radio shielding?

[Mad Scientist Jr]

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...

 

[Meat Plow]

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...

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

 

[Mad Scientist Jr]

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

 

[DaveM]

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



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.

 

[John Popelish]

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

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.

 

[Mad Scientist Jr]

Aha - thanks.

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 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 charactersin the
address)

Life is like a roll of toilet paper; the closer to the end, the faster itgoes.

 

[Mad Scientist Jr]

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! !

 

[John Popelish]

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.

I think I'll start with this. Thanks for your detailed input! !

Good luck.

 

[Meat Plow]

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.

--
#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

 

[John Popelish]

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.

 

[Jack Pagel]

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

 

[Bob Masta]

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...

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!

 

[MadEngineer]

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

 

[Mad Scientist Jr]

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

 

[Mad Scientist Jr]

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.

 

[Mad Scientist Jr]

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

 

[Marra]

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...

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.