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Buzzing onto receiver, very odd problems

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Alright folks. Here's my issue:

 

I have a Yamaha RX-A700 receiver (awesome unit, heavily pleased with the purchase) that works great. I hook my iPod up to it all the time and jam out. We have a projector set up in the apartment and we usually hook our laptops up to it and watch movies, play around on Youtube, etc, and that worked fine on the previous Onkyo receiver that the Yamaha has replaced. Just today, I finished a frankenstein build using an old Opteron 146 system from years past, and the Asus Bravo video card (has some awesome HTPC features) my roommate got from the Christmas contest. Since the projector is across the room from the receiver and so is the desktop, I grabbed a 20ft 3.5mm extension cable to carry the sound over from the opposite side of the room to the 3.5mm to RCA adapter that plugs into the receiver. Plugged it in, and before I even turned on the computer there was an intense buzzing in the speakers.

 

After this, I thought for a minute and thought it might be the hardware as it has sat for a while. I grabbed the iPod and sent it through the 20ft extension, and it worked fine! No buzzing or anything. At that point, I thought it had to be the hardware but I wasn't entirely convinced so I tried a laptop. That too, played fine - until I plugged the VGA cable from the projector into it. Same problem! The buzzing appeared as soon as I plugged the VGA cable into it. Very bizarre, and very confused at this point.

 

I try one step further. Same laptop plugged into a set of computer speakers while on the projector. Works fine. Tried it through the extension cable to the same set of speakers. Also worked fine. Switched back to the Yamaha receiver, and I get buzzing.

 

So - extension cable works fine on the receiver with an iPod but nothing else. Anything else and it buzzes really bad! Also very confused as to why it buzzes on the laptop only while the VGA cable is plugged in. Any suggestions? I think the motherboard might have optical out on it, and I've got a 20ft optical cable at one of 2 residences, so I might resort to that if I can't figure anything else out.

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It sounds like you have a ground-loop problem.

 

Do you get the buzzing problem from the laptops with their respective AC power adaptors disconnected (laptop on battery only)? If not, it would suggest their is a ground-loop problem. If you still do, it might still be a type of ground-loop problem (maybe from a virtual earth), I'm just most familiar with ones with different audio equipment (mains powered) connected together.

 

You could try experimenting with putting them on the same (or different) power outlets - this is a free, but also often annoying trial-and-error process. You can buy ground-loop isolators (2x RCA in and out) from some electronics and ICE/auto shops - this isn't free, but is usually a plug-in-and-works solution.

 

I just looked at the Yamaha page for the receiver... if it's just for the HTPC, I think an optical / coaxial SPDIF cable would make sense anyway?

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I think you may be right. Yeah, the laptops were plugged into AC at the time and I didn't try otherwise. However, they did not suffer from the buzzing until I plugged the VGA cord in. Not sure why that happens :P I got the 3.5mm cable because I already had it and we had originally used it for laptops and whatnot on the old Onkyo which finally died. Tried an SPDIF cable. Worked great, thanks for reminding me about ground-loop isolation though!

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However, they did not suffer from the buzzing until I plugged the VGA cord in. Not sure why that happens

Basically, small currents/noise from the earth/shield/ground of the projector and the power outlet it's plugged into, is transferred to the laptop or PC via the video ground, and then on through the audio signal's ground... if you plugged the projector, computer, and receiver into the same power strip (and therefore all devices are powered from one wall outlet), you'd probably find the hum goes away, as they'd all share the same earth (different outlets have slightly different small currents on the earth due to electromagnetic induction - the earth wire acting as an antenna of sorts)

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