The Sennheiser HD 505 is 120Ω at 98.7 dB/mW; the Sennheiser HD 600 is 300Ω at 91.8 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 HD 600 needs 6.9 dB more than the HD 505 to reach the same loudness — 6.6 mW against 1.3 mW.
| Specification | HD 505 | HD 600 |
|---|---|---|
| Impedance | 120 Ω | 300 Ω |
| Sensitivity | 98.7 dB/mW | 91.8 dB/mW |
| Sensitivity (voltage) | 107.9 dB/V | 97.0 dB/V |
| Power for 100 dB | 1.3 mW | 6.6 mW |
| Voltage for 100 dB | 0.40 V | 1.41 V |
| Current for 100 dB | 3.4 mA | 4.7 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 15.00 Ω | 37.50 Ω |
| Sources that reach 100 dB out of 280 | 280 | 261 |
19 of the 280 sources have the power for the HD 505 but not for the HD 600 — Apple USB-C dongle (US), Apple USB-C dongle (EU), JadeAudio JA11, 16 more.
2 sources are low enough in output impedance for the HD 600 under the one-eighth rule but not for the HD 505 (Schiit Valhalla 3, Arturia AudioFuse Rev2).
261 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: Sennheiser HD 505 and Sennheiser HD 600. Every figure on this page comes from what Sennheiser published, or is derived from it — what that maker publishes, counted.