Bolt from the Blue: RAAL-requisite SR1a "EarField™ Monitors" - Headphone Review

Review written by Ian Dunmore (@Torq)

Introduction

One of the most interesting, and unique, recent entrants into the Headphone (RAAL-requisite call them "Earfield™ Monitors) world is the SR1a. They’re the first headphone using a true ribbon-driver (known for being extremely resolving and very fast), which was developed by RAAL, in Serbia - who are probably the most well known, and well regarded, manufacturer of ribbon-drivers for speakers.

The combination of a new driver type, the unusual space-age looks, requirement for a speaker amplifier to drive them (you cannotuse a conventional headphone amplifier) and the suspended driver approach had me extremely intrigued. Apparently, that had the same effect on many others as well, as I had tried to hear these at CanJam @ RMAF 2018 - only to find that the RAAL-requisite booth was perpetually busy!

That left no other option but to get hold of a set myself - so the set of SR1a that I am reviewing here are my personal set, purchased directly from RAAL-requisite, after some excellent discussions about them with their CEO, Danny.

I’ve now owned them for a bit over six months, but it only took a few days to know I’d be keeping them. What I didn’t realize at that time was how disruptive these would prove to be …

Note that this will be a rather longer review than usual both due to the unique nature of the SR1a as well as their requirement for a power-amplifier to drive them and my desire to include some comparisons among options there.

The gear chain/equipment used in this review can be found here, and the music I use in my reviews and evaluations is listed here.

Specifications

• Style: Earfield™ open baffle design. Floating near ear, does not enclose ears.
• Impedance: 0.2 Ω with cabling. Used in series with Ribbon/Amplifier Interface.
• Ribbon/Amplifier Interface: 6 Ω. Use with any loudspeaker amplifier.
• Amplifier Power Rating: 100W
• Sensitivity: 91dB/1W
• Max SPL : 111 dB
• Frequency Range: 33Hz-30kHz
• Weight: ~425grams

Technology & Design

In addition to using ribbon drivers, the SR1a differ from conventional headphones in that they do not have an ear-cup, are an open-baffle design and the drivers are positioned differently to most headphones. This is interesting for a number of reasons.

By not using a traditional cup (or baffle), and with the full-range ribbon drivers being held away from the ears, they’re able to bypass a lot of the tuning challenges imposed by both the cup (resonances, internal reflections, phase issues), and the more typical headphone-to-ear driver relationship. At the same time, their output is subjected to the full effects of the external and internal ear structures (which shape frequency and time-domain uniquely - as all our ears are different). The simpler version of which is that they can be tuned to be neutral, and will then be perceived by the listener with the same principal response as a flat-tuned speaker.

There are differences in how the Head-Related Transfer Function (HRTF) works here vs. speakers, as the sound is coming from in front of the ear, not in front of the head, but there is still a natural level of crosstalk between left and right channels, and the result is not only a more natural frequency response, devoid of room interactions, cup interactions and ear-to-driver positioning issues, but also with a much more natural, and dimensional, stage rendering (actual projection of depth).

Package

The SR1a package is a little different to that which typically accompanies a set of headphones. It’s shipped in a very sturdy hard-case, with custom-cut foam lining, that contains the headphones themselves, a 7 ft headphone cable, the necessary interface box and a set of speaker cables to connect it to your speaker amplifier.

The headphone cable is terminated with a female 4-pin XLR connection to prevent you from accidentally connecting it to a conventional balanced headphone amplifier - if you did, it’d see essentially a dead short and either trip it’s protection or fail. Longer cables, and extensions for existing ones, are available from RAAL-requisite.

Build

The construction of the SR1a is quite interesting, and rather clever. The drivers hang from a resilient, but light and flexible spring-steel band, and can be rotated 360 degrees front-to-back. The headband is a simple, wide, leather affair that is adjusted using a simple dual pin-buckle arrangement. The vertical pads are secured with a tongue/groove arrangement, and the temple-pads buckle on - both are quick and easy to replace.

