Fidue A91 Sirius Review
Headfonics 2016

Fidue A91 Sirius Review

Sound Impressions


The A91 projects one heck of a cohesive and balanced presentation with very little by way of odd tuning quirks or annoying peaks from top to bottom. It is beautifully controlled in that respect with excellent sub bass extension and just the tiniest bit of 5-7k emphasis; just enough to bring in some welcome sparkle to a natural sounding and very spacious signature.

I really love how Fidue has tuned the A91, keeping the bass deep but resisting the temptation to go for a crude midbass slam and ensuring the BA’s have enough space and separation so they do not close the whole stage right in.

Speaking of staging, this is one of the most holographic soundstages I have heard in ages from a universal IEM with very good imaging indeed. It is certainly superior to other hybrids such as the UM Merlin which also has a large soundstage but sounds a little less dynamic and much warmer in comparison.


The A91 has virtually no roll off in the sub-bass and hits a subtle but sustained peak around 40k which is only 2-3dB higher than 20hZ and keeps it right there until around 150Hz where it starts dropping into the lower midrange by around 5dB. The elevation is very subtle so you will not hear any overblown mid-bass slam.

The brilliance of this dynamic driver it more felt than heard right down sub-100Hz. Bass fundamentals are excellent, with a mild and natural decay. The A91 bass has a very nice sense of PRaT and musicality, it never sounds thin as a result.

That post 150hZ drop is very slight, nothing scooped out or uneven but just enough to prevent the bass from sounding boomy and bleeding into the lower mids.

Initially, you really won’t find the A91’s bass response omnipotent and heavy handed. That lack of mid-bass elevation means trance and higher pitched EDM perhaps will not have the same type of kick of say a JVC FX-850 or the general thickness and weight of the Andromeda from Campfire Audio.

However, when it’s called upon in tracks such as AES DANA’s “Peripherics” and especially in lots of RnB bass mixes, it really makes its presence felt much like a long throw 15″ in the back of Fiat Uno.


The lower mids of the A91 dips ever so slightly from 150Hz to around 800Hz before a sustained and plateaued elevation from 1-3k. Dialing down the lower mids energy helps keeps the A91 clean and clear though it does mean instrumental weight is more neutral than rich sounding.

Outside of that 5-7k treble peak the 1-3k elevation is on par with the bass performance and combined with the lower mid dip it gives a nice sense of forwardness to its vocal performance without it sounding artificially boosted.

It is subtle thing, not as pronounced as the Andromeda which drops by 2dB from its more prominent bass performance and that helps the A91 keep everything nicely coherent.

Fidue has also pulled back the upper midrange very slightly on the A91 by around 1-2dB from 2-4k allowing percussion attacks to sound smooth and sibilant free and relatively neutral sounding.

This can swing both ways depending on your preferences. For me, I am very sensitive to trailing percussion particularly splashy cymbal work but others might want a bit more presence in this region. Female vocals that hit this range also sound very clear and smooth with excellent detail as well as being beautifully sibilant free.


Fidue has wisely resisted giving the A91’s lower treble presence region any artificial booting. If anything they have actually dipped it out a bit more by around 3-4dB compared to the upper treble which sounds more prominent as a result but not overly boosted in its own right.

As a result, the treble performance has a better upper treble brilliance and sparkle with a smoother sibilant lower treble that is essentially non-fatiguing.

It doesn’t have the same level of elevation as Andromeda’s brilliance, and it does fall away a little quicker in that respect, but those looking for a slightly more relaxed treble offering should find the A91 to be more than accommodating.

Fidue A91 Sirius Review



The A91 is rated at 13 ohms and 113dB which is just on the right side of sensitivity for me and unlike say the Campfire Audio range it is not so sensitive as to give you hiss on even nominally efficient amping solutions.

On the flip side, it does not need a whole lot of juice to power it but I have noticed that tonally the A91 is a little source dependent. How much of that quality sub bass and midrange you want to shine, or how warm or how neutral you want it will be will be somewhat at the mercy of your source.


FiiO X7 vs Cayin i5

As a contrasting example, two quality DAPs I did most of this review with, the FiiO X7 and Cayin’s new i5 produced two different tonal presentations with the A91.

The X7, with the AM3 balanced module, allowed both single ended and balanced output testing sourced from that great ES9018 DAC they stuffed into the main X7 unit. The Cayin i5 uses an AKM AK4990 solution which is a really nice musical DAC and slightly warmer than the more neutral Sabre in the X7.

The X7 with the AM3 gave a very smooth and evenly balanced presentation with nothing overly emphasized right across the range and a nice clean and clear midrange and a smooth and unfatiguing top end with a very nice level of sparkle indeed.

If you are looking for “audiophile” then this pairing is pretty good at the mid-fi level. The realism is top notch, the timbre is accurate and only a minor hint of sibilance from time to time on the vocal delivery.

What it is missing though it a little bit of low-end slam on the A91 with the X7 that I know it is capable of delivering and this is where the Cayin i5 comes in with its more musical presentation.

