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Topic: Wiring up a stereo cell

Steve Guttag
We forgot the crackers Gromit!!!

Posts: 12814
From: Annapolis, MD
Registered: Dec 1999

posted 02-23-2005 11:36 PM

Randy, you need to remember several things, actually.

First Pin 1 on an XLR is signal ground, not pin 3. Even Bozos that make Pin 2 "-" still keep Pin 1 ground.

Next, until a signal is referenced, it is a differential signal and thus has good CMRR. Techncially it is not "balanced" since that implies a reference that is balanced about the reference.

A solar cell, that is not referenced to ground may be wired in differential fashion to achieve the benefits normally assoicated with a balanced input. Note, Dolby has always provided a + and - terminal for the cell input "E" was always just for the shield.

Other, lesser designs had one wire the cell in an unbalanced fashion by connecting one of the leads of the solar cell to ground. These systems are normally hallmarked by the snaps and pops in the sound as each piece of apparatus is turned on (either manually or via automation).

XLR or TRS connectors normally imply differential that is often also balanced. Thus pin-1 is ground (signal). There is also the often forgotten terminal on XLRs that is the chassis ground (the odd funky tab that is often connected to the release button on an XLR). Shield wires would normally benefit by connecting to chassis rather than signal ground so a good drain is established. Then again, in many systems, signal and chassis ground are one and the same. In fact, if you read the various RANE notes, they connect chassis ground and pin-1 on all of their XLR connectors.

Steve

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Randy Stankey
Film God

Posts: 6539
From: Erie, Pennsylvania
Registered: Jun 99

posted 02-24-2005 01:19 AM

I knew about the XLR pins. I've soldered up many of them.
What you mentioned about Rane grounding Pin #1 rang a bell, although I didn't remember it, per se.

What I still have to remember is about how balanced lines work vs. unbalanced. I know about not having a reference to ground and about common mode rejection. About Pin #1 being signal ground and/or chassis ground sounds counter intuitive. If balanced lines are good because they don't have a reference, then why ground them and give them one? Unless that reference is held precicely at earth ground, thus making the reference 0.

I need to hit the books some more!

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Steve Guttag
We forgot the crackers Gromit!!!

Posts: 12814
From: Annapolis, MD
Registered: Dec 1999

posted 02-24-2005 09:02 AM

Randy,

Think about it like this....something can not be Balanced or Unbalanced until there is a reference to balance or unbalance to!

Until there is a reference, you merely have a potential difference or a differential output...not the same but similar to balanced. Some would also call this "floating."

By definition, a "balanced" line is balanced about its reference (normally ground). An "unbalanced" line is unbalanced with respect to its reference (again normally ground).

Ground makes a great noise drain is also quite available. It can be quite safe and sooner or later something in the system will be referenced to it. So, if everyone can't agree to the reference when interconnecting systems, you will create problems. So ground it is.

Steve

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