With the i5, Fosi Audio is entering the planar magnetostat segment for the first time, targeting discerning listeners who are looking for high impulse fidelity and detailed reproduction at a moderate price. To date, Fosi Audio has primarily attracted attention with affordable, technically clean headphone amplifiers and compact DAC solutions in the lower to medium price segment. With the i5, the company is venturing into planar headphones for the first time, with the aim of serving the widest possible audience with a neutrally tuned, high-resolution system.
The project is being financed via a Kickstarter campaign, which is clearly aimed at enthusiasts. Depending on the package, the price is around 342 euros (excluding shipping), which puts the i5 in direct competition with devices such as the HIFIMAN Sundara or the Monolith M1070. The launch is strategically well chosen: With the popularity of planar-magnetic drivers in the community and the interest in linear sound profiles, Fosi is offering a product that clearly positions itself conceptually.
Important preface to the reviews
In this test report, for the first time I have deliberately chosen a different approach to that familiar from classic hi-fi reviews. Instead of just talking about “silky highs” or “musical mids” in vivid poetry, I am concentrating instead on comprehensible, measurable and reproducible parameters. This time, the focus is clearly on combining objective data such as frequency response, burst decay analysis and spectral decay with my own precisely categorized auditory impressions. The music comes at the very end.
My declared aim is to create an overall picture that is comprehensible to everyone, which harmonizes technical characteristics, sonic effect and practical correction options and avoids any hi-fi voodoo. All evaluations and recommendations – especially for digital equalization using the APO equalizer – are derived directly from these correlated observations. I would be delighted if you would share your impressions and comments in the forum after reading this article.
Scope of delivery
Fosi Audio’s scope of delivery is pleasingly practical. The i5 is delivered in a large, magnetically lockable box, in which the headphones are well protected in custom-made foam.
Two different cable assemblies are included: a balanced 4.4 mm Pentaconn cable and a classic 3.5 mm jack cable including adapter to 6.35 mm. Depending on the bundle selected, the packaging also contains an additional pair of ear pads and a higher-quality 4.4 mm cable. A padded carrying bag and documentation are not (yet) included. Nevertheless, for a crowdfunding product, the scope of delivery seems well thought-out, almost complete and practical – without frills, but functional.
Technology, wearing comfort and connectivity
The unboxing is unspectacular, but neat: the first impression is characterized by a clear structure. The headphones themselves appear larger than expected and are visually reminiscent of a mixture of HIFIMAN HE560 and classic Audeze models. The housing of the i5 consists of a combination of plastic, coated metal and a real wood application in walnut look.
The Fosi Audio i5 is a circumaural, open-back headphone with a planar-magnetic drive system designed for maximum detail and controlled transient response. At the heart of the i5 is a 97 millimeter planar driver whose diaphragm has been manufactured to a thickness of just 2 micrometers using the target sputtering process. In conjunction with 44 symmetrically arranged N50 grade neodymium magnets, this results in a strong, precisely controlled magnetic field that drives the diaphragm with minimal inertia and virtually no delay. The acoustic rear chamber is equipped with targeted damping to suppress standing waves and uncontrolled modes, which is reflected in the burst decay measurements by an overall short settling time.
The Fosi Audio i5 is electrically specified with an impedance of 28 ohms and a sensitivity of 98 decibels per milliwatt at one kilohertz. The frequency range extends nominally from 10 hertz to 50 kilohertz, with the usable acoustic energy being most pronounced in the range between 30 and 18,000 hertz. The distortion factor is below one percent at 100 decibels sound pressure, which is appropriate for the driver size and magnetic control. In practice, this results in a moderate amplifier requirement, whereby an output of at least 100 milliwatts makes sense, but 250 milliwatts or more is recommended in order to fully utilize the dynamic reserves.
The mechanical design allows the ear cups to be swiveled and tilted generously. These are swivel-mounted and connected to the headband via a self-adjusting silicone strap system. The headband adjustment is precise and tight, but smooth-running.
In terms of price, the workmanship is above average: no rattling, no burrs, clean transitions. The design is functional, but with a visible claim to quality.
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With a weight of around 520 grams, the i5 is clearly in the upper range of what is still considered comfortable in everyday use. Nevertheless, the load is evenly distributed thanks to the wide, softly padded headband.
The ear pads consist of a combination of velour, perforated artificial leather and textile surfaces, which not only ensure comfort but also sufficient ventilation. The contact surface is generously dimensioned, the inner diameter also fits larger ears. The i5 can also be worn for longer sessions as long as the head is not overly sensitive.
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Connectivity is classic via dual 3.5 mm inputs on the ear cups. This makes the i5 compatible with almost all common aftermarket cables. Both balanced and unbalanced connections are possible without any problems, which also makes the headphones attractive for stationary setups with balanced cabling.
The i5 positions itself as a technically ambitious headphone with high optimization potential, which performs solidly in terms of measurement technology, but only reveals its full strength with tuned correction. Compared to the Sundara, it is more direct, compared to the LCD-2 Classic more pragmatic – and in any case a serious entry into the world of planar magnetic headphones. And that’s exactly what I’m going to test and prove today.
Technical data
Fosi Audio i5 Brochure
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