Does the Bottlehead Crack 1.1 drive the Fostex T50RP Mk3?

Computed from measured specifications · target SPL 110 dB · load 300 Ω

Adequate

The Bottlehead Crack 1.1 adequately drives the Fostex T50RP Mk3 with limited margin (~4.3 dB headroom at 300 ohms; needs ~123.0 mW, delivers 333 mW). Note: this OTL tube amp's high output impedance (120 Ω) interacts strongly with this low-impedance (54 Ω) headphone, which can audibly alter its tonal balance. OTL tube amps are best paired with high-impedance (250 Ω+) headphones; this is a less ideal match.

Headroom
4.3 dB
Required (110 dB SPL)
123.0 mW
Delivered (@ 300 Ω)
333 mW
Required voltage
2.57 Vrms

Power Headroom

Headroom: 4.3 dB 0 · target 4.3 dB below target marginal adequate ample -14 dB +10 +20 +36 dB

Scale shows dB headroom above the 123.0 mW required to reach 110 dB SPL. Amp delivers 333 mW at 300 Ω (+4.3 dB above target).

Fostex T50RP Mk3 Planar · over-ear
Impedance (rated) 50 Ω
Impedance (measured) Source: reference-audio-analyzer.pro/en/r… 53.65 Ω measured
Sensitivity (rated) 91.0 dB/mW
Sensitivity (measured) 89.1 dB/mW measured
Driver Planar
Spec basis Measured
Measured by Reference Audio Analyzer (https://reference-audio-analyzer.pro/en/report/hp/fostex-t50rp-mk3.php) · Source ↗
Bottlehead Crack 1.1 Desktop Tube · Single Ended
Output impedance manufacturer 120.0 Ω
Power @ 300 Ω 333 mW
Power @ 600 Ω 166 mW *est.
Power source Manufacturer
* Loads marked "est." are derived from the published voltage ceiling at a lower-impedance load (V = √(P·R) → P_higher = V²/R_higher). All derivations are upward only.
Bottlehead product page (bottlehead.com/products/crack-1-1-otl-headphone-amplifier-kit): output impedance 120 Ω (published). Power: '~10V into a 300 ohm load' → P = 10²/300 = 333 mW; the '~' qualifier makes this approximate. 600 Ω derived from 300 Ω voltage ceiling (~10 V). Designed for 300–600 Ω headphones only. · Source ↗
Impedance caution

This OTL tube amp's output impedance (120 Ω) is high relative to this headphone's 53 Ω impedance (ratio 0.4×). OTL tube amps are best paired with headphones ≥ 250 Ω; this combination may cause audible tonal coloration.

Planar driver note

Planar-magnetic headphones can have high current demands that are not fully captured by the power headroom model. Even with adequate headroom, some solid-state amps may struggle to deliver sufficient peak current for transient dynamics. Prefer amps rated for low-impedance planars.

Where to buy

Bottlehead Crack 1.1 on Amazon ↗ Fostex T50RP Mk3 on Amazon ↗

Disclosure: links above are Amazon affiliate links. As an Amazon Associate this site earns from qualifying purchases — at no extra cost to you. Affiliate commissions do not influence verdicts; all analysis is computed independently from the data.

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