RAAL-requisite HSA-1b - Switch Drive Amplifier Review


The HSA-1b is RAAL-requisite’s latest headphone amplifier, featuring something they refer to as “Switch Drive”. This provides direct-drive amplification for their incredibly impressive SR1a ribbon-driver “Earfield™ Monitors”, as well as driving your conventional dynamic/planar headphones and efficient speakers. This is a unique capability from a single box and offers unprecedented flexibility and convenience vs. every other current amplifier on the market.

Since it is likely that, for most, an interest in the HSA-1b stems from interest in/ownership of the SR1a, there’s a good chance you’re already familiar with them (if not, stop here and read the following review) and their unique drive requirements:.

  • A 100W into 8 ohms (or 150W into 4 ohms) speaker/power amplifier.

  • The amplifier connects to the headphone via an “interface” box that a) adapts their native 0.2 ohm impedance to something that won’t instantly short out, or trip the protection on, said amplifier and b) applies a compensation curve to account for the frequency response of the step-baffle ribbon headphone driver.

The HSA-1b joins the previous HSA-1a and Schiit Audio’s “Jotunheim R” in offering a “direct-drive” way to power the SR1a. “Direct Drive” means that neither a separate power amplifier nor the impedance-matching/step-baffle compensating interface box are required. Instead you connect the SR1a directly to the HSA-1b, which provides both the necessary amplification and applies the appropriate frequency compensation curve, while also removing the need for speaker cables.

The unit I am reviewing here is my personal HSA-1b, purchased directly from RAAL-requisite. I’ve also had the original HSA-1a on loan, and previously owned the updated HSA-1a (see “History and Iterations”).

This review focuses on listening with headphones; both SR1a and various conventional models. Speaker-drive will be covered in a follow-up, including comparisons across various appropriate, high-efficiency, models.


  • Load Impedance: 0.3 Ohm – infinity
  • Output Power (one channel driven): 10W /8Ω, 20W/4Ω, 40W/2Ω, 55W/1Ω @ 1khz/sine
  • Ribbon Headphones Drive: 1 x RAAL-requisite SR-1a, 1 x Balanced Headphones (Dynamic & Planar)
  • Freq. Response: 100mv/ in/sin. 15hz – 500kHz
  • Power Bandwidth: 10Hz(-1dB) – 100kHz (-2dB)
  • Distortion: < 0.5% at 1khz
  • Signal to Noise Ratio: Better than 90dB
  • Input Impedance: 40 kOhm
  • Sensitivity: 0.5V/40kOhm
  • Input: IN1- RCA, IN2 – XLR

For driving conventional headphones, the remaining specifications are as follows:

  • Output Impedance: 5.3 ohms
  • Power (via conventional 1/4” TRS and 4-pin XLR): 250mW into 32 ohms/26 mW into 300 ohms.
  • Power (“Ribbon” SR1a Output via 4-pin XLR Balanced Adapter): 1900mW into 32 ohms/200mw into 300 ohms.

Features & Functionality

While the HSA-1b is, first and foremost, a direct-drive amplifier for the SR1a, it incorporates full support for conventional headphones and speakers. It is effectively an integrated amplifier with a bias towards headphone output.

All switching/control is done from the front-panel, which is an improvement over the HSA-1a:

  • The first switch determines if the step-baffle compensation for “SR1” headphones is engaged. You want this in the “SR1” (up) position if you’re listening via the SR1a, and in the “HP” (down) position for any other headphone or for speaker listening.

  • Next up is the output selector, which toggles between “HP” (headphone) and “SPK” (speaker) listening. In “HP” mode, all three headphone outputs on the unit are active. The male 4-pin XLR “Ribbon” connection is for the SR1a, and possible future ribbon-driver headphones. It can also be used, with an included adapter cable, to drive inefficient/power-hungry headphones (such as the JPS Labs Abyss AB-1266 Phi TC and HiFiMan Susvara). Next is a standard 4-pin XLR balanced output, which will work with any standard 4-pin XLR headphone, and last, but not least, the ubiquitous 1/4” (6.35mm) single-ended TRS socket.

  • There are two selectable inputs, labelled “XLR” (balanced) and “RCA” (single-ended).

  • Finally, there is the stepped-attenuator volume control, and internally the option to apply input-level attenuation, both of which deserves a more detailed discussion:

Volume Control

The HSA-1b features a 24-position stepped attenuator for volume control, using a discrete ladder configuration and covering a 60 dB range (the bulk of which is in 2 dB increments, with the first few steps being larger). This allows for perfect channel matching, even at the lowest settings, and offers the cleanest possible signal path.

