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DC Protection
started on January 1 st , 2015


Any serious solid state amplifier need a protection circuit because a fault on output transistors can destroy the loudspeakers.

I have integrated in the same circuit 3 different functions:

1) a soft-start to eliminate shock on electric environment during switch on
2) a push button input to have a beautiful chassie using a vandal resistant push button switch
3) a dc protection with fast shutdown

The dc protection have 4 inputs to be used with bridged and circlotron amplifier with no pole to ground.

The optoisolators ensure the isolation between the power amplifier and protection to avoid ground loops.

This circuit does not disconnect the loudspeakers but switch-off the amplifier to save all the system.

With a simple modification you can add more sensors like thermal switches.

The pcb is available at low cost on this Ebay shop.

R1                   1K  1/4W
R2                  10K  1/4W <<< new value
R4,R5,R18           4K7  1/4W
R8,R6,R3             1K  1/4W
R13                 2K2  1/4W
R14,R15,R16,R17     10K  1/2W
R21                 4K7  1/4W <<< new value
R22                 10K  1/4W 
R7,R9,R10,R11       4.7  5W
R19                  10 or more  5W

C1                  47uF   16V    (to verify)
C2                   1uF   63V    MKT
C3                 100uF   16V
C4                2200uF   16V   (to verify)
C5                   1uF   63V    MKT
C10,C11,C12,C13    220uF   100V

D1,D4,D6,D7          1N4007
D9-D20               1N4148
OK1,OK2,OK3,OK4      SFH610  
IC1                  4013
U1,U2                Relay Fider mod. 41.61  12V
U3                   LM311
T1,T2,T3             BC337
TR                   Block  PT 4.5/2/9 with primary voltage 230V or
                     Block APT 4.5/2/9 with primary voltage 115V



Vandal Resistant  Push Button Switches

RS Stock No. 174-6381

Manufacturer Apem Manufacturers

Part No. AV021003C900

The panel cut out  is 22mm, it is the same size of noval socket, so this is the ideal choice for the 10mm front panel