Fosi Audio i5

Luxury Meets Performance: The Fosi Audio i5 is a guilty pleasure

The Fosi Audio i5 is an open-back planar magnetic headphone that delivers a premium experience far beyond its price point. With build quality, comfort, and aesthetics that rival headphones in the $1000-$2000 range, the i5 represents an exceptional value proposition in the competitive planar magnetic market.

So, having listened i5 both in its ‘original’ configuration for a few weeks and now for the past 2 weeks after applying the port seal ‘mod kit’ I have a pretty good idea of what you can expect via the Kickstarter. Without spoiling the rest of the review, I would say you get a lot of ‘headphone’ for your money, but that is not to say it is perfect, I compared it to some headphones in a similar price range that tonally much more accurate headphones but there is something special about the i5 that made it my goto evening headphone, as I said it in title, it is a guilty pleasure.

But before I get onto details of the sound and what you get with the i5, I thought it might be worth mentioning some of my past experience with Fosi Audio.

Fosi Audio: A Company That Listens

One of my earliest amplifier reviews a few years ago, I pointed out some issues with one of their amps, and instead of ignoring me (after all, I am basically a ’nobody’), they both respected my review, but quietly shared a prototype of their first ‘TPA3255’ chipset amplifiers and the TPA3255 chipset was a ‘game changer’ in the lower cost class D amplifier world.

So I was again impressed with Fosi Audio handling of the i5 when the early reviews came out the i5 mentioning the extreme sub-bass roll-off, Fosi Audio quickly designed a solution in the form of a mod-kit that addresses this issue.

So I received mine without the mod-kit, so I got to experience the original ‘sound profile’ but once I did receive the mod-kit, I upgraded and I will go through that process below. But the funny thing is there were many tracks that sound better to me before the mod-kit was installed. But in general it is an improvement with the majority of music I tried. The obvious yet odd thing with music (‘Circle of confusion’), alternative Headphone (and speaker) tuning’s can bring out more details in certain tracks so can be fun alternatives to have in a collection.

Anyway, I thought I might take you through that process, so you can see the internals of the i5.

Note: I believe everyone receiving one via the Kickstarter program will actually get a swapping ‘ring’ so can actually decide for themselves which sound profile is the best:

The Mod-Kit Upgrade Process

This is the purpose of the mod-kit - it is basically a ‘seal ring’ around the drive to stop the bass ’leaking’:

The mod-kit comes in a small box with all necessary parts:

To install the mod-kit, you first need to remove the earpads, which are attached with velcro:

Note: I am not a fan of Velcro, but it does the job here. I do hope Fosi Audio or some third party vendors provide alternative earpads using Velcro as quickly changing the sound profile of a headphone is one of the pleasures of the hobby.

This reveals the driver and the screws that need to be removed:

Note: exposing planar drivers like this can be a bit ’nerve racking’ but all went well.

I thought this feature was a little odd ( and I do wonder if this causes some of the midrange unevenness you will hear me talk about later):

The mod-kit includes a special ring that prevents bass leakage:

The frequency response measurements before and after the mod-kit installation clearly show the improvement:

But I will get onto the sound and the frequency response in detail a little later in the review, but first assuming you recieve the i5 via the Kickstarter with the mod-kit already installed what will your unboxing experience be like:

Unboxing and Accessories

The i5 arrives in premium packaging that sets the tone for the high-end experience to follow:

Opening the box reveals the headphones nestled securely inside:

Removing the foam covering the headphones:

The cables are thoughtfully packaged in separate compartments:

The included cables are high quality, though if I was to nitpick these cables are slightly on the heavy side:

The connectors are robust and well-made with both 3.5mm and 4.4mm cables:

And a simple 3.5mm to 6.35mm adapter is also included:

So what is the headphone itself like:

Build Quality and Design

I found the build quality of the i5 to be exceptional, rivaling headphones that cost several times its price. The materials used throughout exude luxury and durability. This feels and looks like a $1000 headphone from the moment you pick it up.

