Specs & measurements

Output impedance

Also called source impedance, Zout

Output impedance is the resistance an amplifier presents to the headphone or speaker it drives, in ohms. A low value, ideally under an eighth of the load’s impedance, keeps the tonal balance and bass control the designer intended.

Output impedance and the headphone form a voltage divider. For example, a 32 Ω headphone whose impedance rises to 100 Ω in the bass gets about 3 dB more bass from a 30 Ω output, but stays flat from a 1 Ω output, under an eighth of its impedance.

A headphone’s Impedance changes with frequency, often rising in the bass. If the amplifier’s output impedance is a significant fraction of it, the voltage divides unevenly across frequency and the bass or treble shifts. Multi-driver IEMs with wild impedance swings are the most affected.

Most modern Solid-state amplifiers measure under 1 Ω. Some tube and vintage designs are 30 to 120 Ω, which audibly changes low-impedance headphones; some owners like the result, and it is one reason the same headphone can sound different on two amplifiers.

For speakers the same idea is expressed as Damping factor.

Related terms