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radio shielding?
I am building an amplifier
(for schematic see page 5 he http://makezine.com/09/crackerboxamp/ ) but when I turn up the gain, I get buzz and can actually hear some radio broadcasts on it. I assume I need to shield the circuit, or parts of it... Would lining the inside of the project case (in this case a cardboard cracker box) with aluminum foil work? Incidentally, does anyone know if someone makes a type of "shielding" spray paint or primer, that you could just spray on a plastic or cardboard project box, that would provide shielding? Any help appreciated... |
radio shielding?
Did you use shielded wire to connect the controls and jacks?
Thanks for your reply... No, just normal stranded wire. Is that where the most interference would be leaking in? I found shielding paint (see below) - would it help to paint the inside of the project box with this? http://www.lessemf.com/paint.html SUPER SHIELD Reduce or Eliminate EMI / RFI Interference Super Shield A general purpose EMI/RFI shielding in a handy aerosol spray for use on most substrates. Especially good for RF shielding plastic electronics enclosures. Consists of a tough, durable acrylic base pigmented with a high purity nickel flake. One to two mil coating provides 40dB - 50dB shielding across a frequency range of 5 to 1800MHz. About 1600 inē/can coverage at 1.5 mil. Contains no CFC 'S, NO HCFC'S, ozone friendly. 340g (12 oz) aerosol can. Dries to a dull gray color. Click to see MSDS * Surface Resistivity ~0.7 Ohm/sq * Dry time: 10 minutes at room temperature * Recoat time: 5 minutes * Excellent adhesion to most plastics * Tested as per IEEE Std. 299-1997 * Underwriters Laboratories Recognized File No.: E202609 * NO CFC 'S, NO HCFC'S, Ozone Friendly * RoHS Compliant Flammable aerosol, ships by ground only. Super Shield (Cat. #A285) ....................................... $24.95 add to cart On May 30, 1:26 pm, Meat Plow wrote: On Wed, 30 May 2007 10:18:26 -0700, Mad Scientist Jr wrote: I am building an amplifier (for schematic see page 5 he http://makezine.com/09/crackerboxamp/ ) but when I turn up the gain, I get buzz and can actually hear some radio broadcasts on it. I assume I need to shield the circuit, or parts of it... Would lining the inside of the project case (in this case a cardboard cracker box) with aluminum foil work? Incidentally, does anyone know if someone makes a type of "shielding" spray paint or primer, that you could just spray on a plastic or cardboard project box, that would provide shielding? Any help appreciated... Did you use shielded wire to connect the controls and jacks? -- #1 Offishul Ruiner of Usenet, March 2007 #1 Usenet Asshole, March 2007 #1 Bartlo Pset, March 13-24 2007 #10 Most hated Usenetizen of all time #8 AUK Hate Machine Cog Pierre Salinger Memorial Hook, Line & Sinker, June 2004 COOSN-266-06-25794 |
radio shielding?
"Mad Scientist Jr" wrote in message
oups.com... Did you use shielded wire to connect the controls and jacks? Thanks for your reply... No, just normal stranded wire. Is that where the most interference would be leaking in? I found shielding paint (see below) - would it help to paint the inside of the project box with this? http://www.lessemf.com/paint.html SUPER SHIELD Reduce or Eliminate EMI / RFI Interference Super Shield A general purpose EMI/RFI shielding in a handy aerosol spray for use on most substrates. Especially good for RF shielding plastic electronics enclosures. Consists of a tough, durable acrylic base pigmented with a high purity nickel flake. One to two mil coating provides 40dB - 50dB shielding across a frequency range of 5 to 1800MHz. About 1600 inē/can coverage at 1.5 mil. Contains no CFC 'S, NO HCFC'S, ozone friendly. 340g (12 oz) aerosol can. Dries to a dull gray color. Click to see MSDS * Surface Resistivity ~0.7 Ohm/sq * Dry time: 10 minutes at room temperature * Recoat time: 5 minutes * Excellent adhesion to most plastics * Tested as per IEEE Std. 299-1997 * Underwriters Laboratories Recognized File No.: E202609 * NO CFC 'S, NO HCFC'S, Ozone Friendly * RoHS Compliant Flammable aerosol, ships by ground only. Super Shield (Cat. #A285) ....................................... $24.95 add to cart For $24.95 plus shipping, that's some mighty expensive shielding. Why not spend about half of that and buy a proper aluminum chassis for it. They offer inherent shielding. You'll have to do a bit of easy drilling to mount the components, but it's well worth the effort, and it looks so much better than a cardboard box. Surf over to www.mouser.com and search for Bud mini-box. There are quite a number of sizes available; one is sure to fit your needs. Cheers!! -- Dave M MasonDG44 at comcast dot net (Just substitute the appropriate characters in the address) Life is like a roll of toilet paper; the closer to the end, the faster it goes. |
radio shielding?
