Strange PC Audio Problem

[jstraw]

Ok, a little background...

I do a little bit of webcasting from time to time...I fill in for a friend's weekly show when he's unavailable.

Here's my rig:

Everything goes into an XP box running a shoutcast server...

Working backwards we have:

Edirol UA-30

Behringer Xenyx 802

Into that I have an effects loop with just a Behrenger DR100 Diigital Reverb usedmaily for some verb on the mike though I can go for that Boss Radio sound on the tunes when I want to.

Into the board I have:

Audiotechnica AT4033a

PreSonus Bluetube preamp <- Gemini PMX100 turntable mixer <- 2 TT's

Edirol UA-1EX (we'll get back to this) <- HP dvr6000 series laptop

The laptop runs iTunes for playing tracks from CDs and Windows media Player for .wav and .mp3 files.

I have a switchbox that I built so that I can choose TT cueing or the Headphone Output on the Behringer mixer for headphone source.

Here's the thing....

When I have the laptop powered by its AC adapter I get terrible noises into the UA-1EX's channel on the board. If I run the laptop on battery, it's silent.

I've tried plugging into another circuit. I've tried isolating the UA-1EX by running it through a USB hub (since I thought it might be due to its being powered by the laptop). Nothing has had any affect.

I've even tried feeding the board audio form the 1/8" stereo micro headphone/line jack...same noise.

The only thing that quiets it is running the laptop off of battery.

Any ideas? Anyone ever had a similar experience?

[Sckott]

You have a ground loop problem, more than likely.. The sad thing is, it COULD be caused by the power supply you're using, motherboard, the audio circuit....

It's a paaaain to track down.

[jstraw]

You have a ground loop problem, more than likely.. The sad thing is, it COULD be caused by the power supply you're using, motherboard, the audio circuit....

It's a paaaain to track down.


I've isolated the problem...not a ground loop. If I JUST plug the Edirol into the USB port, plug headphones into the Edirol and play a file...with AC power noise, without no noise.

It's a very high pitched, granular, variable noise that sounds like internal componants drawing power, the hard drive, etc...

Why would that noise be sent to the USB port? Maybe I need to totally isolate the power pins on the USB port. I thought the hub experiment would do that.

The problem just seems to be that I have a crap HP laptop.

[Sckott]

Do you have a desktop to recreate the problemo?

[doodlebug]

I've fought similar problems and it is, indeed, a ground loop. Given the symptoms you're reported so far, I would suspect that the laptop's switching power supply is putting a lot of extra noise onto the grounding circuits. This would explain why it works ok on batteries.

To confirm this is the case, _temporarily_ use a cheater plug - one of those little AC adapters that is designed to keep the 3rd prong, a safety ground, from connecting to the AC outlet. Now, _temporarily_ operate your setup and see if the noise goes away. If so, then your power supply is the culprit.

Now, the biggest problem is that neither me nor the Underwriter's Labs nor the Federal Gov't nor your local electrical inspector will permit the removal of the safety ground. Its there for a reason as mentioned above that, with the right set of circumstances, all the metal in your setup can become 'hot' or have the full 120v on it. All it will take then is for you to touch the right places and you'll be seeing more than just the stars.

The best thing to do is to get another power supply and make sure it works correctly. I suspect there's some serious problems with the one you have.

Cheers,

David

[jstraw]

I've fought similar problems and it is, indeed, a ground loop. Given the symptoms you're reported so far, I would suspect that the laptop's switching power supply is putting a lot of extra noise onto the grounding circuits. This would explain why it works ok on batteries.

To confirm this is the case, _temporarily_ use a cheater plug - one of those little AC adapters that is designed to keep the 3rd prong, a safety ground, from connecting to the AC outlet. Now, _temporarily_ operate your setup and see if the noise goes away. If so, then your power supply is the culprit.

Now, the biggest problem is that neither me nor the Underwriter's Labs nor the Federal Gov't nor your local electrical inspector will permit the removal of the safety ground. Its there for a reason as mentioned above that, with the right set of circumstances, all the metal in your setup can become 'hot' or have the full 120v on it. All it will take then is for you to touch the right places and you'll be seeing more than just the stars.

The best thing to do is to get another power supply and make sure it works correctly. I suspect there's some serious problems with the one you have.

Cheers,

David

I always feel like an idiot when I'm caught leaving information out.

It does the same thing whether I'm using the OEM power supply or the aftermarket unit I have. I guess they both could be doing the same thing...

I'll try the cheater and report back. I'm very grateful for any suggestions.

[jstraw]

The "cheater" fixed it. Underwriters Laboratories can kiss my hinder!

Thank you.