Even the drivers are easily user-replaceable, since the actual ribbon elements are contained on a quick-swap cartridge that can be exchanged in under a minute.

I’ll borrow an analogy from another local SR1a owner in regards to their build; the SR1a are built like a Formula 1 car … technical, purposeful, directed, with the focus being on the technical performance and what’s needed to get it, vs. say, the Focal Utopia, which are built more like a Bugatti or a Bentley … still high performance, but with much more attention being paid to the aesthetics and finish involved.

While they appear light and spindly, the actual construction is pretty solid and over six months of very heavy use show absolutely no signs of wear or weakness.

Comfort

This will somewhat depend on the size/shape of your head. For me, the SR1a just disappear when I am wearing them. They fit such that the drivers sit perfectly vertical and perpendicular to my ears, and on the 2nd holes of the band they are perfectly aligned. When first putting them on, I’m aware of the temple and vertical pads touching my face, which is entirely comfortable - but also something I cease to be aware of after a minute or so of having them on. And while they are supplied with a band that is supposed to secure them from sliding forward (which goes around the back of your head), I do not use that myself, as for me they stay put without it.

I have done multiple 12 hour listening sessions with the SR1a with no discomfort at all. And because they’re completely open, there’s no chance of heat-build up. They even work seamlessly wearing glasses as the vertical pads are very soft and there is no cup or pad that needs to be sealed for them to work properly.

Weight will be a non-issue for most as they come in at 425g (0.93lb), which is in the same ballpark as things like Focal’s Utopia, and about 100g heavier than the Sennheiser HD800S, and it is very well distributed due to the wide headband.

However, if you have a larger head, the fit might be less comfortable, getting the drivers to sit perpendicular might be a challenge, and if your noggin is on the smaller side you may encounter issues with them staying put.

Sound

These are simply the fastest, cleanest, most resolving, dynamic and realistic headphones I have ever heard. Strictly speaking, it’s probably best not to even think of them as headphones – RAAL-requisite call them “Ear-Field™ Monitors” for a reason; they deliver a far more speaker-like listening experience than any conventional headphone I’ve tried. And they image, with depth, more like speakers than any headphone I’ve come across.

They’re not going to be for everyone, and some people may require the application of EQ to get them to their favorite signature - as fresh out of the box they’ll be a bit light in the sub-bass and a bit energetic at the top-end, but otherwise are instantly impressive in their neutrality and technical abilities, while being chameleonic in their delivery with a little EQ.

Tonality & Timbre

The SR1a exhibits a “studio neutral” tuning – specifically one that is similar to a typical near-field studio monitor; which is the usage these were originally designed to address. Due to the compact nature of such speakers, most audiophiles will hear this tuning as being a bit bass-light, with progressively more roll-off below 33 Hz and into the sub-bass, and a bit bright - while being resolutely neutral in between those extremes. Depending on the position of the drivers, sub-bass (20 Hz) will play at -6 to -10 dB, not dissimilar to many non-floor-standing high-end speakers.

Timbre is the most natural of any headphone I can name, with no hints of coloration nor smearing from driver or enclosure (e.g. there’s nothing here like the “plastic” timbre that sometimes gets associated with some planar cans). Open-baffle speakers, or those with on-board corrections, are the only things that I can think of that reproduce instrumental timbre in quite as lifelike a fashion, shy of the actual instruments themselves. Even the subtlest timbral changes that occur when varying the intensity of ones play on a piano or the gradual descent into harmonic overloads as brass is pushed to, and beyond, it’s natural limit, are rendered clearly.

You cannot help but hear when a drum skin is hit in a spot that isn’t as tight as another, brushes across brass are so clearly resolved that you can hear the wire bundles split and separate when pressed more enthusiastically, and even the type of string on a violin can be discerned if you know what you’re listening for.

