Power-off thump

[Digsy]

I've heard of power-on thumps, but not a power off one. :conf:

My head unit thumps the speakers when you switch it off. Very odd. The shut-down sequence is:

1) Key off (or press the off button).
2) Music stops immediately but head unit stays live until the arial is retracted.
3) Head unit switches off (display goes blank).

(3) Is when the thump happens. It's not consistent. Sometimes it is loud and sometimes it is very soft (soft enough not to be a concern).

Any ideas what is causing this and how I could get around it? If it can't be cured at source then I was thinking of making up a box with two miniature 5A DPDT relays in it that isolate the speakers as soon as the ignition is switched off.

[Bob]

Do you have any amps installed? Was the headunit installed with the Autoleads adapter, or through the stock amp?

[Digsy]

The head unit is 4x35W and has no external amp. The head unit is wired directly to the speakers.

I used the autoleads adapter to convert the Supra loom to an ISO block (I have been using this adatper with a different stereo for a few years without any issues).

The only difference I can think of is that between the ISO block and head unit there is a noise filter consisting of a voltage and current stabiliser. The voltage stabiliser is a 1000uF capacitor and the current stabiliser is a toroidal choke. The thing is that these are in the permanent 12V line so the capacitor should never discharge, so I don't think it's that.

[Bob]

Sounds like the head-unit is still feeding a signal to the speakers when it shouldn't be. Very strange.

[Digsy]

I've done a quick Google and this seems to be quite a common issue with any audio gear, but I've seen no clear description of what causes it and several proposed solutions, ranging from relays (as I suggested above) to bridging the speaker wires to ground using high-value resistors.

Surely someone else will have come across this and fixed it?

[Matt T]

Make sure you have a good ground, always the first thing to check with these.

One of my mates units does this, he's had it looked at and its a fault with the actual unit, could be the same with yours? :(

[Digsy]

I suppose there is that possibility. It is a second hand unit. Works fine apart from this niggle though.

I'm going to try it tonight with the hard disk drive detached (I think this powers down after the head unit). if that cures it then maybe I just need a better ground on that. Currently it grounds through the 8-pin DIN cable that supplies it with power from the head unit.

[Digsy]

I discovered something else tonight. If I turn the volume right down to zero before I switch off, it doesn't thump. Does that tell you anything?