Build a 6.5W Loudspeaker Amplifier with Negative Feedback

Description

The amplifier circuit utilizes negative current feedback, resulting in the load current being primarily influenced by the input signal rather than the impedance of the loudspeaker. The inductor current flowing through R7 generates a voltage which is then applied to the inverting input of IC1, thereby establishing negative feedback. The overall gain of the circuit depends on the impedance of the loudspeaker, H1 [ Zo ], and the value of R7. The gain equation within the circuit is (Zo+R7)/R7, specifically (8+0.5)/0.5 = 17. The output connection of IC1 is connected to ground, however, this configuration ensures that the base current of transistors Q1 and Q2 is supplied by the power lines and not by IC1. The quiescent current of the Q1 and Q2 transistors is typically between 50 and 100mA, operating in Class A, and is determined by resistors R2, R3, R5, and R6. It is crucial that transistors Q1 and Q2 are matched to prevent significant current and voltage deviations. Adjustments to resistors R2, R3, etc., can be made to achieve symmetrical bias. With full drive, the emitter current of Q1 and Q2 is approximately 500mA. The Total Harmonic Distortion (THD) is 0.01% for the output power, delivering 6.5W into an 8R impedance and a ± 18V power supply. This circuit was originally featured in Elektor’87.

NE5534

The NE5534 is a versatile JFET operational amplifier known for its high input impedance and low noise characteristics. This integrated circuit offers excellent performance in a range of audio and instrumentation applications. Its JFET construction provides high input impedance, reducing loading effects on the signal source. It is a popular choice for amplifier circuits requiring precision and low distortion. It is a bipolar junction transistor (BJT) integrated circuit, providing a complete amplifier stage in a single package. It's often employed in audio applications due to its low noise and distortion characteristics.

BD242C

The BD242C is a general-purpose NPN bipolar junction transistor that is widely utilized in diverse electronic circuits. This transistor is known for its robustness and reliability, making it suitable for various applications, including amplification and switching. Its construction ensures stable operation across a range of conditions. It is a commonly used NPN transistor, renowned for its good performance and affordability. It's frequently found in audio amplifiers and switching circuits due to its balanced performance.

BD241C

The BD241C is a general-purpose NPN bipolar junction transistor, commonly used in a broad array of electronic circuits. This transistor is appreciated for its dependability and adaptability. It is a commonly used NPN transistor, renowned for its balanced performance. It's frequently found in audio amplifiers and switching circuits due to its balanced performance.

Circuit diagram

Part List