Hard audio quality problem: Noise / Ground loop?

Burat · 01-01-2004, 02:04 PM

Noise in audio signal

I built in my CarPC this week. The fact that it is working finally is quite arousing, the sound quality however is rather disappointing.

To put everything into context, first a schema of my installation:

[CarPC] -> grounded on chassis in trunk
|
[Headunit]
|
[Amplifier] -> grounded directly on battery
|
[Speakers]

There is a noise in the audio signal that follows the rotation speed of the engine. I thought that this would probably be a ground loop, causing the magnetic field of the alternator to produce a little current causing this noise.

The sound of the headunit however is of a very high quality - I really enjoy my CD's

- so no problem with a ground loop here.

I tried the following things:
1) I grounded the CarPC on the same (high quality) wire as the Amplifier, directly on the battery. The didn't remove the noise - it actually got worse: I know hear noise when the harddisk moves.

2) I broke the ground-connection in the signal wire from the CarPC to the headunit (3.5 mm <-> ISO Line-in). As I expected, the alternator sound as background noise disappeared. The bad news is that with the sound the same noise appeared - now volume dependent. The louder the sound, the louder the noise.

3) As a test, I connected the CarPC directly to the amplifier. Result: perfect, high quality sound!

Conclusion:
1) It has something to do with grounding.
2) The problem is the connection between the CarPC and the headunit. CarPC <-> Amplifier is perfect, Headunit <-> Amplifier is perfect.

How could I solve this? I'm really running out of ideas right now!

BTW: I have a similar problem with the graphics on my Lilliput TFT: The graphics are not very clear. A lot of ghosting and blurriness. I used a double shielded high quality VGA cable.. So I suspect the ground again..

psyclobe · 01-01-2004, 02:20 PM

Quote: Originally Posted by Burat

Noise in audio signal

I built in my CarPC this week. The fact that it is working finally is quite arousing, the sound quality however is rather disappointing.

To put everything into context, first a schema of my installation:

[CarPC] -> grounded on chassis in trunk
|
[Headunit]
|
[Amplifier] -> grounded directly on battery
|
[Speakers]

There is a noise in the audio signal that follows the rotation speed of the engine. I thought that this would probably be a ground loop, causing the magnetic field of the alternator to produce a little current causing this noise.

The sound of the headunit however is of a very high quality - I really enjoy my CD's

- so no problem with a ground loop here.

I tried the following things:
1) I grounded the CarPC on the same (high quality) wire as the Amplifier, directly on the battery. The didn't remove the noise - it actually got worse: I know hear noise when the harddisk moves.

2) I broke the ground-connection in the signal wire from the CarPC to the headunit (3.5 mm <-> ISO Line-in). As I expected, the alternator sound as background noise disappeared. The bad news is that with the sound the same noise appeared - now volume dependent. The louder the sound, the louder the noise.

3) As a test, I connected the CarPC directly to the amplifier. Result: perfect, high quality sound!

Conclusion:
1) It has something to do with grounding.
2) The problem is the connection between the CarPC and the headunit. CarPC <-> Amplifier is perfect, Headunit <-> Amplifier is perfect.

How could I solve this? I'm really running out of ideas right now!

BTW: I have a similar problem with the graphics on my Lilliput TFT: The graphics are not very clear. A lot of ghosting and blurriness. I used a double shielded high quality VGA cable.. So I suspect the ground again..

Try running a seperate ground for the carputer directly to the battery like the amps are. Grouding of components usually should go to the car chasies, in the same general vicinity of each other. Not one to the chasies, one to the battery, etc.

-Jason

liquid_smoke · 01-01-2004, 02:22 PM

You need a ground loop isolator or inline filter. I just fixed this problem myself yesterday. I bought a ground loop isolator from RadioShack. And for the monitor problems, try to use some ferrites (i think i spelled it right). I also had this problem and fixed it with these, also available at RadioShack. If you dont know what they are, they are little clamp things that go at the end of the vga cable to reduce interference.

here is a link to the groundloop isolator
http://shop.store.yahoo.com/xmfanstore/grlois1.html

The one i got from radioshack was slightly different, the ground loop isolator from RadioShack looks more like the inline noise filter. Either one should take care of the problem.

Here is a link to what the ferrites look like,
http://www.allelectronics.com/cgi-bi...i?category=235

note: i just found that link from google, i never bought anything from that site before. They seem cheap there though, there about 4 bucks at RadioShack.

MikeH · 01-01-2004, 02:26 PM

Here's the RadioShack thingy.

