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Thread: A lot of audio noise interference in new CarPC

  1. #1
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    A lot of audio noise interference in new CarPC

    I just finished up my CarPC install. I used a 5.1 soundblaster sound card and used the line level outputs from the card to feed the amp input. I had to use an external amp to drive my center channel because the factory amp powered that speaker directly. I have zero interference in that line, which runs from the amp in the trunk, to the center channel in the dash. The 4 other channels are line level coming from the PC going up to the dash, and tied into what was the factory connector into the head unit. From there, they go to the factory amp and to the speakers. It is picking up a lot of noise, especially when the car is running (engine noise proportional to RPM). I am using regular speaker wires for those line level inputs. Should I rip those out and replace it with shielded wire?

    *EDIT*

    I forgot to add that the input into my stock amp has 6 pins. Front-Left, Front-Right, Rear-Left, Rear-Right, Signal Ground, 12+ remote
    Coming out of the PC, I have all 4 channel grounds tied together and connected to the signal ground of the amp.

  2. #2
    Variable Bitrate brucedog's Avatar
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    How's it going?

    You are right. Abolutely replace the line level input "speaker wiring" with shielded cable. You see, with your current amlifier audio input wiring configuration, those unshielded input wires are kind of like antennas, picking up any stray electromagnetic noise (EMI) and radio frequency noise (RFI).. The mobile environment is full of rfi and emi, and all of that noise is feeding right into your amps...

    Devices that produce low-level audio output, such as tape decks, mp3 players, carpc sound cards...must have their low-level outputs connected using shielded cables to the audio amplifiers that drive speakers in the car or wherever.

    Here is an example from my own carpc experience and installation:

    My carpc is located in the large rear cargo compartment of my Saturn wagon (see my pictures). I use a Creative Sound Blaster Extreme PCI card, with fron, rear, and sub outputs. I use Monster audio cables to interconnect my sound card to my audio amps inputs; 1/8" mini stereo plugs on the sound card end, stereo RCA plugs on the audio amps end.

    I had some noise on only one out of my three audio amps that I was unable to correct with changes in grounding, cable, re-routing and such. So, I bought and inserted a Radio Shack in-line ground loop isolator (GLI), cost $16, and the slight noise not only went away, I had NO reduction in overall sound quality.

    I tend to buy a great deal of my amps, cables, etc, etc for audio hobbying from
    Parts Express http://partsexpress.com
    Good prices, great tech support, what I need...

    Best of luck and blessings. Keep learning, keep asking...
    Happy CarPC'ing!

  3. #3
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    Thanks for the help! I will start by converting all my lines to shielded wires, and fixing the respective grounds. If that doesn't work, I will pick up a ground loop isolator (2 actually because I have 4 channels). I will just have to cut the RCA ends off though since my factory amp inputs are direct wire. But again, since my amp only has 1 signal ground for the 4 channels going into it, I will have to tie all 4 grounds together after both ground loop isolators (2 channel per isolator)?

  4. #4
    Constant Bitrate bl3nd3r's Avatar
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    Quote Originally Posted by brucedog View Post

    I had some noise on only one out of my three audio amps that I was unable to correct with changes in grounding, cable, re-routing and such. So, I bought and inserted a Radio Shack in-line ground loop isolator (GLI), cost $16, and the slight noise not only went away, I had NO reduction in overall sound quality.

    I also was having a great deal of noise which was fixed through a ground loop isolator. What I found out is that it is very different then a sound filter which blocks out a certain sound range. A GLI works if you have two different ground points in your car for your audio system. In my case I had a ground for the computer in the trunk, and a ground for the amplifier that was in the dash. The loop was completed by having the audio out coming from the computer and running into the amplifier. Once I installed the GLI in line with that wire it broke the ground loop and fixed my problem.

    So I would personally just try a GLI from Radio Shack first. If it doesn't solve your problem then just take it back and then replace your wires. This way it may save you a lot of work.

  5. #5
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    Quote Originally Posted by bl3nd3r View Post
    I also was having a great deal of noise which was fixed through a ground loop isolator. What I found out is that it is very different then a sound filter which blocks out a certain sound range. A GLI works if you have two different ground points in your car for your audio system. In my case I had a ground for the computer in the trunk, and a ground for the amplifier that was in the dash. The loop was completed by having the audio out coming from the computer and running into the amplifier. Once I installed the GLI in line with that wire it broke the ground loop and fixed my problem.

    So I would personally just try a GLI from Radio Shack first. If it doesn't solve your problem then just take it back and then replace your wires. This way it may save you a lot of work.

    I'm actually going to try the wiring first, because with my setup, I don't use RCA plugs...so I would have to cut off the ends of the GLI and splice the wiring into it, so I will do that if my wiring (with shields) doesn't work.

    I will post back, as I hope to attempt this fix after work this evening.

  6. #6
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    OK....frustration is really starting to set in.

    I cut the cable that goes into the amp (shielded) which is a 6-conductor cable with pins as shown in the pic below. I connected the 4 speaker signals to my new wire (shielded) and the signal ground to another wire in my new bunch, and of course the 12v+ remote signal wire. I anticipate tying the signal ground from the amp to all 4 channel grounds coming from the sound card. I also tied the shield of my amp cable to the shield of my new cable.

    I have the new cable tied completely tied in to the amp cable (plugged into the amp) while the other end is connected to NOTHING (except I gave it 12v remote power). Immediately, with the cable sitting mostly outside of my car, and the amp turned on, there is a TON of noise. Now WTF?! The wires aren't even run, but its picking up noise. I used an alligator clip and temporarily tied the shield to a grounding bolt and it cut the noise in half, but still very noticeable. From what I gather, this is a little more than just a ground loop. I'm at a loss and open to any ideas.


  7. #7
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    Ground Loop Isolator (GLI) solved my problem

    To who it may concern - thank you.
    I have an '04 Mitsubishi Montero. There's no tape or aux on the factory Infiniti.

    I just wanted to play my iPod.....

    I got a TomTom Go720 GPS and picked up the iPod interface cable. The TomTom has a built in FM transmitter. But like all transmitters I've tried, static is an intermittent problem.

    I bought a Pyle PLMD9 wired FM modulator (goes in line with the antenna). Hooked everything up and the static is gone but there's a whistle/hum that is new. After a little trial and error, I narrowed the problem to the 12v power supply from my accy outlet to the TomTom. If the TT ran off its' internal battery, no whistle. But plug it in and there's the whistle as plain as day. After some research, ground loop looks like a very likely culprit since its' definitely rooted in power supply to the TT. So here I am $18 later with the Radio Shack GLI installed and my problem is GONE.

    Thanks again to everyone who takes the time to register, post, and share their experiences.

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