On Apr 24, 12:11*am, "Phil Allison" wrote:
"Father Haskell"



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.

** Lemme guess - *you have the IC in a plastic box with no shielding of any
kind ?

Wooden box, open, power supply board and transformer laid
on workbench and connected with jumpers. Testing that everything
works before final assembly.

Ideally, there needs to be a metal box that is connected to the negative
supply, pin 4 of the IC plus the metal parts of any pots. Alternatively,
connect the metal fames of each pot to the negative rail ( pin 4) and add a
cap of about 2200pF across pins 2 and 4 to bypass radio frequency energy.

Easily fixed by covering the inside of the box with foil duct tape.

The volume pot wiper needs to be de-coupled from pin 2 by a series
combination of a 4.7 kohm resistor and a cap of about 0.1 uF. *This stops DC
voltage from pin 2 appearing on the pot and helps with RF suppression.

http://www.national.com/ds/LM/LM386.pdf

.... *Phil

http://fluxmonkey.com/electronoize/386amplifier.htm
modified by adding 25 ohm volume pot to the output end,
with the wiper connected to the speakers ( 2 x 3.3 ohm, series).