This time, I’m reviewing Fosi’s revised version of their debut headphones, the Fosi Audio i5 Rev2. It sells for $549,99 USD.
Disclaimer: Fosi Audio is not related to Headfonia. They sent me the second i5 free of charge to be featured on Headfonia.com.
Fosi Audio
The Shenzhen-based brand Fosi Audio entered the market in 2017 and has since provided the audio market with a steady stream of high-value products, from DACs and headphone amps to dedicated speaker power amplifiers.
The name “Fosi” embodies the four core principles of the company: foresight – thinking further ahead; openness – embracing users’ voices; sincerity – serving you wholeheartedly; and innovation – upholding technical excellence.
So far, Fosi Audio have got a good reputation here with their V3 monoblock class-D power amplifiers impressing Nanotechnos enough to call them “a steal” for the price Fosi are asking. We’ve also reviewed Fosi’s higher-end offshoot Mamoritai Audio’s headphone amp, the Euphony, which, on the other hand, didn’t exactly steal Nanotechno’s heart.
At over USD 500$ retail price, the i5 headphone is one of Fosi Audio’s priciest offerings. For now, it’s only available from the company’s Kickstarter campaign.
The first sample of the i5 we got from Fosi showed a lot of potential, but I agree with my colleagues from other outlets – the low-end response suffered from a noticeable roll-off.
Enter the Fosi Audio i5 Rev2
The basic construction of the Fosi Audio i5 Rev2 is largely unchanged. It’s an open-back planar magnetic or orthodynamic headphone with an infinite baffle acoustic formula. This means that the diaphragm is coupled to the ear. It can be felt when putting the headphones on as the diaphragm flexes subtly when the earcups compress.
In the previous version of the i5, the Velcro that holds the pads to the earcups didn’t seal and the working principle was more akin to an open baffle. Thus the low-end response of the headphones ended up being compromised and rolled off after the driver’s fundamental resonant frequency of around 80Hz.
According to Fosi, all i5 versions from now on will feature the extra sealing ring to improve the low-end response of the headphones. This means that the bass-lite version will likely become quite rare as it has mostly reached a couple of super early backers and most reviewers who, like me, will have to update their reports.
Features:
- Open-back over-ear headphone, infinite baffle acoustics
- 97mm, push-pull planar magnetic driver, 2um diaphragm
- 10 – 50 000Hz advertised frequency response
- 28 ohm purely resistive load
- 98dB/mW or 113.5dB/V
- Weight: 550g without a cable
- Input connectors: 3.5mm TRS
- Cable jack: 4.4mm TRRRS, 3.5mm TRS (6.35mm adaptor included)
- Cable length: 1.5m
Sound Signature and Technicalities
My main testing system for the Fosi i5 was the Topping x Holo Centuarus driving the DROP + SMSL HO150X, Feliks Euforia EVO, and Aune S17 Pro EVO.
The driver of the Fosi i5 remains unchanged, therefore, nothing changes in terms of driveability – most headphone amplifiers and all-in-ones will do the trick. OTL tube amps can be used, however, the low impedance means only the most powerful will be able to source enough power at the 28-ohm load the i5 presents. My Euforia EVO was able to drive the i5 quite loudly without clipping, but it’s not my favorite pairing.
Overall Tonality
The improved Fosi i5 proves that a relatively simple fix can transform how a headphone sounds. By maintaining a seal all the way from the driver diaphragm to the eardrum, the i5 finally achieves true full-range bandwidth. This has a profound effect on the overall sound and makes the response less top-heavy.
Bass
Previously, I stated that the rather conservative approach to the low-end tuning is what dictated the rather analytical tuning of the initial i5. Well, this has changed! There’s a bottomless extension I’ve come to expect from planars. At the same time, the i5 doesn’t go full Harman with the sub-bass boost and keeps it rather tactful.
Make no mistake, this is still orthodynamic bass with fast and clean transients. There’s almost no bloom, and decay dies off into the blackness quite quickly. With electronic performers like Trentemoller, the i5 shows both its muscle and the capability to turn on a dime.
The addition of full-range bass transforms the i5 into a much balanced and less introspective listening experience. Its musical genre bandwidth has expanded tremendously, and there’s finally a good bit of heart to the previously all-brains tuning.
Midrange, Treble, Technicalities
There are no changes to the rest of the spectrum, and I’m glad that there aren’t. Often, physical modifications to an acoustic system incur unintended changes, but not this time. Only the low-end has been affected, and the rest remains the same – a highly technically capable window into recordings. Only the balance has been affected, and now it’s very neutral with every tonal region getting the same amount of attention.
Comparisons
Sennheiser HD6XX
The classic from Sennheiser is the go-to as a tonal balance reference. At the same time, it’s not an exaggeration to admit that, technically, its 40mm dynamic driver is starting to show its age. The Fosi i5 highlights it with devastating precision.
With the fixed low-end response, it walks over the HD6XX when it comes to low-end extension and technicalities. The HD6XX can be a breathtaking experience when paired with a capable setup, but the i5 achieves the same without the heroic efforts in accompanying electronics.
Moondrop Para
The entry-level planar from Moondrop took me by surprise. It delivers a highly technical performance that’s unheard of at the price point. Sure, the build quality is a bit hit-miss, and its 8-ohm, low-efficiency design is a mainstay in my stable as an amplifier torture device, but I admire the effort nonetheless.
At almost the same price as the early-bird deal Fosi i5, the Para feels like a DIY project in comparison. It’s still a highly technical sound with most of the planar benefits, but the improved bass of the i5 is the coup de gras. If you have to choose, go for the i5 every time – it improves on every aspect of the Para.
HarmonicDyne Baroque
Finally, a worthy opponent! At $559 USD, it’s a good bit pricier than the early-bird Fosi i5, but after that deal is through, the prices become almost the same. The Baroque is an open-back design with a freshly developed 60mm (!) ceramic-metal diaphragm dynamic driver. It’s also quite well-made and wears rather comfy.
Both headphones are on par when it comes to technicalities. What differs is the way they’re presented. The Para feels more ethereal, even electrostatic at times, while the Baroque hits harder and delivers a more visceral performance. It’s evident that there’s some bass-boost going on with the Baroque, but never at the expense of the critical midrange.
Conclusion
When I was writing about the first revision of Fosi i5, I was THAT close to giving it the Headfonia Recommended award. The only thing that kept me was the deficient bass, as I knew that many would yearn for more booty. It’s 2025, and it’s okay for audiophiles to like low-end!
I wasn’t the only reviewer who pointed out the potential problem in the lows, and Fosi responded with a fix that’s both simple and brutally effective! Now the i5 truly does it all, and I can safely recommend it for everyone looking for an open-back under $1000 USD. It will be a good deal even after the early-bird period, and I can’t wait to see what Fosi can do in the future.
Audeze, Hifiman, DCA, and other planar specialists should keep their eyes peeled!
Summary
Pros:
- Supreme clarity across the audible band
- Neutral mids and treble
- Accurate imaging
- Very comfortable
- Flagship grade technicalities
- Takes a lot of EQ
- Superb build quality
- Some of the best bass for the price
Cons:
- An ear-gain region dip makes vocals a bit distant-sounding
- Your neck might notice the weight
- No carrying/storage case
- With Kickstarter pricing, the value for money can fluctuate




