Challenge for Electrical Engineers

[Taketwo]

I refuse to say I “need help” because I am a very competent computer user. But this is a problem that I can’t get to the bottom of and goes beyond the scope of most the idiots I have talked to. That is why I am posing this question as a challenge since I know I will most likely just have to ignore it. I’m going to post all the details and maybe someone out there will humble me who knows what to do.
The Situation: I have a Sony laptop (VGN-SR540G), an Acer P244W monitor, a Sony Bravia home theater speaker system. Naturally, there is nothing more I would like to do than use all three at once.
However, when my laptop is connected to both the monitor and the sound system there is audio noise.
Here are the details:
The noise is a low frequency hum. (Sounds a LITTLE like this maybe not as low frequency and not as creepy sounding http://www.youtube.com/watch?v=O_CL24funM4) The video is borrowed from something else I didn’t bother to look at
This noise occurs when my laptop is plugged in AND when my laptop is unplugged.
The audio system has a two prong (ungrounded) plug.
The monitor has a three prong plug.
The laptop has a two prong plug.
The audio system noise is there when the monitor is plugged in through the HDMI output AND when it is plugged in through the VGA output.
Audio output goes through my headphone jack.
Audio noise DOES NOT occur if I am plugging headphones into my laptop HOWEVER if I plug in my audio system to the laptop and then plug in the headphones to the audio system THEN you can hear interference through the headphones.
Audio NOISE is MINIMIZED when LAPTOP SOUND is MAXIMIZED. In other words audio noise does not depend on laptop volume only amplifier volume.
The noise occurs even if the laptop is muted
I have tried a few “moving around the plugs” to deal with ground loop but it really has done nothing at all.
The noise comes from all speakers not just the bass.
If the monitor is not connected to the laptop then there is no noise.
If the monitor is connected to the laptop there is noise.
If the monitor is OFF but connected to the laptop there is noise.
If the monitor is unplugged but connected to the laptop there is not noise.



Okay kids have at it and tell me if you need more details.

 

[C1]

Monitors are bad (and so are notebooks) as they generate a lot of RF. You could be picking up RF thru the audio cables (acting like an antenna) and/or line or sweep frequency noise from the monitor thru the HDMI or VGA cable. Advice is :

1. Use a high quality shielded monitor cable (expect to pay $30)

2. Use high quality shielded audio cable to the audio system. Also use Radio Shack's audio isolation transformer & keep the audio cable run length short as possible.

PS: I do this all the time but using various different type input devices.

 

[Taketwo]

Are you sure? The noise is extremely consistent, no fluctuations or anything I feel like if it was that then I would hear changing distortion.

 

[alyarb]

you can probably vary the interference by varying the field, but if everything is humming along nice and steady then the buzz should be consistently soft. shielded cables are going to be the next thing for you to try.

 

[Red Squirrel]

Do you have any dimmers in the house that are dimmed? Given your circumstances I don't think it would be that, but if you do, it's worth checking to see if it's that by turning them off, or max them.

 

[Taketwo]

No but I live in a dorm room where one outlet is used to power pretty much everything


That couldn't be it cause if the monitor is on but discounnected there is no noise. Right?

 

[Red Squirrel]

Is the sound a 60hz sound, like a square or triangle wave? It does sound like some kind of electrical interference. We had a similar issue at our church with the sound system and rectified it by turning up the record volume at the amp, and reducing it at the computer, and disabling "mic boost". Different setup then you though, but maybe it's a similar situation. Have you tried a different monitor? It almost sounds like it's the monitor itself, most likely it's power supply, might be slightly going bad but not to the point where it does not work.

Oh and I'm far from a EE, just guessing here.

 

[Swivelguy2]

This is why they invented digital audio transmission.

Shielding the audio cables could help, but shielding the video cable probably won't help that much - a lot of the interference is likely to be coming from the body of the monitor.

 

[Taketwo]

Okay first off to the guy who said varying the field what do you mean.
To Red squirell my noise is minimized by doing the opposite of what you did (maximizing laptop minimizing amp to minimize noise)
And lastly, too check monitor interference I disconnected the cable but left it in the same area. still sound.

Left the monitor plugged in diconnected audio plugged it into mp3 next to laptop still sound.

Interesting update: When I touch the hdmi cable to the audio cable the noise produced is identical. Maybe its leaking?
In other words no sort of radiating interference. I Think the leak is through the laptop. (IE when both are plugged in there is a path for noise from the hdmi to go to the audio cable.
And why does it only happen with the audio system and not with headphones

 

[corkyg]

My guess is an impedance mismatch in the audio components, primarily the cables.

http://en.allexperts.com/q/Home-Recording-2858/2009/4/recording-15.htm

 

[C1]

Yes, corkyg is correct. I checked the "line in" specs of my ONKYO TA-2047 cassette deck, for example, and it is 50K ohm. (This is probably very typical for line input to a stereo component, but Ive seen the impedance spec'd as low as 2K ohm.) Remember, a headphone is spec'd typically 8 to 32 ohm which is basically a good a match to the notebook output.

So the first to do is ensure use of a high quality shielded audio cable. You will want to do this no matter what - dont use junk cable. If this doesnt work, you may have to use a matching transformer or try the isolation transformer I mentioned,

 

[Modelworks]

You got a ground loop.
Your rooms ground is at 0 volts.
You plug in the monitor which makes the casing, all shields on the cables and anything metal on the monitor also at 0 volts.
Connecting the laptop now makes anything metal in the laptop casing, connectors, at the same ground voltage as the monitor. Everything is okay unless you provide another path to ground that is different from the monitor.
You plug in the home theater which connects the laptop to another ground source. You may be thinking, but the home theater only has a two prong cord . This is where it gets interesting. Look at the schematic below for a typical home stereo power supply.

Notice that the ground for the sound system power supply has a direct link back to the neutral of the wall outlet. Noise is supposed to be filtered out by the section in blue but that is only for noise coming in from normal usage, not for filtering out issues from grounds coming from other sources.
Neutral inside the circuit breaker box is connected to the ground bus bar. So you are forming a loop from the monitors ground , through the laptop, through the stereo , out the stereo neutral and back into the circuit breaker box.


The solution is pretty easy, get rid of the ground on the monitor.
http://www.radioshack.com/product/in...LAID=422931493

Use one of those and do not connect the tab to the screw on the outlet.

This is the reason why I like older audio gear. They use transformers in the power supply instead of switch mode supplies . Transformers are heavy, cost a lot more to make a power supply with, but are immune to ground loops.

 

[C1]

Maybe.

I can tell you that I do this all the time, but with different combos of devices. Combos include, VCRs, notebooks, desktops, super high end stereo gear (yes, all transformer), battery/portable plug-in recorders/players (using diode bridge rectifiers), etc.

Depending on the combo, you will get interference from the damndest & most elusive sources/places. Always follow the rules:

- High quality cables
- Minimize cable run length
- Keep switching components (dimmers, etc) off the home electrical line & turn off/minimize use of RF level devices (eg, LCD/CRT monitors/TVs, etc) in the vicinity
- Have access to an in line isolation audio transformer (you'll end up having to use it more often than not)

 

[DominionSeraph]

You got a ground loop.

Yup.

And the reason the noise is minimized when the laptop output is maximized is just because you're using less amplifier gain for a specified output.