I believe my stance on sources and dongle especially are quite apparent by now if you’ve read my other reviews. Many sound nigh identical due to the adoption of SOCs which encompass the entire audio path. Indeed, implementation may vary, but as the all-in-one unit contains so much of the system, you really are looking at very incremental changes amongst different models. The S9 Pro evidently shakes things up a bit with its discrete amplifier chip, or at least, it’s unique implementation of ESS’ chipset. I am happy to confirm real world benefits in listening, this one is a clear step above the norm.
Frequency Response –
Testing Methodology: RMAA via Startech External Sound Card
The S9 Pro has a linear frequency response suggesting that it represents audio with great fidelity. Due to the quality of my sound card, I am unable to reliably test other measures such as distortion and crosstalk so they will be used as a personal reference only. Qualities here can also impact the sound as I will detail via subjective listening.
Output Impedance & Hiss –
Testing Methodology: SPL volume matched comparison through an inline splitter to THX789 + SMSL SU9 to Campfire Audio Andromeda and Ara
Hidizs don’t state these specifications on their website, but they really should because it is would be a selling point of the S9 Pro. Empirically tested, I would estimate the S9 Pro to have a sub 1-ohm output impedance. I tested using the Campfire Audio Ara which has an ultra-low 8.5-ohm impedance and a mechanical crossover that can discern even between 1-ohm and sub 1-ohm sources.
Compared to my THX 789 (1-ohm), the S9 Pro gave it a slightly smoother sound suggested a sub 1-ohm output impedance. It showcased a similar character as my low output impedance sources with other sensitive IEMs, an excellent result. Noise handling is equally impressive, being dead silent on both the Andromeda, which is renowned for its sensitivity to source noise. The S9 Pro had a black noise floor with all of the gear I tested it with no matter how sensitive, another excellent result.
Testing Methodology: SPL volume matched comparison through an inline splitter to THX789 + SMSL SU9 to Soft Ears RS10 (flat impedance). Powered by Xperia 5 II with Poweramp Pro via high-resolution output.
As was my experience with other ESS9038-based dongles, the S9 Pro has a very linear and balanced foundation to its sound. This is carried through to the final output with aplomb, offering an almost perfectly even-handed rendition with minimal tonal colouration. I am hearing a slightly revealing top-end but nothing that would hugely affect synergy or polarise for most personal preferences. Rather, it imbues just a little vibrance and zing into its overall image that some may enjoy as it draws greater focus to fine details. Compared to a high-end source based on similar componentry such as A&K’s SE180, transients aren’t quite as defined, and its imaging isn’t so immersive. But the S9 Pro is definitely one of the best hyper-portable sources/dongles I’ve tested when taken as a whole. Surely, the power output is coming into play as well as the S9 Pro does have quite a robust low-end and is able to keep up with even my least efficient IEMs and many headphones too.
Balanced vs Single-Ended Out
It should be noted that balanced isn’t a feature you should feel is necessary, as it doesn’t necessarily bring benefits, but rather solutions to issues that are not present here. That being said, if you do have a balanced earphone or the cables to take advantage of a balanced output, then benefits are to be found. With double the output power and a huge jump in channel separation, this is more true here than on many other sources. I still found the single-ended output to provide a highly impressive sound, in fact, my very favourable comments below were using this output. However, balanced does improve the listening experience further. Staging was the biggest difference to me, you get a noticeably wider and airier stage with greater separation. The background was more resolved and stretched further, creating a more multi-dimensional sound. I would recommend using the balanced output on this dongle if possible, if not, you are still receiving a very accomplished source.
Desktop sources will always be the best foundation of judgement for bass performance as dynamics are tied directly to the power supply, and a larger one invariably has a higher capacitance ceiling. Compared to my THX789, I do notice a drop off in the sub-bass on most portable sources, but the S9 Pro gets impressively close considering the form factor. You won’t get huge physical slam at the very bottom nor the utmost affirmative note attack relative to a larger source; bass drums aren’t as authoritarian nor rumble as powerful.
However, this remains a tight and snappy low-end with no shortage of audible sub-bass kick. Bass is linear with no apparent emphasis working towards a well-separated and defined response. Control is well above average for a dongle, note definition impresses as does its ability to discern texture in the mid-bass. As note attack isn’t too aggressive, this does provide the impression of a smoother bass texture. In turn, its assets are more its dynamics in-class combined with its texturing as you will find sharper timing elsewhere.
Like the bass, the listener is instantly able to appreciate a superbly linear midrange rendition that contributes to versatile synergy and overall separation. The S9 Pro has no huge tonal deviations as you’d expect from any decent modern source, though small qualities in its note presentation do imbue certain qualities. For instance, I am hearing a tinge of dryness in its voicing, lacking the liquidity and delicacy of higher-end sources. Similarly, resolution of fine details operates at a high level but won’t replace your desktop stack. Specifically, the S9 Pro prefers a slightly denser voicing relinquishing articulacy and separation whilst prioritising coherent and well-structured notes.
