[LAU] XLR mixer -> TRS audio card connections

[Fons Adriaensen]

On Sat, Aug 15, 2009 at 02:04:22PM -0500, 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.

You don't need impedance matching. The only place where it
is still required for audio signals is when you're driving
long rented telephone lines form e.g. an OB van to a broad-
casting center. In that case you need to match both the output
and input to the line impedance. But probably nobody is still
doing that today, it's all digital.

Also when the input is balanced you won't need a transformer
except when the two units being connected are on separate
power systems and you could have a ground difference. 

> 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.

That's how all pro audio equipment has been designed for the
last 40 years or so. Output impedance < 100 ohm, inputs > 10k.
It's perfectly OK to connect the two. The high impedance ratio
means the output won't 'feel' the input, and you can split the
signal to more than one input without any gain change.

Ciao,

-- 
FA

Io lo dico sempre: l'Italia è troppo stretta e lunga.