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Unbalanced line levels, AC cables, long distances


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I've got my sampler (which has unbalanced inputs/outputs) running a fairly long distance to my patchbay (probably about 9 feet)... Unfortunately, either from the distance the cable has to run or from the proximity to other kinds of cables (there's a long SCSI cable running from the sampler to the hard drives in the closet, plus there's numerous AC cables), I'm picking up a low (60 Hz?) hum. Likewise... I've got an Audiomedia III card (which also has unbalanced inputs/outputs) running from the closet to the patchbay... While I realize some noise is unavoidable being that the audio inputs/outputs are located on the PCI card itself, I think I'm picking up some slight hum from other cables or from the distance that the cable has to travel over an unbalanced connection. BTW -- all of my gear is running off the same power outlet, so there's no weird grounding issues from having different pieces of gear running from different power circuits. In fact, if I move the sampler a bit closer to the patchbay/mixer and use a shorter cable to plug it in, the sound is fine... No hum. So I have two questions: 1) If I need to run a long cable (8 to 10 feet) and the initial device has unbalanced outputs, what can I do to minimize the potential for interference? I don't think I'd want to use direct boxes, as what I'd probably lose in fidelty from going to line level to mic level then back up to line level would probably be worse than the hum I get right now. (Plus... how many batteries would I be replacing every month? :D ) Or is this assumption incorrect? 2) Assuming part of the problem is with AC cables... How the heck does anyone keep the AC cables far away from the audio cables? I haven't been able to come up with a creative solution for this yet. Given the way the room is and where the equipment has to go, it's almost an impossibility to keep things apart... I should mention that the problems I'm describing aren't so bad that I can't record -- I can only hear hum if I crank things up really loud. And for my purposes, the fidelity is fine... This is more along the lines of fine tuning my modest home studio. :) (Still waiting for the day when hard drives have no moving parts and stuff doesn't need to go in the closet... :( ) [ 12-13-2001: Message edited by: popmusic ]
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[quote]Originally posted by popmusic: [b]...a fairly long distance to my patchbay (probably about 9 feet)...[/b][/quote] Hmmm...I really wouldn't call this a "fairly long distance"...even for unbalanced, which is considered "safe" up to 18 feet. I have snakes at 25'...had no choice...have no problems...(fingers crossed!) :D I think it might be the proximity to other cables...probably AC power cords. Try this...thake an even longer cord...maybe about 15 feet and then you can move it easily in various directions... Does the hum increase/decrease as you change the proximity to other cables? What other equipmnet does the sampler's audio cable come close to? It might be the power supply from something else that is introducing the 60Hz hum...? I know a good witch doctor that makes house calls...that might be the final solution... ;)

miroslav - miroslavmusic.com

 

"Just because it happened to you, it doesn't mean it's important."

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I would not rule out a ground loop. Even though your gear is on the same outlet or breaker, one or more pieces can be out of phase with others depending on how their power supplies are designed. Try lifting the ground lug on your sampler. Also, your sampler could be picking up induced hum from a nearby piece of gear. An audio cable should be able to run along side an AC line, assuming it is a quality cable and properly shielded.

GY

 

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Well, I don't really think a good quality direct box will hurt your signal much, but a cheap one probably will. If you don't want to spend many hundreds of dollars on a pair of high quality direct boxes, and you're sure the sampler is putting out line level (-10), you can get an interface amplifier for about $100. These are specifically designed to take -10 unbalanced signals & convert them to +4 balanced. Scott
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[quote]Originally posted by popmusic: [b]Assuming part of the problem is with AC cables... How the heck does anyone keep the AC cables far away from the audio cables? [/b][/quote] As someone else already said, this is probably the likeliest culprit (along with ground loops). I use velcro cable ties to tie audio cables in one bundle (or more), and power cables in another bundle. Then keep the two bundles as far from each other as possible, and try to make anyplace where audio and power cables must cross a perpendicular crossing. Running audio and power cables parallel to each other is the best way to induce hum into your system.
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Look out for things like TV's, computer moniters and other sources of magnetism and 60 cycles. Try gently looping like type cords around one another. Watch out for single coil pickups and other unbalanced line inputs.

He not busy being born

Is busy dyin'.

 

...Bob Dylan

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[i]As someone else already said, this is probably the likeliest culprit (along with ground loops). I use velcro cable ties to tie audio cables in one bundle (or more), and power cables in another bundle. Then keep the two bundles as far from each other as possible, and try to make anyplace where audio and power cables must cross a perpendicular crossing. Running audio and power cables parallel to each other is the best way to induce hum into your system. [/i] It is indeed advisable to run power lines "[i]perpendicular[/i]" to audio/signal cables. As far as bundling together, I'm not sure. I'm still planning my little studio, and the perpendicular thing is what I hear about alot....will have to find out what I'll need to do when time comes myself goodluck :)
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A "ground loop" results when a device has two paths to ground. This can result when an audio ground gets to earth trough another piece of equipment while also haveing a ground path through the equipments own ground connection, usually the third prong on the AC plug. You might try using a ground lift adaptor on some of the equipment and see if lifting the ground on certain devices makes the situation better. Some people lift the ground on all the gear and then attach ground wires to the chassis of each piece and connect them to a common Star Ground point. This is usually only necessary when the equipment in on several different AC circuits and there are different potentials on the grounds. You might be able to get less hum by just lifting the ground on some of your equipment. It's usually a trial and error thing. There was someone making a 16 channel line balancing device at one point but it was expensive. I've used one on several occasions though and it helped when connecting a bunch of synths to a +4 balanced line input.

Mac Bowne

G-Clef Acoustics Ltd.

Osaka, Japan

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[quote]Originally posted by usicMa: [b]...the perpendicular thing...[/b][/quote] You can run 'em parallel, you just need to keep them about 12"-18" or more appart, and you shouldn't have any problems. If you have to cross power & audio wires...that's where "the perpendicular thing" comes in...as opposed to a more lenghty, parallel crossing. I've always kept my audio, power and MIDI cables at a safe distance from each other (here's the catch)...WHEN POSSIBLE. Let's face it...it all looks great on paper, but when you get into those racks...there is often a point where it seems that ALL the different cables need to go to exactly the same place! :mad: Well,...don't drop a turd over it! :p Use common audio sense and do the best you can. After you turn the equipment on...you can address any problems individually...IF THEY COME UP. You'll be surprised how often something that looks like it might cause problems...doesn't. But then that "fantastic" wiring job you did...is nothing but noise! :mad: Go figure... :confused: [ 12-14-2001: Message edited by: miroslav ]

miroslav - miroslavmusic.com

 

"Just because it happened to you, it doesn't mean it's important."

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