Installing a suppression capacitor

[ErictheBastard]

I have a newer model Blaupunkt (London MP38) in my '05 Elise. It has an aux in on the front panel that works fine with my iPhone UNTIL I plug the iPhone into the 12v power adaptor. Then I get RPM-synchronous whine. The AM signal is also a bit noisy (RPM-synchronous as well).

I understand that a suppression capacitor can eliminate this, but I've never installed one on any of my previous cars. Where in the electrical circuit would it go? Also, the ones I've searched for online are these giant, heavy, blingy monstrosities covered in marketing puke. Any other alternatives?

[Cthulhu]

I wonder if a ferrite bead around the end of the power cable would help

Ferrite bead - Wikipedia, the free encyclopedia

[Chris Mackey]

I just bought one of these to put in my Highlander - 30 minutes ago. The guys on the Tundra forum say it works. I won't know for another week or so.

PAC Audio SNI-1/3.5 Ground Loop Isolator & Noise Filter - eBay (item 190342937640 end time Oct-25-09 20:26:58 PDT)

[TimMullen]

Originally Posted by Cthulhu

I wonder if a ferrite bead around the end of the power cable would help

I know next to nothing about them, but Radio Shack has then pretty cheap. I would think that you would snap it around the power lead to the radio.

[Cthulhu]

If it only whines when charging, I'd think you'd snap it around the end of the power cable.

[holmz]

Originally Posted by Cthulhu

If it only whines when charging, I'd think you'd snap it around the end of the power cable.

Or belt-n-suspenders... wrap around both the radios power cable and the iPhones.

Some phones use their earpiece cable as the antenna - so it might not aid in making a call if the cord is used, and you put a choke on it.

[Cthulhu]

Originally Posted by holmz

Or belt-n-suspenders... wrap around both the radios power cable and the iPhones.

Some phones use their earpiece cable as the antenna - so it might not aid in making a call if the cord is used, and you put a choke on it.

I work at an electrical engineering company (we do analog transmission line simulators among other things). I'll ask our chief scientist what to do when he comes in today . He'll probably draw me a schematic on a post-it or something...



My thought is that something is acting as an antenna, but with the disclaimer that I'm a geologist, not an electrical engineer, myself. If the OP put a bead around the power cable, that would attenuate noise picked up by the power cable.

It sounds like the OP is using the aux jack with or without the charger connected to his iphone, so the only difference is the power cable, no?

Edit: chief scientist isn't coming in today, but the current consensus is that you need some type of filter and that a ferrite bead MAY work on the power cable; people use them on phone lines to cut out audible noise. They're really good at attenuating RF, but if the noise is audible it's closer to DC. Another possibility is ground loop noise, I wonder if there's an easy way to make your ipod and head unit share a common ground. You're picking up alternator or distributor noise somehow, it would seem.

I can mail you a bead, we have snap-on ones.

[ewalberg]

Someone else on this forum who was an installer said that the only time they ran into noise issues was when the head unit and amp DIDN'T share the same ground point. Point being that you could try hooking up a cigarette lighter up to the wires coming out of the head unit to see if that helped...

[ErictheBastard]

I didn't think it was possible to have a ground loop issue in automobile wiring, as the chassis and engine provide a common ground. I also have always associated ground loop issues with AC (60 cycle hum inducted into an adjacent audio path). I suppose the alternator would be the AC source in this case.

I'll try the ferrite bead on the power source first (thanks Cthulhu for the offer to send one, but I think I should be able to source one through one of my local suppliers here in LA - I'm a concert sound engineer by trade). Then I'll try lifting the ground on the audio connector (cheap, on-the-fly rig to Chris Mackay's suggestion).

And to confirm that Cthulhu is correct, yes, the only variable is the power cable.

[Cthulhu]

Geologist FTW...

Our chief scientist confirmed that my original though was correct:

The alternator is constantly arcing, which radiates energy all over the place. Unshielded wire will pick it up, and the lowest level wire is most vulnerable, because the signal has to be amplified the most. If you only pick up noise when the power cable is connected to the ipod, then the power cable must be picking up the noise and injecting it into the signal path.

He suggested using a ferrite toroid, though, rather than a snap-on bead. It's basically a big ferrite donut. You thread the power cable through the center and then loop it around the outside and feed it back through the center again. He said a bead *might* work, but that beads are generally better at attenuating RF. If you loop the power cable once around the toroid it should fix the issue.

BTW: His background is in analog passive filters. Convenient, eh?

[Cthulhu]

I was measuring a USB cable when I realized that you aren't using a USB to ipod power cable, you're probably using a cigarette lighter power connector, eh...

Anyway, it looks like you may have to go snippy-snippy:

http://indashpc.org/downloads/docs/i...e_assembly.gif

They're probably only using pin 2 and 23 on the ribbon cable to provide power, but who knows. I figure you must have a multimeter, and the wires in the cable are colored, I'm sure. Hopefully if you go this route, it won't be a huge pain in the butt. Here are some toroid cores, none are big enough to accommodate the big fat ipod connector, I checked.

Toroid Core

Our chief scientist recommends the largest one you can find, and said you can always wrap it around 3 times. He also mentioned that since they're so cheap, you may want to buy a few of them and see what works.