Need A Hum Filter For Audio Into A Computer. Any Advice??

[Scott Reppert]

Just moved a studio and the new location has great audio/video feeds for the video production side of things, but the audio into the computer that houses Audition has some serious hum on it. Turning off some lights helps, but does not eliminate the hum fully ("Hello. I'm Hum Fully. In the news tonight...").

Any advice or input as to which filtering device is better than the rest to use in a situation such as this would be appreciated.

Answer and talk away. I'll be here. And, thanks...

[Tom Taggart]

First thing to try is simply plugging the computer into the same bank of outlets as your audio source. If the electric is very elaborate, you may be looking at a ground loop, or similar kind of in-balance. For example, if they have a 3 phase power system, you may have the computer plugged into one phase and the audio sources are powered by a different phase--and the electrical grounds are causing the problem.

If that is too difficult to do, or doesn't cure the problem, you need to check to see if the source already has hum on it. Disconnect the audio feed and clip-lead headphones to the feed. More likely, you just need to keep a balanced feed from being grounded on one side, and the computer input is unbalanced. A simple solution is to find a good 600:600 ohm transformer; the computer input side can be grounded if the audio card is unbalanced, but avoid grounding the audio feed side.

Of course, it's not so simple finding a good audio transformer any more unless you have a good junk box...although Surplus sales http://www.surplussales.com/transformer ... ans-1.html might have something that would work.

Another, more expensive cure is a Henry Matchbox, or the comparable item from Broadcast Tools, which accepts a balanced feed and provides an electrically isolated unbalanced feed.

[Scott Reppert]

First thing to try is simply plugging the computer into the same bank of outlets as your audio source.

Can't do this because it's on the complete other side of the studio. I do realize that the interference could be coming from the fact that we are running about 20' of audio cable from the audio source over to the computer, but we did use a higher quality/better shielded cable for that application.

If the electric is very elaborate, you may be looking at a ground loop, or similar kind of in-balance. For example, if they have a 3 phase power system, you may have the computer plugged into one phase and the audio sources are powered by a different phase--and the electrical grounds are causing the problem.

The audio source is plugged into the building's "original" electrical system...which was in place when we got here. The computer is plugged into a "newer" run from whatever was originally in place here over to where the computer and editing station is.

...you need to check to see if the source already has hum on it.

This is not the problem. It's clean as a whistle. When we extract the video which carries audio from the same source, it has no interference at all. Really clear and clean.

More likely, you just need to keep a balanced feed from being grounded on one side, and the computer input is unbalanced.

Everything "appears" balanced, although that still could be the problem.


...the computer input side can be grounded if the audio card is unbalanced, but avoid grounding the audio feed side.

Don't think that's the problem, but that is some amazingly good logic that I will remember for future reference!

Another, more expensive cure is a Henry Matchbox, or the comparable item from Broadcast Tools, which accepts a balanced feed and provides an electrically isolated unbalanced feed.

If the problem persists, this is close to what was going to be our solution. Which leads me to tell you this:

Did a project yesterday afternoon into last night that involved one track of some audio recorded almost three years ago (very clean, no hum) and two tracks featuring voiceover stuff recorded yesterday in that editing area on the computer in question. All the while I was cutting, pasting, multitracking, and mixing I kept thinking to myself: "there's so much hum...this is really gonna suck". Took the finished product out to the car to listen to it, so that I knew that I would be devoid of any additional interference from playing it in the same room. Sounded perfect. No hum. Airable quality right off the bat. I know that there is interference going into the computer because I can "see" it in Audition when I record "nothing". I do wonder, however, if more is added to what I am hearing out of the speakers because they, too, are powered and on the same bank of outlets as the computer.

Anyway, this project is finished and ready to go, but there is still a problem that we need to address...