Have you ever seen those tunable earphones? Some have switches that can alter the frequency response and some have replaceable nozzles. In this review, I’m looking at one of the most affordable tunable IEMs on the market, the TRN STM.

The STM is a dual-driver hybrid in-ear monitor with one dynamic driver and one balanced armature driver. It also comes with 3 sets of interchangeable filters that effectively give you 3 different sound signatures. At times it can be found for less than $20 online. Sounds too good to be true right? Let’s find out.

Disclaimer: This sample was provided for the purpose of an honest review. All observations and opinions here are my own based on my experience with the product.

TRN STM

Specifications

• Driver unit: 1DD+1BA hybrid driver unit
• Impedance: 18Ω
• Earphone sensitivity: 106dB/mW
• Frequency range: 20-2000Hz
• Earphone interface: 2Pin Interface
• Cable :6N OCC Copper cable

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Packaging & Accessories

The TRN STM comes in a standard budget-style small white box. Inside there is only the bare essentials which include the STM earphones, detachable cable, tuning filters and 3 pairs of silicone eartips.

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Design

A combination of the alloy faceplates and coloured transparent housings make the STM a fairly handsome IEM. I’ve actually had a few people comment on the appearance of these, although I myself thought they looked pretty ordinary.

There is a star-shaped pattern on the faceplates to add visual interest. The actual earphones are very lightweight but feel nice and sturdy, especially given the low price. The nozzles aluminium and double as the acoustic tuning filters. I found the threading to be excellent and the filters are very easy to change. I’ll cover how they affect the audio in the sound section below.

Cable

The included cable is pretty much stock standard for an ultra-budget IEM. It’s near identical to the older brown KZ cables and similar also to ones that TRN has provided in the past. It’s a 4-core 6N OCC type with a tight braid.

The cable adopts the popular Type-C 2-pin connectors and the connector housings are transparent plastic. The Y-split is rubber and the plug is a right-angled aluminium 3.5mm termination. In terms of handling the cable is pretty good and has no noticeable microphonics. However, like KZ cables, it’s thin above the Y-split and prone to tangling.

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Sound

Gear used for testing includes:

• iPhone with Shanling UP4
• PC -> iFi Audio Zen DAC
• iBasso DX120
• Aune X5s -> Feliks Audio Echo

The TRN STM has a light V-shaped signature with a clean, light presentation. Its low frequencies are enhanced and the midrange and treble bands are fairly neutral. This is a pretty chill sounding earphone which comes from its laid back and spacious style. That is not to say I find it dull; on the contrary, I find the STM to perform really well for such an affordable model.

In terms of power requirements, STM is easy to drive and will work with just about any source whether it be a smartphone or low-powered DAP.

Filters Explained

STM comes with the gold filters attached by default which makes sense as they are the balanced/reference ones. The red filters are for added bass and the blue filters for enhanced/resolving treble.

In reality, the filters make little difference to the sound but having said that, I do hear more of a difference than the graph below would suggest. The red filter sounds a tad warmer and the blue opens up the top end ever so slightly. But it is very slight so don’t expect 3 different sound signatures; rather just minutely nuanced ones. Still, having the option and the aluminium nozzles being so well machined frankly amazes me considering the price.

*The sound description below is based on the gold/balanced filters which I prefer the most.

Bass

STM’s bass is elevated but sits slightly behind vocals. Nevertheless, the bass is not overblown or dominant although it does enjoy a subtle boost. It has a clean, fast attack and medium decay speed. The sub-bass and mid-bass transition is linear, making neither one stand out more than the other.

The sub-bass rumble is tidy and smooth without distortion even at high volume. In Cubering’s “Dejavu”, the endless drone of the bass notes is clearly heard but it doesn’t overshadow the midrange or show any excessive bleed. STM isn’t for bassheads or someone looking for large, impactful bass. It’s a mature and reserved type of bass rarely heard on budget earphones.

Mids

The midrange is clean-sounding with a spacious, light feeling that’s never cloying or in your face. Note size is neutral with just enough body to not sound anaemic but lower mids could use some extra warmth for naturalness. The stage position is pushed back somewhat making instruments and vocals slightly diffuse in density.

Hardcore aficionados may cringe at this but I found the STM quite good even for classical music. Songs like George Frideric Handel’s “Keyboard Suite In D Minor, HWV 437: III. Sarabande” sound far better than I would expect for a ‘pocket change’ IEM. The strings and horns sound clear but smooth and large in scale. Both stand out clearly above the drums in the background and each instrument is easy to distinguish from the others.

Treble

With the gold filters in place, STM’s treble is a tad light in quantity and slightly diffuse. It’s not overly soft but it has a rounded quality that’s not likely to offend anyone. Unlike a lot of ultra-budget hybrid earphones, the STM treble doesn’t have a raw, metallic or artificial edge to it.

This is an airy treble that avoids overt brightness by dipping at 10kHz and lifting again in the upper treble. It feels open and transparent and although it’s not pinpoint precise, it still manages a fair amount of detail retrieval.

Soundstage

The soundstage is fairly large in dimensions and has more depth than width. Despite the stage being a little narrow, stereo imaging is pretty satisfying. Instrument separation is actually quite good although it does begin to deteriorate during busy segments.

Comparisons

KZ ZST ($16)

KZ’s ZSTx is also a dual-driver hybrid IEM. Right off the bat it has a brighter overall tonality and more forward presentation. It sounds slightly forced compared to the STM which is more laid back and effortless.

Bass on the ZSTx is more accentuated and has faster decay than STM but both follow a similar curve relative to the core midrange. STM’s upper midrange is more forward compared to the ZSTx which reaches more into the lower treble region. As a result, ZSTx’s upper mids are brighter and the lower treble is more energetic giving it a more contrasted sound.

ZSTx has a forward treble which can at times sounds a bit metallic or artificial albeit more precise. STM’s treble, on the other hand, is more rounded and warmer in comparison. ZSTx has a wider soundstage but STM’s stage is cleaner with less resonance in the spaces giving it a blacker background.

KBEAR KS2 ($23)

The KBEAR KS2 is a dual-driver hybrid like the STM. It is more v-shaped, meaning it has increased bass and treble presence with a recessed midrange. The first difference I notice is the KS2’s extra bass. This gives it more warmth and low-frequency impact. In addition, it also adds more warmth to the lower midrange.

As a result of the added bass and warmth, KS2’s midrange has thicker notes. STM sounds leaner and clearer in comparison, albeit more analytical. KS2 sounds dynamic and exciting compared to STM’s more linear and reserved sound.

KS2 has a more forward treble region, which is necessary in order to contend with its enhanced bass. Just like STM, the KBEAR KS2 has a very high-quality treble in the context of the earphone’s price. Despite being forward and lifted, there’s no harshness or sibilance but plenty of air and detail.

Conclusion

Overall, I’m impressed by the TRN STM. It has a somewhat reserved but very mature and clean sound. Although the filter system only makes minimal changes to the sound, it does work which adds versatility and customizability.

The build quality is good, the fit is comfortable and it performs really well, in my opinion, for a budget IEM. Furthermore, STM has a better quality treble than most TRN earphones. Sure, it’s not as energetic or exciting as something like the V90s but it’s a treble that doesn’t cause fatigue and still reveals ample detail. I think this is one of TRN’s best IEMs yet (in the context of its price point) and it has me looking forward to seeing what they do next.