[sdiy] Request brain dump on balanced lines

[Magnus Danielson]

From: media.nai at rcn.com
Subject: Re: [sdiy] Request brain dump on balanced lines
Date: Mon, 14 Oct 2002 12:21:57 -0400

Hi!

> At 12:41 AM +0200 10/12/02, Magnus Danielson wrote:
> >this.
> >
> >The standard way to wire XLRs is
> >
> >Screen	Chassi
> >1	Signal Ground
> >2	Signal "Hot"/+
> >3	Signal "Cold"/-
> 
> I'm a bit confused what you mean here.  Yes, pin 2 is the in phase signal
> (hot), and pin 3 is the current return (cold), but why do you list the
> "screen chassis" and "signal ground" as being separate??  Do you mean the
> shell or case of the connector itself??

I was waiting for someone to ask this question. Thanks for asking it.

The metalic case/chassi of both male and female connectors is bound to be tied
to the protective ground of the chassi when hooked into a box. This is not
equalent to the signal ground, despite popular confusion. Somewhere the chassi
ground and signal ground comes together, and that is the powersupply, but
other than that you keep them separate. They should not vary much (or you run
into SERIOUS trouble, I've had first hand experience of that) but they should
not be shorted together either, because then your cables will short whatever
ground potential you have, so you want to cut them _some_ slack.

So, allways keep in mind that protective ground and signal ground is two
differnt forms of ground, and incorrect usage and arbitrarily swapping between
them might not yield the intended result.

> The reason for pin one is so it connects before the other two pins (which
> is the exact opposite of what is glaringly wrong with RCA connectors).

Exactly. This is certainly how it is supposed to work. This is when people have
considered electrical safety routines.

> >There is a broken way to wire it, but I won't mention it.
> 
> Yes, both the NAB and the EBU had their own ideas :)

Actually, I think this is correct against both AES and EBU. I can check the
details later... I have the relevant docs around here somewhere...

> Btw, according to the IEC today is "World Standards Day".
> 
> >Please note that Screen on the XLR is usually not wired to the shield of
> >>the cable, it is infact a bad idea.
> 
> Huh??

Yes... screen as in chassi. This is also what I say. Your messing with the
protective ground and you don't want to do that on mike/line cables.

Cheers,
Magnus