AFigure 5 (A) is a crystal oscillator. I used a CD4011 quad two-input NAND but any CMOS inverting
gate or buffer should work. R1
limits the crystal current; too much
drive could damage it. R2 provides
DC bias to keep the gate input in
the linear range. I breadboarded
the circuit and find that it works
okay at nine volts VCC and above,
but the frequency is way low at five
volts. Perhaps HCMOS would be
better at five volts. If you don’t
need crystal accuracy, the hysteresis
oscillator using a Schmitt-trigger
gate will be cheaper; see Figure 5
(B). The hysteresis is not well
controlled so you will have to
tweak it to get the frequency right.
Also, the frequency varies with
supply voltage. I breadboarded the
circuit and got 2 MHz with 4.7K
and 47 pF.
NOISY AUDIO
QI am a new reader to Nuts & Volts and thought of a question that you might be able to help with. My wife
and I recently moved into a new
home, and I purchased a floor-model
Samsung DLP. However, one thing in
particular bothers me. It seems to
inject a low frequency “noise” into
the AC line, which my 10” powered
sub dutifully picks up and broadcasts
to the annoyance of both myself and
my visitors. This hum or drone
(depending on your opinion) is very
noticable, and only goes away when
the TV is turned off or the sub is
unplugged — neither of which
enhances the full theatrical effect.
My Rotel pre-amp and amplifier
seem to be completely immune to
the noise. I can’t hear it at all from
my bookshelf speakers. This leads me
to believe that either a) my
inexpensive powered sub wasn’t
really worth getting; or b) the sub (or
TV) could use a power-line filter.
Do you think such a thing would
be worthwhile? I enjoy working with
electronics and would be willing to
try making a circuit to provide
“conditioning” if practical. However, I
haven’t got much experience in
dealing with mains voltage or
designing notch filters. For that
matter, I’m not even sure if I would
know how to go about identifying
the specific frequencies being
injected by the TV. Any advice or
guidance you would be willing to
share is certainly appreciated!
— David Passarelli
AI suspect the problem is caused by the fact that your equipment is plugged into different AC outlets
and the safety grounds are at
different AC potential. Try plugging
the TV and sub into the same outlet.
If you’re already doing that, then you
have a ground loop formed probably
by the coax cables that connect the
system. Make sure the grounds don’t
close on themselves and make a
loop. If you’re still having issues, send
me a diagram of the system. NV
24
May 2010
