Explainers
Impedance, sensitivity and amplification: do your headphones need an amp?
“250 ohms: amp required.” The rule is repeated everywhere, and it is half wrong. What decides how much volume a device can draw from a pair of headphones is the combination of two figures, and a little sixth-form maths is enough to settle the matter.
Two figures, two questions
The spec sheet of a pair of wired headphones gives two values that describe their relationship with the device driving them.
- Impedance, in ohms (Ω): the resistance the headphones present to the current. It ranges from 16 Ω for earbuds to 600 Ω for some studio headphones. It tells you in what form the headphones ask for their energy: a lot of current if it is low, a lot of voltage if it is high.
- Sensitivity, in decibels: the sound level produced for a given amount of energy. It tells you how much energy is needed.
Very sensitive 300 Ω headphones can play loud from a phone; insensitive 30 Ω headphones can remain timid on the same device. Hence the mistake of looking only at the ohms.
The trap of the two sensitivity units
Sensitivity is expressed in two ways, and manufacturers do not always say which:
- in dB/mW: the sound level for one milliwatt of power;
- in dB/V: the sound level for one volt of signal.
The two figures cannot be compared with each other. You convert from one to the other using the impedance Z:
dB/V = dB/mW + 10 × log₁₀(1000 ÷ Z)
For 32 Ω headphones, the gap is 15 dB; for 300 Ω, 5 dB. Headphones advertised at “100 dB” therefore mean nothing until the unit is known. Since headphone outputs behave like voltage sources, the dB/V value is the more telling: it says directly what you will get from a given device.
Doing the sums for your headphones
You need a target: to listen comfortably to dynamic music, the peaks must be able to reach about 110 dB, even if the average level stays well below that (fortunately for your ears). The voltage required is calculated as follows:
voltage (V) = 10 ^ ((110 − sensitivity in dB/V) ÷ 20)
| Typical headphones | Impedance | Sensitivity | Voltage for 110 dB | Verdict |
|---|---|---|---|---|
| Sensitive in-ears | 16 Ω | ≈ 125 dB/V | 0.2 V | Any source |
| Closed-back studio headphones | 80 Ω | ≈ 107 dB/V | 1.4 V | Computer, a good adapter |
| Open-back hi-fi headphones | 300 Ω | ≈ 97 dB/V | 4.5 V | Amplifier recommended |
| Demanding planar headphones | 60 Ω | ≈ 95 dB/V | 5.6 V | Amplifier required |
On the other side, what do sources deliver? A phone's USB-C to 3.5 mm adapter supplies between 0.5 and 1 volt; a laptop's headphone output, 1 to 2 volts; an audio interface or a desktop amplifier, from 3 to more than 10 volts.
The conclusion can be read from the table. With a 1-volt adapter, the 300 Ω headphones top out at 97 dB on peaks: they work, you hear music, but with no headroom. Loud passages are squashed and the sound seems flat, as if it “never takes off”. This is exactly the situation in which an amplifier transforms the listening.
What an amplifier does, and does not do
A headphone amplifier supplies the voltage and current that the source lacks. If your headphones are short of level or distort on peaks, the gain is obvious. If they were already playing loud enough, and cleanly, the amplifier will add neither detail nor “musicality”: a good amplifier is transparent by definition.
The DAC (digital-to-analogue converter) is a separate matter: it turns digital data into an electrical signal. The one in a recent phone or computer is generally beyond reproach to the ear. You replace a DAC when the output hisses, crackles or lacks power, not to gain resolution.
The one-eighth rule
Every headphone output has an impedance of its own, known as its output impedance. If it is too high relative to that of the headphones, two faults appear: the bass loses control, and the frequency response is distorted.
The second effect mainly concerns headphones whose impedance varies widely with frequency, notably in-ears with several balanced armature drivers: plugged into an unsuitable output, they literally change signature. The practical rule is simple:
The output impedance of the source should not exceed one eighth of the impedance of the headphones.
For 16 Ω earbuds, that means an output of 2 Ω at most. Phone adapters and modern amplifiers do far better; it is old hi-fi systems and some valve amps, with outputs of 100 Ω or more, that cause problems.
Why do high-impedance headphones still exist?
In a studio, a single amplifier once drove several pairs of headphones in parallel: high impedances kept it from being overloaded. A coil of finer, longer wire also lightens the moving assembly (coil and diaphragm), which benefits precision. Finally, high-impedance headphones are not very sensitive to the electronic hiss of the source. Many studio models come in several versions (32, 80, 250 Ω): the lowest for portable use, the highest for a fixed workstation.
In practice. All of this concerns passive wired headphones only. Wireless headphones contain their own amplifier, sized for their drivers. The demanding models are mostly open-back hi-fi headphones: see the audiophile headphones buying guide.
Sources and references
- IEC 60268-7 — Sound system equipment: headphones and earphones
- IEC 61938 — Multimedia systems: guide to the recommended characteristics of analogue interfaces
Frequently Asked Questions
Will 250-ohm headphones work with a smartphone?
They will work, but often without enough volume or dynamic headroom, because a phone's adapter delivers about 1 volt at best. It all depends on the sensitivity of the headphones: above 105 dB/V, the result is acceptable; below 100 dB/V, an amplifier is recommended.
Does an amplifier improve sound quality?
Only if the original source was not powerful enough for the headphones. In that case, the amplifier brings volume, dynamics and better-controlled bass. If the headphones were already properly driven, a transparent amplifier changes nothing audible.
What is the difference between dB/mW and dB/V?
They are two ways of expressing sensitivity: the sound level for one milliwatt of power, or for one volt of signal. You convert from one to the other by adding 10 × log₁₀(1000 ÷ impedance) to the value in dB/mW. The value in dB/V lets you compare headphones of different impedances directly.