The Fosi i5 is a full-sized planar magnetic open-back headphone. It was launched on Kickstarter in the spring of 2025 and is now available through regular retailers at a price of around 550 USD / €470 before tax.
As Fosi’s first headphone, the i5 marks the company’s entry into the full-size headphone market, an area where many audio brands are expanding. Having been impressed by the K7, Fosi’s most recent external DAC/amp, I found that the i5 carries forward the same level of performance to price ratio.
The Fosi i5 features large 97 mm planar magnetic drivers powered by 44 N50 neodymium magnets. The diaphragm is only 2 microns thick. Large drivers help improve dynamics, while the thin diaphragm enhances detail retrieval.
Fosi also emphasizes its sputtering technology, which deposits an atom-thin coating on the diaphragm for improved uniformity and responsiveness. In addition, acoustic damping chambers are employed to control resonance and vibrations.
Note: Pre-production units of the i5 sent out for early reviews had a design flaw that caused the deep bass to roll off. Fosi corrected this by adding a sealing ring that closes the gap between the driver and the Velcro-mounted ear pads.
I have listened to both versions, and the difference is substantial. If you encounter early reviews mentioning a severe lack of sub bass extension, it was most likely due to the missing sealing ring.
FOSI AUDIO i5 SPECIFICATIONS
- Open-back circumaural wired headphones
- Driver: Planar magnetic, 97 mm
- Impedance: 28 ohm
- Sensitivity: 98dB/mW @ 1kHz
- Maximum Sound Pressure Level >120 dB
- Frequency Response: 10Hz-50kHz
- THD < 1% @ 100 dB SPL
- Recommended Power > 250mW
- Minimum Recommended Power > 100mW
- Weight: 550 g
Current price and availability:
BUILD AND COMFORT
The Fosi i5 is a very large, full-sized headphone made mostly of walnut wood and metal, featuring oval-shaped cups, large deep pads, and a suspension strap headband made of real suede leather. It weighs 550 g, making it only 62 g lighter than the heavyweight champion among popular headphones, the Audeze LCD-X.
Despite its size, the i5 feels well balanced and not overly heavy to wear. The headband uses flexible memory metal that adapts to different users while retaining its shape, combined with stepless silicone adjustment for smooth and precise strap fitting. I find this system works very well.
The included cable is a detachable 1.5-meter cable with a 3.5 mm termination plug on the amplifier side. There’s a 3.5 mm to 6.35 mm adapter included. The cable connects to the headphones with 3.5 mm connectors, making it compatible with a wide range of cables.
Fosi offers two different upgrade cable with a 4.4 mm jack (one standard and one more flexible) as well as a 4.4 to 4-pin XLR adapter similar to FiiO’s adapter. However, you can also use most third-party cables with 3.5 mm connectors for the headphones, if they suit your needs better. I found the included cable a bit stiff and short, so I mostly used a nice custom made 3rd party cable.
The large earpads are made of breathable memory foam, but since the contact area with the skin is quite large, heat buildup may be a concern. They have mesh fabric on the face and fenestrated pleather on the inside and outside. There’s also available 29 USD “upgrade pads” from Fosi that are non-fenestrated and will increase bass presence – and presumably tune the sound a bit differently. I haven’t heard them myself.
The pads attach with Velcro, and while this system makes them easy to remove and replace, it is also the reason Fosi had to add a sealing ring, but it seems to work well.
Several other manufacturers of planar magnetic headphones, such as Audeze and Dan Clark Audio, have been known to glue their pads in place to ensure a proper seal. However, Dan Clark Audio also uses Velcro on the Ether 2 system, though it is a much finer type than what the i5 uses.
Hifiman’s planar magnetic headphones, by contrast, generally do not rely on a full seal and feature easily detachable pads, making pad rolling simple and enjoyable.
On the i5, the pad dimensions, shape, and mounting system make third-party pad swaps very difficult. I prefer having the option to swap pads easily to fine-tune the sound. With proprietary systems like this, trying alternatives becomes challenging unless a specific solution is made for the i5 by third party manufacturers.
MEASUREMENTS
The frequency response measurements of the Fosi i5 slow a somewhat unusual curve. Modern headphones generally show more consistent results than both Fosi’s own measurements and those from independent sources, especially in the critical midrange area. If you choose headphones based solely on measurement graphs, the i5 may not be for you.
Below are some curves published by Fosi themselves on Kickstarter. Two curves are shown because the pre-production model lacked a crucial rubber ring that sealed the diaphragm to the pads. The pre-production bass response resembles that of older Stax Lambda models, which also suffered from seal issues. Fosi sent out these pre-production units for early reviews, and after receiving feedback, they addressed the heavy bass roll-off with a proper fix.
There are other popular headphones that measure quite unevenly in the midrange, such as the Sennheiser HD829 and the Hifiman Sundara Closed—many closed-back headphones show similar irregularities. The Fosi i5, however, is open-back and is an outlier in this regard.
