I'm designing an audio mixer and currently i'm working on a cue section. The whole point is that i have 2 or more channels and want to mix them in summing mixer. You can turn off the channels you don't want to hear - simple switch -> ground the channel you don't want to hear or connect it to the summing resistor.

When i turn off the channel the summing resistor is floating. Floating input of an op amp is usually not a good idea. Putting relays that ground the floating inputs when unused sounds like a lot of hassle. Any other ideas? Or maybe i can just leave this way? But i'm afraid that floating inputs will pick up a lot of noise.


  • 1
    \$\begingroup\$ What is the signal source that becomes hard-grounded when the switch is in the shows position? what about turning the switch to ground to inputs of the summing amplifier? Depending on the (unknown) signal source I would consider to use separate coupling capacitors for the two inputs... \$\endgroup\$
    – aschipfl
    Jun 12, 2019 at 8:33

1 Answer 1


Floating input of an op amp is usually not a good idea.

Yes, but you are not causing the "real" input to float...

Open circuiting the 47 kohm resistor that feeds the virtual earth summing input is not going to be a problem. The 47 kohm feedback resistor shown in your diagram will keep the op-amp biased at the right point.

Shorting to ground the signal that feeds the 47 kohm input resistor may not be such a good idea and you can probably get away with a switch that just disconnects the signal from the 47 kohm.

  • \$\begingroup\$ thank you very much! \$\endgroup\$
    – cubix
    Jun 12, 2019 at 8:41
  • \$\begingroup\$ Can you please explain why shorting the signal to ground is a bad idea in this case? I was thinking about connecting the relay in shunt mode, it will short the signal to ground. Is that really worse than leaving the open circuit? \$\endgroup\$
    – cubix
    Jun 19, 2019 at 17:28
  • \$\begingroup\$ If the input signal comes from a passive signal then it should be okay. If the signal comes from an amplifier or other active circuit then you might do it damage. \$\endgroup\$
    – Andy aka
    Jun 19, 2019 at 18:06

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