64 Audio Solo Review: Chasing After the Planar Train

Introduction

For CanJam SoCal 2024, 64 Audio has decided to release a brand new IEM: the $1,399 64 Audio Solo. What’s particularly interesting about the Solo is that it’s a planar. 64 Audio specifies that this is a 14.2 mm planar driver, which is coincidentally similar to the 14.2/14.8 mm class drivers found in most of the recently hyped up planar IEMs such as the Letshuoer S12 or 7Hz Timeless.

Of course, drivers and driver size don’t mean much on their own. But it is noteworthy that 64 Audio may be looking to leverage the same technology that their Chinese counterparts found so much success in. And they aren’t the first Western brand to do so; Campfire Audio has already released a couple of their own planars. In today’s review, I’m going to be taking a look at 64 Audio’s approach to planar IEMs and how it stacks up in the market.

What’s in the Box

The packaging of the Solo is standard 64 Audio, so no surprises here. We get:

• A black round 64 Audio carrying case
• A puck of foam and silicone ear tips (9 pairs in total)
• Cleaning tool, cleaning cloth, and shirt clip
• Two 2-pin copper braided cables - 3.5 mm and 4.4 mm
• The Solo itself. It’s made fully of metal with a slightly coarse feel instead of a completely smooth polish.

Personally, I think the Solo is one of 64 Audio’s most beautiful IEMs. I love the copper grill set within the bold black metal shell and repeating 64 Audio logo accents. The copper cable it comes with completes the overall aesthetic. It’s also the best stock cable that 64 Audio has ever included with their IEMs, though the pessimist in me will say that it’s not any different to a $25 aftermarket cable from Aliexpress.

The comfort of the Solo is quite good. Since it uses the standard 64 Audio shell, it’s ergonomic for my ears. Like the other 64 Audio IEMs, there’s also no pressure build-up at all due to the way they’re vented. However, I would recommend using other tips than the stock ones as I find they don’t seal quite as nicely in my ears.

64 Audio Technologies

64 Audio likes to advertise their products with their proprietary IEMs technologies. To their credit, most of them are actually useful instead of just marketing fluff. The Solo continues this tradition with a mix of some old and new tech. I’ll try to simplify it here for those unfamiliar.

Apex Core

64 Audio is the only IEM company I know that focuses on alleviating pressure build-up in their IEMs. They do this through their Apex technology that acts as a vent. In their other products, it comes in the forms of Apex modules that also allow you to control the bass quantity. The Solo doesn’t have Apex modules, but instead uses something called Apex Core. This makes the Solo feel rather open-back as it’s so vented it can feel like you aren’t even getting a proper seal. I can also hear music coming out the back of the shell if I bring it close enough to my ears. However, 64 Audio has clarified on Head-Fi that the Solo is not “technically” open back like the Duo is, but rather that the isolation is more on the open side.

LID

LID stands for Linear Impedance Design, which means that 64 Audio designs their IEMs to not be affected by changes in output impedance between devices. This is a very nice quality of life because it means that you will have a consistent listening experience regardless of what you listen to the Solo out of. While it might not seem important for a typical listener, it can have a big impact if you’re a musician juggling between many different source amplifiers, such as body packs or stage monitors.

tia

While most IEMs use acoustic tubing to direct the sound from the drivers out to the nozzles, 64 Audio eschews this with their Tubeless In-Ear Audio (tia) system in their IEMs. To be completely honest, I don’t actually think this is that important, and in some cases, can actually lead to undesired effects in the treble depending on how it’s implemented. In the Solo, there is a single bore and micro-acoustic chambers that funnel the sound.

Dual Helmholtz Resonators

This is 64 Audio’s newest technology introduced with the Solo. There are two 3D printed Helmholtz resonators in the Solo that help to control its sound, presumably in the treble. This isn’t new IEM technology by any means; the Sennheiser IE 900 has three! Helmholtz resonators tend to be quite effective in the products I’ve tried in the past, and as we’ll see in the treble section, it is an important contributor to the Solo’s sound.

64 Audio technologies in the Solo. Credit to the 64 Audio website.

Sound

To be completely honest, my first impressions of the Solo was not great. I was using the stock tips, and it sounded overly warm and soft. But after playing around with a number of different tips, adjusting the fit, and getting more used to its sound, it became a more enjoyable listen. For reference, I’m using small AET07 tips with the Solo to get consistency with the fit due to its highly vented nature.

Here is the calibrated frequency response of the Solo taken using my IEC-711 clone coupler.

And here is the raw graph.

If you’re familiar with planar IEMs, you’ll notice that the Solo has a couple of distinct features that tend to be found in those 14.2/14.8 mm planars. There’s a large bass shelf that slopes into the midrange and an ear gain peak around 2 kHz that plateaus in the 3 - 4 kHz region. The other tell-tale sign of planar that’s missing is a sharp treble peak, though that’s almost certainly because of the dual Helmholtz resonators in the Solo doing its job.

