Back of the bus...

[ColinD]

Remember when you were on the bus and your walkman (or walkcoffin) used to pick up the engine note, the interference from the alternator or whatever...

...well it's back

Although this time using different technology. I have an aux device (battery powered) feeding into the stero, all is fine at this point.
Now if I connect a interface (powered from car ciggy lighter) to allow me to forward and cycle tracks etc I pick up the hum as described above.

So this is a 12v device, connecting to the battery device (which is fine) instantly adds hum(at engine pitch) to audio.

Any ideas for fixing?

Inline supressor of <insert capacitor size> on one of the lines?
Earthing, it's plastic so thats a no go.
Rip it all out and sell on Ebay buy RX8.

Thoughts?

[SimonT]

Is it a constant pitch hum, or does it increase with revs? If constant, I'd recomment sticking a 'scope across the (input? output?, hmmm) and matching a capacitor.

If it varies, then you might end up with quite a complicated bandpass filter

[ColinD]

arh bugger...It's variable. Looking up our old haynes manuals I could whack a cap over the alternator, but you know what, I don't think I dare on these things.

Nothing inline on the powersupply to help?

[ColinD]

or

I finally decided to try and ground the ground of my audio out. That did the trick. Don't understand it really, but it sounds great!

Next question...yipee, how do I do that?

[Quinten]

I think you suffered a so called groundloop. Don't exactly know what it is, other than bad.

[ColinD]

Also am I right in thinking DC connectors now default to a standard negative earthing and / or that negative earth is the outer sheath of any plug connector...got a cunning plan :S

[humsk]

Originally Posted by ColinD

Also am I right in thinking DC connectors now default to a standard negative earthing and / or that negative earth is the outer sheath of any plug connector...got a cunning plan :S

Most do but unfortunately it’s not guaranteed– just depends the manufacturer. Have you got a mains adaptor for it? They’re usually labelled up with which terminal is which polarity. Best bet is to check it with a voltmeter.

In very basic terms, the hum was caused by you injecting electrical “noise” into the stereo from the ciggy supply and / or the wires were acting as an aerial picking up electric fields inside the vehicle (from all of those ECUs and the like). Adding the ground connection referenced your “aux device” to the vehicle ground giving a path for the “noise” to go to ground.

Quinten, a ground loop is where there is a path (intentional or not) for electrical current (noise) to flow via the grounding into a sensitive piece of equipment eg car stereo. The result is audible noise. For more detailed info, do a search on “groundloops”.