low hum from soundcard to board

[Norm Mast]

At our church we sometimes run the audio from a video thru our main sound board. I only run the board on my Sundays, I did not hook things up. But. the hum bugs me,and it comes thru the house until the actual audio starts then it gets covered up. From the PC built in sound card a 1/8" stereo plugs run out, terminating into a dual RCA plug, this in turns connects to a dual RCA cord that goes to a 1/4" mono plug. This then plugs into a Radio Shack Low/hi impedance converter before running to the board where it connects via a XLR plug into the board. I've re-routed this cable to avoid all electrical lines, even unplugged every device in the video booth, nothing seems to eliminate the hum. What do I do?

[kozikowski]

You're just begging for trouble. Converting from stereo to mono and unbalanced to balanced.

I bet if you got the show from a laptop on batteries rather than a PC, all the problems would vanish.

The ground or shield in a soundcard is not well-behaved. It's not ground and you can get into trouble assuming it is. So back at the XLR, the console thinks pin 1 is stable ground and it's nothing of the sort.

Can you do that for one week? Borrow a laptop for the show--or just as a test.

Sometimes you can help by actually disconnecting the ground in the XLR. You good with a soldering iron?

Koz

[Norm Mast]

I can handle the soldering iron no problem. You're saying remove the 'ground' at the XLR end....and see how that behaves.When they do run video it's loaded on a desktop PC that send to projectors which display that on screens...I get the audio from the PC sound card that comes to my board. Be something to try....

[flynwill]

Have you tried the experiment of unplugging the XLR at the mixer board and verified the hum goes away?

The other thing you can try is a "ground lift" on the power cord from the PC supplying the show. Basically buy one of those adapters intended to allow you to plug a 3-prong plug into a 2-prong outlet, but be careful leave the little bit that's meant to go under the cover plate screw hanging in the air.

You might also unplug the mini-phone connector at the computer and plug in a reasonably sensitive set of headphones and see if the hum is present in the soundcard itself (not likely).

If the problem is with the ground then the best fix would be a un-balanced to balanced audio interface like the Henry Matchbox at the PC end. (Unless that is what the Radio Shack box you have already is...)

[kozikowski]

All those things. Pin 1 at the XLR is intended for protective shield or ground. The console is expecting the sound to appear between pins 2 and 3, 2 being primary or "right-side up." A proper balanced system will work perfectly well (on short runs) without pin 1 connected. There are some studio transmission systems that intentionally break the shield to avoid problems similar to what you have.

Land-line telephones work balanced for miles with little or no degradation in voice quality.

Do you have part numbers on the Radio Shack adapter? Do you have the data sheet you can scan and post? Web page?

Somebody must have a laptop you can run on batteries just long enough to see what happens when you carefully unplug the PC soundcard and plug into the laptop headphone connection. Don't let the laptop touch anything during the experiment (put it on a towel) and run on batteries. Play anything. Internal music, YouTube music (from WiFi).

Since you have a much larger system, it could turn out everything works just fine as long as the video projectors aren't connected. There are few problems that proper engineering can't make much worse.

Koz

[flynwill]

Indeed I realized after that post that the projector would probably need a ground lift as well as it will be tied to the computer's ground via the shield in whatever cable connects it to the computer and almost certainly has a 3-prong plug.

The other thing that occurred to me: Is there any chance this Radio Shack "Impedance adapter" is actually an attenuator? Can you get us a good picture of the mixer board so that we can see what the gains and input selections are for the channels accepting the computer input? If that input is in "Microphone mode" vs "Line mode" then that would mean the problem can probably be fixed with different unbalanced to balanced adapters.

[kozikowski]

A transformer maker whose name escapes me used to make devices to do this exact job. The lead on a lecture I went to said he won't leave the house without a pair of them to interface between his computers and the house.

So yes, that's why some quality info on that adapter is in order. Assuming you don't have hundred foot cable runs, one of the solutions might be stop using the adapter.

Koz

[kozikowski]

Jensen Transformer

http://www.jensen-transformers.com/pro-audio/

They weren't particularly cheap. They knew what they had.

Koz

[kozikowski]

Try the ground lifter thing first. You can do that for 39c usd. You are looking for the problem to either go away or change. If it changes, post back what happened.

Koz