Audio System Noise

[overlord311]

I know this forum is not necessarily about car audio, but I wanted to post a question here in hopes that some of the knowledgeable people could give me some advice. Im an electrical engineer at Cornell University, and have installed the audio system in my car. I am using a pioneer head unit which drives boston acoustic 5 inch-ers in the front, and pioneer 6 by 9s in the back. My head unit has 3 sets of preouts, a front, a rear, and a sub output (200hz, I believe). I am using the head unit to feed a signal to a full range Rockford fosgate 100x2 (into 4 ohm) amp. The RF amp drives a pioneer 12 sub in a sealed oak enclosure that I made for it. My problem is that I am picking up a whine (alternator, I think) in the signal line from the head unit to the amp in the rear of the car (Isuzu Rodeo). Heres the list of scenarios that I have worked through to try to fix the problem:

$$ First off, everything works pristinely when the car is not on, and is just in accessory mode. No alternator running to make noise.

$$ The whine varies with engine speed. I think this narrows the whine to alternator, not ignition noise or anything like that.

$$ For a power wire, I am using unshielded 6 gauge braided copper. I run the wire the length of the car, from the battery, through the fire wall, to the back to the amp. I know this is one of the causes of the problem, as the long straight wire essentially acts like a giant antenna, picking up electromagnetic signals from various components in the car (ie. the alternator)

$$ For the line level rca cable, I have used both shielded and unshielded cable. More is outlined on this below

$$ First thing I did was hook the amp up to a 10amp car battery charger from the wall to try to isolate the faulty component. There is, of course, inherent fault in this practice, as the relative ground of the car (which is not really a true ground) and that of the wall socket are different. Thus, the ground of rca cable is at a different ground than that of the amp, and a ground loop is created in the rca cable. Accordingly, when I hooked it all up like this, the whine was still there.

$$ I tried shortening the power wire by moving the amp and speaker to the front of the car and using the same gauge for the wires, connected it with a piece 3 or 4 feet long. The rca cable ran basically out the car through the passenger door to the amp. The whine persists.

$$ I moved the amp back to the back of the car, and used a larger wire for the ground wire. Of course, resistance is inversely proportional to cross sectional area. I did this to lower the resistance of the ground between the amp and the battery to make the level closer. It helps a little, but not enough.

$$ I unbolted one of the bolts in the back of car, which gave me access to the undercarriage of the car from the inside. I crawled under there, and found a piece of metal that was attached to the frame, what looked like ran the length of the car. I ground it clean with a dremel tool, and ran the ground through the chassis underneath the car. After it was done, I had shortened the ground wire to 18 inches in length. It was better than before, but still a lot of whine.

$$ Finally, in a last ditch effort, I went to Radio Shack and picked up a 10 amp noise filter. This did absolutely nothing.

$$ I decided to move to the rca line, since I seemed to get as far as I could with the power line. There was absolutely no difference in shielded and unshielded cable.

$$ Like I did with the power line, I rearranged the routing of the wire, to no avail. I tried all the preouts on the head unit, all with the same results. The only thing that I found to help was to attach the outside (ground) of the rca cable to the same ground attached to the amp. Thats the best that I could get it.

So that covers the things I have tried. Obviously, there is a ground loop in the system somewhere, and I cannot work it out. I know that there are a handful of companies that make rca level noise suppressors, but I am skeptical of them. I love the sound of the system with just accessories on. If anyone has any idea on what I could do to work out the whine, let me know. I might have already tried it, because I know I am forgetting something here, but I could really use any help I can get. I am really stumped. Thanks for your time!

Brandon Richter

[stormin]

Overlord, Well its been quite a few years but i use to be into car audio pretty heavy. I use to run alpine head units with the older rockford amps. It was a great combo to say the least. Sounds to me like you have isolated the grounding problem to the head unit. Have you chasis ground the head unit? Also,what have you done with the ground wire that comes out of unit? Another idea have you changed around which fuse the head unit is being powered up by? Reason i ask is because i have had the head unit be the cause of my alternator noise which in turn was just amplified by the power of the amp and sensitivity of the speakers. Just trying to give some ideas for you to take a look at. Hope this helps. Goodluck!!!

[Frzninvt]

I have been in car audio for over twenty years and have troubleshot many an engine noise whine. Alot of it is due to grounding, sometimes the noise cannot be totally eliminated. It seems like you tried alot of the methods to eliminate the noise. First off I would run at least 8 gauge power and grounds to the amplifier. A simple solution to eliminating the noise is to use a set of twisted pair RCA's (Stinger Dream Series) as they have the best noise rejection ability and worked well in 1995 Buick Riviera that I could not eliminate the noise in due to the high volume of electronics and wiring in the car. But using the twisted pair RCA's removed it. Also make sure that the RCA's are run on the opposite side of the car that you have run the power, remote turn-on, and speaker leads on, also try grounding the deck and amplifier to the same grounding point, try to keep the grounds as short as you can. You mentioned an in-line noise filter from Radio Shack, you should have tried a Ground Loop Isolator (RCA version). I would definately try the twisted pair RCA's first I am almost certain they will eliminate the noise. Email me directly for more troubleshooting tips, I have competed in the IASCA 1-150W class for a few years awhile back and have alot of experience. Good Luck

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If it's too loud, your too old

[overlord311]

I forgot to mention something critical from my first post. When I have the amp powered up in the back of the car (from the battery), with the car on, I can unplug the rca inputs, and the noise disappears.

stormin,

I have not grounded the head unit...that didn't even occur to me. The logic seems to be right, though. Currently, it is using the same ground that the stock radio had, a fairly small wire that comes out of the saddle. Of course, I have no idea where it goes. I'll give a try in a couple of weeks when I make it home from school.

