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!

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

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

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

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

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