I just discovered this. And I also found that the AC to USB power adaptor does not have a metal ground pin.
So would this AM interference be gone after getting a better AC to USB power adaptor?
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It's double insulated and therefore does not need a protective grounded plug.
Yes. What you're hearing are harmonics of the 20-50Khz switching frequency in the power supply. These are suppose to meed FCC Part 15 incidental and conducted radiation specifications. Except for name brand manufacturers, most of these wall warts do not meet the specs (by my measurements). There's no way to easily predict which wall wart is any better than any other, leaving you with trial and error to make your selection. Fortunately, USB power adapters are cheap and common, so you should be able to find one that's fairly clean (and return the others).
Also, note that your unspecified model AM radio may also be contributory. My guess(tm) is that you have both the USB power adapter and the radio plugged into the same outlet or power strip. Conducted radiation, through the power lines is another component of the problem. Your radio may be susceptible to junk pickup from the power lines, or for really cheap junk, use the power cord as an antenna. Clamp on ferrite beads (more like ferrite blocks) are available to block conducted radiation and pickup.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
The adapter may be a switch mode supply and you are hearing a harmonic spur of the frequency the supply uses to function on. A switch to non- switch mode supply would cure your problem.
-- Live Fast Die Young, Leave A Pretty Corpse
Thank you both
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