This is a definite improvement over the HSA-1a, which for some users with very efficient conventional headphones could exhibit some low-level channel imbalance.

Most stepped attenuators employ a resistor-series (sometimes called “serial”) approach. This means that for each channel you have 24 resistors, along with their solder connections, in the signal path at all times . In a ladder design, as used on the HSA-1b, each step is completely discrete. There are only ever two resistors in the attenuation path, per channel, regardless of setting. This means fewer components and fewer solder joints (every connection in any signal path adds low-level reflections) and thus less noise and distortion in the signal path. It is also a more expensive implementation, which is why the series-approach is seen more often - so it is refreshing to see RAAL-requisite using the higher-quality approach here.

Input Level Attenuation

The HSA-1b’s inclusion of adjustable input level attenuation allows proper matching between both the higher output levels of professional/studio gear, which is where the RAAL-requisite products were initially targeted, and lower-level outputs typical of consumer audio gear. Allowing higher-level signals to be passively attenuated, rather than actively boosting consumer level signals, permits the implementation of a lower-noise input stage.

Out of the box, the HSA-1b is configured to apply 10 dB of input attenuation, in accordance with typical pro-level outputs. Depending on your desired listening level, and sources, you may need to reduce this (i.e. if you cannot get the volume levels you want with the default setting).

And this is done using DIP-switch settings, located on the input board, positioned inside and towards the back of the unit (in the picture above, it’s the blue block with white switches - but this block may be red in some units), and the actual settings for these switches are as follows:

While internal adjustments are not as convenient as externally accessible ones, you should not need to change the settings unless you change your source.

Build & Aesthetics

As with the SR1a EarField™ Monitors, the build of the HSA-1b follows a simple, understated and solidly-tool-like path rather than that of a lifestyle piece. The solid fascia, in black-anodized brushed aluminum, adopts the same design language as the SR1a - finished with crisp white legends/labels and rich, red, highlights. This is mated to an easily removed (for changing DIP switch settings), textured and painted steel chassis, and solid back-plane. Solid connectors, with no play or movement at all, combined with the unit’s near 6kg mass, means no issues with plugging or unplugging cables (no need to hold the unit in place) no matter how snug the connections are. And running heavy IEC-type AC cables won’t be an issue, either.

Switch gear is crisp and positive, and their effects are internally actuated via relays - including power-on/off delays. The individual steps, on the generously sized volume dial, engage solidly; you’re not going to knock the dial and change it accidentally. There’s a reassuring, alternating, clunk/tick from the mechanism (rather than through the headphones) for even/odd positions as they engage.

Only the speaker terminals give me pause; not because of any solidity issues - I’m just used to 5-way binding posts and the connections on the HSA-1b only accept 4mm banana-plugs. This is a trade-off due to the expected number of actual speaker users vs. others and the need for low-profile, insulated, space-efficient, connections.

The HSA-1b is now shipping with a set of 4mm banana-plugs in the box, so those with speakers terminated differently (e.g. bare wire) can easily connect them.


An optional, hard, carry case is available that holds both the HSA-1b and a set of SR1a is available and can either be ordered with the amplifier or purchased separately later:


Initial impressions of the HSA-1b, whether it is driving the SR1a or conventional headphones, are of a rich, fast, punchy presentation with excellent dynamics and “slam”. The midrange is lucid and liquid, the bottom-end kicks like a mule, and there is a hint of added tonal weight/harmonic richness and warmth to the delivery. The result is an incredibly engaging, slightly romantic, musical portrayal vs. the more relentless and technical delivery one gets from other solid-state amplifiers when feeding the SR1a (either direct-drive or via the interface box).

As such, this is the best I’ve heard the SR1a and represents the most enjoyable listening sessions I have had with them - at least without resorting to much more expensive, and much larger, solid-state or tube-based speaker amplification.


Bass lines are taut, deep, fast, tuneful and deliver some serious slam. This low-end kick was one of the first things I noticed with the HSA-1b driving conventional cans. It’s present with the SR1a of course, but sub-bass is less impactful there simply by their nature. This slam is not because the bass-level is elevated … in terms of level its very similar to the Jotunheim R (slightly more prominent than via the interface box), but in terms of how solid it feels and how hard it hits. Pair the HSA-1b with the Abyss AB-1266 Phi TC and the bottom-end seems, well, bottomless … and even more visceral than I am used to with those cans. There is some serious low-end grip at work here.

