Amplifiers, loudspeakers and ohms

The relationship between amplifiers and loudspeakers is often described confusingly, with talk of "matching ohms". A clear picture starts with the amplifier as a voltage source.

The amplifier is a voltage source

A power amplifier tries to put out a voltage that follows the input signal, regardless of the load. Its output impedance is tiny, usually below 0.1 Ω. The loudspeaker then draws whatever current its impedance requires:

power = V² / Z

For the same output voltage, a lower impedance draws more current and more power. An amplifier rated at 100 W into 8 Ω can deliver about 28.3 V RMS; into 4 Ω, the same voltage gives 200 W, if the amplifier can supply the current. The limit is the amplifier's current capability and heat, not a "match".

What the speaker's ohm rating means

A loudspeaker is labelled with a nominal impedance, such as 4, 8 or 16 Ω. This is a rounded value: the real impedance varies with frequency:

• A peak at the bass resonance of the driver.
• A minimum in the middle region, usually close to the nominal value.
• A rise at high frequencies caused by the inductance of the voice coil.
• Dips and bumps from the crossover.

A speaker marked 8 Ω can dip to 4 Ω at some frequencies, so amplifiers must be able to drive it.

Connecting several loudspeakers

Check the lowest total impedance against the minimum load of the amplifier. Use the speaker impedance tool.

Damping factor

The damping factor is the speaker impedance divided by the amplifier's output impedance, for example 8 Ω / 0.04 Ω = 200. It shows how firmly the amplifier controls the motion of the cone. Beyond about 20 to 50 the benefit is small, because the resistance of the cable and voice coil is already larger than the amplifier's output impedance. A long thin cable lowers the effective damping factor, see the wire gauge tool.

Matching is not needed

Setting the amplifier's output impedance equal to the speaker's, as the power-matching theorem would suggest, would lose half the power in the amplifier and leave the cone uncontrolled. See Voltage bridging versus power matching.

Bi-wiring and bi-amping

• Bi-wiring uses separate cables for the low and high sections of a speaker from the same amplifier terminals. Measured differences are very small, and listening-test evidence is limited.
• Bi-amping uses a separate amplifier for each frequency band, either with an active crossover before the amplifiers or a passive one. Active bi-amping has real benefits: the crossover works at line level, the amplifiers drive only the drivers, and each can be matched to its driver.

Practical advice

1. Use cable thick enough that its resistance is a small fraction of the speaker impedance, see the Ohm's law tool and loudspeaker maximum SPL.
2. Do not exceed the amplifier's minimum load.
3. Do not run a loudspeaker with an amplifier that is far too weak: clipping damages tweeters faster than a high-power clean signal.

Draft: written by an AI assistant, awaiting technical review.