Summing resistors in active balanced summing ¿how low?

[JAY X]

Hi Friends!

What is the lowest safe value input resistor to have in a balanced summing amplifier? NE5532, OP275, etc..

I used 22k resistors in feedback and inputs, and noise level when summing 8 inputs is quite high and noticeable. The more inputs, the more noise... !

Is there a way to calculate the noise drop when using lower value resistors? for instance 15k or 10k. Because there should be a safe value to interface to the output of preamps or audio interfaces.

Another important question is about grounding and noise. ¿The better the ground scheme/traces the lower the noise?

Thank you very much,
JAY X

 

[ruffrecords]

You will always get more noise with more sources because in active summing the noise gain is equal to the number of sources.

I am not convinced that bus noise is dominated by bus resistor noise which is usually  much lower than the noise from the preamps feeding it. However, if you want to reduce it by reducing bus resistor values then if you halve the resistor values the noise will go down by 3dB. If you make the resisotr value 10 times lower the noise will go down by 10dB

The noise in a 22K resistor is just a little shy of -114dBu. Since the noise from the preamp driving it is hardly likely to be better than -100dBu the resistor makes no real contribution to the overall noise.

Cheers

Ian