The Etymotic ER4XR is 45Ω at 98.0 dB/mW; the Truthear Zero:RED is 17.5Ω at 99.9 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 Etymotic ER4XR needs 1.9 dB more than the Truthear Zero:RED to reach the same loudness — 1.6 mW against 1.0 mW.
| Specification | Etymotic ER4XR | Truthear Zero:RED |
|---|---|---|
| Impedance | 45 Ω | 17.5 Ω |
| Sensitivity | 98.0 dB/mW | 99.9 dB/mW |
| Sensitivity (voltage) | 111.5 dB/V | 117.5 dB/V |
| Power for 100 dB | 1.6 mW | 1.0 mW |
| Voltage for 100 dB | 0.27 V | 0.13 V |
| Current for 100 dB | 5.9 mA | 7.6 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 5.62 Ω | 2.19 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 Etymotic ER4XR under the one-eighth rule but not for the Truthear Zero:RED (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: Etymotic ER4XR and Truthear Zero:RED. Every figure on this page comes from what Etymotic and Truthear published, or is derived from it — what Etymotic publishes and what Truthear publishes, both counted.