If you are an audio enthusiast like me who loves to build your
own sound system, this circuit is a must. This will protect
your expensive speaker from burning in case your DIY amplifier
breaks.
About the circuit:
The main purpose of the circuit is to protect your speaker from surge voltage of the of amplifier upon power up and when your amplifier outputs a DC voltage offset. The DC offset usually happens when the output stage of your amplifier becomes faulty, this will cause the output to be permanently tied to either positive or negative supply. This fault condition can toast your speaker permanently.
The protection circuit has 3 main parts namely RC low pass filter, RC delay and Relay driver. The low pass filter R1 and C1+C2 is connected directly to the output of the amplifier. The cutoff of this low pass filter is tuned way below 20Hz. This is essentially passing very low frequency down to DC level. Very low frequency signal is unlikely to occur since audio signal is filtered on every audio device not to pass any frequency below 20Hz. For this reason, this low pass filter will only pass DC level that is what we intended. D1-D4 is used to correct the voltage polarity of the signal because DC offset can be negative voltage. Without these diodes, Q1 won’t turn on when negative DC offset occurs. R2 is used to turn on Q2 while C3 is used to delay the turn on of Q2. You can increase the value of C2 to add more turn on delay.
Q2 drives the relay that connects the output of the amplifier to speaker. When a DC offset is detected on the output, Q1 will turn on and will cause the Q2 to turn off and will deactivate the relay that connects the speaker to amplifier.
Update: Corrected the orientation of D3. However, it is not yet updated on the eagle file.
PCB Design
The PCB design is very simple. As usual I made it in eagle. The design is not yet optimized so I am sharing the project files so you can optimized it yourself. 🙂
DOWNLOADS
You can download the eagle project files here: