Ohms

The unit of electrical resistance and impedance, and the number that must be matched between a valve amplifier and the speaker it drives.

A valve output transformer expects a particular load, so a cabinet of the wrong impedance either makes the amplifier run hotter and distort earlier or reduces its output and stresses the transformer. Solid-state amplifiers are more forgiving, though they also have a minimum rating, and going below it risks damage. Cabinet impedance is set by the speakers' own values and how they are wired, in series or in parallel.

Example: two eight-ohm speakers wired in parallel present four ohms, which must match the impedance setting on a valve amplifier.

Ohms

See also