You still have plenty of detail with the A91 pairing but this time, the Cayin i5 tweaks the sub bass with a bit more fervor so the A91 sounds weightier than the X7 combo with a slightly warmer hue. It is a more musical combination than the X7 bringing back those richer tonalities that I missed slightly with the FiiO combination.

Would I trade the i5/A91 pairing for a more balanced and cleaner presentation that I got with the X7? It depends really on the earphone I am using and the genres I select, but not with the A91. I think you are taking away a little of what I love about the A91 and that is its musicality and sub-bass performance.

AK240 vs AK380

Raise it up a notch, say between the AK240 and the AK380 and again the A91 tonal presentation still shifts according to the traits of each DAP and in some ways I had the Cayin/FiiO issue repeat itself.

On the AK240 in balanced mode, you get wonderful detail and incredible imaging but the focus is more on the midrange and a somewhat leaner signature than you will find on the AK380. The bass, controlled in ways, losses that low-end heft and I for one miss it too much to find the AK240 to be a “match made in heaven” as a source/amp for the A91.

The AK380, with its sassier AKM AK4990 dual DAC setup, brings that low end back making the A91 sound altogether smoother and more musical to my ear. Importantly the dynamic driver is delivering right to the very low end as it should be, vocals are richer sounding and treble is less brittle.

I had that same feeling also with the Cayin i5 vs the X7 and the Cayin sports the same DAC as the AK380 so it may well be the A91’s DAC of choice is the AKM AK4990.

Fidue A91 Sirius Review

Select Comparisons

UM Merlin

Close Competitors

The UM Merlin I have is the original custom version released way back in 2011. Today you can buy the universal UM Merlin for a similar price of $879, very close to the A91 and it is also a hybrid single dynamic and quad BA design. It is probably the closest in terms of price and design to the Fidue A91 Sirius.


First and most importantly the sub bass on the A91 has more presence and hits harder than the warmer and politer UM Merlin.

At last, a 5 driver hybrid that takes away the sub bass crown from my old Merlin. The A91 dynamic driver is also a little tighter for me than the Merlin with a touch more pace. Both have a very natural sound to them but the decay on the Merlin is a little longer.

Mids & Treble

Mids and treble on the A91 are cleaner and slightly more forward sounding than the more laid back and warmer tones of the Merlin. The Merlin does have some edge in the 5-7k range but its relatively mild and in keeping with its more relaxed nature.


The soundstage on the A91 is also wider and deeper than the Merlin, in fact, the Merlin sounded a little narrower in the mids in comparison to A91 which I never really got an impression before in other comparisons.

One thing to note though the A91 is slightly less forgiving on bad mixes and matches, that warmer tonality makes the Merlin a safer choice if you throw anything sibilant at it.

Campfire Audio Andromeda

The Andromeda is an all BA design (5 drivers) and retails just a bit higher at $1099. In my review of the Andromeda, I stated it has one of the best tonalities I have ever heard in an IEM.

That opinion remains unchanged but the A91 can rightly take a seat to the left or right of the Andromeda because it feels like a natural complement to Andromeda’s signature. It is like a tradeoff between the two of them in some respects.


The bass performance on both is huge. Really impressive how a BA design keeps up so well with the dynamic driver.

To me, the A91 has the more controlled and natural sounding bass with that awesome sub bass reach but the Andromeda, in return, comes back with a gloriously weighted and thicker meatier sound that really rumbles right the way up to around 300Hz and a few dB higher than that of the A91.


It is indeed a thing of beauty and works wonderfully for a wide range of genres where you require a bit of grunt. But here is the thing, I actually prefer the A91 with genres such as RnB where I want mid to upper bass control and where the sub-bass is more prominent as a result.

It creates a lovely contrast with strong vocals that RnB tend to have and sound better with the slightly warmer tone of vocals coming from the A91.

The Andromeda masks that contrast over a bit more due to the greater mid-bass energy and weight it throws out. I just prefer the A91 separation a bit more in that respect and would veer to the Andromeda for rock and dubstep instead.


Treble on the Andromeda is world-class with incredible extension and detail. The A91 treble is good, nice smooth, and sibilant-free but not as airy sounding or articulate as the Andromeda. It is an area that the Andromeda is hard to beat.


One final note is the Andromeda is much more source-dependent than the A91 and more sensitive to boot. The one Achilles heel of the Andromeda, as a result, is hiss and background noise which will pop up on any mildly inefficient setups which the A91 doesn’t seem to suffer from.

Sennheiser IE 800

Build & Fit

Sennheiser’s flagship IEM is a $750 single dynamic driver in a tiny shell that doesn’t really fit in your ear that well and has an awful cable that serves no worldly use.

I got used to it, added a pair of custom tips on it and we motor on. In stock form, the A91 is far more comfortable with a far superior cable with accessories and has a far better seal. With a pair of Snugs Custom tips, the seal on the IE 800 is world-class custom level. Just a pity the stock cable is not detachable on the IE 800’s.


