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64Audio Duo Review – Fun, Forgiving, Fabulous

Pros –

Great build quality, Comfortable with zero wearing pressure, Powerful and impactful bass, Coherent midrange, Airy and extended top-end, Deep soundstage, Linear impedance design

Cons –

Below average isolation as expected, Separation leaves to be desired, Midrange isn’t as resolving as many competitors

Verdict –

If you enjoy a coherent, bass-forward sound, are averse to IEM wearing pressure but don’t want to miss out on top-level treble extension, the Duo is without equal.

Introduction –

I think by now the vast majority are familiar with 64Audio. Even if you’re a newcomer in the hobby, you’ve likely seen the drool-worthy reviews of models like the U12T topping lists and recommendation charts around the net. More budget-conscious buyers can also delight that the company has gradually begun to implement the technologies pioneered by their flagships into far more affordable models. I recently reviews the U6t which proved this can indeed be a winning formula as far as price/performance is concerned. However, though powerful and engaging, that earphone was an all-BA design that some may not favour. The new Duo is now the company’s cheapest universal IEM and the first all-new model built from the ground up from 64 Audio this year. It introduces a dual-driver hybrid design whilst retaining their signature tia tweeter. In addition, the Duo assumes a semi open-back design that represents a divergence from what we’ve seen from the company before. Whilst those expecting a planar design may be a little disappointed, the resulting product is certainly no less special or unique in terms of overall approach and feature set.

The Duo just launched for $1199 USD. You can read more about it and treat yourself to a set on 64 Audio.

Disclaimer –

I would like to thank Logan and Charlie from 64 Audio very much for their quick communication and for accommodating this review of the Duo. All words are my own and there is no monetary incentive for a positive review. The Duo was provided on a loan basis and is to be returned to 64 Audio following review.

Contents –

Specifications –

  • Drivers: 1x DD low/mid, 1x tia high
  • Frequency response: 20 Hz – 20 kHz
  • Sensitivity: 98 dB
  • Impedance: 9 Ohms
  • Crossover: 2-way passive
  • Isolation: -12 dB (internal apex)

Behind the Design –

tia driver + tia bore

Image provided by 64 Audio

You’ve seen tubeless balanced armature drivers and anyone who has heard a good implementation will tell you there are indeed benefits to this approach. 64 Audio takes this one step further with tia, considered by many to be an industry-leading innovation in BA tech, by removing the entire roof of the driver. The company reason this provides a lower resonance delivery of sound whilst optimising transient response. The driver is then placed within the single-bore nozzle with no obstruction to the sound path. They reason that the short, uninterrupted sound path to the ear enables them to deliver a more linear and coherent frequency response. Combined with the unique driver, you get an extended, high-resolution image. This definitely isn’t just a marketing ploy as anyone who has experienced tia IEMs in person will agree the proof is in the pudding.

Wave sync

As tia drivers only cover the high and ultra-high frequency range, the crossover point with the dynamic driver is especially high relative to most competing hybrid designs. Wave sync was designed to optimise phase coherence under these circumstances by using an all-pass filter. An all-pass filter essentially allows the company to adjust phase and time-alignment across the spectrum without affecting the frequency response. In so doing, the company was able to achieve phase coherence between the two drivers at their optimal crossover point and do so without altering the physical geometry of the drivers. This provides the best-case scenario for both time and frequency domain tuning.

apex core

There are many challenges when designing in-ear monitors, one that is becoming increasingly spoken about is pressure relief. Increased wearing pressure is an especially large issue with in-ears as they form the tightest seal with the ear canal. This can alter the perception of sound over time via the tympanic reflex (causing tension on the eardrum) that results in decreased sound transmission to the inner ear – effectively reducing the perception of dynamics, bass and treble, and resulting in a much less resolving sound. Standing for air pressure exchange, apex is a similar style of technology as ADEL and ATOM, alleviating pressure on the middle ear. The core aspect means this differs from the usual vent system used in other 64 Audio IEMs. Instead, the Duo introduces perforated faceplates for less obstructed airflow and reduced physical fatigue. Of course, this also means they are no longer adjustable in this regard.


