I’m in the process of building myself a single-ended class A MOSFET amplifier consisting of two monoblocks, and I don’t want to use a coupling capacitor between the amplifier and the speaker. So I needed a circuit that would protect the speaker against DC voltages; I have given it a dual role:
- DC protection in the form of a device to disconnect the speaker in the presence of a DC voltage greater than ±1 V, using an LM358;
- speaker connection delayed by around 5 s after powering the amplifier, using a 555.
Innovative Protection Circuit Design: A Unique Creation
Drawing inspiration from various online sources, I’ve formulated a protection circuit that appears to be one of a kind. While the internet provided the foundation, this circuit’s specific configuration and functionality are distinctive. For stereo systems, it’s crucial to implement a protection circuit for each channel, ensuring optimal performance and safeguarding the equipment.
Clever Signal Handling and DC Voltage Detection
The output signal from the audio amplifier, lacking an output capacitor, is directed to the normally-open contacts of relay Re1. Simultaneously, the signal enters a DC voltage detection system comprising an RC integrating network and comparators. This network incorporates a low-pass filter (R6/C4) that selectively permits positive or negative DC components while attenuating the audio signal. Dual comparators, IC2a and IC2b, evaluate these signals. Their outputs, governed by the voltage thresholds set by resistors R7, R8, and R9, enable the relay control mechanism. By judiciously combining LM358 outputs through a wired-OR gate (utilizing diodes D4 and D6), the relay is disabled when either positive or negative DC voltage is detected.
Timing and Relay Activation
A monostable 555 timer circuit, inverted by transistor T1, introduces a time delay upon power application. After approximately 5 seconds, the relay is energized through T2. Notably, the choice of 2N1711 for T2 is due to its availability and superior gain compared to alternatives like BD139. Despite the transistor’s heat generation, it remains well within its safe operating temperature.
Power Management and Voltage Regulation
The protection circuit operates on mains power, regulated by two symmetrical regulators (IC3 and IC4). If the transformer’s secondary voltage is high, modest heatsinks are recommended. To accommodate fluctuations in AC line voltage, the voltage range is wisely limited to 15 to 22 V. A low-current LED for D8 conserves power, allowing the adjustment of R10 to reduce the current to 2 mA when the diode is illuminated.
During testing, it’s essential to apply a DC voltage source to the protection circuit input, as open circuit conditions deprive the opamps of bias voltage, ensuring accurate evaluation of their functionality. Careful testing and attention to the circuit’s unique elements are crucial for successful implementation.