Like all amplifiers, a high-current amplifier takes an audio signal and increases its size and power. A high-current amplifier's unique capability comes in the type of power that it delivers. While all amplifiers have power that is a combination of amperage and voltage, high-current amplifiers can generate high amperage. This allows them to both drive more challenging speakers as well as to drive more speakers at once.
A high-current amplifier's value only becomes clear with a basic understanding of what makes up power. The signal that an amplifier delivers, usually expressed in watts, is made up of a combination of voltage and amperage. Voltage measures how much "pressure" the power flow has while amperage measures how many electrons are actually flowing down the wire in a given period of time. Calculating wattage is done by multiplying the voltage by the amperage. For example, while a low-current amplifier may generate 100 watts by sending 4 amps of current down the wire at 25 volts, a high-current amplifier would send 16 amps of current at 6.25 volts.
Impedance is the key variable in determining whether or not a high-current amplifier will work best in a given situation. Typically measured in ohms, impedance measures how much a speaker resists a signal that is sent to it. Speakers with lower impedance require a higher current power supply, while speakers with higher impedance demand relatively less current and relatively more voltage. A sound system with low impedance speakers will typically benefit from a high-current amplifier.
Low impedances typically occur in two situations. The first is with high-end speakers, many of which, such as large electrostatic "ribbon" speakers, are designed specifically with low impedance. It also occurs when many speakers get connected to one amplifier channel. Two speakers with 8-ohm impedances, which is the norm for consumer-grade home electronics, connected in series will present a 4-ohm impedance to the amplifier. Connection of multiple speakers to a single amplifier channel is also a frequent occurrence in public address (PA) systems.
Generally speaking, low-current amplifiers are smaller, lighter and cheaper to build. High-current amplifiers will usually be larger and heavier as well as more expensive. In exchange for the additional cost and bulk, however, they provide better performance into low impedance loads. They can also use their extra current capacity to deliver more power into medium and high impedance loads to better reproduce volume peaks in music and other material.