The 64 Audio U12t is 12.6Ω at 108.0 dB/mW; the 64 Audio U4s is 11Ω at 107.0 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 U4s needs 1.0 dB more than the U12t to reach the same loudness — 0.20 mW against 0.16 mW.
| Specification | U12t | U4s |
|---|---|---|
| Impedance | 12.6 Ω | 11 Ω |
| Sensitivity | 108.0 dB/mW | 107.0 dB/mW |
| Sensitivity (voltage) | 127.0 dB/V | 126.6 dB/V |
| Power for 100 dB | 0.16 mW | 0.20 mW |
| Voltage for 100 dB | 45 mV | 47 mV |
| Current for 100 dB | 3.5 mA | 4.3 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 1.57 Ω | 1.38 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 U12t under the one-eighth rule but not for the U4s (FiiO M11S, Singxer SA-1, FiiO M15S).
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: 64 Audio U12t and 64 Audio U4s. Every figure on this page comes from what 64 Audio published, or is derived from it — what that maker publishes, counted.