Absolutely buzzing - but not in a good way.

[Owen]

Now and again I do some work in a studio which is local to me. Like all of us I like to hear more of me than the communal foldback will allow me to have. I take a small mains powered desk with me, plug the cans feed into that as well as a link from the DI. I can then mix myself as loud as I like. Unfortunately this places a nasty buzz in other peoples cans. Is there some sort of isolatiing thingumywotsit-ma-bob I can get to make all this earth hum-ness go away?

Edited April 9, 2010 by owen

[AlanP2008]

[quote name='owen' post='801298' date='Apr 9 2010, 09:52 PM']Now and again I do some work in a studio which is local to me. Like all of us I like to hear more of me than the communal foldback will allow me to have. I take a small mains powered desk with me, plug the cans feed into that as well as a link from the DI. I can then mix myself as loud as I like. Unfortunately this places a nasty buzz in other peoples cans. Is there some sort of isolatiing thingumywotsit-ma-bob I can get to make all this earth hum-ness go away?[/quote]

This will be a good ole' earth loop problem. The most direct point to tackle this will be at the connection between the cans feed and your desk (although there will be other ways to approach it too).

The problem is that the common/earth/screen wire on the cans cable is being connected to ground via your desk (and maybe via your DI cable as well - depending upon how that is organised). Without going in to why that is a problem, basically you need to break that ground connection. You could try simply disconnecting the screen on the cable you use from the cans socket to your desk - that is likely to stop the buzz in everybody else's cans, but might well end up with a worse buzz in your personal monitors. However - it might be just fine, so if "trial and error" is an option, you could try that first.

A small improvement might be to use a small resistor - say 10ohms or so - instead of the original direct connection you just broke between the common/screen to your desk. That might be somewhat better, or might make no difference at all...

You could try running the cans signal (signal and common) into a balanced mic input (+ and -, no connection to ground on the XLR pin 1) on your desk, but there is a good chance that the signal level will be too high, and you will need to pad it. You could try a 1k resistor in the signal lead to the + on the mic input, common to the - on the mic input, and a 100R resistor between + and -.

That has an excellent chance of working - however, the odd possibility remains that the level of hum signal on the common of the cans feed is high enough to exceed the common-mode range of the mic amp input (these days, mic inputs are usually elecronically balanced, and such circuits do not have the common-mode range that a transformer-coupled input of old would have had).

The final option then would be to use an isolating signal transformer between the cans feed and an unbalanced line-level input.

This one: [url="http://www.maplin.co.uk/Module.aspx?ModuleNo=226569"]http://www.maplin.co.uk/Module.aspx?ModuleNo=226569[/url] is a stereo unit, and you would only need to use half of it. (You would probably need to make some adaptors for at least one of the phonos.)

Hope this helps.
Alan

[Owen]

Alan, thank you very much.
Owen