Ground loop?

[squeak9798 5,000+ posts]

In a unibody car (basically anything that isn't a truck, SUV or van built on a ladder frame) your best ground is going to be a large gauge wire running back to the battery.

Two SUV's were actually a part of that experiment;

http://www.carsound.com/cgi-bin/UBB_CGI/ultimatebb.cgi?ubb=get_topic;f=1;t=022342;p=2 (about 3/4 of the way down the page)

Anyways....as the correct answers in this thread eluded to...it comes down to application. In some vehicles with high power applications, grounding directly back to the battery would be the best possible grounding location. In other vehicles, in not-so-high power applications, grounding to the chassis may be adequate. In other vehicles, grounding to the frame rails may be adequate.

It all comes down to the specific situation you are dealing with.

 

[helotaxi 5,000+ posts]

Interesting, I do see that point, but if you have the ladder frame it's still better to mount it to the frame, simply because it will give you the lowest point with out having extra capacitance and inductance from the cable which will give you extra unnecessary ground resonance.

Please tell me that you are kidding about the negligible capacitance and inductance actually affecting the quality of the ground. First we are dealing with DC here and the tiny amount (not measurable using basic equipment) is not going to affect the flow of current a bit. DC resistance is really all that matters. Getting the ground potential to as close to zero as possible should be the end goal. If using the chassis is the best way to do that for your particular situation, then that is the route you should take. Chances are that it isn't the best case and something else is needed i.e. a large gauge cable run back to the battery.

 

[joetama 5,000+ posts]

Right, getting it as close to zero is the important thing... However if you have resonations in your cable they will read out close to zero but might actually not be. Where do you think ground loop noise comes from? When you have a potential rise from one point and it oscillates out even in DC unless it’s in this “Perfect” world that doesn’t exist... They might be tiny small and insignificant in most cases, but in a few cases they can make a huge difference, and the more current you draw the bigger those oscillations will become voltage oscillations consequently raising the ground farther and farther from true zero. When this happens you are voiding the whole point of sure grounding. I’m not saying that this is true in every case, but in a few cases it is, that’s why common source grounding is important. Disagree with me if you want, but I’ve seen and heard the difference especially in high frequency electronics.

 

[helotaxi 5,000+ posts]

In high freq electronics, you have just that, high freqs. AC. You have to have reversal of voltage for miniscule capacitance and inductance to have a noticable effect. I could see issues with HAM gear or such where you have to deal with high freq carriers and so forth, but we are talking low voltage DC here. I know that it isn't perfect DC but I can pretty confidently say that if capacitance and inductance were truly an issue, that the welds and glue joints in the chassis of the modern automobile would prove incredibly troublesome. //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif

 

[joetama 5,000+ posts]

Since when do your amps output DC? I sure hope they output AC. Other wise you are burning up alot of speakers!!! The enemy is voltage difference and making sure you have a clear return path that flows smoothly. In some cases running the cable is the best, but conversly frame grounding is the best in other cases....

 

[TypeRBass 10+ year member]

Manville Smith of JL audio did some very extensive testing of chassis grounds on different vehicles. He found that on most unibody cars using the car sheetmetal in for your amp ground with a trunk mounted amp and the amp grounded next to the amp the resistance between that ground and the battery was equivalent to a single run of 4 ga wire. If your amps (all of them total) are drawing more power than you would supply with 4ga, then using the sheetmetal as a ground is a chokepoint in the current delivery path. In a unibody car (basically anything that isn't a truck, SUV or van built on a ladder frame) your best ground is going to be a large gauge wire running back to the battery. In a vehicle with an actual frame you can ground to the frame, not the sheetmetal and get a very low potential ground. For this to be applicable, you have to ground the alt/engine block similarly to complete the path with the alt and be able to get max current with min voltage drop from the alt.
The basic rule I would put forward is that if you are building a system in a unibody car and your primary power wire from the battery to the amp is larger than 4ga, you should strongly consider running an equal sized cable back to the battery neg terminal or the battery ground point.

very well put and true. i have a unibody car (ford escape) and if you gruond to the body it is equivalent to a 4awg ground...so like you said, run from the front battery to the back...like i did.

 

[squeak9798 5,000+ posts]

Since when do your amps output DC? I sure hope they output AC. Other wise you are burning up alot of speakers!!!

I'm not even sure how or why you are bringing this up and into the conversation ??

As far as I was aware, no one was discussing the output of the amplifier.

 

[helotaxi 5,000+ posts]

And since when did you ground your speakers? I though that died off with the AM mash button car radios...

 

[smd4life 5,000+ posts]

So the chassis is the ground is it? What happens when the day comes that a chassis is no longer metal? A chassis can not be ground UNLESS a ground wire is attached to it from the battery. Not too hard to understand.
According to MECP, I must have a certificate to be a installer. What about those of us that were trained 15 years before MECP was even in existence? MECP for the most part is doing this, you pay them, they do the Jedi Mind Trick to you, you get piece of paper that is totally worthless but looks good on a wall.

FYI, a good ground is not about the amount of metal in the return to the battery. It is about the combined resistance through it. With unibody cars that are a combination of glued together panels, crappy spot welds and blended metals, most of them are a horrible high resistance return to the battery. This is simple, electricity is an algebra equation, what you do to one side, you must do to the other. If you want to draw 100 amps of current and you have a build up of resistance on the ground, what do you think is going to happen to that amp in time? A good ground is one that is as low in return resistance as possible and is not always the chassis. If a amps manual also told you that jumping off of a bridge would not kill you, would you believe them and run out and do it or would you consider the fact that their info just might not be accurate?

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