Bass effects, such as interference waves, gel nicely but still permit delineating the individual tones as they cycle in and out of phase. Sub-bass has real presence and control, with the deepest church organ notes having life and presence not often heard via headphone sources yet they do not intrude upon, nor upset, the amplifiers poise and dexterity when overlaid with faster, lighter, melodies.

And EDM, while not an out-of-proportion component of my listening or library certainly got rather more than its fair share of play time when I first started listening to RAAL’s new amp. I found this to be unusually addictive. And this was true both driving the SR1a and conventional headphones (in particular the RAD-0 and AB-1266 Phi TC).

The midrange is a sonorous, liquid, and highly lucid, affair. The tonal weight, or harmonic richness, imbued by the HSA-1b is most notable in this region. I won’t describe it as being tube-like, though there are traits shared there, a level of euphony being one of them, but it draws you in and seduces you in much the same manner.

Choral pieces swell together, yet you can still pick out, and follow, individual performers. Vocals retain their full detail and depth, and the “human-aspect” generally most notable in a live, acoustic, setting carries the life and emotion of the piece - perhaps enhanced by some harmonic glow.

And, while perhaps not totally colorless, I have not yet come across a case where the hint of romance delivered by the HSA-1b hasn’t been both welcome and beguiling.

The overall delivery from RAAL’s new box is creamy-smooth across the spectrum, but this is particularly noticeable in the treble region (and beyond). Existing SR1a owners know that they take no prisoners when it comes to revealing upstream deficiencies, and any upstream roughness or sibilance there can become startlingly apparent. The HSA-1b manages to preserve all the detail, texture and bite here, without exacerbating or contributing any harshness or artificial grain.

The extreme upper frequencies seem to roll-off slightly and, as mentioned, the result is a less relentlessly-in-your-face performance than I’ve heard with other amplifiers, which with the combination of SR1a’s native “studio neutral” tuning is a generally good thing for extended listening-for-pleasure scenarios. However there is no apparent loss of sparkle, there’s air aplenty, and discordant overtones still carry appropriate bite. The brassy shimmer of cymbal and brush work is delivered in a suitably delicate and well-delineated fashion, is full of texture and never becomes steely. While the crash and clang of a full-on strike delivers instantaneous energy and impact followed by ringing-decay that is easily followed all the way down into nothingness.


The rendition of piano here is as convincing, and beguiling, as I’ve heard from a desktop amplifier. Some test recordings I did of my own piano, a couple of years ago, easily resolved the difference in sound between when the curtains in the room were open vs. closed, and when furniture had been moved around between sessions and the HSA-1b stays faithful to that. Spatially (well, lateral spatiality), tonally and in terms of timbre it was a pleasure here through all of my own recorded play and everything else I fed it. No grating moments, and no flashes of “that’s not quite right”. Might not sound like a big deal on the surface, and it may be something I am unusually attuned to, but it is something that I’ve had other well-regarded gear fall foul of.

It’s not just piano, even though that is a particularly challenging instrument to convey properly; timbre is natural and believable regardless of instrument. And it is not just the gross elements of timbre that have a rightness to them. Whether it is the subtlest differences in timbre arising from a different strike-point on a drum-skin, a change in finger pressure or the slide-and-bite of a bow on a string, or the shifts in harmonics through the gentle, reverberant, decay from the wooden body of an acoustic guitar or a cello, all are easily discerned and eloquently rendered.

Mozart’s Serenade for Winds, K361, 3rd Movement, begins one of my favorite ways to assess both timbral delineation and instrumental layering/separation. Via the HSA-1b/SR1a combination … the combined, gently throbbing, pulses of basset-horns and bassoons, overtaken by a single, pure hanging, oboe note that is picked up and embellished with the sweeter tone of a clarinet are fully cohesive while still retaining an ease of following and identifying instruments, and their variations, that keeps them clear and distinctly individual.

Resolution & Detail

At no point during my listening did I feel that I was losing any detail driving the SR1a vs. the Jotunheim R, and in general detail exceeded that I got using speaker amps via the standard interface box. It took a $12,500 solid-state amp via the interface to best the resolution of the HSA-1b with the SR1a. Every tiny nuance was relayed, with nothing being glossed over. The violinist’s breathing in “Introduction et Rondo Capriccioso, Op.28 for Violin and Orchestra (in B minor)” (EMI, Saint-Saëns, Walter Süsskind) is clearly resolved and more easily audible here than pretty much every other unit I’ve tried with the SR1a.

