The Audio-Technica ATH-M20x is 47Ω at 96.0 dB (no reference unit printed); the Audio-Technica ATH-M40X is 35Ω at 98.0 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 2.0 dB more than the ATH-M40X to reach the same loudness — 2.5 mW against 1.6 mW.
| Specification | ATH-M20x | ATH-M40X |
|---|---|---|
| Impedance | 47 Ω | 35 Ω |
| Sensitivity | 96.0 dB | 98.0 dB/mW |
| Sensitivity (voltage) | 109.3 dB/V | 112.6 dB/V |
| Power for 100 dB | 2.5 mW | 1.6 mW |
| Voltage for 100 dB | 0.34 V | 0.24 V |
| Current for 100 dB | 7.3 mA | 6.7 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 5.88 Ω | 4.38 Ω |
| Sources that reach 100 dB out of 280 | 280 | 280 |
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.
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 ATH-M20x under the one-eighth rule but not for the ATH-M40X (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: Audio-Technica ATH-M20x and Audio-Technica ATH-M40X. 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.