The headband features premium materials with excellent padding:

The extension mechanism is smooth and sturdy:

The combination of wood, metal, and premium synthetic materials creates a cohesive and luxurious aesthetic:

The wooden finish on the ear cups adds a touch of elegance:

When placed on a headphone stand, the i5 makes a striking visual statement:

Overall, I am super impressed with what you get for the money, I can’t think of any headphone that looks and feels like the i5 in this price range:

Comfort and Fit

The also i5 excels in comfort with its plush earpads and well-distributed weight. Despite being a substantial headphone at 550g, the weight is well-balanced, making it comfortable for extended listening sessions.

The earpads are exceptionally luxurious and comfortable:

They provide ample depth to prevent your ears from touching the drivers:

Almost hard to get from these photos how luxurious deep these earpads are:

Here are some internal dimensions:

My only worry would be for someone with a smaller head, but my wife, who has a tiny head, was very impressed with the comfort,though she did laugh when she looked in the mirror, as all she could see was the headphone.

I took some photos on a dummy head so people might get an idea for the size of this headphone, though this is at maximum extension, so I cheated a little in this photo:

Sound pairing

Before I get into the sound, I thought I would share some of the amplifiers I was using with the i5 over the past few weeks. I have been typically using a Schiit Midgard with my headphones evaluations for the past year, and while I did use my beloved Midgard again with the i5, I have also been evaluating a few exceptional clean and powerful SMSL headphone DAC amplifiers in the past few weeks.

The i5 looks right at home in a “working from home” desktop setup, where I am currently using an SMSL C200 Pro amplifier (review coming in a few days):

Though it dwarfs the C200 Pro, the C200 Pro has easily enough power to drive the i5 exceptionally well.

As you will see later in the specification section, but the i5 is relatively easy to drive for a planar.

I also used it with my Macbook Pro and it drove it without concern, Apple have done a good job with the headphone output on the recent Macbook Pro’s.

For my evening listening I have been pairing the i5 with an even more powerful SMSL DAC the D0400:

Sound Impressions

So my initial sound impressions were made before the mod-kit arrived but since it will be installed as standard, I have mainly focused these impressions on the sound once the bass-seal ‘ring’ is installed.

Bass

With the mod-kit installed, the i5 delivers impressive bass extension and impact (especially compared to without the mod kit), its bass was much more like a planar bass that you should expect. The planar driver provides the characteristic speed and control expected from this technology, with good texture and detail. While not bass-heavy by default, the low end is well-balanced and can be further enhanced through EQ if desired, as I measured minimal distortion.

James Blake – “Limit to Your Love” - With the bass-seal mod-kit in place, the i5 brings out the full gravity of the iconic sub-bass drops in Limit to Your Love. The low-end dives deep with a rich, rumbling presence that feels physical but never sluggish. Each pulse is controlled and articulate, showcasing the planar driver’s speed and authority. The i5 doesn’t exaggerate — instead, it presents the bass with finesse, allowing the piano and vocals to float clearly above, while the sub-bass rolls beneath like an underground current. It’s a good example of how much depth and control the i5 gains with proper sealing.

Midrange

The midrange of the i5 is where I think there is significant room for improvement, to be honest, it is uneven and will not suit a ‘purest’ but this midrange is also where the ‘guilty pleasure’ in my review title comes from, as this uneven midrange does make it an interesting headphone to listen too, as some instruments will be more enhanced while others will sound more distant.

I thought I would pick 2 songs here to illustrate my point:

Beirut – “Santa Fe” - On Santa Fe, the i5 delivers a lush and addictive midrange character. The rich brass and quirky layered vocals sit slightly forward in the mix, with a kind of glowing presence that flatters the track’s nostalgic tone. While not entirely accurate, the i5 adds a certain saturated charm to the sound — some instruments shine with unexpected prominence, making the whole listening experience feel emotionally heightened. It’s not studio-flat, but it’s engaging in a way that makes you keep coming back.