The detail offered by the SR1a, combined with its native tonal profile, could become a little relentless after a while. The HSA-1b somehow tempers that, without any apparent loss of detail. I was listening in longer sessions, typically hours at a time, with no fatigue, and still able to hear every last detail, right down to the edit-points in analog masters/mixes (not uncommon in, say, early Prince albums).

Resolution remains excellent when driving conventional headphones, however listening to them back-to-back against the SR1a, via the HSA-1b, it shows just how much some headphones leave on the table - even when driven with the same sources and amplification. Further comparisons with those same conventional headphones driven from the HSA-1b and a slew of other, high-end, solid-state amplifiers showed it really was a difference between conventional cans and the SR1a, rather than anything related to the HSA-1b.

Dynamics & Transients

In my SR1a review, I said that the 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. That performance is maintained, and even enhanced here, due to the hammer-like macro-dynamic impact the HSA-1b delivers (perhaps down to having 10 raw amps of unfettered current available from the free-running internal PSU). Yet, it is notable in its extreme deftness and micro-dynamic subtlety as well. And the contrast between the two is, at times, striking.

Textures resulting from tiny fluctuations in volume are sometimes glossed over by other amplifiers that have this much kick to them, but they are rendered intact by the HSA-1b. Such micro-dynamic resolution and nuance can be the difference in the emotional presence of vocals (easily heard with, say, Leonard Cohen’s darker works) or proper timbre in instruments (for me, most easily discerned in piano, string and woodwind components).

Stage & Imaging

Assessing imaging and staging is something the SR1a actually makes possible in terms of speaker-like depth-wise spatialization and layering and not just for the typical lateral localization/width and “distance from stage” that regular headphones limit you to. The HSA-1b fully exploits the SR1a’s capabilities here, projecting a deep and solid depth-wise image with the SR1a, making it easy to hear where an instrument is placed in 3-dimensional space (I suspect the sense of height is, as ever, more to do with the tall drivers than anything else). Provided, of course, that your source material was recorded in a manner that preserves the necessary temporal cues; be that from a natural acoustic, stemming from a simply mic’d stereo performance, a binaural production or even a concert recording with individual suspended microphones (panned and leveled appropriately at capture or mixing).

Holly Cole’s “Train Song” (Temptation) 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.

Open, live, venues are presented with a definite sense of physical space. Large churches “feel” cavernous, with the slow, delayed, reverberant echo from a large pipe organ or the mono-tonal chanting from an individual friar, being vivid and obvious. Similarly, intimate chamber recordings or small jazz ensembles maintain a sense of constrained space, with the jazz-club effect of a tight group of players in front, and a bigger expanse behind you, being convincingly rendered.

Cowboy Junkies “Mining for Gold” (The Trinity Session), is one of my preferred tracks for seeing how well a setup conveys the sense of space and ambiance of a venue. And the combination of the HSA-1b and SR1a does this with more vivid dimensionality and air with the than on anything else I’ve heard it via (except well sorted 2-channel set ups).


I would say this amplifier is an exemplification of contrasts. It delivers slam, impact and punch, while simultaneously being deft, delicate and nuanced. It plays deep, with weight and gravitas, providing a very solid musical foundation … without sacrificing speed or dexterity. It luxuriates in its mid-range delivery, yet somehow manages to maintain balance with both upper and lower registers. It can kick like the proverbial mule, with impressive dynamics - even with conventional headphones, yet does not exaggerate or gloss-over the subtlest, gentlest, tiniest reverberations in vocals or strings. It renders music as a cohesive whole, allowing you to delve into, and focus on, the details of any aspect of it - yet without exaggerating the prominence of any individual element.

vs. HSA-1a

I loved the HSA-1a with the SR1a … so much so that I bought one purely on that basis. With conventional headphones, I was not quite as convinced. While certainly enjoyable, I felt it didn’t come close enough to my existing amplifier (at the time, an SPL Phonitor X) to allow me to run with just the HSA-1a for solid-state duties.

The HSA-1b is an improvement over the “1a” in a number of areas, some more notable with conventional headphones. The background is blacker (this was never an issue with the SR1a), channel separation and lateral imaging is superior, midrange is more lucid/liquid and holographic, there is greater refinement in the highest registers, resolution has taken a step up and channel balance is perfect (again, never an issue with the SR1a).

