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So another week goes by and it seems that this isn't quite fixed after all :rant:

I test drove the car last week and it seemed to be perfect. Took it into work on Friday and it seemed fine in the morning. On the way home however I started to get the same interference, intermittently at first and then constantly regardless of radio, CD or USB input. I was stuck in traffic and as the engine got hotter it seemed to be worse and even more so when the engine was under any load.

As I mentioned in the last post I didn't run a dedicated earth from the head unit back to the same chassis earth point as the amp so I've done that today in the hope it might be the final fix. Noise still there and just audible through the tweeters even at tickover.

The aerial is still earthed via the standard earth strap but I tried with the aerial unplugged and no change so it doesn't seem that the interference is being introduced through that.

This is really starting to frustrate me now!!

Logic tells me that it IS an earthing issue because changing the amp earth improved it massively but now I'm stumped again. What should I be trying next?

Not worth starting anything now...🍺

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Possibly a degradation in the electronic ignition? Depends on whether it is a radiated or conducted emission issue. Do you have any bypass capacitors/filters on the stereo power supply? Could the ignition leads be slowly degrading, which could be worsened when hot?

Also, power the Head and the Amp from same power supply point, as noise will be magnified because I think common mode noise rejection will be worsened with different power supply take off points, but again, depends on how the designers have done the power supply internally. Or filter each power supply?

Also, I have no idea of circuitry, but if the Head and Amp are DC coupled on the input/output, this may also cause problems under certain situations, but I have no idea of common practice with these circuits now. You could try DC isolation of the signals between Head and Amp. And might be worth checking input impedance of the Amp, as if it is too high relative to the output impedance of the Head, it may also pick up noise here. As a rough test, you could put say a 10k resistor across the amp input in parallel with Head signal and see of it changes amount of noise.

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The plugs and leads are genuine NGK and were new a couple of years ago, as was the electronic ignition sensor in the distributor. The coil is Lucas and AFAIK is the original.

The noise is definitely ignition related as its a ticking related to engine speed. Just don't know if it's caused by a faulty component or the way the audio system is wired.

The head is currently powered via the original loom and the amp direct from the battery (as per the fitting instructions that came with it). There's also a 'signal' wire from head to amp that tells the amp to turn on when the head is switched on.


Not worth starting anything now...🍺

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When I was stripping down mine it had a  suppressor as in the picture fitted to the fuel pump in the engine bay.   I don't think it was stock but I do seem to remember using these way back in the day on my mk1 astra.   Its probably not your solution as you say its engine  speed related but might be worth a look at.



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