Gramophone Dreams #53: RAAL-requisite HSA-1b headphone/loudspeaker amplifier

Today is Monday. Since Saturday, I have changed the amplifier driving my Falcon Gold Badge LS3/5a speakers three times—from the Parasound Halo A21+ (250Wpc into 8 ohms) to the Pass Labs XA25 (>25Wpc into 8 ohms) to the Elekit TU-8600S (9Wpc into 8 ohms). Now I am listening to the Falcons via the just-arrived RAAL-requisite HSA-1b headphone and speaker amplifier. This unusual, made-in-Serbia amp is priced at $4500. It's specified to produce 10Wpc into 8 ohms, 20Wpc into 4 ohms, 40Wpc into 2 ohms, and 55Wpc into 1 ohm, these values from the loudspeaker output

Just before I installed the RAAL HSA-1b, I was playing an unusual EMI LP called Renaissance Suite featuring music for Joël Santoni's 1974 film La Course en Tête (HQS 1415). This film chronicles, in a strange and wonderfully artistic way, the career of Belgian cycling star Eddy Merckx. What's most strange is that the score is composed and arranged by British early-music specialist David Munrow and played with great vitality by the Early Music Consort of London. Imagine a sports documentary featuring the sounds of droning bagpipes, tromba marina ("nun's fiddle"), and rauschpfeiffen accompanying guys in tight shorts and bright-colored shirts pumping skinny bikes up European hills.

Using the 300B Elekit TU-8600 to drive the LS3/5as, Renaissance Suite displayed a wood-and–gut-strings tone that made Munrow's early music feel authentic, medieval, and trés exotique. Its temper was fresh and celebratory, but the sound was soft-focused.

When I connected the Gold Badges to the RAAL HSA-1b amplifier, the sound became noticeably firmer and better defined. With the HSA-1b, the score's flow and momentum dominated my awareness. The music had more energy. David Munrow's compositions made more sense when the music pumped harder.

When I switched from the wood-box Falcons to RAAL's SR1a wide-open full-range ribbon Earfield Monitors—loudspeakers to ear speakers—I experienced a clearer, more focused "view" of this recording.

RAAL's steel-caged, head-mounted transducers are unique in the headphone marketplace. They are built to last forever and are moderately expensive ($3500). The SR1a's dipole ribbons are ensconced in stainless-steel cages positioned completely off the head. They are true ribbons, and they present amplifiers with almost a dead-short load impedance: 0.2 ohm, including cable. The owner's manual recommends 100W of amplifier power (footnote 1). With sufficient amp-juice, the SR1a deliver levels of focused, uncolored, low-distortion sound that (I believe) exceed those of the super-resolving Stax SR-009S electrostatics as well as my beloved JPS Labs Abyss AB-1266 Phi TC and HiFiMan's Susvara planar magnetics.

The SR1a headphones require so much high-current amplifier power that it seems only logical that RAAL-requisite would make an amplifier that could, at the flick of a front-panel switch, drive their headphones or my speakers.

The HSA-1b
RAAL's new HSA-1b amplifier (footnote 2) is the first solid state speaker and headphone amp I've tried. When I wired it to the floorstanding Stenheim Alumine Three speakers and heard how effectively it could drive a $32,900 pair of speakers, I knew this was a different species of speaker/headphone amp than Linear Tube Audio's 10Wpc (at 8 ohms) Z10e, which I reviewed in Gramophone Dreams #36, or Ampsandsound's 8Wpc (at 8 ohms) Bigger Ben, which I described in Gramophone Dreams #47.


RAAL's HSA-1b is a sturdy little red-knobbed 11.7lb amplifier that measures a modest 3.5" high × 8.6" wide × 12.6" deep. It uses two bipolar junction transistors (BJTs) per channel bolted in pairs to a single 8" × 3.5" × 2" aluminum heatsink.

Build quality is such that inside its chassis, the HSA-1b looks military-spec. On the outside, the high level of fit and finish appears both industrial-strength serious and home-decorator charming.

Besides (and beside) the obligatory IEC power socket, the HSA-1b's rear panel has two line-level inputs: one single-ended (RCA) and one balanced (XLR). There are also two pairs of banana plug jacks for connecting stereo loudspeakers. Below the speaker jacks it says "DESIGNED and PRODUCED for RAAL-requisite by S.A.EQ, SERBIA."


According to Aleksandar Radisavljevic, RAAL-requisite's chief of everything, "S.A.EQ stands for Serbian Audio Equipment. It is the company of a very good friend I've known for 20 years, Mr. Dragan Domanovich. I was impressed with the sound of his speaker amps, and once our headphones were finished, I asked him if he could make an amp for the SR1a that would be branded RAAL-requisite and be exclusively available as RAAL-requisite. He was eager to do the necessary modifications to his preferred amplifier topology, and little by little, over a year of iterations, Dragan got the sound right where all of us at R-r and SAEQ wanted it. Since SAEQ has the capability, they are also doing the manufacturing."

