[sdiy] Audio mixers

Chris Muir cbm at well.com
Fri Jun 17 00:39:38 CEST 2011

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.

I think this article 
On Jun 16, 2011, at 2:30 PM, Richie Burnett wrote:

>> 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,
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Chris Muir                     | “Space is big. You just won’t believe how vastly, hugely, 
cbm at well.com              |  mind-bogglingly big it is. I mean, you may think it’s a long 
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