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?

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







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

"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 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.

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 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.

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 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 characters in the
address)

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

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 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 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.

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.

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

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!