cyamamot@nunki.usc.edu (Cliff Yamamoto) (05/26/89)
Greetings!
What is considered a "ground"? If a piece of electronic gear is
operated on land in a fixed position, the soil is "ground". But in mobile
(ship, plane, auto) environments is the common/negative connection of the
mobile power supply (i.e. generator/battery) *always* considered to be ground?
With audio equipment, the shield (shell) of the common RCA plug/jack
is "ground". But *where* in the equipment is this "ground" referenced to?
They say to avoid ground loops, one should ground everything to a common point.
Is this supposed to connect the POWER SUPPLY grounds to one point OR connect
the SIGNAL grounds to one point? Or are these two "grounds" the same?
Here's the weird problem. I installed a stereo and amp in my car.
The amp is installed and connected to the battery. This scenario is with the
engine running, amp powered up :
When RCA's are disconnected from the amp, no noise. When I connect the RCA's
to the stereo AND power the stereo with a bench power supply, no noise & plays
fine. BUT by just merely connecting the car body ground ONLY to the ground of
the stereo, I get all kinds of motor/ignition noise.
At first I thought the noise was coming through the 12 V. B+ line. But that's
not even connected in this case! It seems by using the bench power supply,
I'm FLOATING the radio above car body ground; ergo no noise. YET the RCA's
are connected between the amp and stereo. Funny thing was I got a small tingle
of a shock just before I connected the car body and the stereo ground together.
It's hard to believe by sitting in a car you can get a shock by touching the
car body and holding a stereo (which is supposedly "grounded" thru the shields
of the RCAs).
Any info on what "grounds" are SUPPOSED to be or how to fix this problem would
be really appreciated!
Thanks in advance :)
Cliff Yamamoto
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UUCP : ...uunet!usc!cyamamotcruff@ncar.ucar.edu (Craig Ruff) (05/27/89)
In article <3917@merlin.usc.edu> cyamamot@nunki.usc.edu (Cliff Yamamoto) writes: > Here's the weird problem. I installed a stereo and amp in my car. >The amp is installed and connected to the battery. This scenario is with the >engine running, amp powered up : >When RCA's are disconnected from the amp, no noise. When I connect the RCA's >to the stereo AND power the stereo with a bench power supply, no noise & plays >fine. 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? -- Craig Ruff NCAR cruff@ncar.ucar.edu (303) 497-1211 P.O. Box 3000 Boulder, CO 80307
dplatt@coherent.com (Dave Platt) (05/27/89)
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
cook@stout.ucar.edu (Forrest Cook) (05/27/89)
In article <3297@ncar.ucar.edu> cruff@handies.UCAR.EDU (Craig Ruff) writes: >In article <3917@merlin.usc.edu> cyamamot@nunki.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. ... >Currently they are grounded to to different points on the frame. ^^^ Currently? maybe you have stray ground currents floating in the frame. But seriously, folks, the problem could probably be solved by electrically isolating the power amp from the car frame using plastic washers and running a heavy gauge ground wire to the receiver or the place where the receiver is grounded. This can be tricky sometimes so I would recommend mounting the amp and checking for no continuity to ground before wiring it up. The amp's +12V line should probably be connected to the same place as the receiver's +12V line if possible. You might even try disconnecting the ground side of the audio line where it enters the receiver. This may make the noise worse but it's worth a try. The goal is to only have one (preferably short) ground path between the amp and the receiver and to remove any other ground connections. Make sure that the speakers wires are not grounded anywhere, they should only connect to the amp. Shielding speaker wires is probably as effective as plating them with mercury in normal environments :-) :-) (see the sci.electronics archives) Good Luck ^ ^ Forrest Cook - Beware of programmers who carry screwdrivers - LB /|\ /|\ cook@stout.ucar.edu (The preceeding was all my OPINION) /|\ /|\ {husc6|rutgers|ames|gatech}!ncar!stout!cook /|\ /|\ {uunet|ucbvax|allegra|cbosgd}!nbires!ncar!stout!cook
bill@bilver.UUCP (Bill Vermillion) (05/29/89)
In article <3917@merlin.usc.edu> cyamamot@nunki.usc.edu (Cliff Yamamoto) writes: >Is this supposed to connect the POWER SUPPLY grounds to one point OR connect >the SIGNAL grounds to one point? Or are these two "grounds" the same? ..... > Funny thing was I got a small tingle >of a shock just before I connected the car body and the stereo ground together. >It's hard to believe by sitting in a car you can get a shock by touching the ..... >Any info on what "grounds" are SUPPOSED to be or how to fix this problem would >be really appreciated! Connect the power supply ground and the audio grounds all to the same point. This should be a wire that runs to "ground" side of the battery. You have a classic ground loop because there is resistance between the point you have connected to the car body, and the point of connection to the battery. That's why you got the tingle. The body appears not to be at ground potential. -- Bill Vermillion - UUCP: {uiucuxc,hoptoad,petsd}!peora!rtmvax!bilver!bill : bill@bilver.UUCP
rdsnyder@mit-amt (Ross D. Snyder) (05/29/89)
Another solution would be to install isolation transformers in the RCA cords between the receiver and amp. This would eliminate DC continuity in the lines and would prevent ground loops through the audio signal lines. Use 50Kohm:50Kohm audio transformers of good quality (like with shielding). I had a similar problem with my home stereo a year ago when I reconnected it, and an RCA plug was loose. I heard noticeable hum from the speakers, but the hum disappeared when I wiggled the RCA plugs. I was thinking about opening (disconnecting) the shield of the RCA cords at one end and mounting threaded studs with wingnuts on the back of my components, which would be connected together in a star configuration with heavy-gauge wire. Wouldn't the double ground connections established by the shields of *stereo* patch cables result in the potential for ground loops in all cases? I should cross-post this to rec.audio to give the MonsterCable fans something to chew on. :-) -Ross