64 Audio Duo

In-ear headphones with hybrid drivers for superior sound standards

In a nutshell

With the Duo, 64 Audio offer great in-ear monitors with a partially open baffle that sound great, even if they are less analytically tuned than the U series. These are high-quality headphones that are a lot of fun to listen to and are especially aimed at discerning home hi-fi lovers.


The most affordable in-ear monitor to date from 64 Audio goes its own way with its hybrid partly open design. The sound tuning is in line with the ideals of the American manufacturer but aims less at audiophile neutrality and analysis, and more at listening pleasure. But don’t worry: these are still headphones that deliver plenty of detail and monitor quality. You can expect the highest level of sound enjoyment in the price range above one thousand euros.

64 Audio is one of the leading suppliers in the field of in-ear monitors (IEM). The company, which has its own development and manufacturing facilities, concentrates on offering high-quality products for professional and audiophile customers. With a price of 1,199 euros, the universal fit model Duo is the manufacturer’s “smallest” product to date. Unlike the U-series, which relies on multiple balanced-armature drivers, here a hybrid construction is used – technology that is also used in the larger models Nio and Trio as well as in the flagship Fourté. In this case, a dynamic bass-midrange driver and a BA driver for the high-frequency range were combined. In addition, the Duo have a partially open rear panel, which allows the dynamic driver to operate more freely. The trick is to keep the bass stable and full range. So we were certainly curious about them …

In practice

The 64 Audio Duo adheres to a no-frills concept: a high-quality, robust and ergonomically shaped in-ear monitor that is placed directly in the ear with one of the numerous fitting pieces which are available in different sizes and materials, meaning it fits comfortably for a long time. The replaceable twisted cable with a 3.5 mm jack connection is routed over the ear to ensure the necessary secure fit.

The elegant design does not necessarily demonstrate its technical finesse: The Duo use the manufacturer’s “tia” system, meaning that the BA and dynamic 9 mm drivers can do without the typical sound-colouring tubes thanks to a deliberate hole in the housing. “Apex Core” addresses the problem of resonance in closed headphones. In this design, sound can escape via a perforated grille on the outside. Accordingly, the external isolation of about 12 dB is comparatively low, which means that the Duo are not suitable for stage use. In a sense, it is a partially open in-ear monitor. Furthermore, built-in “Wave Sync” technology is used, and according to the manufacturer, this ensures the drivers’ output at the correct time in combination with the crossover. Finally, “LID” is included; this, according to the manufacturer, ensures consistency from the headphones independent of the impedance of the sound source. Accordingly, the focus with these headphones is placed expressly on the listening experience.

Sound of the 64 Audio Duo


Our sound evaluation took place with an iPhone 8, a Shanling M3X (review here), as well as with an RME ADI-2 Pro FS (review here).

The 64 Audio Duo delivered a detailed sound image. They sounded balanced, with a high fun factor that was completely unstrained – bold and finely drawn. Depending on the mix, it sounded big, intimate and sometimes rather flat. A tonal assessment was possible, even if not quite on the level of more expensive, open over-ear designs.

In the bass, they delivered contour, pressure, dynamics and reproduced the necessary tonality. I would not describe the bass range as completely neutral but as slightly accentuated. However, this extra boost was not disturbing. Low bass was definitely present, but in this case without any emphasis – here the IEM was discreetly restrained.

Voices like in “Algorithms” by Muse or on Whitney Houston’s “Exhale” were clear, intelligible, full-bodied and had proximity. They were credibly surrounded by the attendant rooms or spatial effects. For example, the ghostly backwards echoes in “Lust” by Tori Amos came across wonderfully clearly.

Speaking of Tori Amos: the abrasive strokes of the double basses on her Schubert adaptation “Star Whisperer” immediately gave me goosebumps. The other classical instruments, from the occasional rumbles of the Bösendorfer grand piano to the other strings and the woodwinds, were authentically located and sounded impressive in the wide stereo panorama.

Pop and EDM also excel in the mid-range spectrum with high detail resolution. As so often, “Arthur Spark” by Yello was exemplary, becoming a colourful kaleidoscope of sound.

I also particularly liked listening to the metal with these headphones. Productions that tend towards the aggressive, like Meshuggah’s “Bleed”, fortunately, didn’t tend to become tinny. At the opposite end of the spectrum, Slayer’s “Repentless”, which is massively characterised by mids, lived up to the intention of the mix when played back.

In the high frequencies, the Duo again operate with plenty of detail and air without developing any disruptive sound from the BA driver. It was not ultra-transparent but always maintained a certain warmth in the upper range. At the same time, the tweeter delivered a speed that, beyond percussion, also made the timing between doubled tracks obvious and ensured a secure location in the stereo image. The Duo resisted annoying harshness but was able to reveal when a recording or mix was overshooting the mark.

There’s no doubt about it: this model will please not only music lovers but will also certainly tempt many a producer. As with any 64 Audio product, I have no hesitation in recommending these in-ear monitors as a replacement or, even more sensibly, as a supplement to studio monitors. The room doesn’t matter in this instance!

The agony of choice – Duo or U6t?

For an extra 200 euros, the 64 Audio U6t (review here) offers a solution that uses six BA drivers per side. Although the Duo uses a hybrid design with dynamic low-mid drivers, the general sound characteristics of both speakers are quite comparable. The U6t is a little more controlled in the bass but less punchy and full than the 64 Audio Duo, and this also partly applies to the midrange. I would describe the U6t as more neutral and analytically tuned and confirm that they have even better detail resolution and stereo projection. This model also offers better external isolation. Here, it is not only a matter of taste but also application.

2 years ago by Ulf Kaiser
  • Rating: 4.5
  • Sound
  • Handling
  • Price/Quality
  • Function

Technical specifications

  • Ear couplingIn-ear
  • Typeopen
  • Transducer principleDual drivers: dynamic & Balanced Armature
  • Frequency response (headphones)20 - 20.000 Hz
  • Impedance9 ohms
  • Sound pressure level (SPL)@1kHz: 98 dB

What's in the box

  • TrueFidelity foam ear tips (S/M/L)
  • SpinFit silicone ear tips (S/M/L)
  • Silicone ear tips (S/M/L)
  • Black premium cable
  • Sticker

Special features

  • Crossover: integrated 2-way passive crossover
  • Isolation: -12dB through internal apex technology
  • Connection: 2pin

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