Amp with Tone Controls & Soft Switching

Built around an LM380, this amplifier includes tone controls and electronic "soft switching". The soft switching circuitry ensures power is built up gradually eliminating the dc thump. The soft switching is enabled by a BD131 transistor wired as a switch in emitter follower configuration. The collector is wired to a permanent supply voltage, the 2H series inductor serves only to filter out power supply hum. More: This inductor is not too important and may be omitted if the DC supply is adequately smoothed. The control voltage is applied to the BD131 base terminal, the 10u capacitor and 10k resistor having a dual purpose:- i) a gradual charge of the 10u capacitor.

The described audio amplifier circuit utilizes the LM380 integrated circuit, which is a power audio amplifier capable of delivering up to 14 watts of output power. The LM380 is known for its robustness and ability to drive speakers directly. The inclusion of tone controls allows for user customization of audio output, enhancing the listening experience by adjusting bass and treble frequencies.

The electronic "soft switching" mechanism is a significant feature of this design, aimed at preventing the undesirable DC thump that can occur when the amplifier is powered on or off. This is achieved through a BD131 transistor configured as an emitter follower. In this configuration, the transistor provides a buffering action that allows for smooth transitions in power delivery to the LM380. The collector of the BD131 is connected to a stable DC supply voltage, ensuring that the transistor can operate effectively.

The 2H inductor serves as a low-pass filter to mitigate any noise or hum from the power supply, contributing to cleaner audio output. However, it is noted that this component is not critical; it can be omitted if the DC supply is sufficiently filtered, which can simplify the design without compromising performance.

Control of the soft switching feature is managed through a control voltage applied to the base of the BD131. The combination of a 10µF capacitor and a 10kΩ resistor is employed in this circuit to create a time constant that allows the capacitor to charge gradually. This gradual charging effect is essential for the soft switching operation, as it ensures that the amplifier's output power ramps up smoothly, thereby eliminating abrupt changes in output that could lead to speaker damage or audible clicks.

In summary, this amplifier circuit design effectively integrates soft switching technology with an LM380 audio amplifier, providing both functionality and protection for connected audio components. The careful selection of components and their configuration plays a crucial role in achieving a high-quality audio output while minimizing potential issues related to power supply noise and abrupt power changes.Built around an LM380, this amplifier includes tone controls and electronic "soft switching". The soft switching circuitry ensures power is built up gradually eliminating the dc thump. The soft switching is enabled by a BD131 transistor wired as a switch in emitter follower configuration. The collector is wired to a permanent supply voltage, the 2H series inductor serves only to filter out power supply hum.

This inductor is not too important and may be omitted if the DC supply is adequately smoothed. The control voltage is applied to the BD131 base terminal, the 10u capacitor and 10k resistor having a dual purpose:- i) a gradual charge of the 10u capacitor 🔗 External reference