Soft start, Speaker Protection by Audio Workshop

Soft start. Speaker Protection. Speaker Delay

We have completed this module this week. It is a custom build by order of a customer. The module has many specific functions. It can be used for speaker protection, speaker delay, toroid soft start and others.

Rectifiers

Four MUR860 per channel are used to make a bridge rectifier. This will be used in a dual-mono power amplifier .

Soft-start

The soft-start circuit inserts four 5W resistors into the primary winding of the transformer. Those are included into the current path for about 500mS which is more than enough to prevent the mains fuse from blowing out. After the initial time has passed the relay triggers and closes around the resistors creating a low resistance bypass path for the mains current.

Speaker protection

Two uPC1237 IC’s are used to perform this feature. The output is monitored for any DC presence in the signal. The relay is broken when more than +/-1.5V is detected.

AC detect

The load is automatically disconnected upon power-down. The IC’s monitor the AC voltage. The goal is to disconnect the load before the current in the power caps is drained out. This will prevent the nasty pops and clicks when shutting down the amp.

Electronic turn-on and turn-on

We all know that routing mains supply to the front panel is not a good idea. I don’t like it. Carrying AC voltage all across the amplifier calls for many troubles like humm, noise etc. The board has the so called electronic power on and power off function. The designer doesn’t have to carry around dangerous voltages any more. Instead only 5V DC pair of wires to the fron panel is used. That’s way better.

The circuit uses a 555 timer for debouncing the tactile push button and one 4013 dual D-type flip-flop.