In fact it has improved enough with conventional cans that it is now my only solid-state headphone amplifier.

vs. Jotunheim R

Jotunheim R has a more strictly-neutral and technical delivery vs. the HSA-1b’s more colored and romantic rendering. The top-end of the HSA-1b is a little smoother, overall tone is somewhat richer, the midrange is more liquid and the bass seems more prominent (though this is mostly due to the inverse effect of slight roll off in the extreme treble) and the end result is that the RAAL unit is less relentless and is easier to relax with. This results in longer listening sessions both without fatigue nor giving up the technical abilities of the SR1a.

In my initial comparison, Schiit’s unit came across as being more incisive and having a little more bite/edge on fast transients, despite how fast and punchy the HSA-1b is. Extended listening drew me to the realization that this is largely down to the Jotunheim R having a slightly sharper/more forward top end - which tends to emphasize transients/attacks rather than them actually being any faster. It’s most notable with percussion, since when listening to bass-heavy pieces it is clear the HSA-1b is the unit delivering a bigger kick and more slam on the low-end, even if the difference isn’t particularly large. A simple, modest, EQ bump in the higher registers will yield the same effect on the RAAL box, if that’s your thing.

Similarly, some of the character of the HSA-1b can be realized with the Jotunheim R using EQ, though this requires multiple EQ points, is more complicated, and doesn’t get you the same slam/impact, smoothness, liquidity/lucidity or staging.

Conventional Headphone Drive, Pairings & Outputs

The HSA-1b can drive regular headphones either directly from its dedicated “Conventional HP” outputs or, if you need/want more power, from the SR1a (“Ribbon”) output using the supplied adapter cable:

In general, you can simply use which ever output you find sounds the best with a particular set of headphones. The “Ribbon” output offers about 8x the power output of the “Conventional HP”; the trade-off is less range on the volume dial and the potential for some low-level hiss with very sensitive headphones (which shouldn’t need the power in the first place).

In either case, with conventional headphones, you’ll want to put the “SR1/HP” switch in the “HP” (down) position. This disengages the step-baffle compensation that is needed for the SR1a. You won’t damage anything using the “SR1” setting with normal cans, but you will notice reduced top-end presence and treble energy.

The signature of the HSA-1b, as described in the general “Sound” section is maintained with normal headphones, so I won’t repeat those descriptions here (though do pay attention to the last paragraph about “Resolution & Detail”). I will emphasize that the incredible sense of grip, dynamics and authority projected by the HSA-1b belies the comparatively modest specified output power from its conventional headphone outputs. The headphones I tried resulted in excellent sound from the standard 1/4” TRS and 4-pin XLR outputs, regardless of source material or how loud I wanted to listen. The only difference between these two connections I was able to discern was a sense of slightly better separation from the 4-pin XLR output.

All that said, I found I had definite preferences for some cans paired with one or the other output - with most favoring the “Conventional HP” output:

Headphones Preferred via the “Conventional HP” Output:

All of my dynamic headphones, and the two more efficient and lower-impedance planar cans, were preferable via the “Conventional HP” outputs. For one thing, you have more range on the volume dial as these tend to be easier to drive, and the already black-background seems even deeper and darker vs. the SR1a output and these cans.

Rosson Audio Design - RAD-0

I didn’t note any specific changes in apparent blackness of the ground using the RAD-0 from the SR1a output. They do appear to kick a bit harder driven that way, but whether that’s a difference worth having vs. getting more range/granularity on the volume control is going to be a personal call.

Sennheiser HD800S

There is good synergy here, with the natural traits of the HSA-1b acting as good counterparts to the occasionally less-desirable aspects of the HD800S’ own signature. There is a little extra meat in the mid-range here, compared to, say, my former Phonitor X, and there is less tension in the treble with borderline sibilant sources, without losing detail.

In general I had come to a point where I would only listen to the HD800S via tube amplification, but that is no longer the case with the RAAL unit here.

ZMF Vérité C LTD

The tonal and technical characteristics of the ZMF Vérité C are a highly engaging match here. Depending on your preferences around warmth and bass, you may find you want to adjust your pad choice to keep that in check (obviously depending on what pads you use to begin with), but the lucid, sonorous, liquid mid-range that the HSA-1b brings to the table here is very well exploited by the ZMF cans in general, and the Vérité C in particular.

Headphones Preferred via the SR1a “Ribbon” output (w/ adapter):

The headphones listed here I found generally sounded better when driven via the SR1a output, using the included adapter cable. General improvements were better dynamics, more headroom/volume and extra low-end “kick”, but a couple of models had additional benefits that I’ll comment on individually:

HEDDaudio HEDDphone

The already highly-dynamic performance of the HEDDphone is kicked up another notch here, response reaches lower, with a near-visceral sense of bass energy and slam and greater percussive impact.

