LM391-Based Power Amplifier with 35 Watts Output

The circuit was designed to drive the power transistors of a power amplifier that will produce a power output of 35 Watts.

• LM391 – an audio power driver designed to drive external power transistors in 10 to 100 watt power amplifier designs with features such as high power supply voltage operation, low distortion, dual slope SOA protection, internally protected for output faults and thermal loads, true high fidelity performance, low input noise, shutdown pin, high supply rejection, user programmable protection for output transistor, and selectable gain and bandwidth
• BD139 – NPN power transistor used for driver stages in hi-fi amplifiers and television circuits because of its low voltage at 80 V maximum and high current at 1.5 A maximum
• TIP2955 – a general purpose complementary power transistor manufactured in epitaxial-base planar technology which is suitable for audio amplifier, linear power, and switching applications because of its low collector-emitter saturation voltage

Based on the operation of IC1 LM391, the power transistors will be driven completely to produce the desired output. It will not require any external components even though it was constructed the inclusion of very few components. To be able to achieve a stable current pf 45 mA, the 10K Ohms trimmer TR1 is used for the proper regulation of the current. The inductor L1 used is made up of 20 coils of wire with 0.9 mm in diameter. This inductor is connected across or in parallel with the resistor R16 to create an RL circuit. This RL circuit is also known as RL filter or RL network which is one of the simplest analogue infinite impulse response electronic filter.

A suitable heatsink should be used for the transistors Q3 & Q4 so that it can be placed directly above the PCB during the packing of the circuit with the presence of the TIPxxxx series of transistors. When the type of packaging used is TO-3, short cables should be used with the corresponding locations on the PCB.

TO-3 is also referred to as metal can which is used for silicon controlled rectifiers, integrated circuit, and very large transistors. The position of the components inside the packaging is mounted on a metal plate with a metal can crimped on top of it which will provide efficient heat conduction and durability. Normally, three leads are used for TO-3 packages but it can be made with four or more leads. There is one lead that is usually attached to the case itself while the other leads pass through the metal plate and a glass seal. For mounting to a heatsink, the design contains two tabs with holes. It can be used in high-current and high-power applications where equivalent components of other cases may be prone to damage.

The 35 W power amplifiers are being used in car radios and they may come in hybrid stereo configuration for better quality. Other forms that may come as portable power amplifier will be suitable for small mobile PA applications where a 12 Vdc is the only power source available in a vehicle system or similar. The amplifier may be attached to a small speaker or other flat surface with mounting bracket.

Source:users.otenet.gr/~athsam/power_amplifier_35w_with_lm391.htm