Alternator noise

[Shoppinit]

Thank you for contacting Kensington.

The problem you are experiencing is due to a grounding issue with your car's sound system and the power socket, not with the LiquidAUX product.

That's a relief. I'll just by a new RS6 then.

A small device called a 'ground loop isolator' resolves this issue and eliminates the background whining or buzzing sounds.

Does anyone have any experience of these filters? Presumably they're just some kind of high pass filter which I don't really want on my audio line.

Is it possible to filter the power supply somehow?

[PetrolDave]

A ground loop ISOLATOR is NOT a filter.

The chassis in a car is not all at the same voltage, due to the high currents and high frequency interference (spark ignition) the chassis in one part of the car can be a few tens or hundreds of millivolts different in potential to another. Without a ground loop isolator this is seen as signal by an audio device, and since the audio signal is 1V or 4V that noise is significant.

With a ground loop isolator you route a signal wire from the ground of the source along with the audio signals themselves to the receiving end, and the isolator measures the DIFFERENCE between these two, so it doesn't see the difference in ground voltages.

[Shoppinit]

OK That makes is much clearer. Thanks.

In this case the iPhone is generating the audio signal which is going directly to the Tuner. The kensington is effectively being used as a charger. When the phone's not connected to the car via the cigarette lighter, there is no noise whatsoever. However, when I plug the kensington in I get the horrible noise, presumably through its charging circuit.

This is why I thought about using a filter on the power supply. Possibly a large capacitor?

PS. When I say tuner, I mean the TV Tuner of the RNS-D

[Shoppinit]

OK. I bought a ground loop isolator to see if that has any effect. There are only RCA inputs and outputs on it. No way of grounding it. I wonder how it works.

[Shoppinit]

Looks like a big ferrite ring to me Can't imagine that working.

Still I'll give it a try in a bit and see what happens.

[PetrolDave]

With RCAs you don't need a separate ground, an RCA has a signal and a ground connection anyway!

The ground on the RCAs on the input side should be electrically separate from the ground on the RCAs on the output side - if it isn't then that isn't a ground loop isolator.

When I was working on car anti-noise systems we used a ground loop isolator on the input to the amp so that we didn't add interference into the car audio system - worked a treat.

If the box you've got doesn't need a 12V feed and has only ferrite rings then it's not a ground loop isolator - it's a high frequency filter.

[Shoppinit]

It's identical to this:

http://www.maplin.co.uk/Module.aspx?ModuleNo=33172

except it's made by US Blaster.

I only picked it up because I saw it on the shelf while I was buying a lead. So despite it saying "Ground Loop Isolator" on the box, it's actually a high pass filter... great! How's that for trading standards?

I'd still prefer to filter the DC supply I think...

[PetrolDave]

It's identical to this:

http://www.maplin.co.uk/Module.aspx?ModuleNo=33172

except it's made by US Blaster.

I only picked it up because I saw it on the shelf while I was buying a lead. So despite it saying "Ground Loop Isolator" on the box, it's actually a high pass filter... great! How's that for trading standards?

I'd still prefer to filter the DC supply I think...

When you read the answers on the Maplin website it's clear that what they're selling is a low pass filter and not a ground loop isolator.

If the problem is difference in ground potential then a DC supply filter will do absolutely nothing - because the noise isn't in the supply it's in the ground.

BTW worst case I've seen was in a BMW a couple of years ago - we saw nearly a volt difference between the chassis at the front and in the boot!

[Shoppinit]

I only found the maplins site after I bought the filter. Needless to say that it says nothing on the box about being a filter and not an isolator! Just says ground loop isolator!

[PetrolDave]

Just says ground loop isolator!

Typical mislabeling by a non-technical sales or marketing person who doesn't know enough about the technology they're dealing with...

[Shoppinit]

Terrible. Funnily enough though, the filter works nicely. I was quite shocked. I can't detect a shred of alternator noise. Question is, how much music am I losing? I'm old enough not to miss those high frequencies :p

[PetrolDave]

Good news that it works.