Mad Scientist Jr wrote:
Did you use shielded wire to connect the controls and jacks? Thanks for your reply... No, just normal stranded wire. Is that where the most interference would be leaking in? I found shielding paint (see below) - would it help to paint the inside of the project box with this? http://www.lessemf.com/paint.html SUPER SHIELD Reduce or Eliminate EMI / RFI Interference Super Shield A general purpose EMI/RFI shielding in a handy aerosol spray for use on most substrates. Especially good for RF shielding plastic electronics enclosures. Consists of a tough, durable acrylic base pigmented with a high purity nickel flake. One to two mil coating provides 40dB - 50dB shielding across a frequency range of 5 to 1800MHz. About 1600 inē/can coverage at 1.5 mil. Contains no CFC 'S, NO HCFC'S, ozone friendly. 340g (12 oz) aerosol can. Dries to a dull gray color. Click to see MSDS * Surface Resistivity ~0.7 Ohm/sq * Dry time: 10 minutes at room temperature * Recoat time: 5 minutes * Excellent adhesion to most plastics * Tested as per IEEE Std. 299-1997 * Underwriters Laboratories Recognized File No.: E202609 * NO CFC 'S, NO HCFC'S, Ozone Friendly * RoHS Compliant Flammable aerosol, ships by ground only. Super Shield (Cat. #A285) ....................................... $24.95 add to cart It would probably be little help. And if you coat the box with anything conductive, it won't help much unless you can make a good electrical connection between the coating and pin 4 of the chip. If you really want to try this, thin brass shim stock cut and folded to fit the box, is probably the best you can get, short of a copper or aluminum box. It is also easily solderable to make the ground connection. If this amp powered by a battery? In this configuration, pin 2 is the most sensitive node, and anything connected to it should be surrounded with shielding. Pins 1 and 8 are also somewhat sensitive so if the gain pot has a metal case, that case should also have a ground wire run to pin 4 to shield the resistive element. Pin 7 is less sensitive, and since this circuit connects nothing to it, it is probably not involved in the noise. |
radio shielding?
Aha - thanks.
On May 30, 6:19 pm, "DaveM" wrote: "Mad Scientist Jr" wrote in ooglegroups.com... Did you use shielded wire to connect the controls and jacks? Thanks for your reply... No, just normal stranded wire. Is that where the most interference would be leaking in? I found shielding paint (see below) - would it help to paint the inside of the project box with this? http://www.lessemf.com/paint.html SUPER SHIELD Reduce or Eliminate EMI / RFI Interference Super Shield A general purpose EMI/RFI shielding in a handy aerosol spray for use on most substrates. Especially good for RF shielding plastic electronics enclosures. Consists of a tough, durable acrylic base pigmented with a high purity nickel flake. One to two mil coating provides 40dB - 50dB shielding across a frequency range of 5 to 1800MHz. About 1600 inē/can coverage at 1.5 mil. Contains no CFC 'S, NO HCFC'S, ozone friendly. 340g (12 oz) aerosol can. Dries to a dull gray color. Click to see MSDS * Surface Resistivity ~0.7 Ohm/sq * Dry time: 10 minutes at room temperature * Recoat time: 5 minutes * Excellent adhesion to most plastics * Tested as per IEEE Std. 299-1997 * Underwriters Laboratories Recognized File No.: E202609 * NO CFC 'S, NO HCFC'S, Ozone Friendly * RoHS Compliant Flammable aerosol, ships by ground only. Super Shield (Cat. #A285) ....................................... $24.95 add to cart For $24.95 plus shipping, that's some mighty expensive shielding. Why not spend about half of that and buy a proper aluminum chassis for it. They offer inherent shielding. You'll have to do a bit of easy drilling to mount the components, but it's well worth the effort, and it looks so much better than a cardboard box. Surf over towww.mouser.comand search for Bud mini-box. There are quite a number of sizes available; one is sure to fit your needs. Cheers!! -- Dave M MasonDG44 at comcast dot net (Just substitute the appropriate characters in the address) Life is like a roll of toilet paper; the closer to the end, the faster it goes. |
radio shielding?
