Fidue Sirius A91: rich cosmic body, and an illuminated stage amongst the void with some anomalies to starboard

Pros:  Well above average soundstage, great instrument separation, excellent details, full bodied, speedy, easy to drive, excellent build quality, beautiful

Cons: Harsh treble and artificial mids on some tracks (minority of tracks), shallow insertion, potential vent blockages change sound, large shell won’t fit all ears, price



Thanks to Tony at HiFi Headphones for loaning me the Fidue Sirius A91 for the purpose of providing an honest review. This review has been adapted from a review originally posted on HeadFi.


Sirius is the brightest star in Earth’s night sky, so bright that under ideal conditions it can even be viewed in daylight. Sirius, an illuminated compass for dividing up the night sky to Polynesians and Ptolemaics. In early Catholicism, the position of Sirius in the firmament was used to guide the timing of nightly prayers. Will this new flagship release from Fidue scorch the inner universe of the ear leaving a glowing ember in the night sky of memory (from Ancient Greek Seirios, glowing or scorcher), or will it be the ‘dog star,’ a cause of cacophony and fevers in the waxing hot ‘dog days’ of summer?

Wikipedia via Hubble Space Telescope National Geographic

The Sirius A91 is Fidue’s entry into the flagship headphone market. Fidue has been casting its light into ever more ambitious markets while also improving on the fit and finish of their lower end releases. Earlier this year I had the pleasure of participating in a tour of the Fidue A65, a beautiful, under $75 headphone released in 2015. The Sirius isn’t in the same constellation, nor should it be. Fidue’s offerings on the high end have long been moving into more complex hybrids IEMs. Last year they released the A73 (2 driver, 1 dynamic driver, 1 balanced armature); and in 2014 they released the A83 (triple hybrid, 1 dynamic driver, 2 balanced armature). I think they’ve been scaling up their capabilities for this release for a long while. The Sirius seeks to establish Fidue as the brightest star in the night sky, with five drivers (1 dynamic driver and 4 balanced armatures) striving to push the IEM to a distant zenith. Will it succeed? Will its bright light be white hot stellar light, or a more localised Martian glow?

Form & Function

Packaging and accessories

The Fidue Sirius A91 comes in a small square box. The box is wrapped tightly in a paper branding band that I struggled to get off without ripping. I had to be slow and patient, characteristics I’ve gained much more experience with from baby mealtimes. After the wrapper is safely removed and set aside, the box is a leather box with foam inserts to aid in safe transport of all the sexy audio innards. The top of the package has really nice foam tray for holding the shells, with a classy blue ribbon used for pulling out the adaptors on the underside. The IEM tip storage is another story, it’s a cheap blown plastic number that the IEM tips don’t want to go back on once you’ve taken them off—a bit of a pain for a review unit that you want to return in pristine condition. Below the tray the aluminum case ornamented in laser etched edges and lettering, with a dimpled waist can be found. Pulling up on the lid reveals the headphone cable. Along with all the other gubbins you’ll find your warranty card, an airline adaptor, a 3.5mm to 6.3mm adaptor, and a guitar pick tool and some desiccant—do not eat—for keeping everything dry and bug free on the long journey from China. The guitar pick tool is used for releasing the MMCX connectors, trust me, you want to use the tool, and not to channel Jimi, but I bet you could play some Little Wing, if you like.

I was both generally impressed with the accessories. I think that every headphone manufacturer should make their headphones balanced, and include adaptor cables. That Fidue has included adaptor cables for standard 3.5mm TRS and for 3.5mm TRRS (HiFiMan balanced) is impressive to me and is a big point in their favour. I wish I could just buy the cable, and adaptors like on the A83. Maybe that will be an option in the future. The selection of tips was limited, but I found the Fidue tips to be of very high quality. I tried the comply foams, but they made the sound too warm for me. My favourite tips with the Sirius were not the ones that came with the IEM.

Build quality

The headphones are very solid feeling with a nice curvature to the back of the IEM. The case has excellent build quality and a beautiful form worthy of flagship status. The case is not large, so you won’t be traveling with a bunch of extra tips. It has just enough room to fit the headphones and a cleaning tool. Fidue’s intended function for the case is clearly to look prestigious—mission accomplished. A little more size to the case would give it a bit more functionality.

Fit & Function

The Sirius has large earpieces, so fit may be an issue for some people, but it was not for my medium sized ears. The earpieces have a shallow fit with a relatively short bore. Another thing to note is that the ports on these are inward facing, so if the port is blocked, you’ll effectively turn the bass up on these. The headphones are comfortable over the ear, and the cables help with this. I found that rotating the MMCX connector towards my head gave a nice tight fit of the cable above my ears.

