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Hidiz S9 PRO and Mermaid MM2 Combined Review

A Killer Combination?

Christopher Coke Updated: Posted:
Hardware Reviews 0

Hidiz has made its name on delivering outstanding sound quality for less. Today, we’re looking at an exciting pair of products that, together, create a portable hi-fi system small enough to carry in your pocket. The first half of this equation is the S9 PRO, a tiny yet high-spec dongle DAC/amp hybrid that pulls audio processing completely out of your phone. The second is the Mermaid MM2 IEMs, an exciting set of earphones with a unique look, swappable tuning filters, and a one-two punch of magnetostatic and dynamic drivers.

This review will be split into two parts to give each one its due. We would like to thank Hidiz for sending these products in for evaluation.

Hidiz S9 PRO Specifications

The Hidiz S9 PRO is a tiny hybrid DAC/amp designed to connect to your smartphone and PC. Its job is simple: take all of the 1s and 0s that make up digital audio and convert it into an analog signal able to be played out of headphones. It pairs that with additional amplification (volume), so it’s able to drive everything from in-ear headphones to full-size over-ears that require a bit more juice. Of course, that’s all true of other dongle DACs, and the S9 PRO sets itself apart in a number of important ways.

Rather than use mystery parts like your average dongle DAC, the S9 PRO opts for top-tier audio components and an aluminum body for durability and a more premium finish. It utilizes a pair of ESS9038Q2M DAC chips for all of its audio processing. This dual DAC design gives each ear piece its own audio chip, reducing crosstalk and improving clarity. The more important part, rather than simply having two of them, is that these chips open the door to outstanding compatibility and sound quality.

The S9 PRO is able to reproduce audio at up to 32-bit/768kHz in PCM. That’s high enough where most users won’t be able to hear any loss in quality, even when listening to an MP3. Of course, it also supports playback of lossless FLAC files too, as well as DSD512, so you won’t be limited when it comes to quality. If you’re more interested in streaming, it supports Tidal’s MQA Master Quality tracks and Apple Lossless Audio. Depending on what you’re listening to, the illuminated Hidiz logo on the device’s face will change color so you can tell at a glance what resolution is being passed through.

While most dongles at this price only support 3.5mm single-ended connections, the S9 PRO expands that to balanced connections with a 2.5mm balanced output. While connected with a 3.5mm cable, it’s able to provide up to 100mW per channel, but that doubles when connected with a balanced cable. At 100mW, the S9 PRO is powerful enough to drive many over-ear headphones, including the relatively inefficient Sennheiser HD6XX. If you’re planning on running planar magnetics, however, I would definitely recommend using the balanced connection for extra headroom and power.

That power does come with extra battery use, however. My battery dropped noticeably faster when using the S9 PRO compared to any other dongle DAC in my collection, including the powerful THX Onyx. I would have loved to see a rechargeable battery in the unit to solve this, but it’s not a big issue overall.

If the rest of the design wasn’t abundantly clear, this is a device that puts audio enthusiasts first, and that translates through directly into how it sounds. The S9 PRO has a neutral sound signature that emphasizes crisp, articulate details. It doesn’t add warmth to the sound, but isn’t anemic either; you’ll hear your headphones exactly as they were intended to sound without the dongle inserting itself in the middle.

That isn’t to say it does nothing – far from it. The S9 PRO renders sound crisply and cleanly. There is definition between the different instruments and sound sources. Headphones that excel at layering and detail retrieval will sound better than ever on the S9 PRO. Bass is fast and tight with a clear punchiness when called for. Textural details throughout the frequency spectrum come through cleanly and articulately. Listening to the Mermaid MM2 earphones, which I’ll get to in the next section, I was able to hear clear, definable differences between the different filters, something that wasn’t as pronounced when I paired them with another source.

If you prefer a jolt of warmth in your headphones, the S9 PRO won’t be for you, but if you already have a great set that you enjoy the sound of, it can be an excellent fit that offers plenty of power.

Hidiz Mermaid MM2 Specifications

Now we come to the second half of this review, the Mermaid MM2 in-ear monitors. They’re an interesting set in more ways than one, and well worth a look if you’re in the market for a pair of IEMs and that allow you to tailor the sound signature to your taste. They’re particularly interesting given their relatively affordable price.

