The Audio-Technica ATH-M20x is 47Ω at 96.0 dB (no reference unit printed); the Audio-Technica ATH-R70xa is 470Ω at 97.0 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 ATH-M20x needs 1.0 dB more than the ATH-R70xa to reach the same loudness — 2.5 mW against 2.0 mW.
| Specification | ATH-M20x | ATH-R70xa |
|---|---|---|
| Impedance | 47 Ω | 470 Ω |
| Sensitivity | 96.0 dB | 97.0 dB/mW |
| Sensitivity (voltage) | 109.3 dB/V | 100.3 dB/V |
| Power for 100 dB | 2.5 mW | 2.0 mW |
| Voltage for 100 dB | 0.34 V | 0.97 V |
| Current for 100 dB | 7.3 mA | 2.1 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 5.88 Ω | 58.75 Ω |
| Sources that reach 100 dB out of 280 | 280 | 275 |
One caution before reading the rows. Audio-Technica ATH-M20x 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.
5 of the 280 sources have the power for the ATH-M20x but not for the ATH-R70xa — Apple USB-C dongle (EU), FiiO BTR11, Steinberg UR22C, 2 more.
32 sources are low enough in output impedance for the ATH-R70xa under the one-eighth rule but not for the ATH-M20x (Zoom AMS-22, Audient EVO 4, ART HeadAmp 4 Pro, 29 more).
275 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: Audio-Technica ATH-M20x and Audio-Technica ATH-R70xa. Each figure on this page is labelled on those pages with where it came from, including the ones Audio-Technica never published — what that maker publishes, counted.