Now, I’m not the biggest believer in drivers dictating the sound of a headphone or IEM. It all depends on how they’re implemented. That said, I understand that people do use it as a shortcut to categorize a certain type of sound, and I’m personally guilty of doing so with planar IEMs with the 14.2/14.8 mm drivers. So yes, despite some small nuances, the Solo largely does sound like the other popular planars that have been out on the market for a couple of years. You’ll see this theme come up repeatedly throughout this review.

Bass

The Solo has plenty of bass without being a basshead IEM. Like many planars however, it does lack some subbass depth at the lowest octaves, thus making the Solo a little more midbassy. For example, kick drums and floor toms have an initial boominess, but the underlying rumble is shy. In other words, the Solo doesn’t indulge much in the decay of drum notes. This doesn’t mean however that the Solo is bad on tracks with these instruments. My usual test track for this is Anberlin’s Losing It All, and the Solo performs as expected on that open sequence with the drums. The most important part is that the weight of these notes are preserved, even if some of the decay is lost.

The tradeoff is an increased sense of quickness in the note articulation, particularly for plucked bass guitar notes. The big bass shelf provides plenty of body so it’s not just a lean midbassy punch. Overall, I’d say the bass quality of the Solo is fine. It’s a bit blunt and nothing special in the market, but also nothing worth complaining about.

Mids

The midrange of the popular 14.2/14.8 mm planars have a distinct shape to them. Looking at the raw graph, they have a slope from the bass into the lower mids, an ear gain peak around 2 - 2.5 kHz, and then a plateau at 3 - 4 kHz. When visualized on the calibrated graph using the preference bounds, you can see how there is excess lower mids energy and the upper mids peaking at 2 kHz. This can create a slight “hole in the mids” effect depending on what you’re used to, but once there’s a bit of brain burn in to adjust, it’s not an issue.

The more apparent effect is what I call a “vocal hump” where the vocals are pushed up because of the peak at 2 kHz instead of 3 kHz, and you get a large dip in the vocal presence region of 3 - 4 kHz instead. This paradoxically makes the Solo sound both warm and vocal forward. It’s very noticeable tonally if you compare to those “new meta” IEMs like the MoonDrop DUSK. Though which is preferable is up to you. Personally, while the “new meta” style tonality is more immediately enjoyable and less colored, I can still enjoy the planar-style midrange tuning.

Compared to other planars, the Solo’s 2 kHz peak is quite mild. Here is the raw graph compared to the 7Hz Timeless and we can see how it’s slightly shaved off so you don’t get quite as much of that “hole in the mids”. That said, I do like that the Timeless has more energy at the 3 kHz mark to soften that “vocal hump” and balance out the warmth.

Treble

From that graph, we can also see that they’ve significantly tamed the treble response of the Solo. Now, this is an IEC 711 clone coupler, so take the treble with a grain of salt. But to my ear, this is the direct effect of the dual Helmholtz resonators that 64 Audio is advertising on the Solo. It makes the treble of the Solo sound extremely controlled. Notes are kept on a tight leash - never splashy nor overly sharp. Yet it doesn’t sound so smoothened out that all definition is lost, nor is it necessarily dark. While I appreciate what 64 Audio is doing here, I usually prefer a bit more brilliance in the hats and cymbals. It comes off as trying a bit too hard to toe a line rather than letting the treble ring out naturally. The other important effect that shaving off the bright spots in the Solo has is that it warms up the overall sound signature more than you might expect.

Presentation

With it looking and feeling like an open-back IEM, I was most obviously interested in how its staging performance would be like. Unfortunately, I’m sad to say that it isn’t anything special. Though it feels open with no pressure build-up and low isolation, the soundstage width isn’t much larger than most other IEMs. Height and depth are similar to other IEMs i.e. negligible. Imaging and layering are alright. Despite its warmth, instruments aren’t overly congested. And this is where the Solo does well. Like many planar IEMs, they tend to have a sense of articulation even in busy tracks. However, this doesn’t mean the Solo is a super resolving IEM. It’s alright in that department, but doesn’t highlight every little detail or nuance like I would expect from a high resolution monitor. That articulation is more for ensuring that the primary notes in the foreground are well defined.

Should You Buy It?

No, not at this price. Like most of the popular planars, the Solo is a pretty good IEM. And 64 Audio has genuinely made key improvements in the treble with the dual Helmholtz resonators. However, $1,400 is a LOT of money to spend for what is, in my opinion, a fairly standard 14.2/14.8 mm planar in a 64 Audio shell. Not only that, they’re about 2 years late to the planar train. It’s why I didn’t include a comparisons section - if you want a planar, there are countless for less than $200. While the Solo might perform a bit better than a lot of them, there isn’t a clear-cut distinction unless you somehow love love love the planar IEM sound profile but are simultaneously very treble sensitive.

And if you were someone looking to spend $1,000+ on some high-end IEMs, let me point you to last year’s 64 Audio U4s instead. It outperforms the Solo in pretty much every metric for me while being $300 less. It’s a proper culmination of everything that made 64 Audio so popular in the IEM space, but at a relatively welcome price. The Solo unfortunately just feels like an aimless step away from that direction.