No audio when connected to vehicle's 12v via a 5V USB adapter

I have no audio, just noise when plugged into the Aux input when the Sansa Clip is connected to the vehicle’s power with a 12v to 5v USB adapter. It’s fine when running on battery only. I can hear good audio at the output of the Sansa while connected to the USB adapter. I wonder if the power needs to be filtered or I should isolate the audio with transformers so there’s no physical connection to the Aux port, just inductive. I’ve read ground loops can cause this condition. 

Yeah its a ground loop. Well technically not a ground loop, since theres no real ground on a battery powered car, but same idea. An isolator will help.

Ok thanks - I wondered how the term ground loop came up. Maybe a Negative Loop would be more accurate. Still, I find it strange the condition occurs. The power supply (USB 5v) and audio output common to the left & right channels are simply sharing the same negative connection which should be fine. I may have  a 600 ohm audio transformer that I’ll try for one channel. If successful will get  second. That would be less expensive than the couplers I saw for around $20.

I’m finally getting away from CDs - I prefer CD-RWs since I like to add/change content frequently, but with this new laptop the quality has been poor. All have skips, even CR-R disks have intermittent clicking sounds like an amp being overdriven. I’ve been using the Sansa Clip with a low power (legal) FM stereo transmitter for listening around the house and outside. The Sansa has absolutlely no trouble connected to a USB adaptor which is connected to an old 12v power supply, perfect audio. The same power supply powers the transmitter.

Out of curiosity I’ll connect the Sansa to an older car that still has only a CD & cassette player and use one of those cassette adaptors. I know - cave man technology. But I’d suspect no ground loop trouble since the audio is being inductively coupled to the cassette player head.

Its because the ground on the audio jack (or rather common) isn’t at the same voltage as the vehicle ground it receives from the USB. Usually this doesn’t matter, but it can cause a problem if the stereo tries to short the common line to ground.

Unfortunately it doesn’t matter now - last night was fooling around with a resistor/capacitor circuit and broke the player. Now there’s no audio. I will buy another. What’s strange is earlier in the day I tried it in my son’s car - was fine with vehicle power. I guess I can use the player as an 8gig memory stick.

Not strange, it depends on the stereo. If the stereo provides coupling cap between ground, it will work. If it doesn’t, it won’t. Don’t screw around with homemade parts. Isolators cost just a few dollars online.

Bought a new 8gig Sansa Clip Zip on sale at Radio Shack. The salesperson knew of no such device to isolate the audio circuit. Same with the Best Buy Mobile store in the mall. I made my own with parts I had on hand. The audio sounds good. No perception of any noise at all when running it on vehicle power.