Fortunately, we don’t listen to pure sine waves. Real music is far more complex— even a single piano note contains a rich mix of frequencies and resonances. That’s why the music doesn’t sound nearly as spiky as the frequency graph suggests. I have to say, I’m genuinely impressed by how natural the Fosi i5 sounds compared to what the measurements indicate.
That said, when a headphone measures very unevenly, especially in the midrange, how it sounds to the listener will vary a lot depending on the music, the track, the instruments, and the vocals. It all depends on which frequencies in the track are more dominant, since they will be either enhanced or reduced according to how the headphone reproduces them. This can lead to very different impressions from track to track, listener to listener and from reviewer to reviewer.
When reviews and impressions differ significantly, unit variation may also be a factor, as two headphones rarely sound exactly the same even when they should. I also found the i5 to be rather dependent on a good amplifier match. I will comment more on this later.
LISTENING IMPRESSIONS
In the following, I’ll share some track-by-track listening impressions of the i5. For this session, I used the (sadly) discontinued Questyle CMA800R amplifier, which pairs very well with the i5.
Associated equipment
- Questyle CMA800R amplifier
- RME ADI-2 DAC FS
The percussion is very snappy, with a strong sense of reverb. The double bass is even sounding and controlled, while wind instruments and piano sound clear and natural. The headphone delivers a wide soundstage with excellent imaging.
Spontaneous Compositorius
The saxophone sounds natural with a slight breathiness. The percussion has a liquid, crisp treble that is clear without being harsh. The double bass is well balanced—never overpowering, yet rich in texture and nuance.
Fuglefangeren by Maridalen
The imaging is excellent, with impressive instrument placement. The saxophones sound natural, featuring a warm yet textured timbre, while the double bass is well balanced—neither overbearing nor too subdued.
The Sun Always Shines on TV MTV Unplugged by a-ha
The prepared piano in the intro is crisp and highly articulate. Vocals are upfront and clear while remaining natural, with excellent instrument separation. The presentation is very dynamic, the bass is rather neutral in terms of presence.
Pink + White by Frank Ocean
The sound is very dynamic. The bass is polite yet precise and nuanced, revealing plenty of subtle details. Vocals are clear and upfront.
Vicarious by Tool
The bass is modest in quantity but excellent in quality. There’s impressive instrument separation and imaging.
Jambi by Tool
The guitars have rich texture, but aren’t especially crisp. Bass is polite but detailed. Vocals are nicely separated and nuanced. I’ve often heard this with more warmth and body, but the i5 isn’t too bright either. There’s occasional sibilance, which is normal on the track.
Escape Route by Boris Blank
The sound is very spacious and dynamic. The bass isn’t massive but remains detailed and even. Percussion is snappy, and trumpets are lively and up front without being overwhelming.
Electrified by Boris Blank
Fascinating dynamics, great detail and imaging.
It Could Be Sweet by Portishead
The bass is tight and far from bloomy. Vocals are upfront but sound slightly different from what I’m used to—there’s something a bit off on this track.
The producer has applied techniques such as double tracking and reverb, which may be more exposed and sound different than usual due to the i5’s frequency response variations. Regardless, instrument separation and imaging are excellent.
As Before by Olga Konkova
The cajón drum lacks low-end punch but conveys a strong sense of reverb. The piano and vocals are clear and slightly crisp.
Midnight City by M83
This is a very busy electro pop/rock track. It’s presented with excellent dynamics and good detail. The bass is present and even, though not very strong. Overall, the sound is spacious with nice layering.
Rambling Man by Laura Marling
Vocals sound natural with good articulation, though a few notes feels a bit more articulated than I’m used to.
Sunrise by Norah Jones
The vocals are upfront, detailed, and quite intense, with a few notes sounding more prominent than usual—likely due to the spiky midrange shown in the graphs. The bass is polite yet detailed, while the piano remains clear without being overly crisp.
Limit to Your Love by James Blake
Lots of detail and texture in the piano and vocals. Vocals are upfront and energetic. The almost violent bass line is definitely not boosted and feels very controlled.
Summer 3 Vivaldi Recomposed by Max Richter
Quite energetic delivery, rather intense sounding, nice detail and imaging.
Mahler’s Symphony No 2 III by Paavo Järvi
The kettle drums have a tight punch and a dry character. There’s great instrument separation and the instrument timbre feels natural. Great dynamics in the mids and highs.
WRAPPING IT UP
Sound Signature
The Fosi i5 delivers well-extended, tight bass with a polite presence. The mids are generally upfront but slightly uneven in level. Treble is always a bit uneven in headphones, and the i5 is no exception; however, I generally find it relatively neutral in presence, neither too bright nor too dark overall.
Treble
The treble is very clean and there often is a liquid character to it. It’s airy and articulate without being overly sharp.
