On Sat, 23 Apr 2011 20:56:03 -0700 (PDT), Father Haskell
wrote:

Homemade, powered by small 13 vct transformer from an old
boom box and LM317T regulator (ps seems capable of 14v clean
power, easily). Sounds like it's picking up the nearest AM station,
especially loud and clear when I touch the volume and gain control
shafts. Touching the heat sink behind the 317 with a fingertip
_quiets_ the interference, though.

Causes? Fixes? Thanks.

The cause is simple rectification of the AM signal at the input to the
amplifier. One method of reducing RF pickup is to insert bypass caps
at the input(s) that form a low pass filter. A bypass cap from Pin 7
to ground will also help. Very long leads is a bad idea. A 0.1uf
bypass between power (pin 6) and ground (pin 4) should help. I can't
offer a more specific solution without seeing a schematic and photo of
the construction method.

You might want to repost your question (with a link to the schematic
and photo of your construction) to sci.electronics.design newsgroup
which is better at offering design and construction suggestions.




--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558