Sixties-style 40 W Audio Amplifier Schematic Circuit Diagram

The Legendary 2N3055 Transistor

In the early 1960s, RCA introduced a transistor that would achieve legendary status: the 2N3055. Utilizing a pair of these transistors, one could assemble an audio power amplifier capable of delivering a robust 40 W output into an 8 Ω load. The circuit outlined here captures the essence of that era. For instance, each channel incorporates only seven active components, showcasing the design simplicity characteristic of that time, which, interestingly, embodies a timeless quality. This ‘retro’ power amplifier pumps out 45 W into 8 Ω with an input signal level of 0.5 Vrms.

Circuit Operation: Signal Processing and Output Stage

The input signal is directed to the base of T1, while negative feedback from the output, attenuated through voltage divider R5/R6, is applied to the emitter of T1. The collector current of T1, proportionate to the difference between input and feedback signals, is sent to the base of T2. T2, drawing current through R8 and R9, provides voltage gain. Capacitor C6, a bootstrap capacitor, maintains the voltage across R9 nearly constant, ensuring the current through R9 remains independent of the amplifier’s output signal level within the audio band.

Quasi-Complementary Push-Pull Output Stage

Transistors T4–T7 constitute a quasi-complementary push-pull output stage. In the 1960s, a true complementary PNP transistor to the 2N3055 wasn’t available. To overcome this, designers utilized a complementary Darlington pair comprising a PNP driver transistor and an NPN power transistor. Diode D1 offers balanced biasing, reducing distortion. Transistor T3 sets and stabilizes the output stage’s operating point; hence, it should be thermally coupled to the output transistors. The amplifier operates on a single supply voltage of approximately 65 V, a characteristic of the 1960s era. Capacitor C1 (4700 µF) transfers the signal to the load and provides speaker protection in case of transistor failure.

Safety Measures and Assembly Instructions

The amplifier lacks output current limiting. Although not critical, caution is necessary. A slow-acting 1.6-A fuse in the supply line provides some protection against potential mishaps. The power supply includes a transformer, bridge rectifier, small capacitors, and a 4700 µF electrolytic capacitor. An LED serves as a power-on indicator. Assembling the circuit involves straightforward steps, with proper heat sinking for transistors T3, T4, and T5 (TO126 package) and T2, T6, and T7 (single heat sink, thermal resistance ≤ 2 K/W). Before applying power, precise adjustment of P2 and voltage checks are crucial. The circuit, while affordable and unpretentious, delivers respectable sound quality with minimal distortion, making it a worthwhile DIY project for enthusiasts.

PCB Layouts and Further Details

The author has prepared two PCB layouts—one for the amplifier and another for the power supply—which can be downloaded in PDF, Gerber, and/or Easy-PC CAD formats from the provided link [1]. This project is an excellent opportunity to delve into the technology of the 1960s, offering both educational value and practical experience.

Internet Link