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Need a bit of community validation to see that mapping of the following circuit schematic:

enter image description here

to this hand-drawn wiring diagram seems correct, and there are no obvious mistakes:

enter image description here

Also, does having 2 (or 3 inputs) instead of the 4 shown in schematic impact the circuit characteristics in any way ?

Similarly, does having more than 4 inputs instead, impact this circuit characteristics in any way ?

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    \$\begingroup\$ Input coupling caps. \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Dec 31, 2015 at 5:24
  • \$\begingroup\$ Also, you probably want a rail splitter instead of tapping in between two batteries. \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Dec 31, 2015 at 5:24
  • \$\begingroup\$ @IgnacioVazquez-Abrams, argh... indeed, I forgot to draw the input coupling caps in the wiring diagram. As for the rail splitter, do you mean "precision rail splitter" IC's ? \$\endgroup\$
    – bdutta74
    Dec 31, 2015 at 5:31
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    \$\begingroup\$ Not seriously, but the batteries could become unbalanced with time meaning that the pair will measure within spec but each will have different voltages. Stick to primary (non-rechargable) batteries if you can't find another op amp for the splitter. \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Dec 31, 2015 at 5:51
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    \$\begingroup\$ An observation : those very high resistance values make great noise sources; I hope you can hear the input signals above the white noise... These are not microphone level inputs, but after the mic gain stages, right? \$\endgroup\$
    – user16324
    Dec 31, 2015 at 11:25

1 Answer 1

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The major issue is that the wiring diagram omits the input coupling caps in the schematic. The previous stage may have an output coupling cap built-in, but you should not depend on that.

Another issue, which is both in the schematic and the wiring diagram, is that the ground connection is tapped off between the two batteries. This could unbalance the batteries, which in the case of secondary (rechargable) batteries could lead to irreversible damage to one of them. I recommend that you switch to using a rail splitter instead; a precision splitter IC would be the easiest solution, but even a simple voltage divider/voltage follower combination would suffice. Failing that, don't use rechargable batteries in this circuit.

As for the number of connections, the input resistors attenuate the input signals and the feedback resistor causes the op amp to amplify them back up to their original combined volume. This scheme is called a summing amplifier since its output is simply the sum of all inputs. The number of connections should not matter provided the input and feedback resistors are all of very similar values.

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  • \$\begingroup\$ Great that you added the reference to 'summing amplifier'. It clears up my remaining doubt. \$\endgroup\$
    – bdutta74
    Dec 31, 2015 at 7:33

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