LM386 chip amp picks up radio interference.
Father Haskell
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.
Phil Allison
"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.
http://www.national.com/ds/LM/LM386.pdf
.... Phil
Jeff Liebermann
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 je...@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
Father Haskell
> "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).
Phil Allison
"Father Haskell"
"Phil Allison"
> "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.
** RFI heaven.
Pun intended......
> 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.
** Maybe so, but making reliable electrical connection to such foil is not
so easy.
Nuts, bolts and solder lugs are essential.
> 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
>
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).
** I'd call that an output attenuator - not a " volume pot ".
..... Phil
Jamie
put it in a die case box and use some chokes on the inputs.
Jamie
Jamie
That would be a die Cast, not case box.
Jamie
William Sommerwerck
Heck, a plain old Bud mini-box will do.
Father Haskell
8 x 1 sheet metal screw with wire end bent 180 and clamped
between two washers.
>
> > 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
>
> 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).
>
> ** I'd call that an output attenuator - not a " volume pot ".
>
> ..... Phil
So you control volume by controlling input?
Phil Allison
So you control volume by controlling input?
** You after help is just like picking fights ?
Cos you are being damn rude.
.... Phil
Father Haskell
>
> - Show quoted text -
Or a bit weak with electronics and thankful for everyone's advice,
including yours. No reason to get your hackles up.