Resolution/Detail:

These are easily the most resolving, detailed, headphones I’ve experienced – besting every dynamic or planar I’ve heard, and even beating the high-end electrostatic cans on custom amplification. The are merciless in how they reveal every last little detail, nuance, edit-point, background noise or artifact in the music they are fed.

Even compared to headphones already well regarded for their ability to render micro-details, such as Focal’s Utopia, the STAX SR-009S or the LB-Acoustics MySphere, the SR1a are clearly superior.

With big orchestral works in particular, the resolving power of the SR1a really stands out. With planar cans I find the more ambitious classical pieces tend to devolve somewhat into a homogenous “wall-of-sound”, where dynamic cans usually fair better in keeping the details intact and the instruments separate - the SR1a retains their vivid individuality to an even higher degree, but does so without spoiling the coherence of the overall piece.

With the tambourine on Prince’s “Tamborine” you can clearly hear the individual zills jingling. Other highly-resolving headphones will do this too, but it is far better delineated and easier to hear with the SR1a.

If detail is your thing, I know of nothing in the headphone world that tops the SR1a.

This can be a double-edged sword, however. Marginal material is laid bare - with all its faults fully intact. You’re almost compelled to listen in an extremely focused manner – and while that’s a crucial factor for a mixing or mastering engineer, it is not necessarily the most relaxing way to experience music. There are times, when it can be too much of a good thing.

Bass:

Bass speed, impact, dynamics, texture and articulation/delineation are all extra-ordinarily impressive here. What’s lacking, somewhat, is low and sub-bass level (or quantity), though it is worth noting this does improve by 1-2 dB over the course of a few days actual listening.

Amplifier choice also has audible implications here.

But … these are clearly not a bass-head can - at least if you’re not bass-quantity-focused.

The quality of the bass reproduction here is so good, however, that it’s worth persevering with. And the level can easily be brought up using EQ. Something the SR1a takes extremely well, as the lack of cups, and open-baffle design, mean that making EQ changes does not result in undesirable resonance and phase effects elsewhere. You can dial in relatively large amounts of sub-bass/lower-bass boost with NO deleterious effects to the rest of the spectrum and with no bleeding into the higher-bass or midrange at all.

One of my bass-test tracks is Beyoncé’s “Partition”. The first segment of that has major bass hits that start at 80 Hz and roll-down to 20 Hz (you can watch them do it on an analyzer, the peak of the bass note moving right to left over the space of about 4 seconds). There is texture to those bass notes that is more evident and well defined with the SR1a than with any of my other headphones.

Early in my listening I had 4-6 dB of sub-bass (20 Hz and under) and 2-3 dB of bass shelf lift at work, though I gradually backed this off as I tried different amplifiers and the ribbons broke-in. But even so, with the application of EQ, I was able to get the bass to within a couple or so percent of my Abyss AB-1266 Phi CC without issue – while maintain the SR1a’s other benefits.

For those not against using EQ, there is amazing ability waiting to be tapped here. If you don’t use EQ, either because you don’t want to, or you can’t, then the SR1a may well be too light on the bottom end for you.

Midrange

Lucid, fluid, immensely detailed, completely natural and superbly coherent. This is the best-midrange performance I’ve heard out of a headphone (yes, that’s becoming a bit of a theme, I know). There’s no coloration I can detect here, but at the same time the music remains full of verve and life. Neutrality without sterility nor veil.

Notes, and melodies, that once seemed to blend into each other are given lives and stories of their own with the RAALs. Yet this is achieved without any diminishment of the musical whole. The massing of strings will swell and then flow, as a glorious band of tone, yet individual players and their instruments are fully present, layered and discernible. Similarly, close vocal harmonies are rendered in a manner in which the combined, sonorous, overtones are vivid and rich, but one can still choose to focus on an individuals voice and follow it without effort.

Vocals are rich with detail, possessing a depth rarely encountered outside of live, un-amplified, performances, or the best studio sessions. Tonal and dynamic nuances are rendered clearly, adding to the depth and sense of emotion present in the singer’s delivery.