At first blush, I was not a fan of the cloth cable. I managed to quickly tangle it, and it took me a while to figure out how to use it and not have it become a Rapunzel nest of fibre. Once I figured it out, I was happy.

From my small understanding of metallurgy, Rhodium is an excellent conductor, but jewellers seem to think it wears out in white gold rings.  When scouring the internet, jewelry sites told me that Rhodium plating on a high friction item like a ring may only last between 6 and 24 months. A more knowledgeable man than me told me that “gold is softer, with a hardness of 2.5 mohs compared to 6 mohs for Rhodium.” Maybe jewellers are just trying to get us to spend money we shouldn’t? The plugs scream quality, a no expense spared ambiance, and allude to the bright radiance of the IEM’s Sirius namesake. They are solid connectors.

Audio sites say that Rhodium is better wearing than gold, whilst jewellers say that Rhodium plating wears off a ring in 6-24 months. I know that my white gold wedding band (often Rhodium) has the yellow gold shining through quite a bit. I don’t know who to believe, but I’d say if you have had the experience of your gold jacks wearing out, giving Rhodium a try isn’t a bad idea, but don’t be surprised if your results aren’t any better. Also, buy your Rhodium plated gear from reputable sources, as a layer of Rhodium plating that is too thin will come off in no time at all. Don’t go hunting from random east-asia source number five on aliexpress.

Audio quality

For my listening time I tried the Sirius on a few sources. I played the Sirius out of my Note 2 (I really should consider upgrading sometime soon) long enough to know that I only needed to turn the volume up to 50%. The Sirius is plenty sensitive. I played the Sirius out of the Questyle CMA600i and I played the Sirius out of the LH Labs Geek Out V2 (low gain) in both balanced mode and single ended. I compared the Sirius to my two lower priced but excellent headphones in my library: the 64 Audio X2 and the Echobox Finder X1. I tried to volume match comparisons using volumes between 78 and 82 dB using white noise and my cheapo SPL meter. I tried using a coupler, but couldn’t get a matched insertion depth across the IEMs, so I pressed the headphones with tips on to approximately the same location and same depth on the foam ball on top and measured. I had to do a bit of tuning by ear on the 64 Audio X2, as the sound wasn’t quite right (I think it is the ADEL technology).

The Sirius claims some serious frequency response, reaching a Cold War era nuclear weapons cache depth on bass and an ‘oh crap supersonic explosions’ level at the top (4Hz to 45kHz). Specs like that paint the picture of having even extension to unfathomable levels at both extremes, and I was pretty sure that I wasn’t going to hear any of that, so I ran some test tones from and Binkster Test CD. I could not hear anything at 10Hz, but could feel the driver moving. At 16kHz the sound started to roll off. I’m not sure if aspects of this insane proclaimed frequency response show up in the sound. One thing that proponents of HiRes will often say is that people hear timing differences to 192kHz (pretty arbitrary frequency to choose), so anything beyond audibility on frequency, may still show up as audible on timing signals, which is really what gives a soundstage its feel. On another make of IEMs that report having frequency response up to 45kHz, the oBravo ERIB and EAMT series, I found that the soundstages were the best of any in ear I’ve heard and comparable to many high end full-size headphones. The Sirius has a good soundstage, with good depth and width, but it isn’t anywhere near the oBravo level of stage.

As folks who have owned more than a couple sets of headphones know, tips can make a big difference on the sound of an IEM. I tested the Sirius with the stock tips, the included Comply foamies, and Spinfits. I used Paul Simon – The Werewolf and Daft Punk – Giorgio by Maroder to do my tip rolling. I found that the stock tips had a nice balanced sound. Comply added a little bass body and warmed the signature up a little more—too much for my liking, and as usual for me, Spinfit was in the Goldilocks Zone (keeping with the stellar theme, the link is to NASA). Spinfit tips tightened the bass a bit, giving it nice diction, whereas before the bass was a bit smoothed, with a little bit of missing detail. The Spinfit also lends a bit more starch to the treble’s collar—I like it crispy, Zankou chicken.

The Sirius has a rich tonality with a deep rich bass, warm rich mids, and clear treble—but with some sound surprises slicing in. It’s a sound signature dripping with a Julia Child size dollop of butter, but every once in a while the matron of French cooking grabs the knife.

Funny the World

With the Sirius I sometimes heard a bit of harshness in vocals. With Keith Greeninger – The Harder That We Love the vocals were pushed forward more, introducing some harshness and making Keith sound shouty at times. On Smoke & Mirrors Percussion Ensemble – Udakrep Akubrad, I had some harshness in the treble and some percussion instruments that sounded unpleasant. I’m a big fan of percussion music, so having glockenspiel sound harsh isn’t good.