The stage is still rendered as wide and laterally separated as ever, and image depth increases even if depth-wise layering hasn’t improved.

This is the best performance I’ve heard with the HEDDphone, from a solid-state amplifier, to date.

HiFiMAN Susvara

These gain, for me, a desirable increase in low-end punch/slam and overall presence vs. running them on various other high-end solid-state headphone amplifiers. They “wake up” nicely with the HSA-1b via the SR1a output, in the same way they do running off good speaker amplifiers. You can, of course, drive the Susvara from the HSA-1b’s speaker outputs as well, and doing so enhances the low-end and presence a little more, as well providing a bit more headroom, with the only real trade-offs being convenience and a slight sense of extreme high-end roll-off vs. the SR1a output.

Driving Speakers

The HSA-1b offers speaker outputs, via standard 4mm sockets (for “banana” plugs). Power output available is 10W at 8 ohms, 20 W at 4 ohms and 40 W at 2 ohms (all at 1kHz), so this calls for higher-sensitivity speakers or lower listening volumes. And, perhaps counter-intuitively, high-sensitivity speakers tend to be larger than you’d expect with most bookshelf/desktop speakers trending towards lower efficiency.

While I have tried the HSA-1b driving a handful of speaker models, of varying sensitivities, and was more than happy with its performance with suitably sensitive models, this was mostly to ascertain if the unit’s general signature/character and presentation was preserved with speakers; and to that I can say it certainly is. Beyond that, proper speaker coverage with specific speakers and comparisons will be covered in a follow-up segment.

Other Notes

There are a couple of things of note here, either specific to the HSA-1b, or that relate to its usage vs. other options, that I felt were worth calling out specifically - rather than burying them in another section:

Bass Level/Quantity

Much has been said about the relative bass-level offered by a non-EQ’d SR1a and various amplification choices. With natural music, and even electronic pieces, the natural bass response of the SR1a is a pretty close match for what’s present in the source material, at least down to 33 Hz (technically the sub-bass region). Tweaking this with amplifiers works, to some extent, but if you’re really wanting “moar-bass” then using EQ is a much more reliable and efficient (and vastly cheaper) way to achieve that rather than “amplifier rolling”.

Yes, the HSA-1a/b and Jotunheim R exhibit a more robust bass response than most speaker amplifier/interface box combinations will yield, without resorting to EQ. And while the HSA-1b (or HSA-1a for that matter) exhibits somewhat more slam and impact in the lower registers than Schiit’s little wonder, it doesn’t really change the level/quantity of bass delivered. In other words, don’t buy an HSA-1b with the expectation that it will meaningfully up the bass levels of the SR1a vs. other direct-drive options.

Noise Floor/Connected Inputs

If an input is left unconnected , or floating, then some low-level hiss may be apparent at the highest volume settings on that input. There’s no issue here in actual use, even with no music playing, with all of the headphones I tested the unit with. Just don’t try evaluating the noise-level of the amplifier with nothing connected to it!


Simply put, the HSA-1b delivers the most enjoyable, immersive, engaging and outright addictive listening experience I’ve had with the SR1a, short of spending more than double the $4,500 asking price on high-end speaker amplification!

In fact, I like the HSA-1b so much with the SR1a that I would have had no qualms about buying it just to drive them, even though that wouldn’t be taking full advantage of what it offers. But since I found it similarly compelling with almost every other regular headphone I tried it with, and it reduced my desktop box-count into the bargain, it was an even easier decision - and better value as a result.

The HSA-1b has a seductively rich, lucid, dynamic and punchy character/signature, which yields spectacular synergy with the SR1a. That signature is present on all outputs, and with some headphone/listener/preference combinations it could conceivably be “too much of a good thing” - though I haven’t personally run into that across my collection of cans.

Negatives? Even with all the flexibility on tap, $4,500 is still $4,500. I suspect some of that price can be attributed to the realities of its smaller potential market. While it’s very hard to beat with them, it would be harder to justify if it wasn’t being used with the SR1a at least some of the time. It is, for sure, an engaging, and still addictive, listen with conventional headphones (and/or the right speakers), but if you don’t need the SR1a “direct-drive” capability then other options exists that have different signature and performance biases and at lower cost.

I think the HSA-1b is a significant overall improvement over the HSA-1a. Not only is the revised control setup much friendlier and the features more flexible/usable, but more importantly the listening experience is also improved - especially with conventional headphones (the HSA-1a was already fantastic with the SR1a).

How much improved?