Joni Mitchell – “A Case of You” - so with this track, for example, the i5 reveals its midrange inconsistency. Joni’s voice — usually intimate and raw — feels slightly withdrawn and less textured than it should. The dulcimer and guitar also lose some of their delicate harmonic richness, making the emotional core of the song feel just a bit distant. The unevenness in this frequency range doesn’t ruin the track, but it does flatten its depth, reminding you that the i5’s tuning leans more toward casual immersion than critical accuracy.

So, normally a headphone with this type of midrange would not get a recommendation, but there is something interesting and hence ‘guilty’ which I think might be more about the overall sense of luxury that this headphone provides, that made me want to pick it each evening for the pleasure of seeing what some of my favourite music sounds like with it.

Treble

The overall treble presentation is clear without being harsh. The treble most extends well and provides good detail, though it shares some of the same peakiness issues as the midrange but most headphones have some treble variations, especially on different heads. I never found it sibilant, and overall I think the treble is well done in this price range.

Agnes Obel – “The Curse” - On The Curse, the Fosi Audio i5’s treble character comes into subtle focus. The track’s delicate piano lines and shimmering strings benefit from the headphone’s upper treble lift, which adds a gentle airiness without ever becoming harsh. While the overall tuning leans slightly dark — keeping sibilance and glare comfortably in check — the i5 still manages to pull surprising detail from the subtle layering in the vocals and room ambience. It’s a great example of how the i5 balances smoothness with enough sparkle to keep acoustic and classical textures alive.

Soundstage and Imaging

Whatever it is about large earpads and large planar drivers, but they definitely provide a much wider soundstage for me. So typically the Hifiman ‘Egg shaped’ headphones like the Edition XS or the Arya Stealth provide excellent sound stage, but the i5 matches these Hifiman in soundstage. Imaging was also precise, allowing for accurate placement of instruments and vocals within the sonic landscape.

The i5 creates an immersive listening experience that works particularly well for orchestral music and well-recorded acoustic performances.

Specifications and Measurements

Specification	Details
Device Type	Open-Back Wired Headphones
Transducer Type	Planar Magnetic Drivers
Maximum Sound Pressure Level	≥120 dBSPL
Frequency Response	10Hz - 50kHz
THD	< 1% @100 dBSPL
Sensitivity	98dB/mW @ 1kHz
Nominal Impedance	28Ω
Recommended Power	≥ 500mW
Minimum Recommended Power	100mW
Weight	550g
Dimensions	16cm(6.30 in) × 24cm(9.45 in) × 11.5cm(4.53 in)

As you can with 98dB @ 28-Ohms there is no problem driving this headphone with my devices, even an Apple Dongle would be ok, but for peace of mind, I would pair it with a more powerful amplifier.

Measurements

All these measurements were taken with my KB501X soft ear pinna with a 711 clone coupler, the Fosi Audio measurements are available on my measurement database: here

Frequency Response

The frequency response measurements show, the significant improvement in the sub-bass after the mod-kit installation:

So here is a comparison after the mod-kit with 2 classic headphones in this $300-$500 price range:

I have highlighted 3 areas which might be of interest from these measurements:

1. Edition XS and i5 have that classic linear bass extension while the HD600 rolls off
2. The i5 has some problems in this area while the HD600 is perfect and the Edition XS goes for a recessed ‘soundstage’ FR
3. In this range, the Edition XS might have too much treble, while the i5 is more relaxed / dark

But all three headphones from 10K onwards were similar with a good treble extension.

Some EQ Recommendatiosn

Until we see more measurements, there is a danger in using one rig measurements. But for now, here would be my recommendation for tweaking the midrange. You can also increase the subbass, but I was focusing more here, on sorting out the midrange which technically can be reliably done as it is the treble that will be different for everyone and would need more personal EQ:

So this EQ is basically about tidying up that midrange area. This area is super important for tonality. The presense region which is were most of the forward vocals ‘presence’ will be handled (for example, whether the vocalist appears in front of the band) is also brought into a more harman ‘preference’ curve FR. I think for me with this EQ it bring the i5 inline with the better headphones for tonality.