It is easy to find oneself wholly sucked into a piece and just stay there, enveloped in the music. But if so inclined, you can step back and dissect the ensemble in its entirety and pull out, and track, any individual element.

Treble

Hugely extended, clean, extremely smooth, precise, conveying an amazing sense of air and space, with absolutely no sibilance or shoutiness at all. Even turning them way up and running through my female-vocal-torture-tracks (Julia Fordham, Heart, some early Annette Strean), they soar with ease - without so much as a hint of brittleness, tension or stress.

This is usually the kind of performance I would expect from the better tuned top-of-the-line IEMs, albeit with much more space and openness here.

Now … I will say that initially I thought I was hearing some low-level “grain” in the highest registers, but this turned out to be micro-details and texture that I’d not heard so clearly delineated on full-size cans previously. This was easier to sort out with natural, acoustic, instruments - where a comparison to the actual, live, instrument was possible.

And while the natural tuning of the SR1a will tend to result in the treble being more present than with many cans, it has never become fatiguing, tiring or uncomfortable, even after marathon listening sessions.

Dynamics & Transient Response

Startlingly fast, with huge impact and presence, the macro-dynamic delivery of the SR1a started off as a wake-up call that kept getting progressively more addictive the more music I threw at them. Whether it’s the incredible immediacy of attack with pizzicatos and plucked strings, something that I often focus on and was still surprised here, or the thwack-and-crack of a drum-strike, the start-stop-start of the SR1a seems instantaneous.

Speed, impact, transient performance (attack) and decay (and the changes in texture as the larger drum skins settle) in “Drum Warfare” (David Fesliyan, Elimination) is staggering - rapid-fire beats are all distinct and individual. Only the very best dynamic and electrostatic setups I’ve heard even come close here and, honestly, I still think the SR1a is faster, more precise and better delineated - while imparting a much more visceral and hard-hitting rendering.

Micro-dynamics are superlative with any track you can think of (that actually has content that requires such things) … big orchestral pieces, simply-mic’ed acoustic jazz, gruffer male vocals and so on. A bow subtly bouncing and dragging across steel has readily perceptible variations in volume (micro-dynamics), as does the progressively ragged breathing of the soloist, something easily heard in “Introduction et Rondo Capriccioso, Op.28 for Violin and Orchestra (in B minor)” (EMI, Saint-Saëns, Walter Süsskind) with the RAALs.

Stage & Imaging

This is one of the standout features of the SR1a compared to normal headphones – they can do speaker-like imaging and project a stage within which you can localize sounds by depth as well as laterally.

This is in contrast to almost all other headphones, which typically present a largely flat, or linear-but-curved, “stage”, either inside the head or at fixed position in front of it. Within which, lateral imaging can vary from the often fatiguing “between the ears” three-blob (left, center, right) to a more expansive, layered, discernible and graduated beyond-the-ears/drivers presentation. And in most cases there is no depth-wise layering, separation or localization/spatialization at all.

 

The diagram above illustrates how I hear the stage projection, and imaging for several competitive headphones. Consider the curved bars to be akin to rows of instruments/performers – and how widely they project laterally, their thickness to be the depth of each row, and the space between them how well they are separated, discernible and how vividly they stand out in image:

• The Utopia has a narrow, shallow, projection. There’s very little depth to it, beyond where the image forms relative to the listener. This is very intimate staging. For the most part it is not possible to discern depth-wise location - instruments/performers essentially appear laterally distributed across a curved plane.

• The HEDDphone has a wider stage than many headphones, and as a result lateral localization and separation is much more apparent. The sound can appear to come from well outside the head/headphones. Instruments are rendered closer to the listener than even with the Utopia, but with a little more depth. It is still not possible to localize instruments or performers in terms of how deep in to the image they are, or whether one is in front of, or behind, the other.

• MySphere has a similarly wide lateral throw to the HEDDphone, but adds clear depth-wise cues that make it apparent when one instrument or vocal is in-front/behind another. It also projects further from the listener.

