[LAU] XLR mixer -> TRS audio card connections

Arnold Krille arnold at arnoldarts.de
Sat Aug 15 16:10:51 EDT 2009


On Saturday 15 August 2009 21:04:22 Brent Busby wrote:
> Is it wrong to use a regular cable (no transformer) to connect the XLR
> main and submix group outputs of a console to the TRS (balanced) inputs
> of an audio card, or should impedance matching be done in that case?
> It is quite common in music stores these days to find cables that are
> XLR male on one end and TRS male on the other.  (I'm currently using
> those on my mixer outputs.)  The cables *are* balanced, but they do not
> contain a transformer at all.
> The impedance of my console's main/submix outputs is rated as less than
> 75 ohms, but the input impedance of my audio card is 10k ohm.  This
> would seem to almost answer the question by itself, it weren't for the
> near impossibility of actually finding a matching transformer that's TRS
> and not TS on its 1/4" end.  I looked at a lot of them.  They're all
> made for hooking up guitars, amps, and mics, and they all seem to have
> an unbalanced plug opposite from the XLR end.

There is absolutely no need to do any transformation in that chain. Balanced 
to balanced is pretty good.

Afaik impedance-matching doesn't necessarily mean that all impedances have to 
be the same, they have to match. So if the output of the mixer has <75Ohm 
compared to the input impedance of the soundcard of 10kOhm, it means that 
almost no energy is lost at the output and all is "lost" at the input. And you 
want the energy of the signal to be lost at the input so you can use it. This 
is when you need the signal, not the power. For speakers (and for solar-
modules) you want the impedances to be about the same so the efficiency is high.

For more I have to get out my "physics for engineers"-book...

Have fun,

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