SS1 Transistor 180w Amplifier
I'm sure like me all of us have a lot of parts laying around. Ideas are always running around and I decided to make use of an output transistor array that was pulled from a vintage Realistic Sta-2290 receiver.
Dare I say that the lights briefly dim when powering up your amplifier? The solution is a simple soft start circuit as you can see in the schematic. When power is first applied it ramps up the power supply for a split second through the three 68 ohm 5 watt resistors in parallel, thus limiting the current inrush. When the voltage rises the relay closes and bypasses the resistors. Signal ground (0V) is isolated from chassis ground through the bridge diode array and parallel resistor capacitor combination. If there is a short to chassis the diodes will conduct blowing the main fuse, and the capacitor helps shunt any high frequency noise to ground. Speaker protection is accomplished using Rod Elliot's speaker protection board. It is a compact design that will protect speakers from any DC potential appearing on the output of the amplifier and quickly disconnecting the speakers and shunting them to ground. Also there is a short turn on delay as the power supply ramps up then the relays are activated connecting the speakers to the amplifier. The output transistor array has a factory temperature safety limit which I decided to connect to the speaker protection board. If the heat sink overheats the limit will disconnect the speakers. An option here would have been to shut down the amplifier in case of an overload, but I figure I would rather protect my speakers as fast as possible in any overload situation.