• SR1a - shown here with the drivers set at 45 degrees (having them wider broadens and somewhat deepens the image, at the expense of reducing bass levels). The image is the widest, and the deepest, is much more vivid and stable and sits further from the listen and clearly lets you hear depth-wise cues and locate instruments and performers both front-to-back and side-to-side. Not quite to the same level as a well-setup near-field speaker rig, but the closest I’ve heard form any “headphone” and dramatically more so than even the nearest rival.

Cowboy Junkies “Mining for Gold” (The Trinity Session), a track that tends to sort the men from the boys when it comes to conveying the sense of space and ambiance of a venue more vividly dimensional and airy with the SR1a than on anything else I’ve heard it via, excepting properly setup speakers. This remained true regardless of where I set the drivers … even fully “in” it’s still the best reproduction of this I’ve heard.

Holly Cole’s “Train Song” (Temptation) also is distinct in the degree of depth to the image. It is projected in front of the listener and every instrument can be placed in the stage effortlessly. This is, again, a more vivid, stable and expansive projection of stage than I’ve heard with any of my other headphones, even if the HD800S and AB-1266 get some of the way there.

Amplifier Requirements & Pairings

At launch, the only way to drive the SR1a was with a speaker/power amplifier, with the ability to deliver 100w into 8 ohms (or about 150w into 4 ohms). This is still the standard way to power them, and they include a special interface box to bring the 0.2 ohms of the ribbon driver up to something that won’t effectively short circuit the amplifier (about 5.6 ohms) and that applies compensation to get to a neutral response from the open-baffle, cup-less driver.

It’s worth noting that the stated power requirements are real, and while you can get away with a bit less in some circumstances, you’re generally going to want to follow the recommendations here. It is not just output volume that suffers when the SR1a don’t have enough juice, the overall quality of the reproduction takes a sizable hit as well.

Since the SR1a launched, another option for driving them has come to market, which does away with the need for the interface box entirely, is much smaller than suitable speaker amplifiers and produces much less heat to boot, and that’s the newly available (1/21/20) Schiit “Jotunheim R” - more about which below.

Before reading further, if you have a suitable power-amplifier already, I would recommend giving it a try before seeking alternatives, as the SR1a perform very impressively even with relatively inexpensive amp options - provided the power is there. Otherwise I would strongly suggest starting off with the Jotunheim R, which a super convenient and very high-performance way to drive the SR1a.

Many headphone listeners contemplating the SR1a won’t necessarily have exposure to suitable amplifiers to drive the SR1a, so what follows are some high-level comparisons among readily available options, at different tiers (this is not everything I’ve tried’ other comments/comparisons can be found in this thread). For these comparisons I used an SPL Phonitor X as the pre-amp and both an RME ADI-2 DAC fs and a Chord DAVE as the DACs.

Parasound NewClassic 2125 v.2

This is 2-channel, THX certified amplifier, rated at 150w into 8 ohms and 225 watts into 4 ohms. I am not normally much of a fan of this type of amplifier, but it was the first speaker amplifier I could lay my hands on when my SR1a shipped earlier than I expected - and I was too anxious to wait.

I was pleasantly surprised, with the NewClassic providing a punchy, dynamic, suprisingly resolving and entirely enjoyable delivery. It was good enough that even this comparatively inexpensive unit (MSRP is $795, but it can be found for $599 from some dealers), coupled with the SR1a, easily matches and even outperforms a number of similarly priced flagships.

Schiit Vidar (1x Stereo & Dual Monoblock)

Using a single Vidar, in stereo configuration, adds some low-end presence to the SR1a, adds body/weight/presence to the sound, while delivering ample power, plenty of punch, has palpable dynamics and very good resolution. This was a notable improvement over the Parasound amplifier, particularly in the bass region and also in terms of tonal weight. In fact, switching to the Vidar allowed me to back off the initial low-shelf EQ boost I had applied by about 1.5 dB.

