45 Watt Class-B Audio Amplifier Circuit Design

Description

Discrete-component op-amps are employed in this design to drive a complementary Bipolar Junction Transistor (BJT) output stage, resulting in Class B operation. This configuration minimizes quiescent current when delivering low output currents, as the op-amp effectively shuts off the transistors. At higher output currents, the power transistors conduct, and the op-amp contributes only approximately 0.7/R11 to the overall output current. The op-amp’s quiescent current biases the external transistors, thereby significantly reducing the crossover frequency.

The concept originated from a letter published in Wireless World, December 1982, page 65, authored by N. M. Allinson, formerly at the University of Keele, Staffordshire. The letter proposed using op-amp ICs as drivers, but considering the required supply voltages (up to +/- 35V) for an amplifier generating roughly 50W, a discrete-component op-amp approach was deemed suitable and ultimately proved successful.

The discrete-component op-amp is based on a Douglas Self design. However, his original circuit featured a Class A output stage. To achieve the desired Class B operation, the circuit was subsequently modified accordingly. The amplifier is powered by a mains transformer supplying a secondary winding rated at a common value of 25 + 25V (or 24 + 24V) and 120VA power. This allows for driving two amplifiers at 45W and 69W output power into 8 and 4 Ohms respectively, exhibiting very low distortion (less than 0.01% @ 1kHz and 20W into 8 Ohms).

This simple, straightforward, and rugged circuit, designed for any high-quality audio application and, above all, to complement the recently initiated series of articles forming the Modular Preamplifier Control Center, is also ideally suited for constructing a good Guitar or Bass amplifier. Enjoy!

Integrated circuits, specifically op-amps, play a critical role in this amplifier’s functionality. Op-amps are used as drivers for the BJT output stage, ensuring a Class B operation. This configuration minimizes quiescent current when delivering low output currents, as the op-amp effectively shuts off the transistors. At higher output currents, the power transistors conduct, and the contribution of the op-amp is limited to approximately 0.7/R11 to the overall output current. The op-amp’s quiescent current biases the external transistors, thereby significantly reducing the crossover frequency. The precise control afforded by the op-amp is crucial for achieving the desired amplifier characteristics.

Circuit diagram:

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Power supply :

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The main design targets for this amplifier were as follows:

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