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I am trying to sum three audio signals using a non-inverting summing amplifier as shown below. If I want to bypass one of those signals, what is the correct way of doing that (let's say with a switch)? Connecting the route to ground as shown?

I assume just disconnecting the signal from the circuit would affect the amplification of the other two, right?

schematic

simulate this circuit – Schematic created using CircuitLab

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Yes, not grounding a disconnected input will change the gain of the other two. The gain of the amplifier itself will remain 3, but the other two signals will be less attenuated into the amplifier.

Note that using a inverting summing amp gets around this issue. The gain from each input to the amplifier output remains the same whether other inputs are driven, disconnected (left open), or shorted to ground.

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If you consider the case when one of the input signals is silent its voltage will be zero. This will be exactly the same as connecting that input to ground. The result: no change in amplification of the other signals.

The output of your opamp will be given by the formula

Vout=(1 + R2/R1) (Signal1 + Signal2 + Signal3) / 3

What this is telling us is that the three signals are summed and divided by three. (Consider the case where two of the signals are silent and at zero volts. The one remaining signal is fed in via a 24 kΩ resistor with two others pulling it down by 12 kΩ resulting in 1/3 the signal level at the non-inverting input.)

To compensate for the division by three, the gain of the amplifier has been set to three also by

1 + R2 / R1 = 1 + 24 / 12 = 1 + 2 = 3
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Connect unused inputs to ground.

If you leave it as an open circuit then it can behave as an antenna and pickup noise.

If you look at the formula for output voltage in a Summing Amplifier you will see that connecting unused inputs to ground won't change the amplification of other inputs.

The amplification is not changed if you connect/disconnect to ground. It just reduces the opportunity for those inputs to acquire noise.

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Good question. What you could do is, instead of switches use FETs and control them digitally.

About the gain question. The gain won't change if the ratio of the feedback and input resistance is not changed.

Over to you. Try this practically.

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  • \$\begingroup\$ What I meant specifically, is it correct to bypass by connecting the line in question with ground like shown? \$\endgroup\$ – jilski Jan 1 '16 at 14:18
  • \$\begingroup\$ This mostly answers a different question, and at best skirts around what was actually asked. Disconnecting without ground one of the inputs does change the gain of the other two. The gain around the opamp doesn't change, which is 3 in this case, but the other signals are less attenuated on the way in. \$\endgroup\$ – Olin Lathrop Jan 1 '16 at 15:15

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