The Fosi i5 is a full-sized, planar magnetic, open-back headphone featuring a large driver with a 97 mm circular diaphragm. Fosi is an emerging company focused on delivering high-value audio at an affordable price. The i5 was originally launched on Kickstarter in the summer of 2025, and is currently available from various vendors, typically priced around 550 USD before tax.
The HD 660S2, released in 2023, is the latest addition to a proud lineage of headphones that began with the HD 580, followed by the HD 600 and HD 650. Both the HD 600 and HD 650 remain in production since their respective launches in 1997 and 2003, continuing to enjoy strong popularity among professionals and audiophiles. They all feature drivers with a 38 mm diaphragm. Like its older siblings, the HD 660S2 is a 300-ohm high-impedance headphone. The price is typically around 500 USD before tax.
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: 550g
Current price and availability:
- Amazon: Fosi i5
SENNHEISER HD 660S2 SPECIFICATIONS
- Circumaural open-back design
- Dynamic driver with 38 mm diaphragm
- Impedance: 300 Ohm
- Sensitivityl: 104 dB/1V
- Frequency response: 8-41,500 Hz
- Distortion factor: <0.04% (1 kHz, 100 dB)
- Weight: 260 g
- Velour ear pads
- Cables: 1.8 m (6.3 mm stereo jack) + 1.8 m cable (4.4 mm balanced) + 6.3 mm to 3.5 mm stereo adapter and carrying pouch
- Made in Ireland
Current price and availability:
- Amazon: HD 660S2
BUILD AND COMFORT
The Fosi i5 is primarily made of wood and metal and feels very solid, though it is heavy. However, the weight is well distributed across the large pads. The HD660S2 weighs less than half as much and is very comfortable, unless the clamp force is an issue. There is a solution: you can extend the metal part of the headband and gently bend it to your preference.
COMPARISONS
Associated equipment:
- Amplifier: Topping A90
- DAC: RME ADI-2 DAC FS
Jambi by Tool
The tonality is quite different between the two, with the mids standing out in particular. The HD660S2 has more midbass and a richer sounding presentation, while the i5 is clearer with a darker background. Overall, the HD660S2 sounds better balanced.
Smile by Pearl Jam
The vocals are not too different here, but I prefer the HD660S2. Both are dynamic, with the i5 sounding a bit snappier.
Almost Like The Blues by Leonard Cohen
The i5 has a very textured and tight bass. The vocals are highly detailed, though the reverb sounds more prominent than I am used to. On the HD660S2, the vocals are much richer with the reverb pushed further into the background, while the bass is less complex and has a thumpier character.
Black Crow by Cassandra Wilson
Once again, the i5 comes across as technically stronger. It sounds cleaner with better articulation. The vocals are good but slightly boxy and intense compared to the smoother and better balanced presentation of the HD660S2.
Some Day My Prince Will Come by Coryell Miroslav Quartet
The i5 sounds very well balanced, with bass and electric guitar that are articulate, detailed, nuanced, and warm. The HD660S2 also performs well, but it has more midbass and a less balanced and articulate presentation.
Bored by Deftones
The i5 is very clear and tight, with snappy percussion and nicely textured bass. The HD660S2 offers thicker bass and vocals that are less upfront and articulate but more natural. Both are very enjoyable.
Desert Island Disk by Radiohead
The HD660S2 sounds warm and intimate while still offering good detail. The i5 is more spacious, with better instrument separation and imaging. On this track, their tonality is fairly similar.
As Before by Olga Konkova
The i5 delivers a tighter-sounding cajón drum and clear vocals, with a piano intro that comes across as very intense. The HD660S2 feels more natural, still crisp but smoother, with vocals that are warmer, less articulate, and not as upfront
It Could Be Sweet by Portishead
The HD660S2 sounds well balanced, with smooth, excellent vocals and a pleasing bass presence, though the bass is not very tight. The i5 offers much tighter, but also leaner bass, along with crisper vocals.
Young Vivaldi by Modo Antiquo
The i5 is very crisp and upfront, while the HD660S2 is a bit smoother, though not laid-back. Both sound nice, each offering a distinct instrument timbre.
WRAPPING IT UP
Sound Signature
The Fosi i5 and HD660S2 have different sound signatures, though not drastically. The i5 features leaner, tighter bass and a more upfront, crisper midrange. The treble levels are fairly similar, with slight variations depending on the track, with one of them sounding brighter than the other at times, but neither are overly bright nor particularly dark.
Treble
Both headphones have good treble presence and neither sounds very bright or very dark. They both deliver good detail, but the i5 is generally more detailed. Which one appears brighter can vary depending on the track.
Midrange
The midrange is perhaps the most notable difference. The i5 has a crisper but somewhat uneven midrange, while the Sennheiser is renowned for its accurate, natural, and neutral midrange. However, the i5 reveals more detail.
Bass
The i5 has a very neutral, slightly dry, well-textured, and detailed bass, while the HD660S2 delivers more midbass, sounding warmer but with less detail.
Soundstage and Imaging
The HD660S2 has a nicely intimate soundstage with good imaging, but the i5 is significantly more open, with a darker background, better instrument separation, and improved layering, resulting in superior imaging.
Detail, Dynamics, and Timbre
The HD660S2 offers good detail, particularly in the midrange and treble, but overall the i5 excels at detail retrieval. Both sound dynamic, but the i5 is more articulate and lively in the midrange and treble, while the HD660S2 is smoother and more laid-back.
Timbre can vary between very different and relatively similar depending on the track. In general, however, I find the HD660S2 to have a more consistently natural sounding tonal balance and timbre.
AMPLIFICATION
So far in this comparison, I have been using the Topping A90 amplifier, which is an impressively consistent all-rounder. It drives every headphone I have tried well, though there are always more optimal pairings depending on the type of headphone. The Sennheiser HD660S2 and the Fosi i5 are very different in their amplifier requirements because they are very different types of headphones.
High-impedance dynamic headphones tend to pair extremely well with good OTL tube amps, while planar magnetic headphones need a lot of power. The HD660S2 pairs very well with the Woo Audio WA2 amp. I tried the Fosi i5 with my Schiit Vidar 2 speaker amp using Hifiman’s HE adapter, and the results were excellent. Having an amplifier that provides maximum control over the diaphragm is key.
When comparing the two headphones with these amplifiers, they sound closer to each other than when both are driven by the Topping A90. The i5 becomes more dynamic, cleaner, and more cohesive, especially in the mids, while the HD660S2 gains clarity and improved dynamics.
CONCLUSION
The Sennheiser HD 660S2 and Fosi i5 take different approaches: 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. Both are great alternatives, but which you prefer depends on whether you prefer a more relaxed and natural presentation or a more precise and energetic one.