The Hidiz Mermaid MM2 are a hybrid earphone, but not in the way you might think. Typically, when we use that term, we’re referring to a combination of one or more dynamic drivers and balanced armatures. The MM2 does indeed have a dynamic driver (and a big one at 10.2mm), but it pairs that with a magnetostatic driver. While I’ve used this driver in the past, my experience has been limited. Hidiz describes it as being a nimble driver that specializes in ultra-high frequencies without requiring the extra voltage you might need for another driver type (like electrostatic).

The MM2 also has a unique sound tuning setup that allows you to dial in the sound signature you would like. The angular metal faceplate of each IEM is broken by a decorative sound filter. There are a total of three sets included with the package that can be unscrewed and swapped out to accentuate bass, treble, or balanced sound signatures.

Image Credit: Audio Reviews

We’ve seen a number of similar systems (switches on the back of the IEM, swappable nozzles), but the MM2’s filters are some of the most effective I’ve encountered. There is an immediate and noticeable difference when changing sets. My own measuring rig is still being set up, but our colleagues at Audio Reviews have put the graphs of all three on a single graph (pictured above) so that you can see the difference each makes.

The sound can be tailored further by swapping the ear tips. There are two sets of S/M/L silicone tips included in the box. The two sets feature different sizes nozzles for more or less bass. Pairing these with any of the tips can be used to moderate or increase its effect. Compared to the filters, the impact of the tips is much more subtle but still present.

Before getting into the sound, let’s talk about the tangibles. The MM2 features lozenge-cut aluminum faceplates with resin shells for a comfortable fit. You won’t need to worry about these becoming fatiguing over time. There’s also a beautiful oxygen-free copper and silver wires that is very soft and not microphonic. It connects to the IEMs with a standard 2-pin connection, so is easily upgradeable if you choose. There’s also a very nice carrying case of faux leather that seems very nice for the cost of these IEMs.

With that done – how do they sound? The Mermaid MM2 has a v-shaped sound signature that should appeal to a wide array of users. The bass is prominent using with the balanced or bass filter, and the upper mids jump up before rolling off again in the treble. The resulting sound is one that’s great for rock, metal, hip-hop, and pop. It’s a versatile IEM!

I found myself bouncing between the bass and balanced filters for this review. In the end, I think the Balanced gives the best overall sound signature for this IEM. Using the bass filter, it can definitely punch and slam, but loses some of the tightness and resolution in the low end, making bass notes feel softer and less defined. At the same time, this isn’t a bad thing if you’re looking to use them for gaming because they can definitely sound cinematic and aggressive with that filter installed.

Detail retrieval is decent, but isn’t this IEM’s strongest suit. There is good separation between the layers, so you can clearly pick out different instruments and audio sources, but some of the tiny textural details get lost in that V-shaped tuning and treble roll-off. Soundstage falls into the same category of being good but not amazing in any way. Sounds seem to exist outside your ears but aren’t stretching in depth or height.

But at roughly $85, these are a pretty good investment for the money. The Moondrop Aria still one-ups them in detail, but the MM2 wins out with its more mainstream, fun-enhancing tuning. The ability to fine tune the sound is also nice and allows you to nudge it toward bass or treble much more so than other headphones I’ve tried. They’re a better match for a wide array of content, including games and movies, and their versatility and pleasing sound make them fun to put on and slip away with.

Final Thoughts

The Hidiz S9 PRO and Mermaid MM2 are each solid products for audio enthusiasts and are well priced compared to the competition. The MM2 in stands out for its fun, tunable sound, and the S9 Pro for its tiny size, ample power, and support for balanced connections. If you’re in the market for either, I would recommend keeping both on your consideration list.

The product described in this article was provided by the manufacturer for evaluation purposes. Articles may include affiliate links from which we may earn a small commission to help support the site. Authors do not earn affiliate revenue or commissions.

7.5 Good
  • S9 PRO: offers plenty of power in a tiny package
  • S9 PRO: Support for balanced earphones
  • MM2: Tuning filters make a noticeable difference to the sound
  • MM2: V-shaped sound signature is good for rock, pop, and a wide variety of other genres
  • MM2: Surprisingly fun to game with
  • S9 PRO: No volume or track controls
  • S9 PRO: Noticeably increases battery drain
  • MM2: Bass can be too emphasized with bass filter/tips with some content
  • MM2: Not the best for detail retrieval (but still decent for this price)


Christopher Coke

Chris cut his teeth on MMOs in the late 90s with text-based MUDs. He’s written about video games for many different sites but has made MMORPG his home since 2013. Today, he acts as Hardware and Technology Editor, lead tech reviewer, and continues to love and write about games every chance he gets. Follow him on Twitter: @GameByNight