A Candid Review of the STAX SR-003: By Boogi3

Who is STAX?

If you’ve clicked into this article, it’s somewhat likely that you already know of this legendary Japanese high-end audio manufacturer. But to those outside of the hobby, it's as an obscure brand. So here's a curated view of their legacy: 

STAX Ltd. is a Japanese company that first established itself in 1938, and first released audio components, like a condenser microphone, turntable tone arms and phonograph cartridges. Soon after, they released electrostatic driver tweeters, and in 1960, a full electrostatic in a headphone style followed. This was the STAX SR-1, and the term earspeaker was soon coined for it.


Along with the SR-1, there was the accompanying energisers that powered the earspeakers. As electrostatic earspeakers do not operate on the same voltage as regular electrodynamic transducers, a special power source was needed. Typically, most 3.5mm headphone jacks on your laptops or phones don’t provide more than 2 volts.

However, on an electrostatic system, like the SRD-1 that was made to power the SR-1, a whopping 150 volts of bias voltage was needed for the earspeakers. Successors to the SR-1 increased to 230V of bias voltage, which soon became the STAX standard voltage

An illustration of how electrostatic drivers work, further explained at https://staxaudio.com/technology,

More audio components followed, with speaker systems, more amplifiers/energisers, and tone-arms. On the earspeaker end, this culminated with the SR-Sigma, one of the most interestingly-designed earspeakers in the STAX lineup

The STAX SR-Σ (Sigma), which set the foundation for future STAX earspeakers

This further development and commitment to improving their electrostatic earspeakers eventually lead to the Lambda series. Introduced in 1979, the SR-Lambda was clearly inspired by the SR-Sigma. And in 1982, the SR-Lambda Pro followed, which also introduced the new STAX PRO Bias standard, operating on a 580V bias voltage. The Lambda series became the basis for the next generation of STAX earspeakers, with the iconic rectangular and boxy housings a mainstay until modern day.

The STAX SR-Λ (Lambda) Signature, part of the Lambda series succeeding the SR-Σ (Sigma)

The STAX SRM-T1S, which featured both the Normal Bias (right-most connector), and the newer Pro Bias (the left-most and centre connector)

But all releases so far have been earspeakers in the headphone-style, with others being full sized speakers. What about those who just want an electrostatic in-ear? 

Enter the SR-001, the “Portable type Electrostatic In-The-Earspeaker System” (a mouthful). it was the first in a line of interesting in-earspeakers that featured their own proprietary connector and a portable, battery powered energiser.

The STAX SR-001

This wasn’t enough, however, as some users wanted more from the new in-ears. They wanted to plug into their more powerful, more meaty desktop energisers, so the SR-003 was released, essentially a S-001 MK2 with a Pro bias connector.

The Setup

And this SR-003 is what I’ll be talking about today. In the expansive world of IEMs, this example, coincidentally as old as I am (1997 babies unite), is still a unique and historic piece in this modern day. In the quarter of a century that it has been out, despite the leaps in driver technology and development, there hasn’t been anything coming close to how this STAX in-ear sounds. There are electret-style transducers out there, but these are slightly different in the way they work. And as some may say, electrets don’t sound remotely close to how a full electrostatic system sounds.
To power this unique in-earspeaker, I’ve opted for the familiar STAX SRM-D10. The first truly transportable DAC-energiser combo from STAX, it is a solid source to pair the SR-003. It’s got an internal battery with a 5-pin PRO bias, pushing 200Vrms, which is enough to decently power most earspeakers in the STAX lineup, and will handle the in-earspeaksers without any sweat. The stage has been set, and enter stage left, the STAX SR-003.


The STAX SR-003

The SR-003 is fundamentally an open-backed IEM, and hence requires a very quiet and conducive environment to hear it at its best. Stax also sells the CES-A1 sealing cover-- a plastic shell cover that creates an acoustic chamber on the rear volume of the driver, and also attenuates external sounds. These covers also affect the overall frequency response of the SR-003, and I’ll cover the notes of both the configurations.

