[LAU] OT(ish): Connecting balanced out to line in

[Arnold Krille]

On Wednesday 08 December 2010 18:00:34 linuxdsp wrote:
> James Stone wrote:
> > On Wed, Dec 8, 2010 at 3:49 PM, Gabriel M. Beddingfield
> > 
> > <gabrbedd at gmail.com> wrote:
> >> On Wed, 8 Dec 2010, James Stone wrote:
> >>> I guess in hardware?
> >>> 
> >>> Thinking more about it, do I need to plug the DI out into a preamp
> >>> before it goes into the line in for the soundcard? (the instrument is
> >>> a bass guitar unamplified).
> >> 
> >> If you can, just skip the DI and plug the guitar straight into the sound
> >> card's line-in.
> > 
> > I've tried that before, but (at least with an electric (non-bass)),
> > the sound was rather woolly and lacking in treble - I though due to
> > the mix of Hi-Z and line level input.
> 
> If you are connecting a guitar to a soundcard input you will almost
> certainly need some kind of DI box to provide the necessary high
> impedance input for the guitar otherwise you will get a dull "woolly"
> sound - as you describe.
> 
> A balanced line has three conductors instead of the usual two.  These
> are the screen, and the two signal wires.  The signal wires carry the
> audio signal, but in opposite polarity.
> 
> In a conventional balanced input stage, the two signal inputs are
> subtracted from each other - and being of opposite polarity, the result
> is the original audio.  However, the important advantage of this method
> is that any noise or interference induced on the wire(s) will be of the
> same polarity on both signal conductors (in theory) and so will be
> cancelled out (this is what common-mode rejection is all about).
> 
> You can get a signal for your single-ended input to the soundcard
> between either signal phase and the screen.  To avoid any phase
> inversions, connect the positive signal out from the DI box (XLR pin 2)
> to the signal in on your sound card input (Tip of the jack connector),
> and the screen (XLR pin 1 ) to the ground on the soundcard input (Body
> of the jack connector).
> 
> This is not the ideal way to convert between balanced and unbalanced
> signals, but it will provide a working solution.  There's some more info
> here:
> 
> http://www.rane.com/note110.html
> 
> As for whether the signal level will need more amplification, that
> depends on the DI box.  If you can feed a line level mixer input, then
> probably not, but try it and see.

If the di-box output is transformer-based, the level and signal-quality will 
be the same regardless whether you connect it to a balanced input or an 
unbalanced one. The key is that normally the transformer would output a 
voltage-difference between hot and cold signal. if you connect cold to the 
ground of the receiver, the transformer will create the same voltage between 
your ground and hot signal. Even if the ground of the receiver is in fact at 
+60V potential it will create its audio signal with respect to that. This is 
the main reason to use transformer-based outputs in devices and di-boxes.

Have fun,

Arnold
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