64 Audio U4s

Audiophile in-ear headphones with four drivers

In a nutshell

The 64 Audio U6s are this manufacturer’s “most affordable” universal in-ear headphones to date. At 1,199 euros, they are no bargain, but they are a top audiophile product with a wonderfully detailed sound image that makes music playback a pleasure. At the same time, this product is aimed at lovers of hybrid constructions who appreciate the lush sub-bass of a dynamic driver. The 64 Audio U4s are capable of delivering a powerful but always controlled foundation. A listening test is highly recommended!

  • excellent sound quality
  • variable tuning due to four apex damping elements
  • warm bottom end

With the U4s, 64 Audio adds another excellent-sounding in-ear monitor to its product portfolio. With a recommended retail price of 1,199 euros, the “smallest” Universal Fit model to date relies on a combination of dynamic and multiple balanced-armature drivers. This top audiophile product delivers a wonderfully detailed sound image that makes music playback a pleasure.

Features of the 64 Audio U4s

The new U4s take 64 Audio, from Vancouver Washington, into the high-quality in-ear market. This model is a universal-fit (custom-fit) variant of the A4s, and as such, they are a hybrid design, using one dynamic driver in the bass and three BA drivers in the other frequency ranges. The suffix “s” stands for stage and indicates that the tuning is not completely linear but has a slight bass emphasis.

The workmanship of the U4s is of 64 Audio’s usual high quality, as is the choice of components. The housings are made of decorative and sophisticated-looking blue anodised aluminium. The black-grey outer decorative plate with the manufacturer’s logo is supposed to look like it is made from an iron meteorite. The twisted black connection cable is not a standard product either; it is made of copper with silver coating with an impedance of just 0.23 ohms.

In addition to the dynamic woofer, the low-impedance 64 Audio U4s use separate Balanced Armature drivers for the mids and highs and the manufacturer’s patented tia-BA (Tubeless In-Ear Audio) driver for the highs, all carefully coordinated by a 4-way crossover.


In contrast to the Fourté, which have a comparable design but are considerably more expensive, here other drivers are used as well as the tweeter. All BA drivers in the Fourté function without sound tubes and so are even more expensive to manufacture.

The housing design also uses variable APEX damping elements (Air Pressure Exchange), which can be deployed to further refine the sound tuning through controlled pressure equalisation. In fact, the external isolation and the low bass level can both be modified; the greater the isolation, the more intense the bass. For the first time, four such damping pieces are included: mX, m15, m20 and the new m12, which is only included in the U4s and makes tuning even more flexible. If you replace the pre-mounted m15 (15 dB external damping) with the m12, the bass is also lowered somewhat.


Finally, 64 Audio’s LID (Linear Impedance Design) circuitry is used, and this is supposed to ensure a constant frequency response and, thus, the best sound results, regardless of the output impedance of the sound source.

The 64 Audio U4s in practice

The 64 Audio U4s are simply in-ear headphones without any other extras. They are completely no-frills and designed for upmarket sound enjoyment as well as professional use. Apart from the cable routing over the ear, which may not be to everyone’s taste but guarantees safety on stage, the U4s offer excellent wearing comfort. The construction is robust and, thanks to an ergonomic design, fits securely and pressure-free in the external auditory canal. As usual, with products from 64 Audio, a wide range of fittings are available in different sizes and materials to ensure the best possible fit. The cable is equipped with a gold-plated 3.5mm mini-jack connector and a 2-pin connector on the receiver side, so it can be replaced at any time, for example, with a balanced version.

How does the 64 Audio U4s sound?

Thanks to the good passive external insulation of the 64 Audio U4s, the listening experience was intimate, unstrained and generally pleasant. The reproduction was exceptionally rich in detail and also wonderfully balanced. It became immediately clear that these headphones are in the audiophile sound category.

The fullness of sound that these in-ear monitors deliver, even at low levels, should not be taken for granted. With other headphones, I usually have to tweak them a fair bit more before I get a taste for them. Quieter levels are also an important argument for hearing protection. In practice, the U4s played smoothly in combination with a RME ADI-2 Pro, Focusrite Clarett audio interface and an ordinary Apple iPhone 8. Neither level reserves nor unsightly distortion were a problem here, thanks to LID.

