Input impedance, the higher the better or not?

[tingtong5]

Hi all,

I recenly built a simple tube based pre-amp with an input impedance of about 470K. I was wondering if higher is always better when it comes to input impedance or maybe not because of higher johnson noise from the resistor from input to ground ? Maybe I should change my input resistor from 470K to 47K. What do you think?

Regards,

Ronald

[ruffrecords]

Originally Posted by tingtong5

Hi all,

I recenly built a simple tube based pre-amp with an input impedance of about 470K. I was wondering if higher is always better when it comes to input impedance or maybe not because of higher johnson noise from the resistor from input to ground ? Maybe I should change my input resistor from 470K to 47K. What do you think?

Regards,

Ronald

The noise coming into the input depends on the parallel combination of the grid resistor and the output resistance of the source. Since you do not want to unduly attenuate the input signal the rule of thumb is to aim for the grid resistor to be at least ten times the value of the source resistance. This means that the parallel combination of the two is almost the same as the source resistance and so it is this that primarily determines the noise at the input, not the grid resistor.


Cheers

Ian