If you have to provide a fan to cool your audio power amplifier final transistor, it better to turn it off when it’s not needed. This solution is good since you don’t have to hear the fan noise when you play the music with low level volume at night, the power transistor won’t get hot. When you start turning your volume control toward maximum level, your power chip/transistor begin getting hotter and this circuit will turn on the fan to cool the heat sink. You won’t hear the fan noise because now your music is getting much louder. Here is the circuit’s schematic diagram:
The temperature sensor (NTC, negative temperature coefficient thermistor) should be placed as close as possible to the power transistor or IC. Installing it on the heat sink is good idea, just make sure a tight thermal contact and locate it close to the power transistor/IC.
If you have a thermometer, you can set the VR1 at heat sink temperature about 70 Celsius degree. Lowering this setting it’s OK but you might hear the fan noise although you only use your amplifier to play music for mild volume level. What if you don’t have a thermometer but you want your power amplifier safe? The following technique can be use but use at your own risk: use your amp for loud volume and use your finger to regularly touch the heat sink and adjust VR1 to activate the fan at heat intensity where it’s the maximum you can stand to touch. The LED indicator will give you the information that the fan is active. Stay cool with your amplifier!