Have a 2000 DSe with the Alpine Harmon Kardon system.

I have just started to have an issue, with the subwoofers "thumping" when the radio is turned off, or switched between functions and radio stations etc..

The Subs still seem to be working and can be adjusted by the head unit.

Im assuming the issue is with the amp or sub unit, as when its unplugged from the headunit everything else still works perfectly.

Anyone had that issue? If so any idea what might be going on?

from what you say, it sounds like the amp is sending an output to the speakers, for that to happen either the amp has developed a problem or the head unit is sending a signal that the amp is amplifying (obviously) the p38 amp is a PITA to deal with there are ones on the well known auction site but many being sold can be DOA, from the main stealer IIRCC its 440 and no returns policy on them.

Out with the electrical tester it is then.

yeah i nearly bought a P38 with a dud amp, but the seller wanted too much for the car, with that and the its other issues, so walked on it, but in the process i did a lot of research into the amp / sat nav / ICE system.

though i used to repair amps in a previous life, i no longer have the bench test equipment to do a repair and am a bit out of touch these days

Popping is usually a small amount of DC which appears at the output at turn on and off. There should be no difference in DC potential at the speaker output terminals, ever. But a 'Pop' means that for a split second, there is. This can be caused by all sorts of things.

It could be internally generated, faulty muting circuits, the start of a fault in the output stage. as a question, changed anything electrically on the car recently ?

on the large amps i used to work with it was a matter of course to turn the gains down before turning the system on or off to prevent any DC signal being amplified.. of course that a bit tough to do with an ICE amp.. does it pop if you have the sound turned right down when powering on/off ?