After a month or so of listening and comparisons, the HSA-1b has fully replaced my SPL Phonitor X/HSA-1a combination to become my primary, and in fact only, solid-state headphone amplifier.

I highly recommend SR1a users make a point of getting their ears on the HSA-1b. That’s regardless of whether they use conventional headphones as well; it just sweetens the already honey-like pot if you do.

At the end of my review of the SR1a I said, “I just don’t know where I would go from here …”

I do now …


History and Iterations - HSA-1a vs. HSA-1b

The HSA-1b is a revision to, and upgrade of, RAAL-requisite’s prior, and somewhat short-lived, HSA-1a amplifier - of which there have been two iterations. The initial release of the HSA-1a had two 4-pin male XLR connections for driving two pairs of RAAL-requisite’s “SR1a” ribbon-driver EarField ™ monitors:

A subsequent revision, an optional conversion to which was available to existing HSA-1a owners, swapped one of the SR1a outputs for a conventional, balanced, 4-pin XLR headphone output. This also included an adapter cable for driving inefficient/power-hungry conventional headphones, such as the HiFiMan Susvara and HE6/HE6-SE etc., from the SR1a output:

The HSA-1b refines the implementation of the HSA-1a, retaining both SR1a (ribbon driver) and conventional 4-pin balanced XLR outputs, adds a 1/4” TRS connection, moves all the switches to the front of the unit, replaces the conventional, potentiometer-based, volume control with a 24 position stepped-attenuator (for perfect channel balance) and adds configurable input level attenuation. The adapter cable for using the “SR1a” output with hard-to-drive dynamic/planar cans is still included.

I have heard all three of these models, at length, in my own system. The first HSA-1a was on loan. On the basis of its performance, I bought one of the second revision units, and have since replaced it with the HSA-1b.


Well, if this is your last full “proper” review for the site then it’s a helluva way to go out!

You make a compelling case for this amp and addressed some of the questions I had regarding it. While on the pricey side this is definitely something to consider given the breadth of cans it can drive and I guess drive well.

I probably would not have been as likely to purchase this without a good HP out option but maybe this is now something to consider, with the SR1a’s, a bit down the road.

For me, this may be one of your best reviews. Happy New Year!


One thing that might be worth mentioning: early HSA-1b units shipped with a 1.6amp fuse. This turned out to be too conservative, and is pretty much guaranteed to blow. On mine, it did so within three weeks of receiving the unit (and using it pretty much every day, which I still do now). RAAL now recommends using a 3amp fuse. I assume more recent units are shipping with the higher spec fuses now. But if you have one with the 1.6amp fuse, the 3amp fuses are cheap from places like Amazon (which is where I got mine from).


Nothing beats going out on top!


the amp does a great job with the sr1a, it is a more refined version of the jotr although the jotr with a balanced in is more powerful and drives the sr1a with far less of a turn of the volume knob…the hsa-1b is less sibilant and far easier to listen for periods of time…it does a nice job of driving both the abyss TC and the susvara despite its modest power rating which is interesting but it must be with the adapter…one thing I really like is that the amp runs really cool to the touch which is great for someone like me who has it in the bedroom near my bed…my formula s/powerman is perhaps a bit more powerful with the susvara or abyss but the hsa-1b is excellent with both…a real nice all in one amp for a person who owns the sr1a and conventional HP’s

If you’re finding that to be the case, you probably have the HSA-1b set to it’s default -10 dB input level attenuation. Switch it to 0 dB (that’s how I run mine, the -10 dB setting is really for when connecting to pro-output-level/studio gear). My review, above, shows the settings and where to make the changes.

I did change it to 0 but the jotr in the balanced out still is more powerful

Thank you for very informative review.
I would like to know how sound MySphere 3.2 with the new amplifier .What is the difference between MySphere 3.2 and SR1a? What do you prefer between them and why?
Best regards



My first post here.

I am demoing the the SR1a and the HSA-1b as I type this, really love this headphone and amp. I tried demoing the SR1a with my Benchmark stereo AHB2 + HPA4 + Matrix Minii- 3 Pro and it was too hot on top. A big disappointment. Love the AHB2 with my speakers.

The HSA-1b sounds so much better. However, the DAC I am using with the SR1a is the DAC I use for my bedroom headphone only system and need to get another DAC. The bedroom is a Topping A90 and Matrix Mini-i 3 Pro + Meze Empy. It sounds good with that combo but the Matrix is a bit hot with the SR1a though useable. I am wondering what DAC would be a perfect pairing with the SR1a + HSA-1b. My top contender so far for a warm DAC is the Halo Audio May KTE. I have not heard it yet.

