Two numbers decide whether a headphone will play loud enough from a given source: impedance, measured in ohms, and sensitivity, measured in decibels of sound pressure per milliwatt or per volt. Together they answer the only question an amplifier is there to solve. Everything else you read about amplifiers is either downstream of those two numbers or is not really about power at all.
Impedance, plainly
Impedance is the opposition a headphone presents to an alternating current. Headphones range roughly from 8 ohms to 600 ohms, and the value is not fixed across frequency, particularly on multi-driver in-ear monitors where it can swing considerably.
Higher impedance draws less current for a given voltage. That sounds helpful until you remember that you still need a certain amount of power at the driver, so a high impedance headphone needs a higher voltage swing to get there. Phone outputs are voltage limited, which is why a 250 or 300 ohm headphone often sounds quiet and lifeless straight out of a phone while a 32 ohm one is fine.
High impedance exists for a reason. It allows a finer voice coil with more turns of thinner wire, which reduces moving mass. It also makes the headphone far less sensitive to the output impedance of whatever is driving it. Studio designs historically used high impedance because a single mixing desk output might feed several headphones at once.
Sensitivity, and why the units matter
Sensitivity tells you how loud a headphone plays for a given input. It is the number that decides whether a source can reach a comfortable listening level, and it is quoted two ways that are not interchangeable. Decibels per milliwatt describes output for a given power. Decibels per volt describes output for a given voltage. For low impedance headphones the two figures are close. For high impedance ones they diverge substantially, and a specification that omits the unit is close to useless.
Listing data on this site illustrates the problem. Sensitivity appears as 98 dB on the Audio-Technica ATH-M40x, 101 dB on the Koss KSC75, 109 dB on the Meze Alba, and 127.259 with no unit at all on the Audeze LCD-2 Classic, which is not a plausible headphone sensitivity under any convention. The same Audeze listing also gives the item weight as 6.64 pounds, which is roughly ten times what a headphone of that type weighs, so the record has more than one bad field. Treat these numbers as a rough sorting aid, not as engineering data.
The rule of thumb, and when it fails
| Headphone profile | Typical impedance | Typical sensitivity | Amplifier needed? |
|---|---|---|---|
| In-ear monitor | 8 to 32 ohms | 100 to 115 dB | No, and a powerful amp can add hiss |
| Portable closed-back | 16 to 40 ohms | 95 to 105 dB | No |
| Studio monitor, low impedance | 32 to 80 ohms | 95 to 100 dB | Sometimes, for headroom on peaks |
| Studio monitor, high impedance | 250 to 600 ohms | 90 to 100 dB per milliwatt | Yes, voltage is the constraint |
| Planar magnetic | 20 to 50 ohms | 85 to 95 dB | Usually, current is the constraint |
Planar magnetic designs are the case people get wrong most often. Their impedance is low and unusually flat across frequency, which looks easy to drive, but sensitivity is often low, so they want current rather than voltage. The HIFIMAN Sundara and the SIVGA P2 Pro are both planar designs, and the P2 Pro is listed at 98 dB sensitivity. Numbers in that region from a phone output usually mean you will run out of volume before you run out of enthusiasm.
Output impedance, the specification nobody prints
There is a third number, and it belongs to the source rather than the headphone: the amplifier’s output impedance. The conventional guideline is that it should be no more than about one eighth of the headphone’s impedance.
When that ratio is violated, two things happen. The frequency response changes, because the headphone’s impedance varies with frequency and the resulting voltage divider varies with it. And electrical damping falls, which loosens bass control. This is why a multi-driver in-ear monitor can sound different from two sources that measure identically on paper, while a single dynamic driver headphone with flat impedance sounds the same from both.
It is also why a very powerful desktop amplifier is a poor match for a very sensitive in-ear monitor. The usable volume range collapses into the first few degrees of the knob, channel imbalance at low settings becomes audible, and the amplifier’s noise floor becomes a constant hiss.
Symptoms that actually indicate you need an amp
- You run the source near maximum volume and still want more.
- Bass sounds thin and slow, and it does not tighten up at higher volume.
- Loud passages compress or lose separation while quiet passages sound fine.
- The headphone is rated above roughly 100 ohms and you are driving it from a phone or a laptop.
Symptoms that do not indicate an amplifier problem include a tonal balance you dislike, a lack of detail, sibilance, and poor isolation. Those are tuning, recording and fit issues. An amplifier supplies voltage and current. It does not add information that was not in the signal, and it will not fix a headphone whose response you disagree with.
Choosing the amplifier
Match the amplifier to the headphone, not to a price bracket. For low impedance planars you want current delivery and low output impedance. For high impedance designs you want voltage swing. Most desktop units publish a table of output power into 16, 32, 300 and 600 ohms, and that table is the only part of the specification sheet that matters for this decision.
Combined converter and amplifier units are the practical starting point for a desk. The FiiO K11 and the resistor ladder variant, the FiiO K11 R2R, cover the common case, and the HIFIMAN EF400 is aimed squarely at planar loads. Tube hybrid designs such as the Schiit Vali 3 are chosen for their character as much as their power, and it is worth being honest that this is a tonal preference rather than a technical requirement. For a laptop bag, a stick converter such as the AudioQuest DragonFly Red provides more drive than a typical laptop output without a desktop footprint.
The wider amplifiers range is the place to compare published power figures against the impedance of whatever you already own. Do that comparison before buying, because the honest answer for most headphones sold today, which sit near 32 ohms with sensitivity around 100 dB, is that a phone drives them to uncomfortable volume already and an amplifier changes nothing you would notice.