The HSA-1b's front panel features a 5/8" diameter pro-audio–type power switch, illuminated by a red light. To its right is a four-pin XLR headphone output connector that is male instead of the usual female so that only RAAL's SR1a ribbon headphones can be connected; the HSA-1b package included a gender-changing female-to-female XLR adaptor so that users can connect power-hungry, low-sensitivity headphones such as HiFiMan's 83dB/mW Susvara to the HSA-1b's high-current SR1a output. To the right of that connector is the female four-pin connector for conventional balanced headphones, and then a 3.5mm (unbalanced) TRS jack. Above the headphone connectors is a row of three switches allowing users to select between SR1a and conventional headphone drive through the SR1a output, headphone or loudspeaker output, and balanced and unbalanced inputs. On the far right is the HSA-1b's big, red, faceplate-dominating volume knob, which controls its click-click, 24-step attenuator.

Radisavljevic told me in an email that the HSA-1b uses bipolar junction transistors because "they are current-driven devices, not voltage-driven devices like MOSFETs." Consequently, "BJTs have higher switching losses but lower conduction losses."

I told Radisavljevic about the "incredible drive" I was hearing from the HSA-1b and that it was driving the 15 ohm, 83dB/2.83V/m–sensitive, Falcon LS3/5a loudspeakers with force and surprising ease. I asked him to explain how his little "headphone" amp could do this. He responded, "The drive is there because that thing is made to supply 7 amps of current into a 0.4 ohms load, so 15 ohms is nothing for it. It's basically a loudspeaker amp that is further optimized for driving low impedance loads." In another email, Aleksander filled me in on some specs not listed on the RAAL-requisite website: "The gain is about 10dB from the RCA input. With XLR input, you can choose to reduce gain if you have a high-output DAC."

Inside the HSA-1b on a small printed circuit board sprouting perpendicularly from the XLR input board is a six-position DIP switch that allows for "internal sensitivity adjustments of the XLR inputs." According to the owner's manual, users have three choices: –10dB, –6dB, and 0dB. My unit came preset for –6dB.

"The output impedance at the Conventional [headphone] output is 5.3 ohms. Power from that output into 32 ohms is 250mW, and 26mW into 300 ohms. Power output at the SR-1a/Ribbon output (when driving dynamic or planer-magnetic headsets) is 1900mW into 32 ohms and 200mW into 300 ohms." If my math is right, that means that with the leftmost toggle switch set to the down position, the ribbon output puts out about 7.75V.

The HSA-1b goes uptown
Elsewhere in this issue, I am reviewing Stenheim Suisse's $32,900/pair Alumine Three floorstanding speakers. These 41" tall speakers look very red-black-and-gray-aluminum-cabinet expensive. Their physical presence is uber-tasteful and trés moderné. Best of all, they reproduce recordings with an astute, nonrefractive clarity that is incredibly beguiling.

Stenheim's website says the Alumine Three is 93dB/2.83V/m sensitive (footnote 3) and recommends a minimum amplifier power of 10W. So naturally, with a little impish persuasion, I tried powering them with the 10Wpc HSA-1b.

I had a difficult time deciding whether to start my RAAL-Stenheim auditions with Champion Jack Dupree or Jacqueline Du Pré. The day was hot and muggy and the air smelled like fish, so I went with New Orleans pianist Champion Jack's 1958 Jerry Wexler–produced masterpiece, Blues From the Gutter (16/44.1 FLAC Atlantic/Qobuz). I was surprised how lush and sweet Dupree's piano sounded and how rich and lifelike Wendell Marshall's bass sounded coming from the Alumine Three's dual 8" woofers. The Champion's loping N'awlins attitude came through completely. My only concern was how or why this normally bright-and-hard Atlantic recording was sounding a little thick and soft.

Then I switched to Britain's most celebrated cellist, Jacqueline Du Pré. Playing her Haydn & Mann: Cello Concertos, from 1969 (24/48 FLAC-MQA Warner Classics/Tidal), the soft was gone and real, authentic bite and presence were back. The strings of the London Symphony Orchestra, conducted by Sir John Barbirolli, came through with just the right amount of corporality and precision. The HSA-1b + Alumine Three combination made a fresh, open, note-clear sound. Violin transients and cello harmonics entered my room with a great semblance of the real thing. This recording has rarely felt this compelling.

During my June-July auditions, I listened to dozens of albums with the RAAL HSA-1b powering the Stenheim Alumine Threes. I rarely felt a need for more power.

The RAAL-requisite SR1a
It is good that the HSA-1b drives the Falcons and Stenheim loudspeakers, but the main reason it exists is to power RAAL's SR1a full-range ribbon headphones without RAAL's amplifier impedance converter. I hoped it would at least equal the sound quality I get when powering the ribbons from my Pass Labs XA25 amp or the amp Schiit Audio makes expressly for the SR1a: the Jotunheim R.

