Marshall Lead 100 Mosfet switch thump

[Carlosraj]

Hi

I am getting a loud popping sound from my amp whenever I turn it on or off. How do I overcome this. Please assist.

Thanks

[Enzo]

So it is either a noisy switch or not. SO leave the switch on, and unplug the amp from the wall outlet instead. Does it still pop, or does that way not pop?

If that way does not pop, then your switch is noisy, and a cap across it ought to help.

If that way does not pop, then either it is something inside the amp.

And consider many amps pop, and this could be perfectly normal.

[Mick Bailey]

I've had a few of these amps with a problem at turn-off. When I've scoped the power transformer secondary I saw a large oscillating peak. Because at the instant of turning off the amp's main filter capacitors are still charged the peak was being amplified. My fix in each case is to wire a class X2 0.33uf capacitor across the transformer primary (UK mains is 240v, 50hz). As the amps are DPDT switched the easiest place to do this is directly on the mains switch on the PT side. This damps the oscillation and eliminates the noise.

Of course, different causes can show the same symptoms and it's best to analyze the situation first