In turn, it discerns textures well and instruments are portrayed with a realistic timbre. This is a nicely layered source too, with great contrast between its background and foreground crafting an impressively wide image. However, it doesn’t discern between individual layers such as vocal harmonisations especially well. My main criticism here is regarding organisation on complex tracks where the characteristics of its note presentation can make it sound busier than some. This isn’t to the extent that the source sounds coloured nor remotely congested, and the S9 Pro remains altogether, a clean, clear and well put together source.
As aforementioned, I am hearing a lick of forwardness in the highs, imbuing just a hint more zing than a dead neutral source – such that I would consider Topping’s high-end gear. To preface, this isn’t to the extent of the infamous sabre glare you may have heard of coming from their previous generation DACs; I do think the 9038-based sources I’ve tried have all been mostly well-behaved, so this is more a repercussion of individual implementation. The lower treble is impressively even and linear, portraying a well-bodied, textured foreground. There’s a hint of middle-treble emphasis above that slightly lifts instrument clarity and draws greater focus to shimmer, air and decay.
The presentation is characterised then by its slightly energetic nature but remains free of glare or grit in so doing. That said, this does detract slightly from the initial transient, I do perceive the S9 Pro to have a slightly softer note attack. This means that though it is a well-detailed and honest representation of music, micro-details can get smoothed over more than some. I would suggest this is responsible for robbing the midrange of some articulation as well. Above, headroom is top level for such a compact source, clearly better than the majority which contributes to a strong imaging performance. It won’t give you huge micro-detail and sparkle nor the highest resolution, but a slightly vibrant expression realised through nigh class-leading extension.
It’s here that the S9 Pro’s sound makes a lot of sense, it images far better than you’d expect for such a compact source. Many sources are able to effectively provide a linear sound and detailed lower-treble but fall short when it comes to soundstage and layering, providing a superficial experience. The S9 Pro is no such creation, delivering out of the head width on the right IEMs and a good amount of depth too, forming a multi-dimensional image. It remains width biased, but I was impressed by its dimensions combined with its neutral note size.
Specifically, there’s a palpable air surrounding each element often absent on competitors. Again, layers are well defined, and the background isn’t super detailed, but does showcase impressive width. Separation is likely its weakest link, with note definition lagging slightly in the treble (somewhat counterbalanced by its tonality) and the bass and midrange being more structured over ultra-concise. It’s not a poorly separated sound, aided by the larger stage, but not the best I’ve heard.
Driving Power –
Fir M4 (6.4 Ohms): The M4 is a very low-impedance hybrid monitor and is highly discerning, showcasing source quality clearly. This was a terrific pairing and a great showcase of the S9 Pro’s ideal environment. Even compared to my desktop stack, I was getting near identical extension and power. Notes weren’t quite as controlled and defined, but very close. The top-end was detailed and well-extended. The soundstage showcased impressive dimensions and proportions, similar between depth and width but both scaled down slightly compared to my larger sources.
Soft Ears RS10 (25 Ohms, 100 dB): The RS10 is deceptively difficult to drive and highly resolving, scaling with sources. The S9 Pro did admirably here, I was only using 7% volume output, bass was almost as extended as my desktop stack but lacking that last ounce of pressure and control. Mids were slightly less revealing and articulate but equally well-structured and layered, but layers were less delineated. The top-end was slightly brighter with more shimmer and air but less transient attack. The soundstage presentation impressed; it was similar but scaled down.
Final D8000 Pro (60 Ohms, 98 dB): The D8000 Pro is a highly resolving planar with special emphasis on bass extension. This pairing does demonstrate where such a small design shows its limits, though was an admirable performance on the S9 Pro’s behalf. Bass was full and robust but not especially well-controlled, being a little fuzzy around the edges. My desktop stack had noticeably more power and weight. The soundstage was the next biggest change, being just as deep on the S9 Pro but quite a bit narrower. I felt the sound was superficially balanced, certainly enjoyable and a far cry from the stock output on my phone and laptop. But for full-size headphones, you won’t get the full experience.
Suggested Pair Ups
The higher driving power and generally neutral note size and tone means the S9 Pro rarely polarises and achieves wide synergy. Though I’m sure there are rare outliers, it drove all of my IEMs with aplomb, delivering outstanding dynamics and soundstage expansion for a hyper-portable source. I didn’t find it too bright on already brighter earphones either, especially as its transient response isn’t to sharp so brightness is compounded upon. Headphones are driven well too, full-size headphones at a stretch but the vast majority of portable headphones with relative ease both from a volume and current point of view. This is especially so from the more powerful balanced output. Where with many portable sources you do feel like you’re missing out on dynamics and power. While it still isn’t ideal here, you don’t miss out tonally but more with regards to note presentation which is less impactful on the overall listening experience.