Overview

The Fostex T500A Mk III is a limited production horn super tweeter featuring a newly developed ring-shaped pure magnesium diaphragm, precision-machined solid brass horn and equalizer, and an Alnico internal magnetic circuit. Manufacturer specs list 7kHz–40kHz (−10dB), 8Ω impedance, and 104dB/W sensitivity, with a recommended crossover of ≥7kHz (−12dB/oct). Despite the premium materials and Japanese craftsmanship, independent measurements show notable deviations from these claims and practical limitations in use.

Scientific Validity

Independent measurements by Joseph Crowe show: (1) impedance sweep with a fundamental ~3.1kHz and a pronounced resonance peak at ~9.4kHz; (2) off-axis directivity maintaining a wide ~120° listening window up to 10kHz but narrowing to ~60° by 12kHz; (3) harmonic distortion with H2 = 2.74% at 10kHz for an 85dB test signal and H2 = 6.09% at 95dB; higher-order components remain comparatively low; (4) IMD around −58dB at 10kHz, worsening to ~−40dB by 20kHz; and (5) time-domain decay that largely dissipates within ~7 periods. Critically, while Fostex publishes extension to ultrasonic frequencies, the measured on-axis response meaningfully extends only to ~20kHz. These data points substantiate significant gaps between marketing claims and observed behavior, keeping transparency below expectations for a flagship super tweeter.

Technology Level

Premium construction—ring-shaped Mg diaphragm, solid brass horn/equalizer, Alnico magnetic circuit—reflects high manufacturing standards. However, the underlying compression driver + horn architecture is conventional, and the implementation fails to translate material extravagance into superior measured performance versus simpler, modern alternatives (e.g., planar ribbon/AMT). Net mass of ~5.15kg underscores build solidity but does not correlate with objective performance leadership.

Cost-Performance

Typical current new-retail pricing is USD 1,262 each (minimum order two) → 2,524 USD/pair. As a functionally comparable add-on super tweeter with 8–40kHz response, adjustable 8/10/12/14/16kHz crossover and attenuation, the Aperion PST MKII Planar-Ribbon costs 399 USD/pair. Cost ratio (cheapest not-inferior alternative vs. target):

(399 USD) / (1,262 USD × 2) = 0.158 ≈ 0.16

For high-sensitivity (≈98–103dB) horn systems where level matching is a constraint, Aperion HST AMT Dual (98–103dB) is the cheapest suitable within the same ecosystem at 1,599 USD/pair:

(1,599 USD) / (1,262 USD × 2) = 0.634 ≈ 0.63

Even under this stricter sensitivity constraint, alternatives remain markedly cheaper per delivered functionality and published bandwidth. Given the T500A Mk III’s measured ~20kHz practical extension and non-trivial distortion/IMD behaviors, its price premium is not justified on performance grounds.

Reliability & Support

Fostex is an established brand and the T500A Mk III’s substantial mechanical design suggests baseline durability. However, limited production may complicate future parts/service availability relative to mass-production products. Warranty/support appears standard for the category but offers no offsetting advantage to the weak objective performance/value metrics.

Rationality of Design Philosophy

The design prioritizes luxury materials and traditional horn loading without commensurate measurement gains. Recommending a high crossover (≥7kHz) constrains integration flexibility compared with solutions that can cross lower and maintain dispersion. The net result looks like market positioning rather than engineering-driven audible improvement.

Advice

Objectively, the T500A Mk III does not earn its asking price. For most systems, the Aperion PST MKII delivers the target functionality (8–40kHz extension, user-adjustable crossover/level) at ~16% of the T500A Mk III’s new-retail pair price. For horn systems needing high sensitivity, the Aperion HST AMT Dual still undercuts the Fostex on delivered capability per dollar. Buyers who prioritize measurable performance should allocate budget to modern ribbon/AMT super tweeters with transparent, verifiable behavior and flexible integration.

References

[1] Joseph Crowe, “Fostex T500A Mk.III Super Tweeter with Horn No.2306,” independent measurements (FR, impedance, polar, decay, distortion/IMD), accessed 2025-08-14.
[2] Aperion Audio, “PST MKII Planar-Ribbon Super Tweeter Pair,” specifications (8–40kHz, 6Ω, 94dB), price 399 USD/pair, accessed 2025-08-14.
[3] Fostex, “T500A Mk III,” official specifications: 7kHz–40kHz (−10dB), 8Ω, 104dB/W(1m), recommended crossover ≥7kHz (−12dB/oct), gross weight 5,150g, accessed 2025-08-14.
[4] Madisound, “Fostex T500A MkIII Super Tweeter – Limited Production,” current new-retail price 1,262 USD each (min order 2), accessed 2025-08-14.

(2025.8.14)