[sdiy] Audio mixers

[ASSI]

On Thursday 16 June 2011, Richie Burnett wrote:
> 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."

I'm sure Mr. Elliot knows this as he talks a bit about different noise 
sources later on, but that statement is only true in a literal sense if all 
inputs see the same noise (noise is fully correlated, like when you have a 
noisy ground).  If each input sees it's own version of fully non-correlated 
noise, the noise gain increases geometrically, e.g with sqrt(N).  In a real 
system you will have a mix of uncorrelated and correlated noise sources, so 
you'll end up somewhere inbetween these two cases.  For instance, ground 
noise is correlated, but thermal resistor noise is not.  The conclusion that 
it is optimal to disconnect unused input resistors from the summing node is 
always valid however.


Achim.
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