Soft Start for Loudspeaker with DC Protection

Figure 1 

The circuit was designed for the purpose of providing protection to the loudspeaker and also incorporating soft touch technology for powering on.

• TL071 – a low noise JFET input operational amplifier with features such as common-mode input voltage range, high slew rate, operation without latch up, compensated internal frequency, high input impedance at the JFET input stage, low noise, low total harmonic distortion, protected from output short circuit, low input bias and offset currents, wide common-mode and differential voltage ranges, and low power consumption
• BC560 – a PNP epitaxial planar Silicon enclosed in a 24-pin dual in-line package or 3-pin TO-92 package typically used for switching and AF amplification due to its low noise and high voltage features
• BC550 – an NPN general purpose transistor with low current and low voltage used for low noise stages in audio frequency equipment
• 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

The protection of loudspeakers occurs after a power amplifier gets damaged. During this time, there might be some DC voltages floating inside the circuit and on the output of amplifier that would create more damage and thus, affecting the loudspeaker. Since the amplifiers are connected to the loudspeaker, the 12 V relay RL1 cuts the line of connection when it gets triggered. This interrupts the operation immediately while stopping the DC voltage to reach the loudspeaker. The relay is connected in parallel to provide 3 seconds of delay time from the instance when the power supply is applied. The delay is essential to the circuit as it protects the loudspeaker from sudden voltage transients or undesirable explosion when switch of the supply has been opened.

The operation of the circuit is indicated by the visual light from LEDS D4-5 where D4 signifies the green for OK status while D5 signifies the red for the delay timing or DC voltage presence. The circuit is being supplied symmetrically by 12 V, which can be sourced from a small independent power supply or from a reduced voltage from mains power supply. The circuit is best constructed for protection for each final amplifier that will be arranged. The quality of the relay contacts should be adequately proportional in terms of the large current that would pass the circuit during the operation.

Figure 2 

The loudspeaker protection circuit may be applied suitably in the cars with signal receivers that have one or more loudspeakers connected to direct-coupled audio power amplifier, to prevent burn out in the amplifier’s output stage and to the loudspeaker. They can simply be mounted between the amplifier and the speaker in the speaker cabinet, then disconnects the speaker upon sensing the presence of a direct current voltage. Some are designed to operate with quad transistor amplifiers and any other transistor amplifier up to 500W RMS.

Source: http://users.otenet.gr/~athsam/Lsp_protection.htm