Ground loop woes.

[clipper]

I have adapted an old blaupunkt radio / cassette to accept a line in (aux) from my mobile / cell phone. Thread here:

https://www.electronicspoint.com/thread ... st-1759797

On connecting the phone to the stereo in the car and reproducing audio from it, there are two issues:

1) When the phone is connected via the 3.5mm stereo jack, the phone audio plays over the top of the radio audio, i.e. I am hearing both audio feeds through the speakers simultaneously. This issue can be "easily" solved however by plugging the charger into the phone. It seems that for some reason which I do not understand, when the phone has a power source, the audio signal is strong enough to "switch" the audio feed selected by the audio processor and silence the radio.

2) The problem with this is that upon connecting the phone charger I am getting a significant amount of engine speed related whine and crackle.

I am not so bothered about the phone needing to be connected to the charger as I like to take the opportunity to charge the phone in the car, but I cannot live with the ground loop interference.

I assumed that the noise was due to the fact that I had the phone charger plugged into the cigarette lighter, whereas the stereo is connected to the stereo loom, so I installed a second lighter socket which takes its 12V+ from the same switched 12v supply as the stereo and also grounded the new socket to the stereo earth (in the dash loom, brown wire), but this has made very little improvement (some, but not enough).

I then suspected a poor earth through the dash loom, so I ran a new wire from the common stereo / charger earth to the car body earth lug (nice and clear bare steel), but this made no difference.

The audio ground of the aux cable is connected to the stereo ground, and I have considered disconncting this, leaving only the positive audio channel feeds connected, but on the test bench this gave a lower volume audio feed from the aux input so I am reluctant to do this, but would it potentially help?

Short of installing a ground loop isolator (last resort), does anyone have any ideas of anything else to try?

[paultv]

I wonder if the charger you are using has horrible cheap chopper circuits which are getting to the body of the car.

Connect a mains (separate from the car ) charger to your phone - and see if it is clean -

Probably no help at all.

I undertook one of these conversions and found it impractical ( on a 90's Pioneer )
for much the same reason, too many switching issues on the control chip.

Paul

[TriggerFish]

I tried this too on my Blaupaunkt SRM37 (I think that's the model) by redirecting the three wires from the tape head to a 3.5mm cable. It works, but like you, there's lots of squeal/background noise. I have up, and just used a bluetooth streamer to the aux in on my amp (the radio plugged into the amp already).

I don't remember it being anywhere near as bad when I was testing with mains power (stepped down to 12v, obviously) inside the house, FWIW.

[clipper]

I wonder if the charger you are using has horrible cheap chopper circuits which are getting to the body of the car.

Connect a mains (separate from the car ) charger to your phone - and see if it is clean -

Probably no help at all.

I undertook one of these conversions and found it impractical ( on a 90's Pioneer )
for much the same reason, too many switching issues on the control chip.

Paul

Thanks, but I have already tried various chargers and cables and all give the same result, except for chargers / batteries which are not connected to the car's electrical system.

The switching on this unit is actually quite good, as long as the phone has a charger connected. The radio plays clearly until an input from the phone is detected and it switches immediately to the phone audio for the duration of the playback, then reverts to radio.

This is great for when I want to listen to the radio, but have Google Maps navigation activated on my phone because Google lady interupts the radio, then when she's finished telling me what to do the radio comes back.

The only issue is that Google lady is accompanied with an alternator whine...

[clipper]

I tried this too on my Blaupaunkt SRM37 (I think that's the model) by redirecting the three wires from the tape head to a 3.5mm cable. It works, but like you, there's lots of squeal/background noise. I have up, and just used a bluetooth streamer to the aux in on my amp (the radio plugged into the amp already).

I don't remember it being anywhere near as bad when I was testing with mains power (stepped down to 12v, obviously) inside the house, FWIW.

Thanks.

Your problem seems to be the connection at the tape head reader wires. Your phone audio was going through the tape pre-amp, which you really don't want.
The aux input feeds need to be added after the tape pre-amp, at the level of the power amp.

Your "squeal" was possibly the same issue as mine, of course in the house the issue is not noticed. In fact, in the car it isn't noticed until the engine is running....

[Satan]

Have you tried a ground loop isolator?

[clipper]

Have you tried a ground loop isolator?

No, not yet, as I said, masking the issue is my last resort, I would rather get to the bottom of it and solve the issue at its root cause.

And to be honest, I am not convinced that an isolator will work, because normally they solve the issue of two compnents with different ground potentials, which I know is not my case as they are both grounded to the dash ground lug.

[Satan]

Lots of equipment come with ferrite cores due to the close proximity of cables close to voltage cables.