The first time a Soundnews reviewer went to a headphone meet, he brought an Audeze LCD-3, and the host was listening on a Fostex TH900. It became the most discussed headphone of the evening, "awakening some subterranean bass notes that I never knew were there in my music." The group nicknamed them "disco headphones" because they were so much fun to listen to. Fostex builds its headphones around drivers of its own design. The TH900 and its siblings use a diaphragm of light biocellulose fiber, which starts and stops quickly and damps its own ringing. The RP headphones are planar magnetic, meaning a thin film held between magnets is driven across its whole surface at once. GREQ Reviews first heard the TH900mk2 at High End Munich and "knew instantly that the hype was real." Fostex suits listeners who want deep, tight bass and wooden cups, and studio users who want a planar for monitoring.

The RP Series is the way in. The T50RPmk4 is a semi-open studio planar, and headphonecheck.com says it is a good match for the low-voltage outputs of audio interfaces and smart devices. Its cable plugs into either earcup, which helps left-handers. The T60RPmk2 adds solid black walnut cups, and Carsten Kaiser of headphonecheck.com heard a warm balance that favors acoustic, jazz, blues and classical recordings. The TH Premium models are the hifi line, and the closed TH900mk2 is the classic. Carroll Moore of MajorHiFi wrote, "If you're searching for that open-back sound in a closed-back headphone, you've found it." These are easy to drive, so a phone or dongle will play them, though Jordan Aaron of MajorHiFi found the TH808 gained dynamics with more power. Headfonia liked the open TH909 best on a Schiit Asgard 3 stack and a Hagerman Tuba tube amp. GREQ Reviews notes that the twist-lock pads come off easily for a swap.

Fostex was founded in 1973, and it dates its RP planar technology to 1974. Home speaker builders know Fostex for its FE full-range drivers. Kari Nevalainen of 6moons wrote that the 4-inch FE103 gathered a small cult following after an article in L'Audiophile, and that dozens of enclosures had been proposed for it, from small bass-reflex boxes to rear-loaded horns. From 1981 Fostex and TASCAM led the move to affordable multitrack recorders, and the battery-powered X-15 of 1983 recorded four tracks on a cassette. In headphones, the T50RP grew a cottage industry of its own, and Headfonics traced the modified versions sold by enthusiasts from Smeggy's Thunderpants to Dan Clark's Mad Dogs and Zach Mehrbach's ZMF. The TH909's cherry birch cups are lacquered by the Sakamoto Urushi Factory, founded in 1900, and the TH616 marked the company's 50th anniversary in 2023.