The HiFiMAN HE-4XX (Drop) is 35Ω at 93.0 dB (no figure from the maker at all); the HiFiMAN Susvara is 60Ω at 83.0 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 needs 10.0 dB more than the HE-4XX (Drop) to reach the same loudness — 50 mW against 5.0 mW.
| Specification | HE-4XX (Drop) | Susvara |
|---|---|---|
| Impedance | 35 Ω | 60 Ω |
| Sensitivity | 93.0 dB | 83.0 dB |
| Sensitivity (voltage) | 107.6 dB/V | 95.2 dB/V |
| Power for 100 dB | 5.0 mW | 50 mW |
| Voltage for 100 dB | 0.42 V | 1.73 V |
| Current for 100 dB | 12.0 mA | 28.9 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 4.38 Ω | 7.50 Ω |
| Sources that reach 100 dB out of 280 | 280 | 231 |
One caution before reading the rows. HiFiMAN Susvara 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.
49 of the 280 sources have the power for the HE-4XX (Drop) but not for the Susvara — Apple USB-C dongle (US), Apple USB-C dongle (EU), Generic CX31993 dongles, 46 more.
1 source is low enough in output impedance for the Susvara under the one-eighth rule but not for the HE-4XX (Drop) (Shanling H5).
231 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.
The gap between these two in decibels does not change with volume — it is a property of the two headphones. What changes is the size of the numbers, and it changes fast: every 10 dB costs ten times the power. The row that matters is the one nearest the level you listen at, and for most people that is nearer the top of this table than the bottom.
The Susvara is more demanding than 99% of the 567 non-electrostatic models in this database; the HE-4XX (Drop) than 83%. Both figures use the same measure as the rest of the site: whichever runs out first against a 2 V, 100 mW dongle, voltage or power.
| dB SPL | HE-4XX (Drop) power | HE-4XX (Drop) voltage | Susvara power | Susvara voltage |
|---|---|---|---|---|
| 80 | 0.05 mW | 42 mV | 0.50 mW | 0.17 V |
| 85 | 0.16 mW | 74 mV | 1.6 mW | 0.31 V |
| 90 | 0.50 mW | 0.13 V | 5.0 mW | 0.55 V |
| 95 | 1.6 mW | 0.24 V | 16 mW | 0.98 V |
| 100 | 5.0 mW | 0.42 V | 50 mW | 1.73 V |
| 105 | 16 mW | 0.74 V | 158 mW | 3.08 V |
105 dB is included because forum power figures are usually quoted there or higher. It is also a level that damages hearing in minutes — how many minutes is its own calculator.
| Source | Price | Headroom, HE-4XX (Drop) | Headroom, Susvara | Output impedance | Noise |
|---|---|---|---|---|---|
| FiiO KA11 | $29 | 15.6 dB | 3.3 dB | 0.7 Ω | 1.7 µV |
| FiiO BTR13 | $59 | 12.6 dB | 0.2 dB | 0.6 Ω | 1.0 µV |
| iBasso DC03 Pro | $69 | 12.5 dB | 0.2 dB | 0.12 Ω | 0.9 µV |
Of the 280 sources here, 231 reach 100 dB on both. That is the easy axis. 66 of them also publish an output impedance low enough for both under the one-eighth rule and a noise figure that stays below the audible-hiss threshold with both — the rest either fail one of those or never printed the number at all, and 163 of them are in the second group.
The cheapest of the 66 is the FiiO KA11, at $29. It leaves 15.6 dB of headroom on the HE-4XX (Drop) and 3.3 dB on the Susvara, its output impedance is 0.7 Ω against a one-eighth limit of 4.38 Ω and 7.50 Ω, and its noise figure puts the hiss floor 28 dB and 40 dB below the 20 dB SPL this site treats as the point where a quiet room stops hiding it.
Cheapest that satisfies three published numbers is not the same as best, and none of it is about how either headphone sounds. To put either model against a source this page does not mention, or to change the target loudness, use the amplifier matcher.
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. 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.