The alternator noise that won't die - Page 2

Spartan117 · 08-01-2007, 07:05 AM

At first that sounded weird, but come to think of it.. just might work.
Maybe I could try grounding the signal's earth somewhere near the headunit as well, it's supposed to be the same, just without the noise, right?

RCA cables beware, I'm coming home soon..

durwood · 08-01-2007, 07:59 AM

Quote: Originally Posted by Spartan117

At first that sounded weird, but come to think of it.. just might work.
Maybe I could try grounding the signal's earth somewhere near the

That might work, basically RCA signal ground or earth technically is isolated from the chassis ground of the vehicle by capacitance or some other isolation form. When companies cut corneres, they tie the signal ground to the chassis ground and if multiple pieces of equipment do the same, that either needs to be broken somewhere by my method (or by a groudn loop isolator), or tied all to the same point.

Try your thing first since all it takes is a small piece of wire and nothing is hacked up.

pancit175 · 08-01-2007, 07:37 PM

Quote: Originally Posted by durwood

Yes, if the inverter was removed from the loop, then it's not your power supply injecting noise. Please try another ground loop isolator. Do you have any car audio install shops nearby where you can get one?

Just to chime in, I'd like to point out that the fact that you isolated that the power inverter (DC-AC) by testing with the house power, it doesnt necessarly mean you isolated the power supply for the computer (the AC-DC link in the chain).

greatwhite · 08-01-2007, 08:04 PM

your blog says "alternator whine". 99% of the time, that's a ground loop. You just haven't found it yet.

Try running off the on board sound instead of the usb card.............

Rob Withey · 08-02-2007, 01:21 AM

Quote: Originally Posted by greatwhite

your blog says "alternator whine". 99% of the time, that's a ground loop. You just haven't found it yet.

Try running off the on board sound instead of the usb card.............

Sounds like a ground loop. When running off the AC power supply, try removing all the grounds from the power supply, screen and PC to the car. If this removes the whine then those grounds are causing your ground loop. 2ohms really is a lot for between ground points.

One reason that ground loop isolators attenuate the signal is that the source is not powerful enough to drive the ground loop isolator. They have a relatively low input impedance, and if you drive them from a line out you are asking for trouble. I use a ground loop isolator with no noticable quality loss, but I also use a unity gain opamp as a kind of line driver to boost the drive capability of the signal so that it can drive into the ground loop isolator (think it was about 300 ohms impedance). You could also use a headphone output instead if you have one, as they generally have a higher drive capability/lower output impedance.

Just my two <insert current here>'s worth.

Spartan117 · 08-02-2007, 11:56 AM

Wahey, now we're getting somewhere. Not quite the results I expected though:

1: Connect the RCA's ground/shield to chassis: no difference
2: Cut the RCA's ground/shield so only the signal wire is connected: no difference (!)
3: After cutting, connect the RCA's ground/shield on the headunit-side of the cable to chassis: alternator whine as good as gone

I'll probably go with the last solution, although I'm not sure why #2 didn't affect the noise or sound at all.
A different brand ground loop isolator should arrive in the mail tomorrow, so I'll try that as well before settling.

Thanks for your help durwood and everyone else

durwood · 08-02-2007, 02:18 PM

Cool. Too many pieces of equipment that tie the RCA ground/shield to chassis ground thus causing a ground loop. Try the new ground loop isolator too and you should get the same results. Glad you got it figured out and have at least one solution.

PugFunk · 08-02-2007, 08:03 PM

Hey champ.

Just a quick (point out the obvious) statement.

Make sure you have a seperate ground for every ground wire. Dont double up on ground points. Also make sure you sand back paintwork at groundpoints as they will not conduct otherwise. Adding vasoline over the ground points also helps conductivity.

Another quick fix would be to check and upgrade your extra ground cable from your (-) battery terminal to chasis. My grounding cable from the (-) is twice as thick as my amp power cables and i have installed gold plated battery terminals. Fixed the trick so to speak.

I personally dont have a computer system. I have a 2 amp 2 sub 12 speaker set up in my peugeot 206 so i dont know exactally what is going on with your system however.

If you do have seperate ground points i would think that your audio cable is to close to your power cable or perhaps trigger wire for either your computer or amp?

Most common place this 'close-ness' can occur is behind your deck. Dont use cheap speaker wire power cable. My speaker cable is almost 2cm in thickness with insulation ontop and i have even grounded my RCA cables.

Dom