The Fostex T40RP MK3 is 50Ω at 91.0 dB/mW; the Fostex TH-X00 (Drop) is 25Ω at 100.0 dB (no figure from the maker at all). 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 9.0 dB more than the TH-X00 (Drop) to reach the same loudness — 7.9 mW against 1.0 mW.
| Specification | T40RP MK3 | TH-X00 (Drop) |
|---|---|---|
| Impedance | 50 Ω | 25 Ω |
| Sensitivity | 91.0 dB/mW | 100.0 dB |
| Sensitivity (voltage) | 104.0 dB/V | 116.0 dB/V |
| Power for 100 dB | 7.9 mW | 1.0 mW |
| Voltage for 100 dB | 0.63 V | 0.16 V |
| Current for 100 dB | 12.6 mA | 6.3 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 6.25 Ω | 3.12 Ω |
| Sources that reach 100 dB out of 280 | 280 | 280 |
One caution before reading the rows. For Fostex TH-X00 (Drop) the maker publishes no sensitivity figure at all. The number in the table was collected when this database was first built and has never been traced to a manufacturer document, so treat every row below it as an estimate rather than a specification.
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 TH-X00 (Drop) (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 TH-X00 (Drop). Each figure on this page is labelled on those pages with where it came from, including the ones Fostex never published — what that maker publishes, counted.