[LAU] ground loop hell

[David Christensen]

On 02/10/2015 05:43 PM, Len Ovens wrote:
> As an aside... I don't think I will need it at this point, The isolation
> boxes that are easy to get are "DI" boxes tha go from high impedance
> unball to low impedance bal. Would these still work to go from a
> ballanced output to an unballanced input?

DI boxes are designed to connect an unbalanced output to a balanced 
input.  I've also pondered running DI boxes "backwards".  From a 
technical view, I'd need to study the schematics of the driving 
equipment, the DI box, and the driven equipment to convince myself that 
it could work and not damage anything.  I might try calling technical 
support.  From a execution view, I'd need the right adapter cables (such 
as XLR-F to XLR-F).  But from a conservative/ safety view, I'd rather 
hook everything up the way the equipment manufacturers tell me to and 
not have to worry about it.


> Or do I need a one to one
> isolator? Or does it depend on the powered monitor?

I'd look in the monitor manual and follow the manufacturer's 
instructions.  If you don't have a manual and can't get direction from 
the manufacturer, I'd use an isolation transformer with suitable 
connectors and cables.


> How does this one look: http://www.rolls.com/pdf/M_HE18.pdf

That looks useful.


> Having "solved" my problem with a single power source, would it still be
> worth while isolating all stage amps from the mixer?

If the mixer has balanced outputs, the stage amplifiers have balanced 
inputs, and both are powered from the same supply, isolation 
transformers should not be necessary.  I'd run balanced cables from the 
mixer to the amps and call it good.  That said, good isolation 
transformers will cause a small lose of signal and fidelity, but they 
might provide protection for certain equipment failures or operator errors.


> I think I will run
> ballanced audio through the snake and do the cable adapting at the stage
> end anyway.

It is best to keep balanced signals in balanced cables for as far as 
practical.


> Right now there is a TS plugged directly into a TRS mixer
> output which means the signal minus is grounded.

I wouldn't plug a TS cable into an balanced TRS output.  First, you are 
shorting one half of the balanced driver output circuit.  Second, you 
are injecting chassis ground noise into the audio circuits.


> There are two unused
> (unusable) snake lines I could use where we have the DVD and computer
> plugged in as line level.
> Hmm, more thought required, the DVD/computer may need changes too.

I assume the DVD and computer are unbalanced.  If those are next to the 
mixer, cables alone might do.  Check your mixer manual.  If the runs 
aren't local and/or you can't find an authoritative wiring diagram, I'd 
use DI boxes.


David