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radio shielding?

Discussion in 'Misc Electronics' started by Mad Scientist Jr, May 30, 2007.

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  1. I am building an amplifier

    (for schematic see page 5 here:
    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...
     
  2. Meat Plow

    Meat Plow Guest

    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
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    COOSN-266-06-25794
     
  3. 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 in²/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
     
  4. DaveM

    DaveM Guest

    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 in²/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.
     
  5. 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.
     
  6. Aha - thanks.




     
  7. 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.
    I think I'll start with this. Thanks for your detailed input! !
     
  8. 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.
    Good luck.
     
  9. Meat Plow

    Meat Plow Guest

    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.

    --
    #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
     
  10. (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.
     
  11. Jack Pagel

    Jack Pagel Guest

    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
     
  12. Bob Masta

    Bob Masta Guest

    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!
     
  13. MadEngineer

    MadEngineer Guest

    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
     
  14. That's a pretty cool trick - so this would mean the radio interference
    is coming in through the guitar and/or guitar cable?
     
  15. 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.
     
  16. 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
     
  17. Marra

    Marra Guest

    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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