[sdiy] Magic 22K

[Tony Clark]

   Hi Lincoln,

> On the subject of noise Im sure someone can answer this: I notice that
> virtual earth mixing amps almost always use 22K input resistors from the mix
> busses. When using a simple opamp circuit the feedback resistor will be 22K
> also. But suppose you try and squeeze some gain out of this circuit by
> increasing the feedback resistor to 100K or try and increase the input
> impedance because you want to make a mixer that can accept (non buss)
> signals of a higher impedance and what happens: A lot more noise than you
> bargained for in my experience.
> For some reason mixer circuits prefer unity gain and additional gain stages
> are added elsewhere. Can someone explain this please? What's magic about
> 22K?

   There isn't anything magical about 22K.  What you are witnessing is 
probably nothing more than some engineer's choice in input resistance 
values.  What you are probably finding is the idea that input resistances 
should always be equal.  22K on the positive input, 22K on the negative 
input.
   Okay, so you have unity gain, and you want to increase it by putting a 
100K in the feedback loop.  Well, what you are doing is increasing the 
gain by over a factor of 4.  Guess what that does to your noise levels?  
It increases by a factor of 4 as well!  However!  It may not be noise 
from the amplifier itself, but just from your input signal!
   Anyone with a studio deals with these issues.  Certain pieces of gear 
just send loads of noise out and cause problems at the mixer.  The best 
thing you can do is to run your gear at the highest possible level out 
and use a graphic (or parametric) EQ to cut the noise out.  You may also 
need to gate the output of the device when it's not making (useful) 
noise, depending on how bad it is.

   Hope that helps,

   Tony

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