Hello, knowledge!

I studied different microphone preamp circuits, all with same topology that you can see in following (simplified) picture. I noticed that they are all about the same, having Rin = Rfb. This gives a minimum gain of 2 given the gain formula A=1+(Rin+Rfb)/Rfb, and a max gain determined by the setting of the potentiometer.

I noticed that some are using 10k, 6k7 or 20k resistors sets.

What difference does it makes? Different input/output impedances? Noise? How?



Also, I studied a mixer schematics that uses many opamps stages (filter, signal routing/switching, buffers, phase reverse, EQ, etc) and noticed that most opamps are followed by a DC blocking cap and a 47k resistor to ground. I'm ok with the cap, but what is the 47k for? opamp loading? additional DC filtering?

Thanks!

 

ooops.
Gain formula should read: A=1+Rfb/Rin

And I guess Rin should be given a better name (RShunt?)
sorry for that.

 

Personally I would strive to use as few DC blocking caps as possible between mixer stages. If you're talking about inter-stage DC blocking, the resistor is probably to provide a DC path for the next stage's input amp.

 

For any given op amp, the resistances on the input pins do have an effect on both output offset and noise performance. The data sheet on low noise amplifiers usually give the designer an idea of where the sweet spot is. As for the coupling cap - usually used to isolate any dc offset from the next stage. To be any more specific would require me seeing the whole circuit in question.