Burat · 01-01-2004, 02:28 PM

Hm I don't know exactly what radioshack is :P - here in Holland we don't have it. I assume that it's a regular electronic component shop?

The noisefilter from the above link, where does it go? Powercable? Ground cable? Sound signal cable?

liquid_smoke · 01-01-2004, 02:30 PM

yes its a regular electronic store, sorry about that, didnt know you were not in the US.

MikeH, that link to radioshack isnt the item i got, although it looks like its ment for the same purpose, but it seems its only for amps. Should work though.

liquid_smoke · 01-01-2004, 02:31 PM

how are you connecting the carputer to the HU, aux in? using mini rca cable?

IF so, this is what you need. this is the item i got at radioshack. If you can find something like this where you are it should fix the problem

http://www.radioshack.com/product.as...t%5Fid=270-054

liquid_smoke · 01-01-2004, 02:34 PM

since the ends of the item above are full rca, you will also need rca to mini rca adaptor. 2bucks
you may or may not need it though, depending on where you put it.
you could connect one end of it directly to the back of your HU, it comes with one mini rca adaptor. and just connect the other end to the cable that goes to the line out on your computer.

Since my problem wasnt actually comeing from my computer, it was comeing from my XM sat radio that was going into the line in on my computer, i connected it inbetween the XM and computer.

MikeH · 01-01-2004, 02:40 PM

Smoke,

You're right. There are two ways to try to attack noise. One is a ground loop isolator (on the audio lines), the other is a noise filter (on the power lines). The latter solved (most of) my noise problems.

liquid_smoke · 01-01-2004, 02:46 PM

Hey, your right too. Not that i know a whole lot about this, i researced and fixed the problem in one day and now have zero noise. I didnt know the inline filter goes to the power lines, so thats good to know in the future. I may need one of those when i put a sub and amp in my car.

Burat · 01-01-2004, 05:43 PM

Hmm I looked around a bit, but only very few shops sell these products around here... But thanks anyway - I'll get back to this topic when I've tried these things out!

Confused · 01-02-2004, 03:27 AM

Have you tried grounding EVERYTHING (HU, computer, amp) to the same point?

As by hooking the sound into the amp directly got rid of the noise, there is a ground loop between the HU and the carputer/amp.

By grounding all my equipment to the same point, I was able to remove all the noise when I used a HU. Or, you could just try a wire from the casing of the HU to the ground for the amp/carputer

Garry

Crav4Speed · 01-02-2004, 03:57 AM

Quote: Originally Posted by Confused

Have you tried grounding EVERYTHING (HU, computer, amp) to the same point?

Exactly... your problem is that you have two different ground points... the amp hooked up to the battery and the comp hooked up to the chassis... Hook up both the amp and the comp to the same ground point on the chassis and not to the battery... ALL your ground should be hooked up to the chassis and nothing else

paulpv · 01-09-2004, 12:36 AM

The thing that I don't get about ground loop isolator is that they say it is used to isolate problems where the devices are not properly grounded.

I bought some ferrite chokes and ground loop isolators today, but I don't understand why I need these if everything is, to the best of my knowledge, properly grounded.

I have all of my devices grounded as perfectly as I know how, and I still get a noise that follows the engine RPMs (I am assuming the noise is coming directly from the alternator).

Here is my wiring diagram:

I should update that drawing to show that the direct connection to the battery is not really there.

All devices are truely grounded to the chassis (not directly to the battery).
In fact, I have two power distribution blocks.
1) +: connects to the battery
2) -: connects to the chassis near the equipment
Every device I have has [more or less] a direct wiring (according to my schematic above) to both distribution blocks.
The amp is not shown in the above diagram, but it's + and - also go through the distribution blocks.

I haven't yet determined the precise recipient of the noise.
I don't know if:
1) The PC is receiving the noise and sending it to the amp's line in
2) The amp is receiving the noise and amplifying it
I will look in to this this weekend.

I just thought: maybe the noise is coming through my amplifier's ACC/power-on input?

Any other ideas where my loop may be?

Pv

euro320i · 09-09-2004, 06:08 AM

Quote: Originally Posted by Confused

By grounding all my equipment to the same point, I was able to remove all the noise when I used a HU. Or, you could just try a wire from the casing of the HU to the ground for the amp/carputer

Garry

Hi Garry,

Does this mean if all the equipment is grounded to the same point, it doesn't matter where the positive is connected? I think this is where the problem is by many installers hooking up a HU with AUX IN. Even if the Amp/CarPC has all a common grounding point, the HU usually has another ground because of the ISO connectors.