[sdiy] Audio mixers

[David G. Dixon]

> I believe that he is talking about removing the summing 
> resistors from the sum node (aka mix point). I agree that in 
> an ideal world that might give the lowest noise, but it is 
> none too practical in many cases. You would have to run your 
> sum node around on potentially longish wires to hit all the switches.
> 
> Here's a thought experiment using a typical line mixer 
> without a master gain like he shows in his figure 4. the way 
> you get a system like this the quietest is generally to turn 
> all the channels down. What does that do in most circuits? It 
> grounds the resistor going to the sum node because the bottom 
> of the pot is at ground. The only thing grounding the input 
> with a switch does is make both sides of the level pot sit at ground.

But if you have, say, a three-input mixer on a filter module or something,
then the input will be floating as long as nothing is plugged into it.  If
something is plugged into it but it is turned all the way down, then it will
still source noise to the opamp.

It might be interesting to try plugging and unplugging things into a mixer
channel turned all the way down to see if there is an audible difference in
noise.