:                   Sablon Reserva 2020 USB

BNC cables:                   High Fidelity Cables CT-2

Ethernet cables:            Sablon 2020, SOtM dCBL-Cat7, Supra Cat 8

DC cables:                      Neotech OCC (DC-3), Paul Hynes fine silver (SR-7)

Interconnects:               Cardas Clear XLR balanced

Headphone cables:       Transparent Ultra cable system

Accessories:                  Synergistic Research Tranquility Base XL UEF with Galileo MPC

                                        Synergistic Research MiG 2.0 footers

                                        Taiko Audio Daiza Isolation Platforms

Acknowledgments

Many thanks to the following companies for supplying cables and accessories to aid in this evaluation: 

• Cardas Audio, for a full loom of Cardas Clear cables.

• Transparent Audio, for the Transparent Ultra headphone cable with a full complement of headphones leads and source terminators.

About the Author

Rajiv Arora — a.k.a. @austinpop — is both a computer geek and a lifelong audiophile. He doesn’t work much, but when he does, it’s as a consultant in the computer industry. Having retired from a corporate career as a researcher, technologist and executive, he now combines his passion for music and audio gear with his computer skills and his love of writing to author reviews and articles about high-end audio.

He  has "a special set of skills" that help him bring technical perspective to the audio hobby. No, they do not involve kicking criminal ass in exotic foreign locales! Starting with his Ph.D. research on computer networks, and extending over his professional career, his area of expertise is the performance and scalability of distributed computing systems. Tuning and optimization are in his blood. He is guided by the scientific method and robust experimental design. That said, he trusts his ears, and how a system or component sounds is always the final determinant in his findings. He does not need every audio effect to be measurable, as long as it is consistently audible.

Finally, he believes in integrity, honesty, civility and community, and this is what he strives to bring to every interaction, both as an author and as a forum contributor.