If this amp powered by a battery?
A 9V battery, although I plugged a 200 mA 9V AC adapter into it and it worked also, though I didn't leave it plugged in that long - maybe if it was in longer it would have fried it. I'm not sure how many mA would be unsafe for this circuit - I looked up 9V batteries to see what the current typically is, but it seems this depends on the circuit. Until I find this out I'll probably just use a 9V "battery eliminator" AC adapter. In this configuration, pin 2 is the most sensitive node, and anything connected to it should be surrounded with shielding. I think I'll start with this. Thanks for your detailed input! ! On May 30, 6:25 pm, John Popelish wrote: Mad Scientist Jr wrote: Did you use shielded wire to connect the controls and jacks? Thanks for your reply... No, just normal stranded wire. Is that where the most interference would be leaking in? I found shielding paint (see below) - would it help to paint the inside of the project box with this? http://www.lessemf.com/paint.html SUPER SHIELD Reduce or Eliminate EMI / RFI Interference Super Shield A general purpose EMI/RFI shielding in a handy aerosol spray for use on most substrates. Especially good for RF shielding plastic electronics enclosures. Consists of a tough, durable acrylic base pigmented with a high purity nickel flake. One to two mil coating provides 40dB - 50dB shielding across a frequency range of 5 to 1800MHz. About 1600 inē/can coverage at 1.5 mil. Contains no CFC 'S, NO HCFC'S, ozone friendly. 340g (12 oz) aerosol can. Dries to a dull gray color. Click to see MSDS * Surface Resistivity ~0.7 Ohm/sq * Dry time: 10 minutes at room temperature * Recoat time: 5 minutes * Excellent adhesion to most plastics * Tested as per IEEE Std. 299-1997 * Underwriters Laboratories Recognized File No.: E202609 * NO CFC 'S, NO HCFC'S, Ozone Friendly * RoHS Compliant Flammable aerosol, ships by ground only. Super Shield (Cat. #A285) ....................................... $24.95 add to cart It would probably be little help. And if you coat the box with anything conductive, it won't help much unless you can make a good electrical connection between the coating and pin 4 of the chip. If you really want to try this, thin brass shim stock cut and folded to fit the box, is probably the best you can get, short of a copper or aluminum box. It is also easily solderable to make the ground connection. If this amp powered by a battery? In this configuration, pin 2 is the most sensitive node, and anything connected to it should be surrounded with shielding. Pins 1 and 8 are also somewhat sensitive so if the gain pot has a metal case, that case should also have a ground wire run to pin 4 to shield the resistive element. Pin 7 is less sensitive, and since this circuit connects nothing to it, it is probably not involved in the noise. |
radio shielding?
Mad Scientist Jr wrote:
If this amp powered by a battery? A 9V battery, although I plugged a 200 mA 9V AC adapter into it and it worked also, though I didn't leave it plugged in that long - maybe if it was in longer it would have fried it. I'm not sure how many mA would be unsafe for this circuit - I looked up 9V batteries to see what the current typically is, but it seems this depends on the circuit. Until I find this out I'll probably just use a 9V "battery eliminator" AC adapter. The reason I asked is that a battery eliminator will probably have some hum in its output, and pin 7 is there to remove most of that interference from getting into the amplifier. See the data sheet for the LM386: http://web.mit.edu/6.115/www/datasheets/LM386.pdf The supply voltage enters the input to bias the output at half of the supply voltage, going past pin 7, which is usually bypassed to ground by 10 uF or more to smooth the bias current from the supply. Even with a battery supply, the voltage will bounce around a little from the current drawn by the output transistors, and bypassing pin 7 helps reduce that effect, too. In this configuration, pin 2 is the most sensitive node, and anything connected to it should be surrounded with shielding. I think I'll start with this. Thanks for your detailed input! ! Good luck. |
radio shielding?
Mad Scientist Jr wrote:
If this amp powered by a battery? A 9V battery, although I plugged a 200 mA 9V AC adapter into it and it worked also, though I didn't leave it plugged in that long - maybe if it was in longer it would have fried it. I'm not sure how many mA would be unsafe for this circuit - I looked up 9V batteries to see what the current typically is, but it seems this depends on the circuit. (snip) The current with the adapter is also controlled by the attached circuit. The 200 mA rating is the maximum current the load can draw, before the adapter risks over heating. The only real risk with the adapter is that, under no load conditions, its voltage will rise above its 9 volt, full load rating, enough to damage the chip. All versions of LM386 can handle a 12 volt supply and are not usually damaged by 15 volts if little sound is being produced. I would check the adapter with no load and make sure it does not produce more than 15 volts. |
radio shielding?
Meat Plow wrote: On Wed, 30 May 2007 15:05:24 -0700, Mad Scientist Jr wrote: Did you use shielded wire to connect the controls and jacks? Thanks for your reply... No, just normal stranded wire. Is that where the most interference would be leaking in? You might want to rewire it with shielded, won't take that long and would eliminate that as a possibility. I built a 50 watt push pull tube guitar amp back in my teens 1970's and had to go back and replace some front end circuits with shielded because of hum. No radio stations but I didn't live close to one like you might. I live a mile away from an AM radio station. Usually it doesn't get into the amps, but there are/were some amps that we could hear the AM radio station coming thru. A quick way to eliminate that is to wind a coil with the guitar cord very close to the plug that goes into the amp. Make the coil small diameter, 2 or 3 inches. That will choke the signal from entering the amp. It see's the guitar cord as a long wire antenna. How many turns? Start with 5 or 6, if that doesn't work try a couple more turns. Hope this may be of some help. Jack Fender Santa Maria Gibson SG |
radio shielding?
On 30 May 2007 10:18:26 -0700, Mad Scientist Jr
wrote: I am building an amplifier (for schematic see page 5 he http://makezine.com/09/crackerboxamp/ ) but when I turn up the gain, I get buzz and can actually hear some radio broadcasts on it. I assume I need to shield the circuit, or parts of it... Would lining the inside of the project case (in this case a cardboard cracker box) with aluminum foil work? Incidentally, does anyone know if someone makes a type of "shielding" spray paint or primer, that you could just spray on a plastic or cardboard project box, that would provide shielding? Any help appreciated... If you are hearing radio broadcasts, it is because of two things: 1) The bandwidth of the amp is too high, and 2) there is nonlinearity (ie rectification) in the amp, typically the input stage. You can probably solve this problem simply with a small RF bypass capacitor across the amp input, just after the input resistor. Set the RC product to a microsecond or so. If you look inside consumer audio amps, you'll find that shielding is not normally used. Best regards, Bob Masta D A Q A R T A Data AcQuisition And Real-Time Analysis www.daqarta.com Scope, Spectrum, Spectrogram, Signal Generator Science with your sound card! |
radio shielding?
You failed to mention what you have on the input, which can be a factor--if you disconnect any input cables, short your input to ground right at the amp and the unwanted stuff is still there, is is very possible your unwanted signals are coming through the 5K gain pot and its leads. Signals on these leads directly modulate the output. Try this: Replace the 5K pot with a resistor (say, 1K--the lower the R the higher the gain) soldered right on the board. Disconnect the pot wires completely. If that cures your problem consider eliminating that gain control and just using the volume pot. Otherwise, you could try shielding those wires, with the shield tied to signal ground, though I'm not completely sure that would work. Use stereo shielded cable with both wires in the same shield if possible. Twisting the two wires together may help. You can also try Mr. Masta's RF bypassing, only on the gain pot inputs as well. GL Glenn |
radio shielding?
That's a pretty cool trick - so this would mean the radio interference
is coming in through the guitar and/or guitar cable? I live a mile away from an AM radio station. Usually it doesn't get into the amps, but there are/were some amps that we could hear the AM radio station coming thru. A quick way to eliminate that is to wind a coil with the guitar cord very close to the plug that goes into the amp. Make the coil small diameter, 2 or 3 inches. That will choke the signal from entering the amp. It see's the guitar cord as a long wire antenna. How many turns? Start with 5 or 6, if that doesn't work try a couple more turns. Hope this may be of some help. Jack Fender Santa Maria Gibson SG |
radio shielding?
Thanks for your reply - that's worth a try.
I was going to try wiring in a foot switch to turn the gain on & off anyway. By the way do you know a good online place to order shielded wire (either stereo or mono)? I had an old cheap shielded guitar cable I was going to cannibalize for this, but I would prefer to find wire that isn't too thick & easy to work with. On May 31, 9:08 am, MadEngineer wrote: You failed to mention what you have on the input, which can be a factor--if you disconnect any input cables, short your input to ground right at the amp and the unwanted stuff is still there, is is very possible your unwanted signals are coming through the 5K gain pot and its leads. Signals on these leads directly modulate the output. Try this: Replace the 5K pot with a resistor (say, 1K--the lower the R the higher the gain) soldered right on the board. Disconnect the pot wires completely. If that cures your problem consider eliminating that gain control and just using the volume pot. Otherwise, you could try shielding those wires, with the shield tied to signal ground, though I'm not completely sure that would work. Use stereo shielded cable with both wires in the same shield if possible. Twisting the two wires together may help. You can also try Mr. Masta's RF bypassing, only on the gain pot inputs as well. GL Glenn |
radio shielding?
I am just plugging an electric guitar directly into the input. I don't
have the actual circuit here with me right now but the schematic is here (see page 5): http://makezine.com/09/crackerboxamp/ I believe the + of the input goes to a 0.01 uF capacitor, the - goes to ground. BTW can anyone explain how to add an XLR out (either balanced or unbalanced) to directly connect the amp to a mixer or recorder? Thanks On May 31, 9:08 am, MadEngineer wrote: You failed to mention what you have on the input, which can be a factor |
radio shielding?
Mad Scientist Jr wrote:
I am just plugging an electric guitar directly into the input. I don't have the actual circuit here with me right now but the schematic is here (see page 5): http://makezine.com/09/crackerboxamp/ I believe the + of the input goes to a 0.01 uF capacitor, the - goes to ground. BTW can anyone explain how to add an XLR out (either balanced or unbalanced) to directly connect the amp to a mixer or recorder? SNIP First of all, any sensitive amplifier needs to be build in a metal box no bigger than is necessary. The box acts as a screen so you have no need to use screen leads inside the box. The circuit you have used has no RF decoupling at the input nor on the supply (the 100uf capacitor on the supply will not decouple very well at RF), I would put a 10n ceramic capacitor across it and, for good measure, a 100n across that! At the input I would put something like 1n to ground (if your input is a guitar then the pickup will be low impedance so there won't be any effect on it's output. I would also be tempted to put an RF choke in series with the input. If, as I suspect, you are picking up AM broadcasts coiling the input cable a few times at the input won't have much effect. These links may be useful: http://www.rane.com/note151.html http://www.benchmarkmedia.com/caig/html/caig06.html Good luck! Charlie. -- M0WYM www.radiowymsey.org |
radio shielding?
I finally got around to replacing the wires from the potentiometer,
rheostat, input and speaker out all with shielded cable which worked - 99.9% of the noise is gone. The only noise left is a slight buzz which goes away when my hand touches the guitar strings. (Which leads me to question whether the guitar is properly grounded. Any advice on how to ensure that it is?) Anyway thanks to everyone for their replies, the shielded wire did the trick. On Jun 1, 6:20 am, charlie wrote: Mad Scientist Jr wrote: I am just plugging an electric guitar directly into the input. I don't have the actual circuit here with me right now but the schematic is here (see page 5): http://makezine.com/09/crackerboxamp/ I believe the + of the input goes to a 0.01 uF capacitor, the - goes to ground. BTW can anyone explain how to add an XLR out (either balanced or unbalanced) to directly connect the amp to a mixer or recorder? SNIP First of all, any sensitive amplifier needs to be build in a metal box no bigger than is necessary. The box acts as a screen so you have no need to use screen leads inside the box. The circuit you have used has no RF decoupling at the input nor on the supply (the 100uf capacitor on the supply will not decouple very well at RF), I would put a 10n ceramic capacitor across it and, for good measure, a 100n across that! At the input I would put something like 1n to ground (if your input is a guitar then the pickup will be low impedance so there won't be any effect on it's output. I would also be tempted to put an RF choke in series with the input. If, as I suspect, you are picking up AM broadcasts coiling the input cable a few times at the input won't have much effect. These links may be useful: http://www.rane.com/note151.html http://www.benchmarkmedia.com/caig/html/caig06.html Good luck! Charlie. -- M0WYMwww.radiowymsey.org |
radio shielding?
On 30 May, 18:18, Mad Scientist Jr wrote:
I am building an amplifier (for schematic see page 5 hehttp://makezine.com/09/crackerboxamp/ ) but when I turn up the gain, I get buzz and can actually hear some radio broadcasts on it. I assume I need to shield the circuit, or parts of it... Would lining the inside of the project case (in this case a cardboard cracker box) with aluminum foil work? Incidentally, does anyone know if someone makes a type of "shielding" spray paint or primer, that you could just spray on a plastic or cardboard project box, that would provide shielding? Any help appreciated... I would firstly put a cap on the input to short out radio frequencies to ground. 220pf from signal in to ground works well for me. Ground the case too if its metal. |
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