Here are those filters:

Preamp: -6.3 dB
Filter 1: ON PK Fc 520 Hz Gain -2.2 dB Q 2.000
Filter 2: ON PK Fc 700 Hz Gain 4.5 dB Q 2.000
Filter 3: ON PK Fc 1000 Hz Gain -5.0 dB Q 1.800
Filter 4: ON PK Fc 2200 Hz Gain 3.8 dB Q 2.000
Filter 5: ON PK Fc 4400 Hz Gain 6.0 dB Q 1.700

Distortion

Distortion measurements show excellent performance, with no concerns even when applying EQ:

Distortion percentage remains well controlled:

Comparisons

Because I feel the i5 is more like a ‘high end’ headphone that a typical good $300-$500. I thought I would compare its frequency response with some of the ‘big boys’ in the more ‘High end’ market. So in this case Meze, Audeze and Focal, but to do this justive I have to switch over to Gadgetry Tech measurements:

Note: since Gadgetry Tech has a different i5 and a different measurement rig than I have, these measurements should only be used for illustrative purposes.

Firstly, at the time of publishing this review, he has no measurements of his i5 with the mod-kit so just comparing mine with his:

So I didn’t want to mix my measurements with his, but ignoring the subbass rolloff (1), I hope you can see some similarities between this i5 and some of these headphones, I specifically highlight the slightly dark treble (2):

But finally some physical comparisons with some similar priced headphones and the new FiiO FT7:

You can see how large the i5 is but also the build quality with only the FiiO FT7 with its carbon fiber feeling as high-end as the i5.

FiiO FT7

The FiiO FT7, while approximately twice the price of the i5, offers a better, more neutral sound signature. However, the i5 arguably has superior build quality and comfort, though the FiiO FT7 carbon fiber body does have significant weight advantages over the i5, and it has a few other tricks to justify its price point. But when I have had both headphones sitting on my coffee table, most people will gravitate to the i5 as the more expensive headphone.

Hifiman Edition XS

The Edition XS is a well-regarded planar magnetic headphone in a similar price bracket. The i5 offers better build quality and comfort, while the Edition XS has a more refined default tuning. After EQ, both headphones perform admirably, but the premium feel of the i5 and probably build quality advantages for longitivity gives it an edge in overall user experience.

Rating

The Fosi Audio i5 earns a 4.5 pragmatic rating for several compelling reasons:

1. Exceptional build quality and comfort that rivals headphones costing 3-4 times its price
2. Luxurious aesthetics with premium materials and attention to detail
3. Responsive manufacturer that quickly addresses issues and implements improvements and a good platform for alternatice earpads
4. Outstanding value for the overall package in the kickstarter (even the late pledge is excellent value)
5. Something a bit different It is a funny thing to say but I can’t think of another headphone in this price range like it.

But I knocked it down a 1/2 a star as the default tuning will benefit from some EQ adjustments, whether this is a minor or major consideration is probably more subjective given the headphone’s strengths in every other area. The i5 represents a pragmatic choice for anyone seeking a premium headphone experience without the premium price tag.

Conclusion

The Fosi Audio i5 is a fantastic headphone that offers exceptional value, particularly for those who appreciate premium build quality, comfort, and aesthetics. While the default sound signature might not suit everyone, especially in the midrange, the application of some basic EQ can transform the i5 into a truly outstanding performer.

After the bass is fixed with the mod-kit ring, I would still recommend applying some EQ to tidy up the midrange unevenness. With these adjustments, the i5 becomes a fantastic addition to any headphone collection – a perfect choice for evening relaxation sessions when you want to indulge in both sonic and tactile luxury.

For the comfort, build quality, and overall package offered at its price point, the Fosi Audio i5 stands as one of the most compelling value propositions in the current headphone market. It’s a headphone that feels like it should cost significantly more than it does, making it an easy recommendation for both enthusiasts and casual listeners alike.