"These are 250 ohm, so you need an amp." That statement is frequently correct and it is not correct because of the impedance figure alone, which is why the same reasoning produces wrong answers elsewhere.
Two numbers together determine whether a headphone plays loudly enough: impedance and sensitivity.
What changed in 2026
- Phone headphone jacks stayed rare. Most listening moved to wireless or to USB adapters with built-in amplification.
- Dongle DAC quality improved. Small USB adapters became genuinely capable rather than merely functional.
- Low-impedance high-sensitivity designs proliferated. In-ear monitors trended toward being very easy to drive.
- Output impedance stayed underspecified. Manufacturers continued to omit the figure that matters for tone.
Two numbers, not one
Impedance is the electrical resistance the headphone presents, in ohms. Higher impedance means less current for a given voltage.
Sensitivity is how loud the headphone plays for a given input, typically expressed as decibels per milliwatt or per volt. Higher sensitivity means louder for the same input.
A high-impedance headphone with high sensitivity may play perfectly loudly from a phone. A low-impedance headphone with low sensitivity may not. The impedance figure alone is not predictive.
Note also the units: sensitivity quoted per milliwatt and per volt are not interchangeable, and comparing across the two produces nonsense. Check which is quoted before comparing.
| Combination |
Needs an amp? |
| Low impedance, high sensitivity |
No — most in-ears |
| Low impedance, low sensitivity |
Possibly — some planar designs |
| High impedance, high sensitivity |
Frequently fine |
| High impedance, low sensitivity |
Yes — classic studio headphones |
The classic 250 or 600 ohm studio headphones sit in the last row, which is where the folk wisdom comes from. Applying it to everything with a high impedance number is the error.
Output impedance and tone
The overlooked effect, and the one that changes sound rather than volume.
The source has its own output impedance. When that is significant relative to the headphone's impedance, two things happen: the frequency response changes, because headphone impedance varies with frequency, and control over the driver is reduced.
The usual rule of thumb is that source output impedance should be well below the headphone's — an eighth is the commonly cited target. A source with high output impedance driving a low-impedance multi-driver in-ear can produce audibly different tonality, typically a change in bass response.
This is why some headphones sound different from different sources in ways unrelated to volume, and it is more common with in-ears than with full-size headphones.
Manufacturers rarely publish output impedance, which makes this hard to check in advance. Where an in-ear sounds tonally different from two sources at matched volume, this is a likely cause.
Diagnose before buying
Two different complaints get treated the same way and have different solutions.
"Not loud enough." A genuine amplification problem. More voltage or current is needed, and an amplifier or a more capable source solves it.
"Sounds bad but is loud enough." Not an amplification problem. Possible causes include output impedance interaction, a poor source, the recording, or the headphone itself. An amplifier will not fix it.
For efficient headphones already reaching comfortable volume, an amplifier changes nothing audible. That covers most consumer headphones, which are designed to work from phones — see DACs and amps explained for what a separate DAC does and does not do.
Common mistakes
- Reading impedance without sensitivity. Not predictive alone.
- Comparing sensitivity figures in different units. Per milliwatt and per volt differ.
- Buying an amp for efficient headphones. No audible change.
- Ignoring output impedance with in-ears. Real tonal effects.
- Assuming louder means better. Volume-matched comparison is the only fair one.
- Expecting an amp to fix tonality. Different problem.
FAQ
Will a USB dongle drive high-impedance headphones?
Better ones will drive many of them adequately. The most demanding studio headphones benefit from something more substantial.
Does higher impedance sound better?
No. It is a design characteristic, not a quality indicator. High-impedance versions exist largely for compatibility with studio equipment.
What is damping factor?
The ratio of headphone impedance to source output impedance. Higher means more control over the driver; the eighth-or-less rule of thumb is a way of expressing the same thing.
How do I know if I need more power?
If the volume control is near maximum and it is still not loud enough, you need more power. If it is comfortable at a normal setting, you do not.
Where to go next
For what a separate DAC contributes, read DACs and amps explained. For the wireless equivalent, Bluetooth codecs explained, and for delay in the audio chain, audio latency explained.