During my initial time with the SR1a, a single Schiit Vidar turned out to be the sweet-spot for value/performance, though today I think there’s a better option (read on). It’s good enough that RAAL-requisite have one as part of their standard show setup.

Right on the heels of my first Vidar, I acquired a second one so I could try a dual-monoblock configuration (which requires a pre-amp with balanced outputs). The changes here, vs. the single unit, were primarily a more vivid stage projection, with increased ability to localize sounds front-to-back, a further slight lift in low-end drive and bass level (another 0.5 dB removed from my bass EQ), slightly better overall resolution and a strong sense of effortlessness/authority.

Schiit Aegir (Dual Monoblock)

Aegir is a lower-power stereo amplifier (20W/channel into 8 ohms), in the same form-factor as Vidar, but is more deeply biased into class A operation and has a different delivery to Vidar. The lower-power available here means you do need to run a dual-monoblock configuration (that’s one amplifier per channel, driven via a balanced, XLR, pre-amp or source but then yields 80W into 8 ohms)) to properly drive the SR1a, and even then it is a little shy on power if you’re pushing things hard.

What you get, here, though, is a slightly smoother and more nuanced delivery than either of the preceding options, with improved instrumental timbre and a slightly more deft and articulate handling of very complex passages in the music, particularly with regards to micro-dynamics.

What you lose, vs. a pair of Vidar is some macro-dynamic punch, impact and slam and the greater sense of ease that is apparent with the dual-Vidar configuration. Big orchestral works, and modern electronic music therefore tend to favor Vidar, where as small ensembles, Jazz, and non-orchestral acoustic pieces tend to play more to Aegir’s strengths.

Benchmark AHB2

This was the primary amplifier that RAAL-requisite used in the development of the SR1a, so it shouldn’t come as a surprise that it makes for an excellent pairing with them, nor that it precisely meets their quoted power requirements. It is typically featured as one of the amplifiers RAAL-requisite use at shows (along side Vidar and, I believe, a Bryston unit).

The AHB2 driving the SR1a yields an articulate, highly detailed, impactful, extremely clean and resolutely impressive sound. It is similarly “effortless” in its delivery to the Vidar, while being a little faster sounding, with slightly zippier transient response (pizzicato’s are excellent here). Micro-dynamic elements are realized extremely well here, too. Timbre is excellent, resolution rivals the best options in this list and the rendering/projection of stage sits somewhere between the single Vidar and the mono-block Vidar/Aegir options.

As good as this is (and it is very good indeed) personally I felt it was a little “dry” sounding and is on the “lean” side for my personal tastes, vs. the “weightier”, more liquid presentation of some of the other amplifiers here (even some of the much less expensive units). While this is a preference, rather than a technical distinction, I personally feel the native tonality of the SR1a really leans towards pairings that are a bit fuller/weightier sounding; your mileage may vary.

McIntosh MA252 (Integrated, Tube-Hybrid)

The tone here was lovely, right out of the gate, with some added warmth and weight/body that really plays well against the SR1a’s studio-neutral tuning. It isn’t quite on the same level in terms of resolution or detail as the solid-state offerings here, and micro-dynamics suffered a bit vs. the AHB2, but tonally and timbrally it was a very nice listen.

Unfortunately, despite it meeting the advertised power-requirements, I found that getting the volume I wanted, with powerful, transient-rich, highly-dynamic music, would start tripping the clipping/protection indicator (tubes flash orange). Though to be clear, I was not hearing any indication of clipping - just seeing the indicator made me not want to push things any further.

This would make a rather nice, and very pretty, way to run the SR1a, given that it is an integrated unit. At some point I may try its newly-released, and bigger/more powerful brother, the MA352 (which also features the signature McIntosh vu-meters and a 5-band tone control).

Chord Étude

Despite trying a number of other solid-state amplifiers in this realm (such as the Linn Akurate 4200, which I found performed similarly to the Chord unit here), this is what I chose for driving my own SR