Listening to Bright Eyes through the Sirius was like listening to his albums completely anew. If the Brakeman Turns My Way had excellent clarity, instrument separation, and soundstage without feeling like any frequencies were given too much weight. Bass had great texture. Hot Knives threw down well defined characterful guitar strums. On Lime Tree echoes in the soundstage haunted while violins emoted with bow and strings suspended in a deep and wide stage. The sound of the Sirius on Bright Eyes was organic and emotionally moving.

Djenting through Animals as Leaders – Ka$cade I was impressed with the speed and articulation of the Sirius. On 2Pac – God Bless the Dead, stage depth and instrument separation peel pack delicious layers of rappers. The Sirius really showcases the rapper multiplicity and depth. I’m nodding my head along in my beat up office chair, wishing I had hydraulics on my work desk.

Some tracks from my listening sessions:

  • Bright Eyes – If the Brakeman Turns My Way, Hot Knives, Make a Plan to Love Me, Lime Tree
  • Animals as Leaders – Ka$cade (Tidal HiFi)
  • Pink Floyd – Wish You Were Here (DSD64 SACD Rip)
  • Smoke & Mirrors Percussion Ensemble – Udakrep Akubrad, Juego de Relojes (DSD256)
  • Roger Waters – Late Home Tonight, Part 1
  • Daft Punk – Giorgio by Maroder (24/88)
  • Paul Simon – The Werewolf (Tidal HiFi)
  • Keith Greeninger – Harder That We Love (DSD128)
  • 2Pac – God Bless the Dead
  • Michael Jackson – Billie Jean (24/96 USA Vinyl Rip, 18DR)


Echobox Finder X1 (Comply)

The Echobox Finder X1 has a smooth natural sound, with good speed, as does the Sirius (Spinfit) on most tracks. Both have good detail retrieval, but given the v-shape sound of the X1, the mids are more satisfying on the Sirius. The soundstage is significantly bigger on the Sirius with more detail and better instrument separation. On Pink Floyd – Wish You Were Here, vocals were more delicate on the Sirius with a touch of sweetness. Vocals and guitar didn’t sound quite as natural on the Sirius. The Sirius had a bad time of it on Smoke & Mirrors Percussion Ensemble – Udakrep Akubrad, though, with some fogginess in the bass and some harshness in the treble.

On Roger Waters – Late Home Tonight, Part One, soundstage is the name of the game, and the Sirius kicks the Finder X1 around the block on this dimension. Again, I find the vocal and string tone a bit sweet, with some hints of harshness in the violins. The sound is really coherent on the Sirius. The Echobox Finder X1’s v-shape doesn’t do it any favours on the mids. Something I’m noticing the more I listen to v-shape signatures, is that they don’t hold up to volume matching. Mids will basically always sound recessed and a bit unnatural. I think v-shaped headphones are made for when you are wandering the street with ambient noise messing up your frequency response. A v-shaped headphone in that situation sounds more balanced. With the comply foams the isolation on the X1 is stellar.

On most tracks, the Sirius was significantly sonically superior, but it didn’t play well with everything I threw at it.

64 Audio X2 (Spinfit)

On 2Pac – God Bless the Dead, the Sirius has a thick, present bassline with excellent kick. The multiple levels of rappers are suspended beautifully, but the X2 just about matches it. The X2 has a touch less resolution on the layered rappers and slightly less depth on the soundstage. I felt like it matched the Sirius on width and height for this track. Something I noted on this track, the Sirius sounded thin and mechanical when the volume was turned down, so I’m not sure that the Sirius will play well with quiet listeners.

On Michael Jackson – Billie Jean, it is apparent that the mids on the 64 Audio X2 are more forward, giving a nice presence to Michael’s voice. The soundstage is great. Drum impact really rocks on these phones. Violins are perfect weapons of emotion rather than being pointy instruments of sonic doom. Switching to the Sirius, snares slam like M80 firecrackers, Michael is a bit further back in the stage and the stage has more width.


Not all stories are written in constellations, poured over by astrologers and other augurs of signs and portents, most stories now are written on the internet. Banal stories on youtube, twitter rants, and journalistic endeavours from talented writers spinning stories of bat and ball, pads and impact on my favourite Seattle sports sites. Producers of high end audio gear tell their stories on the internet, too. I feel compelled to give voice to these stories of industry told through statistics, sensitivity, driver counts, frequency response and small or large doses of marketing stimulants and bromides.

