🚀 go-pugleaf

	From my past high school experience playing with my custom sound
installation (40 watts/channel, 7 band graphic eq, and Sony D-4 portable cd
player) in of all things, an '83 Sentra, I can tell you you should expect a
lot of electrical noise. It is inherent in a car. My first impulse was to
try filtering, then came tunable notch filters, then Radio Shack car
audio patches. Finally, I sat down and thought about it. The brunt of the
noise seems to always come from the car's ignition system. The tunable
notch filter failed because the oscilation frequency depends on the
engine's RPM multiplied by an intergral number (the number of cylinders).
Eventually, I worked it down to a simple circuit with a little back up
protection, noise filtering, and one power regulator transistor for a 9
volt, 500 mA cd player (yes, 4 watts of power! And it was PORTABLE!!!).
	First thing is to select the type of power you want: Constant
(Always on, for example the clock) or Ignition (Only on when key is on). My
car didn't have many problems with either one. Then, once the +12V is
selected, you need to run it in series first through a auto Fuse, then a
reasonable sized inductor (first filtering defense). These are my two new
suggestions. I would also use a 7805 then place rectifying diodes in series
at the output. See circuit below:

+12V   Fuse    L1              1 _________ 2        D1   D2   D3
O-------O~O---UUUUU---+------+---|       |-----+---->|--->|--->|----O  +3V
                     +|      |   | 7805  |     | 
                     ===    ===  |_______|    === C3
Ground                | C1   | C2    |3        |
O---------------------+------+-------+---------+--------------------O Ground

	Where, L1 is say 1 uH, C1 is say 2200uF, C2 is 0.33uF, C3 is 0.1uF,
D1 D2 and D3 are silicon rectifying diodes (like 1N4001). All components
should be rated at least to 16 volts.

	In operation, L1 provides the first filter against spikes. C1 is a
large capacitor to guard against large changes in voltage. C2 is there if
it's far from the battery. C3 provides high-frequency filtering. C3 should
filter out most of the ignition coil noise (assuming you have a
dristributor coil type ignition). Finally, each of the forward-biased
diodes provides a 0.7V voltage drop. The final output should be about +2.9
volts. 

	As a last note, I could never fully filter out all noise from the
system. I had barely audible noise enter the cd player simply from the fact
that I think it was electromagnetically induced into the system. My cabling
(especially the audio cables) were never the best. In MY '85 Honda Prelude
(not my father's car, My own), things work much better with good cabling.
	Please note that this is all for a +12V car system with NEGATIVE
GROUND. You might need to rework this circuit for positive ground and/or 6V
cars. Have fun and Happy New Years!

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Shishin "Squish" Yamada             | Electrical Engineering (Class of '91)
BitNet: Squishy@Casbah.acns.nwu.edu | Northwestern Univeristy
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