Midrange
The midrange is quite interesting. For the most part, it is very pleasant—slightly upfront and often a bit energetic, giving it a lively character. Occasionally, vocals may sound a little different than usual, but nothing extreme. The spikes seen in measurements account for some unevenness, affecting how certain instruments and vocals are presented. That said, the mids remain generally clean and detailed.
Bass
The bass is very neutral, with excellent extension and detail, and a polite quantity. This is mostly how I prefer it, and it is typical of open-back planars: great extension, clean delivery, and a mostly flat response curve.
If you want more bass quantity, you might consider the Fosi i5 non-fenestrated pads (29 USD), or “pads with no ventilation holes for i5 Planar Magnetic Headphones”, as they’re called.
Soundstage and Imaging
The soundstage is large and the imaging capabilities are impressive. There’s great instrument separation and a very black background. At this price, it’s hard to beat.
Detail, Dynamics, and Timbre
The overall detail is very good across the frequency range. Dynamics in the midrange and treble are also strong, but the low-end dynamics are less impressive. EQ can help, though I’m not a seasoned user and didn’t experiment extensively—I still couldn’t get the bass to match the energy of the mids and highs.
AMPLIFICATION
Amplification always matters, sometimes more than others. I have tried the i5 with many amps, and it responds noticeably to differences in amplification.
My ultimate test for planar magnetics is the Vidar 2 power amp. With it, the i5 becomes cleaner and more controlled, especially in the midrange, while gaining improved headroom, dynamics, and imaging. It simply turns into a much better version of itself than on most normal headphone amps.
When comparing amplifiers, the differences aren’t always immediately obvious but rather something that simply “feels” better. The frequency response is largely the same, yet small improvements – like 2% better dynamics, 2% better imaging, or 2% more accurate midrange – add up to a result that is significant as a whole.
Why speaker amps can make headphones sound better is a debated topic. It’s not that the headphones actually require such a powerful amplifier to be driven, but rather that an amplifier capable of delivering sufficient voltage swing and current at the right moment and speed can significantly improve performance. In that regard, certain components in speaker amps are well suited, like the large capacitors.
In other words, it’s certainly possible to get great results from a dedicated headphone amplifier, but for a headphone like the i5 to perform at its best, you need to give it what it needs in the way it needs it.
I won’t go much deeper into amplifier recommendations here, but it is something to keep in mind, especially when reading reviews and impressions.
At the end of the review, I got hold of a Violectric HPA V226, which turned out to be a fantastic match for the Fosi i5; it’s on par with the speaker amp setup. It’s a stunning combination. Violectric amplifiers are known for delivering substantial voltage swing, and in my experience, that matters more for planar magnetic headphones than many are aware of. The Violectric is a pricey amp, but the total cost of the V226 and i5 is certainly justifiable for what you get in audio quality. Consider me impressed.
COMPARISONS
Associated equipment:
- Amplifier: Topping A90
- DAC: RME ADI-2 DAC FS
I’ve spent some time comparing the Fosi i5 to some popular headphones in the same price range: the Meze 109 Pro, the Sennheiser HD660S2, and the FiiO FT7—two dynamic models and one planar magnetic. I have written dedicated comparison reviews of these. There are links after each section.
I used my two Topping A90 amps for the comparisons. While they are great all-rounders, they are not the optimal choice for the Fosi i5. That said, the main impressions regarding the differences between the headphones would remain consistent with other amplifiers.
MEZE 109 PRO vs Fosi i5
I generally find the Fosi i5 to be technically superior, but I often prefer the tuning of the Meze 109 PRO. They are different headphones with distinct strengths. The 109 PRO is, however, much lighter and more comfortable.
Full comparison review here:
Amazon: Meze 109 Pro
SENNHEISER HD 660S2 vs Fosi i5
Like the 109 PRO, the Sennheiser HD 660S2 has a different tuning and character than the i5. The HD 660S2 is smoother, warmer, and more intimate with its famously natural mids, while the i5 is cleaner, more dynamic, and technically stronger with better separation and tighter bass.
Full comparison review here:
Amazon: HD 660S2
FIIO FT7 vs Fosi i5
Compared to the i5, the FT7 has a more neutral and accurate tonal balance, making it the stronger choice for professional work or for anyone seeking highly neutral sound reproduction. It also sounds more spacious, with a slightly drier character in the midrange and treble. The i5 is often slightly warmer, more upfront, and energetic.
Full comparison review here:
Amazon: FiiO FT7
CONCLUSION
The i5 is an excellent-performing planar magnetic headphone for its price, with neutral bass, a lively midrange, and articulate but not overly bright highs. It offers a spacious soundstage with impressive imaging capabilities and detail. There is some unevenness, especially in the midrange, which at times can make the i5 sound less coherent than some competitors. However, with proper amplification, it takes on a lively, organic, upfront, and energetic character, delivering a vivid and engaging experience.
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