Audio Interference Issue?

Posted: Fri May 02, 2008 12:54 am

I think I have what might be some sort of interference problem that has been driving me crazy. It has actually been a problem for quite some time, but only recently since troubleshooting the issue has the problem become severe enough to merit putting everything on hold until the issue is figured out.

The problem was a high-pitched buzz (not low enough to be a ground loop) I kept getting from a Yamaha S90ES keyboard. I finally realized I was only getting noise when I had the USB cable from my computer plugged in to the S90ES; unplug the USB cable and the buzz was gone. I use the USB functionality for sequencing, so it is a must to get this figured out.

The interesting thing was that even after disconnecting power from the S90ES and the computer, the buzz continues as long as the USB cable was plugged in to both devices. I tried several different USB cables with no resolution.

I suspected a Furman power supply in the rack that holds the computer, as it has given me trouble with other devices in that rack in the past, so I started going through the rack putting isolating washers on every device. I also tried a ground lift plug on every device in the rack, as well as the Furman so that the entire rack had a ground lift on it, but the problem continued.

At the same time, I pulled the computer out of the rack (which is in an Antec rack mount case with an Antec power supply) to check and make sure the motherboard was properly grounded to the chassis. While I had the computer out of the rack, I attempted plugging it in to another wall outlet to determine if the problem was with something in the rack, connected up the S90ES via the USB cable, and this time I received an obviously loud ground loop that I was able to resolve with a ground lift. This started me wondering if something is up with the computer and/or power supply.

I put the computer back in the rack, connected up the power and USB cable to the S90ES, and the same high-pitched hum was there. I backed the computer out of the rack just enough that the rack ears didn’t touch the rack rails, and voila, hum gone. I continue hooking up everything to the computer, and as soon as I hook the two monitors (LCD) up to the computer, an awful high-pitched squeal came out of the S90ES. The squeal goes up and down as the monitors turn on and off, so it’s quite obvious THOSE are causing the interference now. Again, remove the USB cable that connects the computer and the S90ES or remove the monitors' DVI cable from the computer, and the noise goes away completely. I decided to try my Access Virus TI which also connects to the computer via USB, but had the exact same problem. I switched over to using balanced cables on the Virus TI, since it has balanced outputs, and the issue was resolved completely.

I tried the same with the S90ES, only to find out it does not have balanced outputs.

I’m really pulling my hair out on this one. Is it possible there is a grounding or some sort of interference issue in the computer, and how does one go about resolving such a problem?

Secondly, how do you deal with a device that does not have balanced outputs (i.e. a keyboard like the S90ES) that must use balanced connections due to interference issues such as this?

Posted: Fri May 02, 2008 1:12 am

Damn, it sounds like you're a better troubleshooter than me, so I thought I'd be able to help you but I dont know! I'll throw out some ideas to maybe jog your mind...I dont know how long this has been an issue, but it may be something strange like the actual length of the cable that is being connected (the ones where when it is connected it causes interference). Have you actually tried swapping cables, tried different lengths? I find sometimes its the simple things I overlook. It doesn't sound like cable to cable interference/noise induction... Is all of this plugged into one rack/outlet/power conditioner?

Posted: Fri May 02, 2008 3:17 am

From your admirably detailed description, I can't quite follow what it is that makes the problem go away at source - was it disconnecting the LCD monitors? If so, can you get hold of an old glass VGA monitor to try to see if everything else works correctly without a buzzing noise? As an itinerant, this may be difficult for you, but I know I could go to my local recycling centre and have the pick of several dozen, such is the headlong scramble to change to flat screens.

If a glass monitor does appear to cure the problem, what then? Well, you spend some time on doing the job the LCD screen manufacturers should have done - suppressing the emission of conducted interference. The main external weapons here are ferrite absorption rings, with several turns of the mains cable taken through one and a separate ring for the DVI cable. By the way, is the problem any different when using analogue (VGA) input to the monitors rather than DVI?

You are evidently a logical thinker and diligent troubleshooter, so I would expect you to be able to get to a workable solution. Good luck!

Posted: Fri May 02, 2008 6:44 am

SYNTHME wrote:

Secondly, how do you deal with a device that does not have balanced outputs (i.e. a keyboard like the S90ES) that must use balanced connections due to interference issues such as this?

