[sdiy] Audio mixers

[Richie Burnett]

> Another question I should have put in my first email.  If I am using a
> switch to remove a particular input from the mix, route it
> somewhere else
> etc, what is the proper way to handle the open ended wire
> when the switch
> is "off"?  Just leave it open?  Connect it to ground?  Thanks again!

If you are using the virtual-earth active mixer then every resistor you 
connect to the mixing bus actually increases the output noise of the op-amp. 
Whilst the gain from any single input to the output is not affected by 
adding more inputs, the noise gain of the op-amp goes up as you load the 
summing node with lots of resistors, even if each input is shorted firmly to 
ground!  So in this case it's best to leave un-used inputs floating rather 
than connect them to ground if you want the lowest noise figure possible.

Rod Elliot gives a more detailed explanation...

"Virtual earth mixers have an interesting characteristic that will seem 
strange at first. Even though the gain for a signal from each individual 
channel may be unity (a common approach), the circuit has a far greater gain 
for noise. This "noise gain" is created because all of the input (mixing) 
resistors are effectively in parallel. So while the signal gain for one 
channel may be unity, the noise gain is ...

  An = Rfb / ( Rmix / N )
  ... where An is noise gain, Rfb is the value of the feedback resistors, 
Rmix is the value of the mixing resistors and N is the number of channels.
For the 3 channel mixer shown, the noise gain is therefore 3, and this 
applies whenever the inputs are connected to a source. Noise gain is 
minimised by disconnecting all mixing resistors that are not being used. The 
signal gain is not affected when channels are connected or disconnected 
because of the virtual earth mixing scheme, and there are no clicks or pops 
provided there is no DC in any of the channels."

quoted from...

http://sound.westhost.com/articles/audio-mixing.htm

-Richie,