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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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LM386 chip amp picks up radio interference.
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. |
#2
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LM386 chip amp picks up radio interference.
"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 ? 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 |
#3
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LM386 chip amp picks up radio interference.
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 |
#4
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LM386 chip amp picks up radio interference.
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). |
#5
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LM386 chip amp picks up radio interference.
"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 |
#6
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LM386 chip amp picks up radio interference.
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 |
#7
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LM386 chip amp picks up radio interference.
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. Jamie |
#8
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LM386 chip amp picks up radio interference.
Put it in a die cast box and use some chokes on the inputs.
Heck, a plain old Bud mini-box will do. |
#9
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LM386 chip amp picks up radio interference.
On Apr 24, 1:00*am, "Phil Allison" 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 ". ..... *Phil So you control volume by controlling input? |
#10
Posted to sci.electronics.repair
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LM386 chip amp picks up radio interference.
"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 |
#11
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LM386 chip amp picks up radio interference.
On Apr 24, 8:51*pm, "Phil Allison" 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 - 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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