Fosi Audio i5 Open-Back Planar Magnetic Headphones

I ordered and received the white box no-frills version directly from Fosi Audio. A small cloth bag for the standard common-ground cable with 3.5mm TRS connector and the 6.35mm adapter was included, but nothing for the headphones. After burn-in and some minor mods and/or EQ settings described below, I find the i5 to be a decent first offering by Fosi Audio. It is a stunning headphone in need of a better voice to match its beauty. More work in sound quality refinement is needed to be competitive with other headphones of comparable price. There are better sounding headphones at half the price, albeit not as well designed.

Note: users are cautioned that modifications to the headphone or the stock ear cushions will void the warranty. An alternative would be to use EQ on the unmodified i5 to tailor the sound signature to your preference. Here are some suggestions from Fosi Audio:
Try a gentle –2 to –3 dB cut near 1.2–1.5 kHz (Q≈1.0) to ease midrange forwardness.
Add +2 dB around 3–4 kHz (Q≈1.0) for presence, and a +2 to +3 dB high-shelf from 8–10 kHz to increase air.
With the gasket removed, if bass becomes too heavy, a subtle –1 to –1.5 dB around 180–250 Hz can help.

First and foremost, while the design of the i5 is undeniably gorgeous and the build quality is impressive, I find the sound signature to be too midrange-forward and treble-relaxed for my preferences. Compared to other planar magnetic, dynamic driver, and electrostatic headphones that I own, including some that are priced significantly lower, the i5 is deficient in upper treble airiness and lacks refinement in resolving details that I typically enjoy. The i5's laid back sound signature has even led me to fall asleep on multiple occasions while listening, which speaks to its soothing nature but not to its suitability for resolving details and nuances in music.

The comfort level is commendable with the adjustable head strap and a comfortable clamping force. It is a heavy headphone, so individual comfort will vary. However, I feel the ear cushions are a bit too thick and deep. A larger open area would help reduce obstruction of the driver and improve sound stage and airiness. I am considering trying shallower ear cushions that place the ear closer to the driver unit, but alternative cushions with the Velcro-like fasteners have yet to become available. I will have to improvise to retrofit similar shaped ear cushions from another manufacturer.

In terms of sound quality, the most problematic issue is unnatural timbre in voices due to unevenness in the 500Hz-2kHz midrange, likely due to driver modes and resonances. This was my immediate reaction upon first listening as evidenced by a shouty and out of phase character in voices, suggesting a quality issue with the driver units' response lacking smoothness and not well matched in this critical midrange region. Additionally, the 2kHz-6kHz range is recessed, lacking some clarity, and there’s further frequency unevenness above 6kHz. Sound source localization and identification on the soundstage is good, especially with balanced audio cables. Vocals are focused and pulled forward. Soundstage size and ambience rendering are adequate. Overall, the midrange-forward sound of the stock i5 is not my cup of tea.

Interestingly, I discovered that the factory installed rubber gaskets, intended to seal the ear cushions against sound leakage, may actually exacerbate the driver modes and frequency unevenness. After removing the gaskets and carefully reseating the ear cushions to ensure a uniform seal, I found an improvement in bass extension and a slight mitigation of the midrange frequency issues, though not a complete resolution. The bass is quite palpable and solid without the gaskets.

Burn-in during the initial 10+ hours of use has helped significantly to stabilize/relax the drivers, further reducing frequency unevenness and midrange timbre issues, and also to solidify bass response. One other modification I made was replacing the driver grill cloth with a more acoustically transparent material (fiberglass screen mesh), which has unveiled treble frequencies and added airiness to the sound, making the i5 more suitable for my non-critical listening enjoyment. The drivers were fully stabilized after about 15-20 hours of use.

In summary, while the i5 headphones are visually stunning and comfortable, the midrange-forward, relaxed treble sound signature may not appeal to everyone, especially those seeking a more detailed and airy listening experience. The i5 is a decent first offering by Fosi Audio, but more improvement in sound quality refinement that is commensurate with the i5's exceptional visual attributes/build quality is needed for it to become a more distinctive, delightful product amidst the competition. For users inclined to perform EQ, the i5 can be an enjoyable headphone.

The i5 is an amazing planar headphone.

It is extremely comfortable, sound stage is enormous, there is a lot of bass and sub-bass but it is not over emphasized or muddy. I would call it a balanced tuning. Instrument timbre is spot on as is note separation.

I have several other planerbmagnetics from hifi-man and foster. The i5 is much easier to drive and is much more resolving.

Only drawback I’ve found is it’s difficult to sleep with them on, my brain is too stimulated listening to the music and hearing new things in favorite recordings!!!

I really did not know what to expect coming from Hifi speakers I just based my purchase on youtube reviews. The Pearl Acoustics guy sold it for me.

What I can say is this Its super duper Comfy, made me hear sounds impossible for Mid range speakes around $1000 below. I just say Orchestra music is divine with this headphones.

My 1 and only grunt that is why 4 stars only is I bought the upgraded cable and im using Fosi GR70 and its not compatible. I was like god damnit I have to order an adaptor?? For real? They sent an adaptor for 3.5mm but the upgraded 4.4mm noo i have to buy an adaptor for the GR70

Aside from this. 5 stars

Fosi nailed it with the i5! Such a good headphone in all aspects! The build is really good. Despite being heavy, the weight distribution is balanced which makes it quite comfortable on my head. And the sound is very enjoyable! I love playing around with eq on these as they react very well! I totally recommend these headphones!

Excellent headphones at a bargain price.