A doughnut shaped transformer commonly used in electronics, speand amplifiers in particular for its low noise, low resistance to current flow, and power output for its size. Most good quality amplifiers use toroidal transformers.

Transformers in general are used to capture power from a wall outlet and convert it to a high-power supply that can be used by the amplifier or electronic device. The transformer is one of three pieces in a power supply. The rectifier follows the transformer and turns alternating current into direct current. The final pieces of the power supply, the capacitors, store energy for latter use by the amplifier or electronic device.

Good quality power supplies need high-capacity capacitors and large, powerful transformers. The transformer should be able to output several times the maximum power output of the amplifier to the speakers. Thus if the peak power of an amplifier is 300 watts, the transformer should be able to output 600 to 900 watts of power. This may be less if the amplifier is class AB, or it may be more if the amplifier is class A.

Toroidal transformers generally weigh around a pound for every 30 watts of output they can produce. Thus a toroidal transformer capable of outputting 600 watts would weigh around 20 pounds. Transformers must be large and necessarily heavy to output large amounts of power for high-powered, high-quality amplifiers. Amplifiers claiming 500 watts output but weighing only 15 pounds should generally be avoided assuming they must have a poor power supply. While an amplifier's weight is certainly not its defining characteristic, most amplifiers with good quality power supplies are fairly heavy thanks to a large toroidal transformer. The transformer and the power supply are vital to a well-designed, well-built amplifier (or any other electronic component).