Please check the Video Link.

https://youtu.be/8BRjHAQC76Y

Inrush current protection

Here’s the Inrush current protection, Low frequency transformer generate very high inrush current greater than 10X the nominal current rating.

 During initial transformer turn ON, the primary winding act as a short circuit across the AC input until magnetic field build up, it may take 2-5 ac line cycles depending on the size of the transformer. The rectifier Bulk capacitor will also generate very high inrush current. Two power NTC will limit the total inrush current.

During start-up, bypass Relay RY1 is OFF, current going to the transformer primary will pass through 2 NTC, the resistance of NTC is high during startup (Lower Temperature) which limits the Inrush current. 

After several line frequency cycles, the Inrush current is much lower and the secondary rectified voltage increases, voltage divider R3,R4,R5 and R10 senses the rectified voltage , if sense voltage is higher than 2.5V reference voltage, U4 comparator output will be High and trigger Q1, driving the RY1, RY1 will bypass the two NTC.

If NTC will not be bypass it will consume a lot of power and temperature will be high. The relay contact resistance is much lower than the NTC so most of the transformer current will pass through the relay contacts.

Low Voltage Rectifier.

The low voltage rectifier will generate +/- 15V  can be used to power the pre-amplifier, +12V regulated output is also available to power the protection controller and other auxiliary circuit such as LED and Display.

High Voltage Rectifier.

Here’s the Power Amplifier Rectifier, the transformer secondary will be rectified by two bridge rectifiers , depending on the transformer secondary voltage, amplifier power and Capacitor the rectifier can generate +/-50V to +/- 100V.

This design it’s capable of up to 1500W maximum power, with maximum bulk capacitance of 30,000uF, Capacitors can be modified depending on your application.

Multi-Function Speaker protection.

Here’s the circuit of the stereo speaker protection, based on UPC1237 controller IC, protections include DC offset, AC OFF Detection and under-voltage protection, two channels over-temperature sensing and comparator to trigger the AC OFF function of the UPC1237 via Q2. The temperature sensor can be place to the amplifier heatsink. Sensing component can be modified to match your amplifier requirements.