Output impedance is the one specification that changes how a headphone sounds rather than how loud it goes. Under the one-eighth rule a source should sit below an eighth of the headphone’s impedance; on a 16 Ω in-ear that is 2 Ω, and plenty of otherwise excellent equipment is nowhere near it.
This sorts by the figure the maker published, lowest first, and counts how many of the 567 non-electrostatic models here the source satisfies the rule for. Price is in the table because the correlation people expect is not there: across the whole database the rank correlation between price and output impedance is essentially zero.
| Source | Price | Published output impedance | Models it leaves alone |
|---|---|---|---|
| IK Multimedia AXE I/O ONE | $130 | <0.1 Ω | 567 of 567 |
| SMSL HO100 | $160 | <0.1 Ω | 567 of 567 |
| SMSL SH-9 | $289 | <0.1 Ω | 567 of 567 |
| SMSL H300 | $296 | <0.1 Ω | 567 of 567 |
| SMSL H200 | $399 | <0.1 Ω | 567 of 567 |
| SMSL HO200 | $399 | <0.1 Ω | 567 of 567 |
| IK Multimedia AXE I/O | $400 | <0.1 Ω | 567 of 567 |
| SMSL DO400 | $520 | <0.1 Ω | 567 of 567 |
| SMSL SP400 | $529 | <0.1 Ω | 567 of 567 |
| SMSL H400 MK2 | $539 | <0.1 Ω | 567 of 567 |
| SMSL SH-X | $699 | <0.1 Ω | 567 of 567 |
| SMSL VMV P2 MK2 | $1160 | <0.1 Ω | 567 of 567 |
| Benchmark HPA4 | $3499 | <0.1 Ω | 567 of 567 |
| Chord Hugo 2 | $2595 | 0.025 Ω | 567 of 567 |
| Merging Anubis | $2790 | 0.035 Ω | 567 of 567 |
The table shows the first 15 of 169. The rest are in the full source list, where every figure is labelled with where it came from.
111 of the 280 sources in this database could not be placed on this list at all, because their makers never published the figure it sorts by. They are not at the bottom of the ranking; they are absent from it. Any list of this kind that does not say so is quietly presenting a fraction of the market as the whole of it.
This is not a ranking of sound quality and cannot be. It sorts one number that manufacturers printed, and manufacturers do not print anything about how their products sound. Every row links to that product’s own page, where each figure is labelled with the document it came from.
Quietest sources for sensitive in-ears · Most power into 300 Ω · Sources that drive everything here · Audio interfaces as headphone amplifiers · Sources under $100 · Hardest headphones to drive · Most sensitive in-ears · Quietest desktop sources · What balanced actually gains · Portable players by power · Most power per dollar · High-impedance headphones