9 replies to this topic

[Selfdestruktor]

This may have gotten better coverage in a forum higher up the list, but it isn't car audio, so....

I've just set up half a dozen 51" plasma TV's on mobile trolleys at work.
They have a shelf to sit a laptop on, and plug in via VGA and a 3.5mm audio cable.

I'm getting a few complaints about a buzzing noise, which seems to be caused by interference on the 3.5mm cable, generated by the VGA cable.
The ferrite core things don't make any difference.
I bought some better quality VGA cables, but that didn't make much difference either, so I was looking at replacing the 3.5mm cables.

I'm looking around on the web, but am struggling to seperate the plain, the overkill and the wank/gold paint/shiny plastic connectors.
They have to be 3-4m long, 3.5mm M-M stereo.

Any suggestions where to get a good deal?
Or am I going to have to make them, out of something like this?: http://www.jaycar.co...w.asp?ID=WB1506
Or, is it going to make little difference?

Tell me if I have the wrong effing idea. I don't really know a lot outside a typical 12v car environment.

Edited by Selfdestruktor, 14 September 2011 - 11:34 PM.

[Captn_Awesome]

So the 3.5mm plug is actually plugged in when its buzzing?
Id be checking the way these people are plugging it in - is the 3.5mm plug seating fully into the socket?

[Selfdestruktor]

Without the cable plugged in at either end, no noise.
With it plugged in at the TV end, a fair bit of noise, made worse if you sit it alongside the vga cable, and worse again if you hang it over the top of the TV or something (near the PSU board I guess).
With it plugged in to the laptop and the TV, the noise is almost inaudible at low volumes, and when you play something at a medium-high volume, it drowns it out.
As soon as you stop what you're playing (e.g. pause Media Player) at a 'listening' type level, you hear this annoying buzz.

Maybe that's just what it's going to do because of the type of cable and there's nothing I can go about it, but I thought there'd be some better quality 3.5mm cables out there somewhere.

Edited by Selfdestruktor, 15 September 2011 - 05:25 PM.

[Captn_Awesome]

Just remove the earth pin from the TV

(Dont actually do this - it will most likely work, its a PA trick - but dont! if someone gets fried you dont want to be responsible)

Definately seems like 50hz ground hum and really an earth lift is the easiest way around it.

You're ALWAYS going to have the noise with the leads plugs into the TV but nothing else.
If its barely noticable with the sound on just learn to live with it. If you mute the sound when theres nothing playing does the hum go away (Im betting still not entirely but probably slightly better?)

If the laptop is running on battery is the hum still there? A lot of laptops power supplies are a bit manky (DC ones) because they use the battery to ripple filter - this could be playing havoc also. Even worse are the AC ones which the laptop has a bridge rectifier internally. Never understood this.

I wonder if you used an external switch box? Or a 1:1 audio transformer...

Edited by ~Sparkles~, 15 September 2011 - 06:49 PM.

[Captn_Awesome]

you could try something like this
http://www.altronics...a=item&id=M1000

or more expencive - but off the shelf
http://www.langtonin...0400004&loc=AUD

[Selfdestruktor]

Yeah, I was thinking something like that.
Fracking heck, though, look at the price for the off-the-shelf one!!!

I can get a cheap Asian sourced one (that looks the same as the one in that picture)from FleaBay for like $8.00 (inc.)
It's only about $25 for a PAC branded one from the US.

Edit: Stuff it, will give it a crack. Just ordered 6 of the cheap ones for $7.66ea. Will see what happens.

Edited by Selfdestruktor, 16 September 2011 - 04:32 PM.

[Captn_Awesome]

Worth a shot. I've used them in a church install where we couldnt get rid of buzz in the powered wedges (another reason I hate powered speakers...) being used as fold backs.
We put an isolation transformer on the unbalanced 6.5mm TRS input to the first box in the loop and no issue ever again. This is a prime example of why balanced signals whould always be used where 240Vac exists...

[Captn_Awesome]

any news?

[Selfdestruktor]

Yeah, the noise surpressor made SFA difference.
Might have to try building an uber-1337 3.5mm cable myself.

[Captn_Awesome]

Concidered a DI box? Not entirely sure how to set that up in a "domestic" AV typer environment but might be worth having a look at. Would need an active unit though or it'll pad the crap out of the signal and you'll have to work hard to get that line voltage back up.

I still think its earth related.

You got a link for those units you got on ebay? I'm confused as to why they didnt work. But it has got me wondering about another possible solution. I might see if I can get in touch with paul and see what he recommends (paul is one of those guys that just knows stuff. He only has one ear but can EQ a system like no one else I've ever met and has this box of tricks (quite literally a box) and nearly always gets us out of trouble when we have issues with bumping in a rig. I've never seen so many odd bits and pieces and concotions of parts and leads, transformers, adaptors etc. Between him and 'doggy' I learned a lot when I was doing rigs - real cluey 'old school' guys.