The HiFiMAN HE-4XX (Drop) is 35Ω at 93.0 dB (no figure from the maker at all); the HiFiMAN Susvara Unveiled is 45Ω at 86.0 dB (no reference unit printed). 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 Susvara Unveiled needs 7.0 dB more than the HE-4XX (Drop) to reach the same loudness — 25 mW against 5.0 mW.
| Specification | HE-4XX (Drop) | Susvara Unveiled |
|---|---|---|
| Impedance | 35 Ω | 45 Ω |
| Sensitivity | 93.0 dB | 86.0 dB |
| Sensitivity (voltage) | 107.6 dB/V | 99.5 dB/V |
| Power for 100 dB | 5.0 mW | 25 mW |
| Voltage for 100 dB | 0.42 V | 1.06 V |
| Current for 100 dB | 12.0 mA | 23.6 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 4.38 Ω | 5.62 Ω |
| Sources that reach 100 dB out of 280 | 280 | 259 |
One caution before reading the rows. HiFiMAN Susvara Unveiled printed a decibel figure with no reference unit, so this page reads it as per milliwatt. If it is really per volt, that model needs far more than the table says. Why the two readings differ by so much. For HiFiMAN HE-4XX (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.
21 of the 280 sources have the power for the HE-4XX (Drop) but not for the Susvara Unveiled — Apple USB-C dongle (US), Apple USB-C dongle (EU), JadeAudio JA11, 18 more.
1 source is low enough in output impedance for the Susvara Unveiled under the one-eighth rule but not for the HE-4XX (Drop) (Shanling H5).
259 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: HiFiMAN HE-4XX (Drop) and HiFiMAN Susvara Unveiled. Each figure on this page is labelled on those pages with where it came from, including the ones HiFiMAN never published — what that maker publishes, counted.