64 Audio Duo

64 Audio Duo Review

64 Audio Duo is a dual hybrid driver IEM featuring an open-back design with apex Core, tia, LID, and Wave Sync technology. It is priced at $1199.

Disclaimer: The 64 Audio Duo sent to us for this review is a purchased unit and does not have to be returned. We thank the team at 64 Audio for giving us this opportunity.

To read more about 64 Audio products we reviewed on Headfonics click here.

Note, this 2-page review follows our new scoring guidelines for 2021 which you can read up on here.

64 Audio Duo
64 Audio Duo
I am fascinated to see where 64 Audio will go with the new Duo technology and improved build quality, be it upwards with more complex offerings that nail the detail or downwards into even more affordable options that would please an even wider audience. Either way, it's a promising and immensely likable beginning.
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This has been a productive year thus far for 64 audio with the launch of the U18s and then the U6t earlier. However, one could argue that both are evolutions of previous older models with new tech and tunings. The new Duo universal monitor though is something completely new.

Not just a completely new monitor but also the most affordable IEM within the current 64 Audio lineup at $1199 and sits just below the U6t in their universal lineup.

So, what does new mean? At a high level, we are talking about 64 Audio’s first-ever open-back designed in-ear monitor not to mention a new apex system, and a very different crossover methodology called Wave Sync. 

I have done open back reviews before but these were the planar driver iSINE10 and LCDi4 monitors so quite a different proposition to the hybrid Duo. 

64 Audio Duo
Copyright 64 Audio 2021

Tech Highlights


The Duo is a dual hybrid driver monitor featuring a 9mm dynamic driver and a single tia high open balanced armature driver implementation.

This is not the company’s first dual-driver IEM with the custom A2 and the universal U2 launched was back in 2015 as part of the company’s switch from their previous name 1964EARS to 64 Audio. However, it is their first hybrid dual driver and certainly the first dual within the Tia range.

The grouping implementation is fairly simple with the dynamic driver covering the lows and the mids, the tia high open BA covering the highs, and an integrated 2-way passive crossover.

The Duo is rated at 9Ω and a relatively current demanding 98dB SPL rating which makes it a bit more juice hungry than some of its peers. That low dB rating is often a consequence of using open vented BA drivers which can be harder to drive than sealed enclosures. You can read more about that in our synergy section on page 2 of this review.

apex core

This is probably the stand-out feature to me and what makes the Duo so different from other current 64 Audio IEMs.

The Duo does not use the swappable apex system, much like the tia Trio does not either. However, the Trio uses an enclosed m15 apex filter whereas the Duo does away with obstructed airflow issues by creating a very striking looking open back perforated grill design. 

Combine that with an entirely tubeless large tia bore design with no obstructions in the front of the two drivers internally and ideally, the Duo should deliver a resonance and distortion-free sound closed to an open-back headphone experience than a typically enclosed driver monitor implementation.

Wave Sync

Going back to that 2-way passive crossover, the Duo has a fairly high crossover point which looks to be around 5-6k on the measurements. Given you are moving from a large dynamic driver to a single BA 64 Audio has implemented what they call Wave Sync technology to keep a lid on any phase control issues.

The key to Wave Sync is a type of “all-pass” filter up the gain in all frequencies equally but controls the phase relationship among various frequencies to allow 64 Audio to correct timing issues without having to physically move the drivers. A particularly important aspect considering the Duo is not using any traditional variable tubing lengths to compensate.


Tia stands for tubeless in-ear audio and is a key unique selling point for the entire tia branded lineup of 64 Audio monitors and first seen in 64 Audio monitors around late 2016/2017. 

At the heart of tia is an open balanced BA design and is pitched as being far more open in sound than a sealed and tubed BA driver which quite a lot of competing monitors still use. Of course, tubeless drivers are now more prevalent in the market but each has its own unique implementation.

With the diaphragm free of obstruction, the contention is that a lot of the vibrations and resonances you get with closed or tubed designs will be gone.

No more tubes and no more dampers leave more room for the chamber design and a switch to a single wide-bore nozzle. Wide bores produce a more natural sound to my ears than individual channels delivered in tubes right up to the tip.


It is not openly pushed on the Duo page apparently an effort to give more priority to the new apex core and Wave Sync technologies. However, the Duo does use LID in much the same way as the other 64 Audio Monitors.

The full name for this feature is Linear Impedance Design. Basically, LID ensures that whatever the impedance level is on any given output you plug the Duo into it should not have a low-Z skew and should play true to its correct tuning.

That means for those holding a 1st gen HiBy R6 or a Shanling M2s with their output impedance ranging from 4Ω to 10Ω there will not be an impedance mismatch and the Duo should sound accurate or ‘true’. This is less of an issue on modern DAPs in 2021 so I am not surprised that LID tech is not front and center for the Duo pitch. 

64 Audio Duo



From afar, the Duo follows the same well-engineered form factor as the other universal IEMs in its lineup with its compact quasi-teardrop style faceplate and smoothly curved black shell and nozzle. 

However, dip in a bit more and you will notice from an engineering perspective the 3-piece Duo has some definite build upgrades, even over the higher-priced tia Trio and U6t. 

