The Fostex T40RP MK3 is 50Ω at 91.0 dB/mW; the Fostex T50RP mk4 is 28Ω at 97.0 dB/mW. This page is only about what they ask of an amplifier — not how they sound, which no specification on either sheet can tell you.
The T40RP MK3 needs 6.0 dB more than the T50RP mk4 to reach the same loudness — 7.9 mW against 2.0 mW.
| Specification | T40RP MK3 | T50RP mk4 |
|---|---|---|
| Impedance | 50 Ω | 28 Ω |
| Sensitivity | 91.0 dB/mW | 97.0 dB/mW |
| Sensitivity (voltage) | 104.0 dB/V | 112.5 dB/V |
| Power for 100 dB | 7.9 mW | 2.0 mW |
| Voltage for 100 dB | 0.63 V | 0.24 V |
| Current for 100 dB | 12.6 mA | 8.4 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 6.25 Ω | 3.50 Ω |
| Sources that reach 100 dB out of 280 | 280 | 280 |
Every source in the database that drives one of them drives the other. On power, the choice between these two changes nothing about what amplifier you need.
1 source is low enough in output impedance for the T40RP MK3 under the one-eighth rule but not for the T50RP mk4 (Shanling H5).
280 of the 280 sources here reach 100 dB on both. That count is about power only — the output-impedance and hiss lines above are separate questions, and a source can pass this one and fail those.
Each model has a page of its own with the full source list, the hiss estimate and the provenance of every figure above: Fostex T40RP MK3 and Fostex T50RP mk4. Every figure on this page comes from what Fostex published, or is derived from it — what that maker publishes, counted.