Current-drive audio amplifiers for hearing loop systems

摘要

The differences between conventional voltage-drive amplifiers and current-drive amplifiers are explained using ideal amplifiers. Examples are then given of the use of freely-available integrated audio-amplifier part, used with induction-type hearing loops in current-drive configurations. Most of electronics is based on applying an input voltage to some circuit and getting an output voltage that is bigger, smaller, or altered in a desirable way. The idea of applying a constant input current to something probably surfaced in the early days of "electronic" television (camera tubes and cathode-ray tubes, not scanning discs). Much use was made of electromagnet coils for focusing and deflecting electron beams. These need feeding with a carefully-controlled current, even though their resistance changes a lot as they heat up. If a constant voltage were applied, then, the current would decrease as the coil heated up. What is needed is a constant-current supply, which provides the necessary current even though the coil resistance changes. Luckily, a pentode valve/tube provided a constant current, proportional to the input voltage, without any special circuitry beyond keeping the screen (g2) voltage constant.