The Audeze LCD-4Z is 15Ω at 98.0 dB/mW; the Audeze LCD-S20 is 18Ω at 93.0 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.
The LCD-S20 needs 5.0 dB more than the LCD-4Z to reach the same loudness — 5.0 mW against 1.6 mW.
| Specification | LCD-4Z | LCD-S20 |
|---|---|---|
| Impedance | 15 Ω | 18 Ω |
| Sensitivity | 98.0 dB/mW | 93.0 dB/mW |
| Sensitivity (voltage) | 116.2 dB/V | 110.4 dB/V |
| Power for 100 dB | 1.6 mW | 5.0 mW |
| Voltage for 100 dB | 0.15 V | 0.30 V |
| Current for 100 dB | 10.3 mA | 16.7 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 1.88 Ω | 2.25 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.
3 sources are low enough in output impedance for the LCD-S20 under the one-eighth rule but not for the LCD-4Z (Astell&Kern HC3, RME Fireface UFX II, RME Fireface UFX III).
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: Audeze LCD-4Z and Audeze LCD-S20. Every figure on this page comes from what Audeze published, or is derived from it — what that maker publishes, counted.