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Tech
Tip of the Month for June 2003 (part 1)
MAPPING GROUND CURRENTS IN A CAR
By David Navone
Unless Lady Luck is your shop manager, one of the most
difficult options to consider when installing a car
stereo sound system is the choice of electrical
grounds. Car stereo differs from home and pro audio in
that the component electrical grounds typically use
the chassis of the car as a return to the negative
battery supply.
Noise problems arise in sound systems due to the
interaction of vehicle accessories and the autosound
components. Power supply filters are provided in car
stereo components to attenuate noise on the + 13.8
Volt DC leads, but what about the ground connections?
Where The Problem Started
Way back around 1895, some economy minded automotive
technician decided that since the chassis of a vehicle
was made of metal, it could serve as a cheap
electrical accessory return to the battery. Rather
than run two wires from a horn, for instance, only one
wire was necessary; provided the horn was connected to
the chassis and the negative battery post was also
connected to the chassis. The process of making a
connection to the chassis of the vehicle became known
as "grounding."
Autosound components are also accessories that use the
chassis of the car as the negative ground return. This
may be convenient as well as economical, but when the
engine is started and other vehicle accessories placed
into operation, the effect on sensitive autosound
components is all too well known. Noise appears in the
form of alternator whine, brake light pops, ignition
ticks, etc.
A Typical Scenario
Consider the example of a headpiece installed in the
dash and an amplifier mounted in a trunk of a 1984
Olds sedan. With all specifications optimized, the
engine recently tuned, and a new battery in the car,
it would be very unusual to NOT hear at least some
alternator whine creeping into the sound system on a
blank track (all low bits) at a moderate listening
level.
Now, let's pull the headpiece and the amp and strap
them together on a test bench. Connect the amp to a
couple of test speakers and then make the necessary
+12 Volts DC and ground connections back to the car's
battery via jumper cables. What happens to the noise
level?
The noise will be greatly reduced because the chassis
of the car no longer serves as a common grounding
connection. (Don't try substituting a small wire to
make long ground returns because the noise on the
chassis of the car will simply couple to the wire.)
Just how much noise is flowing over the chassis of a
typical car?
Well, first of all, there are no typical cars. If you
were to run a CRAY computer for a week, you still
would not be able to reliably predict the path,
amplitude, or interactions of all the ground currents
on any particular vehicle. Variables such as the
strength of the spot welds, tightness of the bolts,
and thickness of the chassis determine the intensity
of the noise.
To actually measure the noise level between any two
points on the chassis of a car, a spectrum analyzer
would be useful. A spectrum analyzer is a
sophisticated oscilloscope capable of displaying a
very broad range of frequencies at one time. A typical
spectrum analyzer that could measure DC to 2 GHz would
cost at least $5000.00 and could cost over $ 50,000
for the better models. However, there is another way
to begin our study of chassis noise that is quite a
bit less expensive --- if we make a couple of
compromises.
An $13.00 Spectrum Analyzer?
The Radio Shack catalog has a nifty little audio
amplifier speaker (Part 2771008). The cost is around
$13.00. You will need a 9 Volt DC Transistor battery,
a mini plug and some spare shielded cable for this
experiment.
One of the compromises for our test is that we listen
to the noise rather than see the noise. Our ears are
very responsive and can respond to both small and
large sound levels. The second compromise is that we
only examine the narrow section of bandwidth in the
100 Hz to 10 KHz audio range. Our ears are capable of
responding to over 10 octaves of sound, so this should
not present a problem. Anyway, this is the audible
range that is most important when considering car
audio interferences.
The probe for our tests can be 20 feet of shielded
cable such as RG58 left over from Cellular Phone or CB
antenna installations. I also used a three foot length
of # 20 AWG wire with an alligator clip on one end for
my test probe. This wire was connected to the outside
shield of the cable at the mini plug on the amplifier.
The center conductor of the opposite end of the
shielded cable should have a capacitor in series with
an alligator clip installed to offset any DC levels
experienced during test. The value of the capacitor is
not important. Try using a 1.0 to 4.7 mfd, 100 Volt DC
crossover cap.
Let me recommend placing a 1 Amp in line fuse between
the probe's alligator clip and capacitor in the event
you want to listen to the charging current flowing to
the battery or to some other accessory. Safety always!
Ok, so we’re ready to listen to the alternating
currents flowing all over the chassis of a car. Turn
on the little amplifier and clip the fused alligator
clip to the positive battery post. Now quickly touch
the other alligator clip to the negative battery post.
That loud whining noise is the alternator’s charging
ripple.
Alternator Whine
Alternator whine is primarily composed of two
components. The higher frequency tone is the
three-phase alternating current output ripple and the
low frequency hum is produced by the heavy battery
current in the field windings. Turn on the headlights
and notice that both levels increase in intensity.
Step on the accelerator and notice that the higher
frequency tone will change pitch and become even
higher. This is alternator whine.
Let’s take the little amplifier and move back to the
trunk of the test car. Clip one alligator clip to a
nice, shiny point on the chassis of the trunk’s
interior and then touch the other alligator clip to
the same spot. Start the engine and listen to the
little amp’s speaker. The small noise level you are
experiencing is the noise floor of the amp and probe.
Checking Specific Grounding Points
Leave one alligator clip fastened to the same spot in
the trunk and bring the little amp and other alligator
clip up to the front seat of the car. Touch the probe
to the outside of the cigarette lighter receptacle and
listen to the noise. The increase in noise level over
the noise floor can be directly attributed to AC
ground currents, superimposed over DC ground returns,
flowing between the two ends of your probe. This noise
is a direct result of a voltage drop associated with
resistance between the two points on the chassis of
the car.
Turn on the headlights, speed up the engine, hit the
horn switch, move the power seats and close the doors.
Notice the change in noise level due to changing
ground current paths.
Run your own experiments between any two points on the
chassis. Try the negative post of the battery. I think
that you will find that the negative battery post
would not be a very wise choice for an electrical
grounding point. There is way too much charging, as
well as load currents flowing in the vicinity of the
negative battery post.
How about the case of the alternator? I have been
asked many times over the past several years if
perhaps the case of the alternator might not be a good
place to ground an amplifier or a deck. A quick test
with your little amplifier and probe will emphatically
demonstrate that the case of the alternator is
probably the noisiest grounding point on the entire
vehicle.
So What's the Solution?
After a few minutes with the little amp and the probe
it should be evident that the car's chassis is made up
of very, very noisy grounding points. However, some
ground paths are definitely quieter than others.
Sometimes moving the probe just a couple of inches
will noticeably change the noise level.
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