Motorola Hi-Fi Amplifier: Build Your 20-80W Power Project

Description

This is a remarkably straightforward, inexpensive, high-fidelity power amplifier. It can be constructed in five different configurations, as illustrated in the table (ranging from 20 Watts to 80 Watts RMS).

Some comments:

Initially, it is essential to measure the gain of the final transistors (T3 and T4), specifically their amplification factor, denoted as hfe or β . A significant discrepancy exceeding 30% will likely result in a distorted audio signal. The components utilized were MJ3001 and MJ2501 transistors, demonstrating an agreement of approximately 5%.

Prior to the initial activation, the inputs of the amplifier must be shorted, and a milliampere meter should be placed on the output. Subsequently, the amplifier is powered on, and the R13 potentiometer is adjusted to reduce the direct current flowing from the output, down to microampere levels, or ideally, to zero. It was possible to successfully lower this current to 10 uA.

More about components

The amplifier incorporates several integrated circuits, contributing significantly to its performance and stability. The most prominent component is the LM386 audio amplifier IC. This IC is a low-power, stereo amplifier chip widely used in portable audio devices and small amplifiers due to its simplicity and low cost. It operates from a supply voltage of approximately 5V and is capable of amplifying audio signals from weak sources, providing sufficient output power for small speakers. The LM386's internal circuitry includes a precision current source, a feedback network, and protection diodes to ensure stable operation and prevent damage from overcurrent conditions. The IC’s performance is highly dependent on the external components used in conjunction with it, such as resistors, capacitors, and the potentiometer, making careful selection and adjustment crucial for optimal audio quality. Its versatility and robustness make it a popular choice for hobbyists and DIY audio projects.

Circuit diagram

I am highly satisfied with this amplifier. It produces excellent sound quality. Have fun with it!