The most obvious changeup is the open back faceplate which is both unique and striking. There are two components to the plate, the anodized black stainless steel 304 grill which is chemically etched for perforation, mold and die-stamped into its shape, and then inserted in the main aluminum plate which includes the plate ‘fenders’ as I have previously described them.

The more nuanced change is the connection of the faceplate to the shell which is almost seamless in comparison to the Nio, U6t, and the tia Trio. It is a very clean finish with less of a perceived ‘fender’ bulge so it rounded the corner even tighter than those previous models. 

To finish it off a thin chrome stainless-steel bezel frame is inserted on top of the grill primarily to keep it secure but it creates a nice aesthetic similar to a headphone enclosure ring. Also, because the Duo uses the apex core system there is no filter port on the plate itself keeping the visual very clean and uniform. 


The shell body itself is cut from a 6063 series aluminum raw square stock and finished in a very similar fashion to the tia Trio. It is not quite as matte black as the U6t shell but not as polished looking as the Nio version.

Probably due to it having only two drivers, the shell is the shallowest I have encountered yet from 64 Audio. The tia Trio is marginally deeper with 3 drivers with the Nio and the U6t being fairly deeper.

64 Audio Duo

Comfort & Isolation

Plenty of comfort but not a huge amount of isolation but better than I was expecting. Given the Duo is an open-back design it should come as no surprise that passive isolation is lower than other 64 Audio monitors at only -12dB which is only 2dB higher than the most open interchangeable apex filter, the mX.

With a swappable filter 64 Audio universal monitors, you can go up to 20dB noise attenuation with the m20. Just bear in mind that the Duo was never designed with big passive noise cancelation in mind so if you need that I suggest you look at the likes of the U6t, Nio, or even the tia Trio.

In your ear, the experience is fairly similar to the tia Trio or the U6t, perhaps a bit flusher to the ear due to the shallower shell. You will still find more of the fitting experience tip-dependent with that fairly elongated plate to nozzle insertion.

The stock foam tips do quite well in terms of keeping the Duo secure and do isolate the best out of all the supplied tips. However, they do apply a bit more pressure than the alternative single bore silicone tips and I find them less comfortable.

Of the two silicone tip varieties supplied, the narrow bore does a better job in terms of fit and isolation compared to the wide bore which I just could not get either a good fit or any isolation at all from.

I would also recommend Final E tips. They do seal better than the stock tips but they will change up the sound signature offering excellent low-end slam but with a thinner mids and feistier treble. 

64 Audio Duo

Stock Cable

I guess the exclusive is exclusive no more with the Duo using the same new Premium cable that came as stock with the U6t. Still an upgrade though on the older high resistance version that came with the tia Trio and having used it with the U6t for a few months now it does handle really well.

This is a larger 26AWG silver-plated OCC copper wire compared to the previous 28AWG SPC geometry though still a 4-wire geometry as with the original.

As you can expect with a larger gauge the cable will offer a better resistance performance and indeed on paper it is rated at 0.23Ω compared to the standard Premium cable’s 0.28Ω specification. 

Finishing & Handling

Aesthetically, the Premium cable has a more typical and slightly bigger aftermarket visual with a soft and shiny black braided jacket. Despite the enhanced girth I actually find this cable more comfortable for handling compared to the original’s harder finish. 

The finishing loses the older right-angle beefy plug and replaces it with a smaller straight 3.5mm TRS jack. A new elongated and branded black aluminum splitter and cinch replaces the older rubbery disc/cinch also.

Lastly, no retentive memory wire which is replaced by the newer softer springy type which I much prefer. That also helps deaden the cable a bit more for microphonics compared to the original.

The Duo cable also does not tangle as much as the thinner Premium cable. Not that the original was memory retentive, if anything both display the same properties in that regard. Rather when rolling and unrolling the new exclusive version just flops out whereas the original got in a heap of tangles and knots. 

64 Audio Duo

Packaging & Accessories

The Duo packaging is very similar to the U6t retail display box, in fact, save for the graphics, it is virtually the same size, layout, and that black finishing. Nothing wrong with that in all honesty since it’s a nice design, with good protection and a decent internal layout. 

The outside is the company’s signature black and white graphical layout with the monitors front and center. Inside you have the customary introduction from Vitaliy under the lid with the case, monitors, cable, and new carry case on full display inside a contoured foam base. 

Aside from the mentioned tips and tray holder, the accessories also include a collar clip for the cable and a small cleaning brush tool. There are no apex modules this time though due to the use of the apex core system inside the Duo.

64 Audio U6t

Carry Case

Yup, it’s the U6t ‘Premium Leather’ case but again, that is not a bad thing as it’s pretty good quality. This is a rounded high-quality simulation leather and black stitching 2-piece puck design. It is soft but strong on the outside with that distinctive 64 Audio logo embossed on the front.

The inside is fairly spacious with plenty of room to fit in a cable or two, the monitors, and even the spider-wed tips tray on top. The sides, top, and inside of the lid are padded with a cloth material so your monitors will not suffer from any undue knocks from day-to-day carrying. 

It is marginally too big to be easily pocketable which smaller plastic containers or soft pouches could offer but it will have no issues being thrown into small bags on the go.

Click on page 2 below for sound impressions and select comparisons

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