Fostex T50RP mk4

The T50RP mk4 represents the culmination of 50 years of Fostex expertise in planar magnetic transducer technology. This semi-open audiophile headphone embodies the fourth generation of RP (Regular Phase) technology, bringing significant improvements in sound performance and ergonomics. Designed for sound professionals and demanding audiophiles, it offers remarkably precise sound reproduction, particularly in the midrange, while delivering impressive bass extension.

Fostex’s Acoustic Heritage

Fostex’s RP technology, first introduced in 1974 with the T50 headphone, is at the core of the brand’s sound identity. This unique approach relies on the use of a fully motorized planar diaphragm that enables regular phase movement (Regular Phase). The T50RP mk4 represents the fourth iteration of this iconic technology, which has shaped the professional audio industry for five decades.

Unlike conventional dynamic transducers, the planar transducers of the T50RP mk4 use a flat diaphragm with a zigzag-printed conductor on a polyimide film. This diaphragm is sandwiched between neodymium magnets, creating a sophisticated magnetic circuit capable of reproducing the subtlest sound nuances.

A Redesigned Fourth Generation

For this new generation, Fostex has completely redesigned its RP planar transducers. Engineers have modified the printed coil pattern to increase and balance the vibration area of the flat diaphragm. The number of magnets surrounding the diaphragm has been significantly increased, optimizing the magnetic flux distribution.

These technical improvements eliminate unwanted resonances and achieve even more precise transient response. The result is increased sensitivity (97 dB/mW compared to 98 dB/mW for the previous generation), a remarkably extended frequency response (10 Hz - 40,000 Hz), and superior transient characteristics.

Impedance has also been reduced to 28 ohms (compared to 50 ohms in earlier versions), making the headphones easier to drive with standard audio interfaces and mobile devices.

Symmetrical Design and Redesigned Ergonomics

The T50RP mk4 features an all-black design, more understated and futuristic than its predecessor, which included orange elements. The most notable change concerns the cable connection system, now symmetrical: both earcups are equipped with connectors, allowing the cable to be plugged in on either the left or right side, depending on your preference or usage environment.

This symmetry goes beyond mere convenience. By making the left and right housings identical, Fostex eliminates the sound characteristic differences typically caused by internal structural variations between earcups. This attention to detail reflects Fostex’s meticulous approach to sound reproduction.

The ear pads have also been redesigned, with increased thickness and low-resilience foam that better conforms to the shape of the head. This circumaural design (completely enclosing the ear) ensures optimal comfort even during long listening or mixing sessions.

Sound Performance and Acoustic Signature

The T50RP mk4 stands out with a warm and engaging sound signature, particularly notable in the midrange, which exhibits exceptional harmonic richness and coherence. This characteristic makes it a valuable tool for sound engineers working on vocals and acoustic instruments.

Unlike most semi-open headphones, the T50RP mk4 is capable of reproducing extremely deep bass, reaching down to 10 Hz, allowing the sub-bass to be perceived with great subtlety. The highs, while slightly recessed compared to some competing models, offer a natural presentation without harshness, ideal for long listening sessions.

The soundstage is particularly spacious, with excellent instrument separation and precise localization of sound sources. This ability to clearly position elements in the sound space is a major asset for studio mixing as well as for video games, where spatial accuracy is crucial.

Versatility of Use

Although initially designed for studio monitoring, the T50RP mk4 proves to be surprisingly versatile. Its reduced impedance and improved sensitivity make it compatible with a wider range of devices than its predecessors.

For sound professionals, it serves as a reference tool for evaluating midrange and mixing vocals. For audiophiles, it offers a warm and detailed listening experience, particularly enjoyable with acoustic recordings and small jazz or classical ensembles.

Demanding gamers will appreciate the precise sound localization, which allows them to quickly and accurately identify the source of sounds in the gaming environment, such as footsteps or distant gunfire.

Comparison with the Previous Model

Compared to the T50RP mk3, the mk4 features several significant improvements. Technically, the impedance has been reduced from 50 to 28 ohms, enhancing compatibility with portable devices. The frequency response has been extended from 15-35,000 Hz to 10-40,000 Hz, enabling more faithful reproduction of sub-bass and ultra-high frequencies.

Aesthetically, the mk4 adopts an all-black finish, more discreet and professional than the mk3, which featured orange cables. The earcup design has also been modified, with the connector moved to the center and new left/right markings now located on the headband instead of the earcups.

The major innovation, however, is the symmetrical design allowing the cable to be connected on either side, a feature absent in the mk3. Combined with the new, thicker, and more comfortable ear pads, this evolution reflects Fostex’s attention to user experience beyond mere acoustic performance.

Technical Specifications

General Specifications

• Type: Semi-open
• Transducer: RP Planar Magnetic
• Impedance: 28 Ω
• Sensitivity: 97 dB / mW
• Maximum Input Power: 3000 mW
• Frequency Response: 10 – 40000 Hz
• Weight: Approximately 330 g (cable not included)

Transducer Technology

• RP Planar Magnetic Transducer (4th generation)
  • Full-drive planar diaphragm
  • New printed coil pattern for an extended and balanced vibration area
  • Increased number of magnets for improved performance
  • Optimized magnetic flux distribution for reduced resonance
  • Enhanced sensitivity, frequency response, and transient characteristics
  • Optimized soundstage and localization

Design and Connectivity

• Symmetrical housing
  • Allows cable connection on the left or right side
  • Eliminates sound differences caused by structural variations
• Connector type
  • Uses a 3.5 mm 4-pole connector for balanced connections
  • OFC copper conductor cable for optimal signal transmission
  • Optional balanced cables available:
    • Φ 2.5 mm: ET-RP2.5BL
    • Φ 4.4 mm: ET-RP4.4BL
    • XLR: ET-RPXLR

Comfort and Ergonomics

• Thick, low-resilience ear pads
  • Circumaural design for increased comfort
  • Suitable for long listening sessions without discomfort

Included Accessories

• 2 m stereo cable, Φ 3.5 mm (4-pole) ↔ Φ 3.5 mm (3-pole)
• Φ 3.5 mm to Φ 6.3 mm stereo conversion connector