The IE 800 does sound very good though despite its slightly brittle treble and is very suitable indeed for classical and orchestral works. I see it as more of a niche IEM and not an IEM that I can just apply liberally to every genre.

It is very much a hit or miss with the IE 800 but with more hits than misses with a relatively warmer than neutral and detailed source such as the AK380.


Both have excellent and spacious bass responses, however, the A91 is the slightly better sub-bass performer with the IE 800 preferring to show off its prowess a little further up around 80-100hZ giving it a slightly more pronounced mid-bass elevation and a warmer sheen to its bass. When truly tested at that 50-100Hz level the Fidue has the edge.


Mids are the polar opposite with the IE 800 showing a scooped-out recessed midrange from 1-4k and the A91 going in the opposite direction with a subtle elevation with a more forward mid-performance bringing the stage in a touch closer.

The treble on the IE 800 is much thinner and more brittle sounding but it does have excellent extension and articulation with great detail.


Treble on the A91 perhaps doesn’t have the same level of air as the IE 800 but it is a touch smoother and less energetic. In comparison, the A91 is the most forgiving of the two and I would be reaching for it over the IE 800 for lossy formats or modern genres such as RnB.



Sort of a left of field comparison given the A91 is about 3 times the price and has a very different design but when I first heard the A91’s sub bass response it reminded me so much of the FX-850’s single driver’s low-end signature I had to go back and take another listen.

Yup the FX-850 also has an excellent extension with a really good sub-bass to midbass performance but the A91 is even better with a more tangible presence, better body, and just slightly quicker than the FX-850.

Both want to reach deep but the FX-850 just slightly rolls off earlier than the A91 which has a more sustained plateau. Further up the FX850 has a more elevated and warmer mid-bass than the A91 which trades that off for a mid to upper bass which is a bit more subdued but with better clarity

Mids & Treble

In other aspects, the FX850 fares less well with a recessed and less dynamic midrange that lacks detail compared to the Sirius and a sparkling top end that is a bit peakier sounding than the A91’s smoother more detailed treble response.

Vocals on both are very good but the JVC is a little more distant and softer in comparison to the greater accuracy generated by the A91. Both have spacious soundstages but the A91 is the most holographic of the two with superior imaging.


It is a single dynamic driver and with it does come a few natural limitations that the hybrid design of the A91 can overcome such as speed and detail. The fact JVC has sought fit to tune a v-shape won’t be everyone’s cup of tea either though I do love the natural-sounding timbre of the driver.


Our Verdict

It is kind of hard to take in the Andromeda and then a few weeks later the A91 and figure out which is the best, luckily I do not have to but if you have a grand in your pocket and looking for a universal IEM then these two should be on your radar.

What the Andromeda doesn’t have, the A91 seems to have, and vice versa. I had no real intention of making this a versus-type comparison but as I sign off on this review it’s what comes to mind. For some, beyond technicalities and tonal qualities though the more competitive pricing of the A91 might tip the hat in its favor.

On its own, the A91 is incredibly good, and very smooth, with wonderful sub bass, excellent soundstage, and world-class accessories. It looks fantastic, comfort is excellent and the swappable cable extensions make this a very useful quality cable system.

Nitpickers might point to the shallow depth of the nozzle, the weight of the cable, and perhaps a lack of truly great treble extension as areas to be concerned. I get that, nothing is perfect in this world but if you enjoy musical-sounding universal IEM this one takes some beating.

Mike said it in the Focal Elear review and I feel it is worth mentioning that sometimes value doesn’t fit the product, to be frank, sometimes it is painful to see where prices are going but in the case of the Fidue A91 Sirius I actually think the price to value equation is very balanced and the asking price is worth it.

I think you get plenty for your money sound-wise, features, and accessories wise and I am pretty sure we will see the A91 in our top 10 lists end of the year as a result.

Fidue A91 Sirius Technical Specifications


  • Drivers: Single Exclusive Super Bass 10mm Dynamic Driver / Four Custom-Made Balanced Armature Drivers
  • Frequency Range: 4Hz – 45kHz
  • Impedance: 20Ω
  • Sensitivity: 113dB
  • Max Input Power: 30mW
  • Distortion: <1%
  • Plugs: 2.5mm/3.5mm Balanced and 3.5mm stereo, Rhodium plated
  • Main Cable: 1.3m, 2.5 mm TRRS balanced (including 2 conversion cables: 3.5 mm TRRS balanced and 3.5 mm TRS single-ended)
  • Conversion cables: 0.18m


  • 1X Pair FIDUE SIRIUS A91 Reference Class 5-Way Hybrid In-Ear Monitor
  • 4X Silicone ear tips (XS/S/M/L)
  • 1X Cleaning tool / disassembling tool
  • 1X Comply foam tips (M)
  • 1X Flight adaptor
  • 1X ¼ inch stereo jack adaptor
  • 1X Luxury aluminum carrying case

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