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Soft start, Speaker Protection by Audio Workshop

Soft start. Speaker Protection. Speaker Delay

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.


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


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.


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  1. Boyan

    If you can only show me the schematic it would be great .

  2. I am looking forward to buy. Thanks to advise the pricing.

  3. dennis

    Good article guys.

  4. Andrew

    greetings everyone my first post here i need full schematic of short circuit
    protection using ic UPC1237 can this short circuit protection be used on high power
    ab class amps running on dc 95 volt rails +/-
    warm regards

  5. Tony

    Hello Ventsislav,

    Is the board available? Either populated or blank.


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