What is an amp, and do you need one?

If you’re shopping around for headphones, you might also see recommendations to purchase an amp (short for amplifier). But if you can plug your headphones into your smartphone and they work, you might wonder why you’d ever need one. So what is an amp, and do you need one? And what difference do amps make to the listening experience?

We’ll cover what you need to know about amps and whether you’ll ever find yourself in a situation that requires one.

An amplifier (or amp) is the last step of producing sound from a device. A digital-to-analog converter can recreate analog signals, but these are low power and, therefore, insufficient to drive speakers or headphones. They require a component to increase their power, which is where an amplifier comes in. The resulting sound would be very quiet if there were not an amp.

To get a comfortable listening volume out of your headphones, we need to increase the power of the signals they receive and avoid introducing extra noise. Therefore, all playback devices already have a built-in amplifier, including smartphones with a headphone jack. External amps are available if you’re having trouble getting enough volume out of a pair of headphones with your device.

If you can plug in a pair of headphones and get a good range of volume out of them, your device’s built-in amp is doing its job. Of course, if you’re using wireless headphones, this doesn’t apply, and any volume issues you have are from another source.

Why would you need another external amp if your device already has an amplifier? An external amp is helpful if you have a pair of headphones with very high impedance or low sensitivity. These headphones need more power to drive them to get an acceptable range of volumes. Furthermore, some common symptoms may indicate your headphones need an amp. The biggest one is low audio levels during playback, no matter how much you increase the volume. We’ll have to discuss impedance and sensitivity to understand why.

Impedance is any device’s inherent resistance to electricity, measured in ohms (Ω). An amplifier has to output a powerful enough signal to overcome this. The sensitivity indicates how much volume a given pair of headphones can produce at a particular power level, measured in decibels (dB).

You only really enter that territory with an impedance rating of around 100Ω. Audiophile headsets like the Sennheiser and Beyerdynamic models in this chart are where this starts being a concern.

An amp should only factor in if you’ve ruled out other reasons your headphones aren’t loud enough. Check to make sure you’ve set your device to an audible volume. If possible, try using another pair of headphones to see if all of them are quiet. If that’s the case, it might indicate a problem with the device. Ensure that you aren’t using equalizer settings that might make your content sound too quiet. Furthermore, it could be that the original creator of the content recorded it with deficient audio levels. In that case, there’s not much you can do other than turn up the volume.

If you have ruled out other possibilities, or you’re looking at a new pair of headphones, here’s how to determine if you need an amp. As a rule of thumb from the chart above, we can see that an amp is only necessary as we tread into high impedance ratings.

Anything below 100Ω probably won’t need an amp. In the fuzzy area between 100Ω and 200Ω, you may need an amp depending on how loud you like to listen to music. You may have to max out your volume slider at these values to get a satisfactory listening experience. Once we go above these values, an amp might be necessary if you want to use your headphones with consumer devices like a smartphone or media player. Some headphones reach the 600Ω range, but you likely won’t find these at your local big box retailers.

The mathematics of headphone amps

You will have to do some math to determine if a pair of headphones will need an amp, using this formula:

The power value comes from the sensitivity rating of your headphones. The complete specification is dB of sound pressure per milliwatt (dB/mW), and a milliwatt is one-thousandth of a watt. For example, an 85dB/mW rating means one milliwatt is required for the headphones to produce 85dB. Decibels are logarithmic, so if we want to increase that to 95dB, we need ten times more power, or 10mW. Note that if your headphones specify their sensitivity at 1Vrms, you’ll have to convert that number to dB/mW to work with this equation using an online tool like this one.

For example, if we take the Beyerdynamic headphones from our chart above, we’ll use 1/1000 as the value for power (because 1mW is one-thousandth of a watt), 250Ω for impedance, and we get V_rms = 0.50 to achieve the full 102dB rating of these headphones. For comparison, the Audio-Technica ATH-M50x need only 0.19 volts to reach their 99dB rating, and the Sony WH-1000XM5 need only 0.22 volts to get 102dB. This is also why the Sennheiser HD 660S is an edge case because it needs 0.39 volts to reach 112dB (converted from the 104dB at 1V_rms value given). As you can see, the high-impedance cans demand more voltage than the low-impedance consumer models. Meanwhile, the edge case may require you to turn the volume up high to hear your tunes.

But if all of this seems too detailed, you’re probably fine. Most consumer headphones these days don’t require an amp because they’re made to work with devices like phones and laptops. Even some audiophile cans are now built this way, too. In short, you probably won’t need an amp unless you have high-impedance headphones and want to use them as daily drivers.