Fidue’s website is difficult to navigate, and insufficiently populated with data on the Sirius A91 for my liking—it’s your flagship, put it on the front page and have good links to information on it—so I have relied on HiFi Headphones for this section of our tale.

Equipped with a 5-way hybrid driver combination, including four custom-made premium armature drivers plus one exclusive dynamic driver, the FIDUE SIRIUS A91 IEM offers reference-class sound with outstandingly accurate and detailed imaging.

As the Flagship model of the FIDUE range, the SIRIUS A91 uses four custom made balanced armature drivers along with a custom built super bass dynamic driver unit. This unique design results in a superbly detailed, accurate and neutral repodruction with clear and enhanced trebles while the custom made dynamic driver adds great warmth in the low frequency spectrum without any distortion. The detachable cable is an 8 core, silver plated, ultra-low internal resistance audiophile balanced cable, with FIDUE’s 4th generation MMCX locking design.

With beautifully sculpted full metal shells with a striking, finned design. A selection of single, dual-flanged silicone and foam eartips provide a snug fit and excellent noise isolation for unparalleled comfort even in long listening sessions.

Price $899 (£749 at HiFi Headphones)
Drivers Single Exclusive Super Bass 10mm Dynamic Driver / Four Custom-Made Balanced Armature Drivers
Frequency response 4Hz – 45kHz
Impedance 20Ω
Sensitivity 113 dB
Rated power 30 mW
Distortion <1%
Main cable 1.3m Braided 8 core silver-plated ultra-low internal resistance audiophile balanced cable (2.5mm Rhodium plated jack plug) with a 4th generation locking MMCX connector
Adaptor cables 2.5mm TRRS female to 3.5mm TRRS male (balanced) Rhodium plated plug

2.5mm TRRS female (balanced) to 3.5mm TRS male (unbalanced) Rhodium plated plug


The Sirius is a stunning package with some special capabilities in soundstage and resolution that make it worthy of flagship status. However, this dog star for me had too many howling dogs in July and not enough beacons in the night. On some vocalists and instruments there was harshness. The bass could be temperamental and I’m not convinced that I didn’t have some times where my ear partially occluded ports and changed the signature slightly.

If you like a warm rich sound, the Sirius might be right up your alley. For those looking for a more neutral/bright signature, which is more my style, it probably isn’t the flagship for you. It looks, feels and sounds premium. It has an excellent soundstage and great clarity and detail while having a rich tonality in a very coherent presentation.

That said, it wasn’t really my flavour. The rich sound was a bit too rich for my blood, and compared to the Echobox Finder X1 and 64 Audio X2, I found the sound quality to only be about a 5% improvement in ideal conditions, and on songs that faired poorly with the Sirius, less than that. I understand that when we get into flagship territory, diminishing returns have already ravaged your wallet, flipped it over and ravished it again, but this year, there are other headphones I would put on my buy list at lower prices than this one for my leaner sound preferences.

3 thoughts on “Fidue Sirius A91: rich cosmic body, and an illuminated stage amongst the void with some anomalies to starboard

Add yours

  1. Hello,

    This is an excellent review, I really enjoyed reading through and made me felt as myself had been through your journey of listening the A91!

    Since I own the Echobox Nomad, which is the big brother of the Finder and sharing the same PEEK driver, I felt that a couple of points can be considered to your review where applied to the Finder X:

    1. Finder’s cable is stock cable which is no where near the cable of the A91, so perhaps on this subject alone Finder could have performed much better? (I swapped Nomad’s stock cable to A83’s balanced cable and resulting improvements are phenomenal)

    2. I found the Nomad hard to drive unless using a dedicated DAC/AMP – which I am now very happy had found AK’s XB10, not sure if this is the case also for the Finder? Maybe a good DAC could drive Finder even better?

    Anyway, on something else, most importantly – can you do a review on Echobox’s Nomad one day? It is priced at $399 and apparently have a frequency range of 5-60k (pre-production unit was on 15 – 35?)! After reading your review on the A91, it seemed Echobox’s Nomad easily can rival Fidue’s $899 Flagship? (I am sure Echobox would happily loan one to you and I think it is a great IEM not many people realise of its existence.)



    1. Hi Alex,

      Thanks for the comment! I’m really glad you enjoyed the review. I’ve talked with Echobox about reviewing the Nomad but it hasn’t happened yet and our review queue is loaded, so I’m not being pushy.

      Maybe it will happen in the future. I no longer have the Sirius, but it didn’t impress me hugely, so I wouldn’t be surprised if the Nomad beats it.



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