I would describe the general characteristics of the U4s as high-resolution, agile and fast, excellently dynamic, spatially responsive and non-fatiguing. In our test, these headphones performed well in all genres.

Dynamic driver for low bass

The bass was striking. It was reproduced in a defined, dynamic and tonally comprehensible way down to the lowest registers and had a certain amount of warmth. The dynamic driver had advantages in low bass reproduction. However, it did not sound superficial; rather, it added the lowest frequencies as a matter of course. With Nick Cave’s “Galleon Ship”, the bass really shook. It also supported an acoustic bass drum when listening to jazz, for example, delicately but quite effectively, and moved additional air for a fuller sound impression. Depending on the material, real deep bass was a little too pronounced, for example, in “Pan Blue” by Yello. However, the effect can be reduced by using a different damping element.

Three balanced-armature drivers for the mids and highs

In the midrange, the 64 Audio U4s enchanted us with a convincing performance. For example, Anne-Sophie Mutter and the Berlin Philharmonic on “Beethoven’s Violin Concerto in D Major” inspired us with the authenticity of the orchestral timbres. Added to this, the intimacy, the tonal richness of colour and a sense of passion in the high registers of the solo violin. Character and realism were evident in the reproduction of Benny Andersson’s Fazioli grand piano, which sounded seamlessly embedded in the space of his Stockholm recording studio.

Even with dense pop arrangements, detail resolution remained at the highest level: On Daft Punk’s “Get Lucky”, it was possible to clearly hear the precisely placed and sometimes slightly animated positions of the percussion in the mix. At the same time, the dynamic driver showed its strengths by tracing the electric bass with a lot of depth and confident contours without disturbing the midrange. Whitney Houston’s flawless voice on “Exhale (Shoop Shoop)” was also wonderful, surrounded by a refined spatial effect. Listening to rock also showed excellent results: “Fire Your Guns” (AC/DC) pushed forward, sounded open and did not have disturbing harshness. The listener can’t help but bob along to the music while, at the same time, listening deeply to the mix. Rarely have I heard Cliff Williams’ bass so clearly. “Bleed” by Meshuggah offered a prime example of current, densely arranged metal with modern sound tuning. The U4s mastered this task coherently and separated the complex patterns of guitars from the bass drums without sounding tinny. In contrast, Exodus’ “Shovel Headed Kill Machine” sounded a touch too busy, and Slayer’s more explicitly mid-emphasised “Repentless” was less radiant. The U4s bring out these differences in a discernible manner.

In the highs, the U4s revealed the radiance and transparency that one desires from high-quality headphones. At the same time, I didn’t detect any unwanted harshness; instead, there was a boundary that could be clearly judged, meaning that these miniature format headphones would also be recommended for use as full-range monitors for sound engineering work.

In this context, the very good transient behaviour should again be emphasised. It leads to a high level of detail, an excellently illuminated stereo panorama and a clear perceptibility of the smallest timing offsets, for example, in vocal doubling.

64 Audio U4s in contrast to the 64 Audio U6t

The final question is, how do the 64 Audio U4s compare to the 64 Audio U6t, which costs around 350 euros and only has balanced-armature drivers. Well, the sound character is different. The hybrid design delivers fuller and deeper sub-bass via the dynamic driver. In contrast, the pure BA drivers sound more analytical, leaner and less in danger of audible transition between the driver types. But this is all a matter of personal taste.

12 months ago by Ulf Kaiser
  • Rating: 4.75
  • Sound
  • Handling
  • Price/Quality
  • Function

Technical specifications

  • Ear couplingIn-ear
  • Typeclosed
  • Transducer principle1x dynamic, 3x balanced armature
  • Frequency response (headphones)10 - 20.000 Hz
  • Impedance@1kHz: 11 ohms
  • Sound pressure level (SPL)@1kHz: 105 dB

What's in the box

  • TrueFidelity foam ear tips (S,M,L)
  • SpinFit Silicone Eartips (S,M,L)
  • silicone ear tips (S,M,L)
  • 3.5 mm black premium cable
  • Apex modules: m20, m15, m12, mX
  • cable clip
  • cleaning tool
  • sticker
  • 64 Audio premium leather bag

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