Electronic – Reduce interference over 3.5 mm to cassette tape adapter

audiocellphoneemfinterferenceshielding

I have one of those cheap 3.5mm > cassette adapters, to use in my car. I've purchased high-end ones (around $40) and I've purchased cheaper ones like this ($20) but I always get static/feedback from these things, no matter the cost (think of that noise you would hear when a mobile phone was about to ring around a tv/radio).

I did notice that if I hold the wire with my hand, as pictured below, the noise is drastically reduced, usually eliminated completely.

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I assume it's EMF/RF interference from my phone's antenna causing the noise. If I'm wrong, maybe someone can point me in the right direction. I am technologically and mechanically savvy, but by no means an EE. I was thinking maybe I could wrap a section of the cable with aluminum foil (picture below), creating a bit of shielding, but the noise is still there.

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Does anyone have any better ideas as to how I can go about [cheaply/easily] creating my own shielding or eliminating the noise? Or maybe you know of a company that makes one of these adapters, which isn't prone to interference.

Best Answer

Looks like this will stay open, so:

It sounds like you are getting RF interference from your cell phone. There's a few ways to go at it.

  1. Simplest and cleanest is to get a split ferrite bead, make a few loops in your cable, and clip the bead on it. The bead will block RF traveling down the cable.
  2. Connected that foil around the cable to the chassis of the car. Without the ground (car chassis) connection, the RF hits the foil and the foil re-radiates it. With the ground, the RF hits the foil, follows the wire down to the chassis, and goes away from your electronics.
  3. Ground the shielding in the cable. Your cable is made of a couple of shielded wires, like headphone cables. You could carefully remove a spot of insulation from each of the pair, and run a run from the shield under the insulation to ground (car chassis.)

Number 2 is ugly.

Number 3 is ugly and if you aren't good with stripping wires and soldering you could mess up the cable beyond your ability to repair.

Number 1 is the cleanest and probably best way to go.

You might find an old ferrite bead on an old wall wart cable that you could scavenge.

If you don't have a ferrite bead (or to kill time while waiting for one to be delivered) you could try this:

Wrap the cable around some soft iron (like a nail) just an experiment. It may well reduce the interference enough that you don't need anything fancier.

If wrapping it around a nail doesn't help, that doesn't mean a ferrite bead wouldn't help. It just means nails can't do the job of a ferrite.

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