all. Digital signal processing is
employed to filter and shape the
signals for a greater effect. You can
purchase a pair of noise cancelling
headphones for under $300.00. The
same technology has been developed
on a larger scale using large
loudspeakers and high-powered
amplifiers to cancel unwanted noise
in a room environment.
[2041 — February 2004]
Does anyone have a schematic for a well regulated and efficient 120
VAC, transformerless power supply (no step down transformer of any kind
— just diodes, resistors, caps, and voltage regulators) with an output of 9
or 12 VDC at 40 mA?
#1 An efficient power supply schematic is shown, though it is dangerous,
because you have a good chance of getting the hot and return sides of the line
reversed. This power supply is a current source, so a parallel voltage regulator
The 2 µF series
get you close to,
if not quite up to
40 mA. Pick a
that will equal
1 volt, then
multiply by 20 to
get R1's resistance. For 9 V, R1 would be 160 Ω; for 12 V,
it would be 220 Ω. In these examples, the power through
the resistor is on the order of 0.5 W, so use a full 1 W part.
Alonzo E. Fuller
Grants Pass, OR
#2 The circuit below will supply a maximum of 40 mA.
Keep in mind that this circuit is connected to the 120 volt
power and is potentially lethal. The supply and the load
must be enclosed in such a way that the user cannot be
The technique works well for
predictable types of noise. Blocking
out the unpredictable noises found
around the home — like those loud
bass sounds you described — requires
more sophisticated DSP software,
which is currently not available for
#2 A physical barrier — such as
insulation or a wall — is still the best
way to block street noise. Large scale
devices to cancel noise are used in
some industrial settings, but only
noise canceling headphones (such as
Sony MDR-NC20) are available to
consumers. The theory of operation
is relatively simple, but the
implementation is difficult. Since
sound is a pressure wave, it can be
canceled out by an "opposite" wave.
When the pressure of the noise wave
is higher than the background
pressure, the canceling wave must
have a lower pressure and vice versa.
Circle #48 on the Reader Service Card.