Audio Systems: car amplifier, power supply connections, loud thump

Question
hi, I have a 3000watt mono block amp running 2 axis subs and when i turn the car all the way off it makes a loud thump noise and sometime it doesnt stop doing it. Im running 4 gauge wiring and the ground is conneted to the back dash on the metal. And ive read about amp delay modules do the also delay the amp when you turn it off.

thanks.

Answer
If it is a "turn-off" thump, it could be from any of the following;

ONE:  The amp has two power lines to it. One is the main supply power, big wire, low resistance - such as the 4 gauge wire you mentioned.  The other is the turn-on supply which comes from the ignition or the head unit, depending on how you have hooked it in the system.  When turning off the internal switching usually lifts the power supply connections to the output transistors before it switches off the 12 volt feed (internally to the amp).

TWO:  If the turn-off system inside the amp is not functioning properly it will give the discharge thump when power drains to zero.

THREE: IF you are switching off both 12v lines feeding the amp, the turn-off circuit internally is losing voltage at the same time as the main supply and cannot switch it down slowly.  So, you will always get the off thump.

FOUR:  If the 12v main power is being switched off and the 12v ON line is not being switched you will always get the OFF thump.


The thump is created when the output stages of the power amp lose voltage.  It is caused because as the voltages discharge down to zero it reaches a point where the output transistors (or fets) go out of bias and they turn on - one side sooner than the other, creating a short duration discharge into the speaker load.

So, when you are turning the car "all the way off" it will create a thump if indeed you are interrupting both 12v feeds to the amp.

I don't know what you mean by "it won't stop doing it".  The only way I can see it continuously thumping is if the current draw is causing a very large voltage drop across the  wiring and making it go into a low frequency "motorboating" condition.

Let me know more and maybe we can figure out what is happening.

cleggsan