# Car stereo wiring and L+ L- and GND

Discussion in 'Electronic Basics' started by [email protected], Mar 16, 2006.

1. ### Guest

This question is part of a larger undertaking that currently has led me
back to brush up on very basic electronics. To make a long story short,
I am wiring a stereo plug (ie, red, white, common) into a connector
that has, R+, R-, L+, L- and GND for ground among other things.

My question is, and please pardon such a basic question, why are there
not just L, R and GND? I've seen a buch of instructions on home made
wiring for a stereo plug but quite honestly, I am confused. Can this
even be done in such a case where the inputs are as such? (L-, L+, R-,
R+ and GND I mean accomodating a stereo plug for this setup.

What is the basis behind having +, - and ground?

Thanks a bunch.

2. ### BobGuest

First, keep in mind that the concept of GND is really just a circuit common
point. Any "common" is usually just one of the power supplies nodes.

A simple car stereo runs off the 12V power supply of the car. The minus
side of the battery is connected to the car's chassis, and this is normally
referred to as GND -- even though a car doesn't have a direct connect to the
ground (unless you rip one of your tires off its wheel).

Without any fancy electronics (e.g., a switching power supply or an output
transformer), the most that any given single amplifier output can deliver,
with a 12V supply, is a signal with a 12V peak-to-peak voltage range. In
older car stereo amplifiers, this 12V p-p signal was converted to a signal
that would swing up (positive) 6V from GND and down (negative) 6V from GND
via a simple series capacitor. Really cheesy (IMHO), and it limits the low
frequency response of the amplifier/speaker combination.

Nowadays, two separate amplifier outputs are used for any given side. Two
separate outputs for the left channel and two for the right. Each side's
outputs are driven out-of-phase. The net result of this is that, if you
connect the speaker between these two outputs (as opposed to one output and
to GND), then you can get 24V p-p across each speaker. Even though this
takes twice as many amplifiers (four total for stereo), you can take full
advantage of the 12V supply that's available and (here's the real point) you
can get up to four times the power, for a given speaker impedance. This
configuration is commonly known as a "bridged" configuration. Even some home
stereos use it.

So, that 's why there are two active outputs per channel. DO NOT ever
connect one of the speaker leads to GND (chassis) in this configuration. You
will, then, not be able to pass go, and you will not collect your \$200.

Bob

3. ### Ralph MoweryGuest

Without seeing the wiring hook up, here is the best guess. YOu do not hook
either speaker wire to ground. The + and - indicate the phasing of the
signals. Speakers may (should) have marks on them also. This means that
when the sound comes out of the speakers the voice coils move in the correct
direction to reproduce the sound as it was recorded. If the speakers do not
have the marks on them, you can do it yourself with a battery. Hook a
small (low voltage) battery to the speaker terminals and see which way the
cone moves. When two or more speakers are moving the same way, mark one
terminal with a + and hook it to the + of the amplifier output.

4. ### Guest

Thank you very much for both responses. I am getting the hang of it.