If I were to use a shorter, thinner wire for the ground on the headunit and a longer, thinker one for the amp (in the back), I wouldn't need any kind of rca level isolator device, would I?

I appreciate all the help.

[Frzninvt]

Try using a different grounding point other than the one on the wiring harness. Connect an extension wire from the back of the deck and try a few grounding points and see if one works. But since you said the noise goes away when you unplug the RCA's from the amplifier tells me it is being introduced through the RCA's. As previously stated get a set of twisted pair RCA's (GSI, Stinger, Phoenix Gold all make them) to run from the Head Unit to the Amplifier and you wont need an isolator or alternate grounding point. If you can't find any let me know, I just removed my system and am getting out of car audio and I have pair of 20' Stinger Dream Series Twisted Pair RCA's that I can sell you at a reasonable price.

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If it's too loud, your too old

[WMcD]

I can best offer some theory you can appreciate and brute force solutions. You're already working in this direction. So please excuse the obvious.

It sure sounds like a "ground loop."

This occurs when there is current flowing in the ground lead of the signal line. I believe there are two outcomes.

First, the field induces a voltage in the center, signal, conductor. So we have to ensure that can't happen.

A second issue may be that the ground connector of the signal like doesn't have enough resistance to keep the respective chassis at zero volts. The input is considered how much the center of the RCA is above the ground/shield input. If the ground is noisy, the the input of the amp reads noise even if the center channel is at zero volts.

I would make sure that all electronic components are grounded though a copper connection independent of the chassis. But then I'd connect that bus to the battery negative.

Again, it sure sounds like a ground loop . . . but where is the hash coming from in the first place.

There is something to be considered in the relation between the alternator and the battery. My understanding is that the alternator is actually a three phase a.c. generator (alterator). The raw output is the superposition of the three rectified sine waves. The battery serves somewhat as a capacitor to integrate out the ripple. In truth, the lead acid battery should act as a nearly a perfect voltage source, rather than a integrator.

It may be that a power supply lead to the electronics is not connected directly across the battery. That is allowing some hash into the power input to the head unit. Perhaps something like a 20 uF capacitor from the positive power lead to ground at the head unit will reduce the hash on the power lead. (Watch for polarity of an electrolydic cap, naturally.)

Overall, again, I'd make certain all grounds are through a bus dedicated to the electonics.

One technique need some discussion. Make sure the RCA connection's shield is not connected on both ends. That ensures that no current can flow in the shield. This might work best if the "ground" of the electronics really are at a common battery ground.

Best I can think of right now.

Gil

[PAR69]

I would suggest you try that RS noise filter on the head unit. I know every time I have installed a new car stereo, I used a cheap one every time and have never had a problem.

And I would also follow Gil's advice and lift the shield on one end of each of the RCA cables.

Paul

[Guest BobG]

I'm a bit out of my comfort zone on this as it's been several years since I did a lot of work with 12 v installation. Gil may be giving one bad bit of advice: do not go to the battery negative post for ground. That is going to add noise. There is a ground loop in your system. Grounding the head unit differently than the amp in the rear is part of the problem. I welcome any body who really knows this complex field to correct me and help solve this problem.

[overlord311]

Why would grounding to the negative terminal of a battery cause noise? Why would a ground loop be created if both the amp and the head unit were grounded at a common location at the terminal, assuming that resistance in the wires is approximately equal? Thanks again.

brandon

[WMcD]

Robert G., I must defend my tarnished honor, Surrh.

I might give advice devoid of all technical merit. Or advice which is unwelcome because it is intuitively obvious to the casual observer and an insult to the intelligence of the reader. But "bad advice"! Bad Advice? Thems fighten words.

Dual trace 'scopes at 20 paces may be the appropriate weapons for dueling. We'll see how far we can throw them at each other.

Please know the above is in the best of good humor. I'm happy read your comments. In truth, there may have been a misunderstanding.

In my defense, please consider that I actually wrote:

"I would make sure that ALL electronic components are grounded though a copper connection independent of the chassis. But then I'd connect that bus to the battery negative.

Bob G., after characterizing my response as "bad advice" , wrote: "Grounding the head unit DIFFERENTLY than the amp in the rear is part of the problem."

By writing "all", I was trying to express that the head unit, the amp, and any other components, should be grounded the same.

A close reading shows that Bob G. and I are in accord on the overall issue.

I also agree ground loops can be a difficult, vexing problem.

Best regards,

Gil