Image provided by 64 Audio

Another challenge of IEMs is that the popular and efficient BA transducer has a limited bandwidth, so multiple drivers (and types) are often utilised to deliver a full-range sound. This opens up impedance variation between the drivers meaning each will respond differently to elevated source output impedance – the rule of thumb being that output impedance should be no higher than 1/8th of the impedance of the earphone, any higher and the frequency response will be altered. Linear impedance design (LID) is as it sounds, a circuit that equalises the non-linear impedance of the multiple drivers in the IEM. This means the Duo has a flat impedance curve and will deliver the same sound signature regardless of source output impedance, an invaluable feature for professionals, especially those who require that consistent sound reproduction. This does not mean the Duo will sound identical from all sources of course, as each can be coloured and showcase different resolving power.

New Premium Cable

Like the U6t, the Duo no longer comes with a generic cable but comes bundled with a new premium cable with a low 0.23-ohm impedance. It has a 7x7x4 multi-twist construction and silver-plated OCC copper wire conductors. It has 26AWG wires making it feel almost like a custom cable in terms of size and sturdiness. Compared to the generic cables of old, the look and feel are in another league.

Unboxing –

The experience here very much reflects the U6t and all of 64 Audio’s newer IEMs for that matter. An outer sleeve with print and specs slides off to reveal a magnetic hard box. Inside are the IEMs within a foam inlet with the cable wound around a card guide below. I am a fan of this packaging as the cable doesn’t come kinked out of the box. Below is a leather 64 Audio branded carrying case. Like the U6t, the Duo comes with a 3D-printed ear tip holder with a range of options available to customise the sound and fit. Included are 2 pairs of TrueFidelity Foam tips, 3 pairs of Spinfits CP145’s and 3 pairs of generic wide-bore tips. A similar premium cable as seen on the U6t is also included, a huge upgrade over the generic options that came before. Finally, 64 Audio completes the experience with a branded sticker.

Design –

The Duo has a silhouette that will be familiar to those returning from other 64 Audio models. That means you get a compact metal shell with a rounded triangular profile. The Duo especially is quite distinct, adopting an entirely new design language otherwise. The wave sync perforated faceplate draws focus first. Constructed from 304 stainless steel with a black coat of paint, it forms striking contrast to the hyper silver rim. This gives the Duo a classic high-performance aesthetic that looks entirely unique yet still distinctly 64 Audio. The main housings are made from 6063 aluminium milled from a single block and noted superbly tight faceplate matching with a barely palpable seam between the two halves. The housings are very uniform and covered in a glossier finish than the sand-blasted U6t. Compared to other 64 Audio models, the Duo is also the slimmest universal IEM in 64 Audio’s lineup giving it a svelte and elegant look.

Up top are non-recessed 0.78mm 2-pin connectors that boast wide aftermarket support. The included cable is identical to that on the U6t which is a huge step up from the generic OFC cables included on former models. In turn, my experience here was much the same, the new cable is sturdy but not excessively bulky or cumbersome. The insulation is flexible with minimal memory but a slightly tacky feel. It is still a solid feeling cable that resists tangles well, just not as well as market leaders. The connectors are metal and angled at the earphone side to route the cable over the ear. The pre-moulded earguides are soft and well-shaped, contributing to a comfortable and stable fit. Though not quite as nice as some high-end IEMs, especially those who have teamed up with custom cable manufacturers, the new premium cable is a nice complement to the Duo and is ergonomically sound in all regards.

Fit & Isolation –

In terms of shape and height, the Duo provides much the same experience as other 64 Audio IEMs. This means if other 64 Audio IEMs give you trouble, the experience here will be very similar. Otherwise, I do find 64 Audio’s universal shells to be relatively accommodating. They are quite compact and all angles are nicely rounded to avoid hotspot formation. The Duo, in particular, boasts their slimmest housing yet achieves a distinctly lower-profile fit than their other models. For my average-sized ears, I was able to wear the Duo for hours on end without discomfort, albeit, this was also my experience on other 64 Audio IEMs. as the housings are height biased, you can twist them to place the bulk in the most deficient area of your outer ear to maximise comfort. Another key difference is isolation which is perceptibly worse due to the semi-open back design.

They aren’t purely open and do block some noise, but aren’t suitable for use in noisy areas. They can be used for commute in a pinch but wouldn’t be the best option explicitly for this. Where this system yields greatest benefit is with regards to comfort, as the Duo has essentially zero wearing pressure besides the actual silicone tips themselves pressing on the ear canal. This means those averse to IEMs may find the Duo to be an adequate experience, they are even more comfortable than the traditional apex sporting models that came before. In addition, the lack of isolation may benefit those wanting spatial awareness such as in the workplace. You can clearly hear your voices whilst blocking out high and low-frequency noises such as air conditioning to a greater degree.

Next Page: Sound Breakdown

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