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RE: Answer....

the thump is caused by a DC offset voltage that occurs across the sum amplifier as it powers up. Some amplifiers have relays that engage the woofer after the amp stablizes - but the plate amps don't often do that.

The DC offset is usually caused because there is a charge held on one side of of the amplifier rails compared to the other - the + and minus rails will will not discharge equally. Other causes could be other capacitors in the circuit path holding a charge - for instance the Low pass filter at the input - stages.

As most say this is usually a benign effect - as woofer can usually hand several volts of DC offset for a short while. My M&K sub does not thump on turn on. A Sub I built with a Dayton plate amp does...

"The hardest thing of all is to find a black cat in a dark room, especially if there is no cat" - Confucius

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