When the Pass Labs XA25 powers the RAAL headphones (via RAAL's impedance converter), recordings sound clear and direct in a way that feels like a raw, unimpeded view of what the microphones captured. The trance music I like has soul.

And color. It is never corny or new-age sentimental. Instead it feels rigorous and smart and kicks up ancient blood memories.


Which is exactly what ambient wizard Steve Roach's 1993 album Origins does (16/44.1 FLAC Fortuna/Qobuz). On Origins' third track, "Clay, Wood, Bone, Dirt," the low, droning, guttural sound of Daniel Nagle's didgeridoo beats all other manmade sounds for instigating blood memories. With the SR1a powered by the HSA-1b, the long, flapping cavity resonances of the didgeridoo and short resonant tones of clay pots appeared, tangible, in a semicircle in front of me.

Footnote 1: You need a special interface—included—to use the SR1a with a standard amplifier. It's a voltage divider that cuts a standard amplifier's output voltage by 97%. The SR1a's require a lot of current but not much voltage.

Footnote 2: RAAL Advanced Loudspeakers, Djordja Simeonovica 419000 Zajecar, Serbia. Tel: (381) 64 144 1111 Web:, US distributor: Requisite Audio Engineering, 2175 Goodyear Ave., Suite 110 Ventura, CA 93003-7761. Tel: (818) 437-0779. Web:

Footnote 3: I estimated the Stenheim Alumine Three's B-weighted voltage sensitivity as 91dB/2.83V/m, which is usefully higher than average.—John Atkinson


thethanimal's picture

As always, I love the journey your articles take us on. Champion Jack Dupree is wailing through my bedroom/office (what a tragedy) speakers now, probably loud enough to raise some collective eyebrows from the wife, toddler, and nanny downstairs. One day I’d love to hear the level of headphone refinement you write about here, but so far I’ve found the DragonFly Red and Sony MDR-7506 combo to be the point of diminishing returns — at $300! I took the Sonys in to the local brick and mortar shop and lined them up against Focal Clears and Stellias at their headphone bar and just couldn’t justify the difference. The salesman did me a solid though by making me try out that DragonFly Red; I was hooked after the first few bars. The Blue had more ease to the sound, but as a percussionist (alas, in a past life at this point) I enjoyed the extra impact I heard through the Red.

PeterG's picture

Great write up, as always! As a Stax 9S owner/lover, I agree that the RAALs are maybe even more resolving. When I tried them, I found them to be thrilling. Actually, for me, too thrilling, I felt like my ear was on the edge of the snare drum or sax bell. But an amazing addition to the handful of super premium headphones--like a Ferrari. Cheers!

tonykaz's picture

Dear Mr.HR,
I was a Stax Dealer and admirer , eventually I sold my personal Stax and quit selling Stax altogether.

It might've been too much of a good thing but was it? maybe it was too much complexity, too specialist, too unique, too fiddly. Maybe it was like a Porsche 911 Turbo for picking up the TacoBell at the Drive thru.

Music is like hanging out with an old friend, Stax Music was like a problem solving visit with my Analyst.

If these RAALs are anything like Stax they will probably be slow sellers compared to gear like Abyss Diana. ( my guess )

Thank you for bringing us this report, I can barely imagine anyone else they could trust with this. ( Tyll is lonnnnnnnng Gooooonnnnnnne ) You always seemed to be Tyll's replacement although un-announced.

Tony in Venice Florida

Ortofan's picture

... did you decide to sell/use in their place?

I've been a STAX headphone user since buying the SR-5/SRD-6 in the mid-1970s. That has been followed by the Lambda Pro/SRD-7 Pro and several other sets.

Which amps were you using to drive the STAX 'phones?
I found that some solid-state amps had trouble driving the load presented by the STAX adaptor. At that time, the SAE 31B seemed to work particularly well, as well as the Hafler DH-200 which appeared at the end of the decade.

tonykaz's picture

Hello again,

I became delighted with Mid Level Sennheisers ( 580/600/650 Series ) and Schiit Class A SS Amplification and Schiit Valhalla 2 with Russian Tubes ( which soar the musical heights with a comfortable ease ).
I've heard the newer outstanding headphones and am now considering the ABYSS Diana but I'm in no hurry.
I have my eye on and am planning for the latest version of Woo Firefly.

Back in my Retail Days, Stax were pretty much the only headphone to own. I had their range of Headphones and their matching Electronics. I was not quite a headphone guy until RMAF 2011 when I met the Schiit Guys, whom I was interested in for their manufacturing concepts and philosophy. I bought into their Products a few months later. I still love Schiit despite their shitty name & power switches on the Rear.

The Sennheisers are better than any Loudspeaker system I've ever encountered. ( even though they are mid level sound quality compared to the latest great stuff : 1266 Abyss.)

Headphones will change with careful Wire selection.

Schiit Tube gear changes with careful Tube Selection.

Tony in Venice Florida

ps. I'm anticipating the FR series of PS Audio Loudspeakers will be must have. Phew -- fingers crossed !