Fostex T50RPmk4 is the fourth generation of the T50RP — one of the longest-running planar magnetic driver platforms in audio, used in studio monitoring environments since 1974. The mk4 is the first generation with a fully redesigned RP driver since the transition to the mk3: new printed coil pattern, increased magnet count, rebuilt magnetic circuit, and a new symmetrical housing with sockets on both sides. Impedance dropped to 28 Ω while sensitivity increased to 97 dB/mW.
What changed in the fourth-generation driver
The third-generation RP driver ran for nearly 20 years — from the mk3 in 2015 through the mk4's 2024 debut. The generational difference is substantially deeper than the mk2-to-mk3 transition, which was primarily a tuning revision on the same driver geometry. The mk4 introduces a redesigned printed coil trace pattern that expands and equalizes the diaphragm vibration area, increased magnet count sandwiching the diaphragm for stronger controlled drive, optimized magnetic flux distribution eliminating self-resonance modes, and a completely new housing geometry. The electrical result is measurable: impedance dropped from 50 Ω (mk3) to 28 Ω, and sensitivity rose from 91 dB/mW to 97 dB/mW — the mk4 drives adequately from an audio interface or portable source without a dedicated headphone amplifier, whereas the mk3 required substantial output voltage in the same context. Frequency response extends to 10 Hz – 40,000 Hz, compared to 15 Hz – 35,000 Hz on the mk3.New cable topology and balanced connectivity
Previous T50RP generations used a left-cup-only proprietary connector. The mk4 introduces a 4-pole 3.5 mm TRRS socket on both earcups — the cable connects on either side with no tonal difference. Symmetrical housing eliminates the acoustic L/R asymmetry caused by differing internal structures between sides. The 4-pole socket is balanced-capable from the factory — optional balanced cables ET-RP2.5BL (2.5 mm), ET-RP4.4BL (4.4 mm), and ET-RPXLR connect without adapters. The T50RPmk4 retains its status as a modding platform: ear pad swaps, internal damping modifications, and acoustic tuning remain viable, with a significantly higher stock performance baseline than the mk3.KEY FEATURES
- 4th-generation RP driver — new coil trace pattern, increased magnets, optimized magnetic circuit
- 28 Ω impedance / 97 dB/mW sensitivity — drives from audio interfaces and portable sources without a dedicated amplifier
- Frequency response 10 Hz – 40,000 Hz — extended vs mk3 (15 Hz – 35,000 Hz)
- Symmetrical housing with bilateral 4-pole TRRS 3.5 mm sockets — no L/R acoustic asymmetry
- Balanced-capable out of the box — compatible with ET-RP2.5BL, ET-RP4.4BL, ET-RPXLR balanced cables
- Semi-open design — acoustically unified with T50RPmk4CL closed-back platform
- Low-rebound circumaural ear pads — optional Ultrasuede EX-EP-RP60 available separately
TECHNICAL SPECIFICATIONS
- Model: T50RPmk4
- Driver type: Planar magnetic RP (Regular Phase), 4th generation
- Design: Semi-open
- Impedance: 28 Ω
- Sensitivity: 97 dB/mW
- Frequency response: 10 Hz – 40,000 Hz
- Maximum input power: 3,000 mW
- Earcup connectors: 4-pole 3.5 mm TRRS (symmetrical bilateral)
- Weight: approx. 330 g (without cable)
- Included accessories: OFC cable 3.5 mm (4-pole to 3-pole, 2 m), 3.5 mm to 6.3 mm adapter