BTW - what a great review.


Hi @Torq, thank you for another outstanding review!

May I ask what are the best speaker amps you tried with the SR1a, regardless of price?


I have not tried it with the SR1a but I think the Luxman m900u would be a top choice. I have demoed that amp a few times, the last time with the TAD ME1 which has a bright top end driver. I think it would sound very good with the SR1a. The CODA # 16 or #8 are 2 other amps I was thinking of getting for my power hungry speakers.

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Thank you for this very thorough review. A tour de force, like the gear that inspired it.


The HSA-1b has the same basic effect on the MySphere 3.2 as it does on the SR1a and other headphones, so the descriptions on sound in the review largely apply there too.

Other than being based on entirely different technology, the best answers I can give will be found in this thread, and probably this post in particular.


So far, the Audio Research Reference 160S on the tube side of things (I tried the VT80SE and found it took a bit too much edge and transient speed off the SR1a).

Solid-state wise it’s been the Chord Ultima 5, which is the fastest, most dynamic and resolving amplifier I’ve tried them with via the interface box.


In the little bit of listening I got to do on the May DAC, there was some sweetness and top-end roll-off in NOS mode (as you’d expect), which can contribute to a sense of warmth. It’s not overtly warm, however … and outside NOS mode it is strictly neutral. Excellent DAC regardless.

Bifrost 2 does have some additional warmth and pairs well with the SR1a/HSA-1b combination. Yggdrasil A2 is similar, but more resolving and dynamic. Rockna WaveDream (and possibly WaveLight) might be options. Possibly the Sonnet Morpheus. A used Pavane (even L1) might fit the bill there also.

But I really slowed down listening to new DACs as even when comparing the best from dCS and MSB (etc.) all I’m really finding is very small, very subtle, differences vs. what I have rather than anything I could say was specifically “better”. Personal preferences could swing that, of course. But for me, since getting the Blu-Mk2/DAVE combination it’s all just been “subtly different, but not better (or worse)”.


Thanks for the feedback. I was considering the Halo May KTE and the AudioByte VOX | ZAP | Hub stack as a future upgrade to my Benchmark DAC3B (which I recently reluctantly sold). Now that I want to get this RAAL headphone and amp combo I wanted something that would be ideal for that RAAL combo. Your DAC suggestions are much more cost effective, and for DACs, I do agree that it is a good place to save money.

The DACs you have mentioned are ones I have head about but never listened to since I normally go for DACs like my Benchmark DAC3B. My currently 1/2 the price Matrix Minii-i 3 Pro is similar to the DAC3B (almost) but I feel it does not pair well with the RAAL combo due to it being a little too strong on top. It sounds rather good on my all-Benchmark 2 channel system, where being very neutral is a virtue. I am really nitpicking now but by buying the HSA-1b I get the opportunity to 2 variants of sound in my setups (with 2 preamps/headphone amps).

I kind of wish I did not hear the RAAL SR1a + HSA-1b, I was perfectly happy with my Meze Empy and Benchmark HPA4 (not selling those 2).

If you’re just wanting a less forward/aggressive top-end, you’re much better off applying some EQ (either via software, or a hardware EQ unit), than trying to DAC-roll (or even amp-roll) to achieve it.

EQ is much more precise, repeatable, easily setup for different preferences and, typically, vastly cheaper.

One way to get that is something like the RME ADI-2 DAC fs, which has excellent on-board EQ capabilities (among other things) and something that, even without EQ or other DSP, I personally prefer over any of Benchmark’s DACs.


You just set off a light bulb in my head. I just paid a professional sound engineer (genius level guy) to create a Convolution file (DRC via DSP) for my 2 channel floor stander that is in a small room. It was an incredible success. I turn off the Convolution DSP in ROON when I listen to headphones but of course I can create some adjustments for a new filter inside of my ROON server for the RAAL only. You may have just saved me ton of money.

BTW - the Convolution file was created remotely once I took some measurements with REW software. It cost me $700 and the process is written up a lot on the AudiophileStyle web site.


I thought I would update this. I went into my ROON DSP settings and created 3 preset settings.

  1. Thiel CS3.7 - All Filters off except Convolution filter (by https://accuratesound.ca/)

  2. Meze Empyrean - Bypass All Filters

  3. RAAL SR1a - All Filters off except Parametric EQ filter

I am going to use the “Brickwall” or “Fast Roll off - minimum” filter on the Matrix DACs filter settings as the baseline and fine tune that with ROON Parametric EQ.