Output impedance is rarely published on headphone-amp spec sheets, so we tested for it on our finalists. Put simply, the lower the amplifier’s output impedance, the better 0.1 to 5 ohms is ideal. Adding impedance (or resistance) can change the values of the filter and thus change the sound of a headphone in ways that can be difficult to predict. Every headphone is essentially an electrical filter, with resistance, capacitance, and inductance that affect the sound differently as the frequency changes. Output impedance is effectively a measure of how much an amplifier might affect the sound of a headphone.Ideally, the sound would be measurably or very noticeably better, but as our testing found, that isn’t always the case. An external headphone amp/DAC shouldn’t make headphones sound worse than the one included in your computer or mobile device.

As for amplifier power, we wanted a headphone amp that had a measured output of at least 15 milliwatts for 32-ohm headphones, which should be more than enough power for most people to listen at a sufficiently loud volume level.

A brief initial setup process is fine, but after that, a good amp/DAC combo should be pretty plug-and-play. You shouldn’t need to fiddle with frustrating software to start listening. We preferred amps that come with adapters that let them work with USB-A or USB-C sources generally, iOS users have to purchase separate adapters. Many are shaped like USB memory sticks, with an integrated male USB-A plug.

Connections are, for the most part, limited to USB.

A battery-powered portable amp should last at least a full workday per charge, and most of them do. Models that draw power via USB are generally smaller and more portable than those with a built-in battery, but because they take their power from your audio device, they drain the device’s battery (though our tests show this drain to be minimal). We considered both USB- and battery-powered amps. We sought options that could easily fit in a laptop bag or a pocket.

If a portable headphone amp is heavy or a pain to lug around, you won’t want to use it. The average audio fan might hesitate to take their amp traveling with them if they paid more than that.

Additionally, since you use these devices on the go, we set a $400 upper price limit. The price of a portable headphone amp shouldn’t be more than that of the headphones you are using.The package includes a USB-A cable for connection to most computers, as well as a USB-C adapter for connection to Android devices it also works with Apple iOS devices through the use of an adapter. The downside of the Hip-dac2 is that it is almost as large and heavy as a typical smartphone, so it probably makes sense only if you own a very power-hungry set of headphones. This iFi model also supports MQA, a controversial technology embraced by some audiophiles that promises to improve performance when you’re streaming MQA-processed audio from Tidal. The Hip-dac2 has both balanced and unbalanced audio outputs, and the balanced output has a bit more power, so it may produce slightly better sound. It offers almost three times as much power as the EarMen Eagle does, and it has an internal rechargeable battery rated for about eight hours of run time, so it won’t drain your phone’s battery.

In almost every way, the iFi Hip-dac2 represents a big step up from the other portable headphone amps featured here. In addition, the Eagle lacks an internal battery and must get its power from your mobile device, but our tests showed that the battery drain is not significant. This headphone amp is missing some features that audiophiles may look for, such as balanced audio output and support for the MQA audio format (more on this below)-but we don’t think those features are essential at this price. The package includes a USB-A–to–USB-C adapter for use with most Android phones, and the Eagle is also compatible with Apple iOS devices-though in that case you need to buy an adapter, which adds to the cost. Resembling a USB stick, the Eagle has a standard 3.5 mm headphone jack on one end and a USB-A plug on the other for simple connection to many computers. Depending on how it’s measured, it produces almost four times as much power as our previous top pick, the AudioQuest DragonFly Red, yet it typically costs about 35% less. Despite its small size, the EarMen Eagle has enough power to get optimum performance from the majority of hard-to-drive audiophile headphones.

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