[LAU] XLR mixer -> TRS audio card connections

[philicorda]

On Sat, 2009-08-15 at 14:04 -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.
> 
> 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.

It's not a problem.
The convention nowadays is for low impedance outputs and high impedance
inputs. 

We are concerned with voltage, rather than power transfer here, so not
loading the output is a good thing.

http://en.wikipedia.org/wiki/Impedance_bridging

> 
> If there is indeed a need transformers on each XLR mixer output in this 
> instance, where can I get one that won't unbalance the connection in the 
> process?  Or is it fine to just use these common XLR->TRS cables that 
> don't have any?  The reason I started to investigate this is because I'm 
> not sure I'm not getting some of the "tone suck" you might associate 
> with a badly matched connection, and this seems a likely cause.
> 
> 
> Addendum:  On the realtime end of things, I'm now achievable a solid, 
> unbreakable 2ms.  I can't seem to do anything that causes an xrun. 

That's a tricky one. You could try switching to a USB soundcard, and
using a hub with multiple devices attached. Removing and adding USB
devices while recording then may help to increase the incidence of
xruns.

If that doesn't work, the only recourse may be to use a slower processor
and an older motherboard. Also, try to find the most obscure and least
Linux compatible PCI and video cards you can, and put as many of them in
the machine as possible.

> That's good at least!
>