Weird earthing problem

[dave]

Before doing anything, try disconnecting pin25 at one end of the ilda cable, if this sorts out your problems then your on the right track.

[The_Doctor]

Craig, do that the other way round. Leave the local grounding intact, and connect all mains PSU's to one source. That gets you a star network that minimises chance of loops. Second, Jem mentioned you'd gone with the original differential drive, which was a good move. Common mode signals should be rejected that way. So long as there is a few hundred millivolts at best between the two local grounds, (on scan amp, and DAC output gnd), when the DAC and scan amp have NO signal interconnect at all, the way then is to wire the two differential lines per channel, and to break the gnd lines between them, leaving the PSU line gnd intact to each. That way you should fix this without compromising safety.

[survey]

Well, it seem's that's not only my laptop (Compaq Presario 2530EU) with these
problem, as they said, that's probably a ground loop.
I resolved my problem with a 3-prong to 2-prong AC adapter (lift ground).

[The_Doctor]

You did exactly what people are NOT advised to do. Granted, you might have a double isolated UL tested PSU design that is appropriate for this, but look again at my advice. I learned that for a reason. If a ground is provided, USE it, especially if you're thinking of doing shows. Ignoring safety features that basic WILL be enough reason for any official who sees it to shut you down.

[Wiring Monkey]

Thanks for all the suggestions, I'll try them out as soon as I can. I have already tried removing pin 25 from the projector and it had no effect, this suggests to me another gnd between the laptop and projector. I'll check all the input lines to make sure there's no grounds there and look into making the grounding a 'star' arrangement.

I don't suppose there are any links in the DAC for differential/unbalanced operation are there?

Craig

[The_Doctor]

I don't know the Medialas DAC, and (Google wasn't very useful either) but choosing between differential and single ended outputs can vary. The simplest scheme is to use a driver that puts out two signals equal but out of phase, and you use those + and - outputs for differential, and either of these and Gnd for single ended. The output wiring diagram should identify those to tell you what might be doable. Also, look at the actual PCB to see if the - output and Gnd are one and the same. On a proper differential output, they won't be.

Also, you might have a common ground through the cable screen and DB25 connector shell. Like audio screens, it helps if you can open one end to allow the supply grounds to remain intact. It should be easier than with audio leads, too.

[Lasermad]

Congratulations all , you have all won a penny chew for having the most incomprehensible thread i have read this year ........... i thought i was getting there , managing to understand things an you lot manage to completetly dumbfound me !

Paul

[Jem]

Paul

Had to smile when I read your post!

I was lost even before I started, this is why Craig is being kind enough to handle this for me. If he manages to crack this one I'll definately owe him a drink or three

Other than the very basics I don't do electronicy things, give me Optics and nice simple lens formulas anyday

Cheers

Jem

[p1t8ull]

simple lens formulas

WOOOOP! WOOOOP! WOOOOP!
oxymoron alert!!
oxymoron alert!!

[Jem]

I just knew that would get a response

Jem