LM386 chip amp picks up radio interference.

F

Father Haskell

Guest
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.
 
"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.
Click to expand...

** 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
 
On Sat, 23 Apr 2011 20:56:03 -0700 (PDT), Father Haskell
<fatherhaskell@yahoo.com> 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.
Click to expand...
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
 
"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 ?
Click to expand...
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.
Click to expand...
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
Click to expand...
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
 
On Apr 24, 12:11 am, "Phil Allison" <phi...@tpg.com.au> 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 ?
Click to expand...
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.
Click to expand...
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
Click to expand...
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).
 
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.


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

Jamie
 
Jamie wrote:

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.


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

Jamie



That would be a die Cast, not case box.
Click to expand...

Jamie
 
"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.
Click to expand...
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 ".
Click to expand...
So you control volume by controlling input?


** You after help is just like picking fights ?

Cos you are being damn rude.


..... Phil
 
On Apr 24, 1:00 am, "Phil Allison" <phi...@tpg.com.au> wrote:
"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.
Click to expand...
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
Click to expand...
So you control volume by controlling input?
 
On Apr 24, 8:51 pm, "Phil Allison" <phi...@tpg.com.au> wrote:
"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.

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 ".

So you control volume by controlling input?

** You after help is just like picking fights ?

    Cos you are being damn rude.

.... Phil- Hide quoted text -

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