Path: utzoo!attcan!uunet!cs.utexas.edu!tut.cis.ohio-state.edu!ucbvax!pasteur!ames!coherent!dplatt
From: dplatt@coherent.com (Dave Platt)
Newsgroups: sci.electronics
Subject: Re: Define "GROUND" (& how to solve a "GROUND" loop prob)
Keywords: ground, loops, earth, ship, plane, audio
Message-ID: <25533@coherent.com>
Date: 26 May 89 19:29:14 GMT
References: <3917@merlin.usc.edu> <3297@ncar.ucar.edu>
Reply-To: dplatt@coherent.com (Dave Platt)
Distribution: na
Organization: Coherent Thought Inc., Palo Alto CA
Lines: 71

In article <3297@ncar.ucar.edu> cruff@handies.UCAR.EDU (Craig Ruff) writes:
> In article <3917@merlin.usc.edu> Cliff Yamamoto writes:
> > BUT by just merely connecting the car body ground ONLY to the ground of
> > the stereo, I get all kinds of motor/ignition noise.
> 
> I've got this same problem.  Luckily, the level of the ignition noise is low.
> I've done the usual, and replaced all noise condensors in the charging/ignition
> circuits.  I don't have any spark noise, so I didn't have to bother with
> the spark plug wires.  I even went so far as to use shielded speaker cable,
> grounded at the power amp ground terminal.  I guess I'll have to continue
> to make changes.  The next step will be to hook the stereo ground to the
> power amp ground with a wire.  Currently they are grounded to to different
> points on the frame.  For reference, my power amp is installed under the
> driver seat, which is within 2 feet of the stereo.  If the amp were in,
> say the trunk, would the noise be worse because of the greater distance?

Probably yes.

The problem here, as you've both guessed, is that the various electronic
components are grounded incorrectly.  Because the car's frame is not
a perfect electrical conductor, it's quite possible for two components
grounded at different locations to "see" a voltage difference between
their ground-points.  If a pair of components have two different
ground-paths (say, one through the frame, and another through the
RCA-cable shields), then it's possible for a voltage difference to
appear between the two ground-references seen by one or both
components (depending on their internal circuitry).  As a result,
the components will tend to pick up electrical noise, distort, or do
other unfortunate things.

Similar problems can occur in home stereo installations;  I had to chase
down ground-loop problems when I installed my stereo components in a
metal rack.

General rules-of-thumb for installing auto stereos:

1) Do not use the frame as a ground reference;  it's not electrically
   clean enough.  Instead, isolate the components from the frame, and
   run a ground-wire back to the battery.

2) Components that are located close together (physically) can be tied
   into the same ground-wire and 12-volt supply.  Components that are
   located some distance apart should run a separate pair of ground
   and (fused) power-supply wires to the battery.

This combination will usually result in a clean connection... what you
want is for each component to have a low-impedence path back to the
12-volt supply, and an equally low-impedence path to the common ground
point.  This ensures that all components are "seeing" the same voltage
references, and will eliminate most ground-loops.

You may wish to apply a third rule in some cases:

3) Do not establish redundant ground connections between components.
   If two components are tied to the same (low-impedence) ground via
   their ground-connections, then the RCA-cable connecting them should
   not establish a second ground.  This means, usually, that you should
   disconnect the cable's shield from the RCA plug at one end of the
   cable... usually at the "downstream" end.
   
   This step is probably unnecessary in most cases, if you've taken care
   to ground all components to a single reference point with a good,
   low-impedence cable.  However, if you're running a long RCA-cable
   (say, from a tuner/control unit in the dashboard to an amp in the
   trunk), then this hack may buy you some additional noise-immunity.

-- 
Dave Platt    FIDONET:  Dave Platt on 1:204/444        VOICE: (415) 493-8805
  UUCP: ...!{ames,sun,uunet}!coherent!dplatt     DOMAIN: dplatt@coherent.com
  INTERNET:   coherent!dplatt@ames.arpa,  ...@uunet.uu.net 
  USNAIL: Coherent Thought Inc.  3350 West Bayshore #205  Palo Alto CA 94303