Anyone who records anything already owns a headphone amplifier: the socket on the front of their audio interface. There are 65 interfaces in this database out of 280 sources, which makes them the second largest category here, and by installed base they are almost certainly the largest one in the world.
They are also the category where one specification goes wrong so consistently that it stops looking like a fault and starts looking like a design brief. The median published output impedance of an interface here is 10Ω. For every other kind of source in this database it is 0.5Ω. That is a factor of 20, and it is not a rounding difference.
The one-eighth rule says a source should sit below an eighth of the headphone’s impedance, or it starts shaping the frequency response rather than just making it louder. The median in-ear in this database is 18Ω, so the ceiling is 2.25Ω.
An interface sitting at the category median of 10Ω satisfies that rule for 0 of the 222 in-ears here. Of the 43 interfaces that publish an output impedance at all, 15 clear it for an 18Ω in-ear. 29 clear it for a 250Ω studio headphone.
That second number is the explanation. These outputs are not badly designed; they are designed for the headphones a studio actually owns. A 50Ω output into a 250Ω monitor is inside the rule with room to spare. The same output into a 16Ω multi-driver in-ear is four times over it, and the in-ear was not on the drawing board.
| Interface | Price | Published output impedance | Into 32 Ω | Models it leaves alone |
|---|---|---|---|---|
| Tascam US-2x2HR | $199 | 66 Ω | 18 mW | 3 of 567 |
| Tascam US-4x4HR | $249 | 51.5 Ω | 45 mW | 6 of 567 |
| Audient EVO 16 | $750 | 50 Ω | 87 mW | 6 of 567 |
| Audient EVO 4 | $129 | 50 Ω | 22 mW | 6 of 567 |
| Audient EVO 8 | $249 | 50 Ω | 22 mW | 6 of 567 |
| Audient ORIA | $3500 | 50 Ω | 979 mW | 6 of 567 |
| Audient iD24 | $599 | 50 Ω | 119 mW | 6 of 567 |
| Audient iD44 MkII | $809 | 50 Ω | 190 mW | 6 of 567 |
The last column is not a verdict on the product. It counts how many of the 567 models in this database that output impedance leaves alone under the one-eighth rule — and for the top of that table the answer is close to none of them. The simulator shows what the response actually does at those numbers.
| Interface | Price | Published output impedance | Into 32 Ω | Models it leaves alone |
|---|---|---|---|---|
| IK Multimedia AXE I/O ONE | $130 | <0.1 Ω | 26 mW | 567 of 567 |
| IK Multimedia AXE I/O | $400 | <0.1 Ω | 115 mW | 567 of 567 |
| Merging Anubis | $2790 | 0.035 Ω | 961 mW | 567 of 567 |
| RME ADI-2 DAC FS | $1399 | 0.1 Ω | 1.2 W | 567 of 567 |
| RME ADI-2 Pro FS R | $2199 | 0.1 Ω | 3.0 W | 567 of 567 |
| RME ADI-2 Pro EX | $2499 | 0.1 Ω | 1.9 W | 567 of 567 |
| Apogee BOOM | $299 | 0.5 Ω | 240 mW | 566 of 567 |
| Apogee Duet 3 | $649 | 0.5 Ω | 1.2 W | 566 of 567 |
The cheapest interface here that clears the one-eighth rule for an 18Ω in-ear is the IK Multimedia AXE I/O ONE at $130, which publishes under 0.1Ω — less than a hundredth of what the most expensive offenders publish. Output impedance is a design decision about what the socket is for, and some makers decided it should be for headphones. Reading down the two tables, the thing that predicts the figure is not price and not brand prestige: it is whether the maker treated the headphone output as a monitoring convenience or as an amplifier.
The median interface here delivers 112 mW into 32Ω against 380 mW for everything else, so on paper it is the weaker half of the database. In practice that rarely matters, because the median headphone needs 1.0 mW for 100 dB SPL. The median interface reaches that on 561 of the 567 non-electrostatic models here, and the weakest one in the category — the Steinberg UR22C at 12 mW into 32Ω — still reaches it on 512 of them. Every interface here clears nine tenths of the catalogue.
Driving everything is a different bar, and only 6 of the 65 clear it, because the last few percent of this database is HiFiMAN planars and low-impedance in-ears that want more current than a monitoring socket was ever asked for. But if your interface sounds wrong with in-ears, power is the least likely explanation on the list.
Output noise is the other half of in-ear matching, and this is where the category is at its worst. 1 of 65 interfaces publishes an output noise figure. Across every other kind of source in this database, 70 of 215 do. The exception is the RME Babyface Pro FS.
So for 64 of the 65 interfaces here, the question “will this hiss with my in-ears?” cannot be answered from the manufacturer’s own documentation at all. Not answered badly — not answerable. This site leaves those blank rather than filling them with someone else’s measurement, which is why the hiss estimate is missing on most interface pages.
If you use an interface with full-size headphones of 80Ω or more, none of this is your problem and the numbers above are trivia. If you use it with in-ears, three things are worth knowing: check your own model on the output impedance checker, remember that an in-line attenuator lowers hiss but raises the impedance the headphone sees — the attenuation calculator shows both effects — and that the full list of interfaces here is sorted by price with both figures beside it.
And if a maker does not publish the output impedance of a socket they sell as a headphone output, that is itself information. 22 of these 65 do not.