[sdiy] Request brain dump on balanced lines

[Magnus Danielson]

From: Grant Richter <grichter at asapnet.net>
Subject: [sdiy] Request brain dump on balanced lines
Date: Fri, 11 Oct 2002 15:09:48 -0500

Hi Grant!

> Can someone familiar with balanced mixing console lines summarize the
> information?
> 
> What I need to know is typical voltage levels and impedance's for TRS jacks
> and XLR connectors.

The reference level in broadcasting is +4 dBm. However, you can see higher
levels than this.

The standard way to wire XLRs is

Screen	Chassi
1	Signal Ground
2	Signal "Hot"/+
3	Signal "Cold"/-

There is a broken way to wire it, but I won't mention it. Please note that
Screen on the XLR is usually not wired to the shield of the cable, it is
infact a bad idea.

Impedance is _said_ to be 600 Ohm nominally. In practice you see a much more
spread variation. For instance, 600 Ohm are quite unnatural for cables, they
are much less. Normal twisted pair (such as for Ethernet) is around
100-120 Ohm. Seeing input impedances of 10k Ohm is not unusual.

Actually, impedence in the audio-range is nothing near as important as in
higher frequencies (say 1 GHz). Up here 600 Ohm impedence is almost as good as
a full open-curcuit!

> I'm trying to figure out a nice (i.e. cheap) way to interface +/- 5 volt
> single ended synth signals to your typical mixing console balanced inputs.

My recommendation is that you look at Rane's excelent website. They have a
bunch of nice app-notes online on exactly that.

> Is a single ended to balanced transformer a good way to go, or is an active
> differential output better?

The key issue is how large common mode voltage you have to remove on the
balanced side. I see no reason why a good balanced setup with active solutions
can't be done. Naturally, Analog Devices with their SSM diff-pair components
argue for their stuff. Also, transformer people like Lundahl
(http://www.lundahl.se/) naturaly argue for their products (also, Lundahl is
a good selection if you have to, or really want, use transformers).

> I get asked about this all the time and don't really have a good answer.
> 
> Thank you for any information or help.

I hope you got kicked the right way. Don't fear to ask more...

Hunting hum and noise out of a PAs is one of the things I've actually done, and
with some succsess I might add.

Cheers,
Magnus