The Internet now offers a huge number of different sound amplifiers, for every taste and color, to suit any need. As you know, even the most reliable amplifiers tend to fail, for example, due to improper operating conditions, overheating or incorrect connection. In this case, there is a high probability that the high supply voltage will end up at the output of the amplifier, and, therefore, will easily end up directly on the speakers of the speaker system. Thus, a failed amplifier drags with it “to another world” the speaker system connected to it, which can cost much more than the amplifier itself. That is why it is highly recommended to connect the amplifier to the speakers through a special board called speaker protection.
Scheme
One of the options for such protection is shown in the diagram above. The protection works as follows: the signal from the amplifier output is supplied to the IN input, and the speakers are connected to the OUT output. The negative of the amplifier is connected to the negative of the protection circuit and goes directly to the speakers.In the normal state, when the amplifier is working and power is supplied to the protection board, relay Rel 1 closes the input of the board to the output and the signal goes directly from the amplifier to the speakers. But as soon as a constant voltage of at least 2-3 volts appears at the input, the protection is triggered, the relay is turned off, thereby disconnecting the amplifier from the speakers. The circuit is not critical to resistor values and allows for variation. Transistor T1 can be used 2N5551, 2N5833, BC547, KT3102 or any other low-power npn transistor. T2 must be composite with a high gain, for example, BDX53 or KT829G. Light-emitting diode in the diagram serves to indicate the state of the relay. When it is on, the relay is on, the signal goes directly from the amplifier to the speakers. In addition to protection against DC voltage, the circuit provides a delay in connecting the speaker system. After applying the supply voltage, the relay does not turn on immediately, but after 2-3 seconds, this is necessary in order to avoid clicks in the speakers when the amplifier is turned on. The supply voltage of the circuit is 12 volts. Any relay can be used with a winding supply voltage of 12 volts and a maximum current through the contacts of at least 10 amperes. The S1 latching button is located on the wires; it is needed to force the relay to turn off, just in case. If this is not required, you can simply short-circuit the tracks on the PCB.
Assembling the device
Amplifiers are most often designed for two channels, left and right, so the protection circuit must be repeated twice for each channel. For convenience, the board is laid out so that it already provides for the assembly of two identical circuits at once. The printed circuit board is manufactured using the LUT method, its dimensions are 100 x 35 mm.
After drilling the holes, it is advisable to tin the paths. Now you can start soldering the parts. Particular attention should be paid to the pinout of the transistors; it is very important not to confuse it and solder the transistors on the right side. As usual, small parts are soldered first - resistors, diodes, capacitors, and only then transistors, terminal blocks, and, last but not least, massive relays. To connect all wires, you can use terminal blocks, the places for which are provided on the board. After soldering is completed, you need to wash off the remaining flux from the tracks and check the correct installation.
Protection tests
Now that the board is completely ready, we can begin testing. We supply power to the circuit (12 volts), after two seconds the relay should simultaneously click and turn on LEDs. Now we take some kind of constant voltage source, for example, a battery, and connect it between the minus of the circuit and the input. The relay should turn off immediately. We remove the battery and the relay turns on again. You can connect a battery by changing its polarity; the circuit operates regardless of what polarity the voltage appears at its input. We perform the same manipulations with the second circuit located on the same board. The protection threshold is approximately 2 volts. Now that the protection board has been tested, you can connect it to the amplifier and not be afraid that the speakers in expensive speakers will deteriorate due to the amplifier breaking down. Happy assembly.