Yeah...you pretty much did everything you could. Sometimes, things are just weird. There IS an answer, and it's probably simple, it's just that the EXACT right combination hasn't been attempted, yet.

As far as balancing an unbalanced signal from your keyboard, you could try something like the following, with a TS cable input, and output with a proper TRS-TRS or TRS-XLR cable:

http://www.ebtechaudio.com/lls-2des.html

They also make purely Hum Eliminators, but I wonder if that's your problem. If you're going to bal/unbal a signal anyway, the Line Level Shifters also have the Hum Eliminator built in. Just look around for the best price. I don't know if anyone else manufactures something like them, but these are a good example for that. It's a passive box. You can probably get a higher grade one somewhere, (and probably pay much more) and some folks here may know what those are, but these have worked well for my purposes.

Good luck taming your "hissy fit". Wink

Kapt.Krunch

Posted: Fri May 02, 2008 10:01 am

Try a DI to balance the unbalanced signal?

Posted: Sat May 03, 2008 1:51 pm

After troubleshooting a bit more, I think I completely eliminated any of the cables from the keyboard to my mixer. I tried balanced and unbalanced cables, DIs, short runs, long runs, and none of them change what’s going on. The only thing that stops the screeching is unplugging the USB cable from the keyboard. Thus, the source of the noise MUST be the computer or the LCD monitors attached to the computer (since, unplugging the monitors from the computer seems to make the screeching go away). Even more strange, if I unplug the USB cable from the computer and leave the other end plugged into the keyboard, and touched it to the chassis of the Lynx Aurora (which is also hooked in to the same computer via an AES16 card) I heard the same screeching sound. So the sound is obviously inundating everything that is hooked up to the computer.

So, thinking it possibly could still be some sort of electrical issue, I put a ground lift on both monitors and it completely resolved the issue. Weird stuff!!!

Thanks for everyone's ideas and suggestions. After wasting DAYS on this issue already, I really needed the motivation to continue troubleshooting and trying different things.

Posted: Sat May 03, 2008 2:16 pm

Try a really expensive Monster USB cable.
ProTool Mbox users have report this issue...
Called the USB whine.

Posted: Sun May 04, 2008 12:34 am

Well, I thought I had it resolved. No more loud squealing, but now I hear what sounds like a very quiet, electronic sounding helicopter noise from the outputs of the keyboard whenever the USB cable is attached. Unplug the USB cable, the fuzzy chopper is gone.

Tried it without the monitors plugged in to power and not plugged in to the computer, which changed nothing (which at least tells me I’ve got the issue with the monitors figured out).

I’ve tried putting a ground lift on every other device that is connected to the computer, including the computer itself, again, no resolution here.

Strangely, when I move the mouse (which is USB also), the buzzing sounds gets louder.

This really has become one of those things where, after you tear your hair out, you continue to tear everything out of your rack piece by piece.

Posted: Sun May 04, 2008 7:46 am

Do you have another computer that you could use to rule out your main computer?

Here are a couple of thoughts...

It almost sounds like a "digital" problem that is somehow getting back into the audio. If you have tried ground lifting everything and it is still there then I would suggest that it is somehow a keyboard issue and you should contact the manufacturer to see if they can be any help.

I had a problem recording a Rhodes piano and no matter what I tried I had hum problems with the unit. I finally got an AC isolation transformer, fed the Rhodes off the transformer and BINGO no hum. It was an extreme solution for what should have been a easy to fix problem.

I also helped a friend of mine do some recording of his keyboard and we were "shocked" literally when we tried to plug in his keyboard into his computer and drew a spark when the grounds got close together. I got out my trusty VTVM and measured 1.9 volts ac between the keyboard audio output's ground and the computer's ground. The keyboard was powered off a WALWART and the computer was plugged into the wall outlet. After some additional trouble shooting we found that the computer's power supply was faulty and was putting voltage onto the computer's chassis from a leaky capacitor that was going from the AC input to the chassis for RF attenuation. We were able to cut out the capacitor and the problem ceased to exist.

If you know someone with some test equipment who knows how to use it then I would suggest you check out your computer for the same kind of problem.

Best of luck!

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