The AKG K612 Pro is 120Ω at 91.8 dB/mW; the Fostex T50RP MK3 is 50Ω at 92.0 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.
In amplifier terms these two are the same headphone. The gap between what they need for 100 dB is under half a decibel, which is smaller than the step between two clicks of most volume controls.
| Specification | AKG K612 Pro | Fostex T50RP MK3 |
|---|---|---|
| Impedance | 120 Ω | 50 Ω |
| Sensitivity | 91.8 dB/mW | 92.0 dB/mW |
| Sensitivity (voltage) | 101.0 dB/V | 105.0 dB/V |
| Power for 100 dB | 6.6 mW | 6.3 mW |
| Voltage for 100 dB | 0.89 V | 0.56 V |
| Current for 100 dB | 7.4 mA | 11.2 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 15.00 Ω | 6.25 Ω |
| Sources that reach 100 dB out of 280 | 277 | 280 |
3 of the 280 sources have the power for the Fostex T50RP MK3 but not for the AKG K612 Pro — Apple USB-C dongle (EU), Steinberg UR22C, Sony NW-A306.
17 sources are low enough in output impedance for the AKG K612 Pro under the one-eighth rule but not for the Fostex T50RP MK3 (Zoom AMS-22, Arturia MiniFuse 2, Arturia MiniFuse 1, 14 more).
277 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: AKG K612 Pro and Fostex T50RP MK3. Every figure on this page comes from what AKG and Fostex published, or is derived from it — what AKG publishes and what Fostex publishes, both counted.