LM386 chip amp picks up radio interference.

"Father Haskell"

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

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.

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.

.... Phil

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     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

any

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

a

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

ps DC

modified by adding 25 ohm volume pot to the output end, with the wiper connected to the speakers ( 2 x 3.3 ohm, series).

"Father Haskell" "Phil Allison"

Wooden box, open, power supply board and transformer laid on workbench and connected with jumpers.

** RFI heaven.

Pun intended......

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

** Maybe so, but making reliable electrical connection to such foil is not so easy.

Nuts, bolts and solder lugs are essential.

modified by adding 25 ohm volume pot to the output end, with the wiper connected to the speakers ( 2 x 3.3 ohm, series).

** I'd call that an output attenuator - not a " volume pot ".

..... Phil

put it in a die case box and use some chokes on the inputs.

Jamie

That would be a die Cast, not case box.

Jamie

Heck, a plain old Bud mini-box will do.

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t 8 x 1 sheet metal screw with wire end bent 180 and clamped between two washers. s

So you control volume by controlling input?

"Father Haskell" "Phil Allison" > "Father Haskell"

8 x 1 sheet metal screw with wire end bent 180 and clamped between two washers.

So you control volume by controlling input?

** You after help is just like picking fights ?

Cos you are being damn rude.

.... Phil

f

ive

ly,

not

ops

.

Or a bit weak with electronics and thankful for everyone's advice, including yours. No reason to get your hackles up.