The STAX SR-003 MK.1, pictured with the CES-A1 covers and SRM-D10

Cover off

The SR-003 without the covers is just like listening to a full-size STAX earspeaker. Apart from the lack of noise isolation however, they do sound fairly different from their larger brethren.
On first notice, the SR-003 has a very wide and open presentation of tracks. This sense of space in such a tiny package is impressive on its own, owing to the earspeaker moniker we've attached to in STAX products. There is also immense detail reproduction, largely thanks to the extremely thin and light diaphragm. Some users even report it’s detailed to the point one can hear recording artifacts that normally goes unnoticed.
While I can’t confirm that myself, I do enjoy the high level of detail that the SR-003 pushes out, and this plays well with a lot of my music. It’s not fatigueing detail, like how some IEMs artificially bolster the upper-mid and treble response to solicit a higher level of perceived detail. This practise is essentially forced detail, whereas the SR-003 has some proper detail in the way it handles the attack and decay of a note. It’s not artificial in any way, and doesn’t go too fast or too slow.
On to the frequency response, the sub-bass is definitely not the strong suit of the SR-003. I don’t get any of the punch and oomph like I do in a solid dynamic driver IEM. The upside though, is that the sub-bass is incredibly detailed and textured, and this gives me the perception that there’s a lot more to the sub-bass than there really is.
It lays the canvas for which the rest of the frequencies can freely dance on. It certainly doesn’t rumble, but it does just enough to remind you that it’s still there, a gentle tap on your eardrums. The midbass has more heft, and adds to the overall warmth of the IEM. It’s not a basshead’s type of bass, though. 
Now the mids to me might be divisive. Some diehard STAX fans might come after me, but I will explain myself well enough. On some tracks, and only those tracks, there is an odd honk to some vocals. I do think it could be attributed to the warmup time for the diaphragm to get charged up, but I do hear some odd hollowness in the male vocal region. It’s not a universal problem though, as most other male vocal-focused tracks don’t have the same issue.
Perhaps like someone who woke up with a bad sinus, the SR-003 just needs a little time to clear the nasally quality out of the vocals, and once warmed up, it breathes freely and incredibly. Otherwise, the mids are beautifully presented. Instruments are incredibly life-like, and the one outstanding quality is that individual instruments and vocals are layered and separated very well, and there isn’t a claustrophobic staging or presentation at all. It’s definitely a warm sounding IEM, and this is the defining quality of the SR-003.
The treble of the SR-003 is just what electrostatics are known for, nice and airy, it extends beyond the limits of my own hearing and doesn’t have any odd peaky, sharp spikes. It’s so smooth at the top end that it’s absolutely easy to listen to these for hours on end. And yet, there is no lack of detail at all, and this proves that you can have treble be detailed without needing to make it fatigueing or overly bright.

With the covers

Now, with the covers on, the sound of the SR-003 largely stays the same, with a couple large changes. I won’t recap everything as most of the above still holds true, just covering what’s changed.

The covers now turn the incredibly open in-ear into one that has a surprising amount of noise isolation. It’s not, dare I say, somewhat usable in moderately noisy environments, although I still won’t think of using these on the MRT or in the bus (not because of the sound, because you need to carry around a somewhat heavy and cumbersome kit to listen to it properly).

The covers also slightly give the bass response more girth, but only marginally. It sounds more energetic, more punchy, and can perform freely now without much interference. I do enjoy a lot of music from the electronic/EDM genre and their subsets, and the addition of the covers turns the SR-003 from an odd choice of a monitor into something that I can bop my head to. The incredible bass resolution and balanced punchiness paired with an inoffensive and smooth treble response makes these a quirky but still incredible sounding IEM on EDM tracks. It's not something that most other detailed IEMs can pull off, with how aggressive and sharp the treble response can be on some of these tracks.

The covers makes the SR-003 into a more versatile IEM, and even at it’s age, it’s beautifully handling songs and genres made long after the generation that the in-earspeaker was born into. 

As for anything else, well, it’s basically the same IEM with some (dare I say?) improvements. The covers are definitely my favourite way to listen to the SR-003, and as elegant the open-back configuration is, my plebeian taste in music currently enjoys the closed, covered up SR-003. But who knows? I might just pivot back into the classics that I do still love.


Older mastering or style of recordings work beautifully with the SR-003 in open configuraton. Think of the late 70s into the early 90s, these are the style of songs that are the most well paired together with the in-earspeakers, and be ready to be transported into a different age with these
A blend of the new and old, of steel and gold
Once you fit the covers on, however, it transforms into something that will deceive you about its age. It devours modern music, and spits it back at you in a way no other IEM of this day and age has yet. Have I ever seen an IEM this versatile? Perhaps there are a few in the modern era, but I can’t seem to recall. And either way, it has its quirks and flaws, but none of them involve the sound much at all. It’s a truly unique in-ear, and it stands alone in a landscape saturated by driver count wars and increasingly complicated designs
However, it’s understandable yet melancholic that this electrostatic in-earspeaker never really took off. The total cost of a system when factoring the need for an energiser to power it, along with the absolute lack of portability, meant that it was a sort of experiment. It was an attempt to downsize the larger STAX earspeakers into a form factor that could fit in the ear. But without anything to drive it home as a portable setup (apart from the SR-001, which many considered largely below potential), the SR-003 wasn’t able to get much traction. Why use an in-ear when the Lambda series of headphones clearly outperform the IEMs, at similar prices, and with arguably better comfort?
Well, the real question to me is, why not? A pure audiophile always has a soft spot for something quirky, and for me, this SR-003 as old as I am is just that.
And for that, it’s just that special. A true STAX special.

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