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!