The Fostex T20RP MK3 is 50Ω at 92.0 dB/mW; the Fostex TH919 is 25Ω at 100.0 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 T20RP MK3 needs 8.0 dB more than the TH919 to reach the same loudness — 6.3 mW against 1.0 mW.
| Specification | T20RP MK3 | TH919 |
|---|---|---|
| Impedance | 50 Ω | 25 Ω |
| Sensitivity | 92.0 dB/mW | 100.0 dB/mW |
| Sensitivity (voltage) | 105.0 dB/V | 116.0 dB/V |
| Power for 100 dB | 6.3 mW | 1.0 mW |
| Voltage for 100 dB | 0.56 V | 0.16 V |
| Current for 100 dB | 11.2 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 |
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 T20RP MK3 under the one-eighth rule but not for the TH919 (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 T20RP MK3 and Fostex TH919. Every figure on this page comes from what Fostex published, or is derived from it — what that maker publishes, counted.