hi,

Coming from a more electrical than car-audio background,
I've been wondering about some of the more common hookup
methods i've seen or have been told about.

I'm in the process of hooking up two amps (decided to go
with bridging the alpine and pioneer last night - thanks
to everyone who replied!). I showed my plan to a car-audio-
enthusiast friend of mine and he mentioned a couple of things.

What I did was to run a properly-rated and fused power cable
(both + and -) straight from the battery to the boot to the
first amp, then patch that to the second amp about 3 inches
away. The length of the patch is about 3 inches.

The head unit is connected in the same way, straight to
the battery and properly fused.

The audio cables are balanced microphone cables - at the ends
i split the shield and used the hot and cold leads as left
and right. self-amalgamating tape acts as an insulator on
the exposed shield.

My friend mentioned that apart from running seperate power
cables, i should also run a ground lead from the amps to
the car body in the boot. Why is this so? I can't
imagine ground loops being a problem as both the source and
amps share the same ground. Having a direct ground to the
battery is also certainly a shorter route than going through
the car body.

How do ground loops occur in a car anyway? There is only one
power supply... or two if you count the alternator and battery,
but they share the same ground anyway.

Also.. thick power cables. If the cables I use are rated ok
for my current and aren't very long (12 feet, so i don't
imagine the resistance causing too large a voltage drop), what's
wrong with that? Is there a reason to go 8GA ... or even 4GA?

The amps I'll be running are "normal" - 35x4 and 25x4, both
bridged.


As with everything technical, there could always be some
obscure reason why things might be different, so i thought
i'd ask first. I just want to seperate the "so it looks good"
reasons from the "it'll work fine" reasons.


thanks!



--

eug k wrote:
[....]

How do ground loops occur in a car anyway? There is only one
power supply... or two if you count the alternator and battery,
but they share the same ground anyway.

Hrm i just realized, it's probably a very long path through
the car body to actual 0V, through a bunch of ground straps
and all, so it sounds possible to get a ground loop if e.g.
your head unit is hooked up straight to the battery and your
amp in the boot is grounded at the boot. Am I close?


--

My friend mentioned that apart from running seperate power
cables, i should also run a ground lead from the amps to
the car body in the boot. Why is this so?

He's incorrect on both counts. There's no reason to run separate power
cables (wire is wire). Just use a bigger wire if needed. As for the
ground, you can do it either way. You'll save yourself some money and some
hassle by grounding to the chassis though.

I can't
imagine ground loops being a problem as both the source and
amps share the same ground. Having a direct ground to the
battery is also certainly a shorter route than going through
the car body.

No, it's not shorter. Keep in mind that the total cross-sectional area of
the car chassis is much bigger than a wire, so it would be a lower
resistance connection.

How do ground loops occur in a car anyway? There is only one
power supply... or two if you count the alternator and battery,
but they share the same ground anyway.

Ground loops can occur if the resistance between the two grounds is
substantially different.

Also.. thick power cables. If the cables I use are rated ok
for my current and aren't very long (12 feet, so i don't
imagine the resistance causing too large a voltage drop), what's
wrong with that? Is there a reason to go 8GA ... or even 4GA?

Often, people tend to try to keep the voltage drop to 1v or under, so they
choose their power cable accordingly. I see no reason for this. The only
reason you'll need to go bigger than current-handling capacity is if there's
a possibility that you'll upgrade your amplifiers in the future. You'll
probably want to run an 8ga to err on the side of caution (and future
upgrades).