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I encountered a little tricky situation in one of my project. I have two audio signals, A and A+B on two different wires. I would like to know if there is a solution to electronically retrieve the B signal alone. Is there any IC or any kind of circuit that could allow me to do that kind of substract ?

Best regards,

Vincent

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  • \$\begingroup\$ Connect them to the two inputs of differential amplifier...You should get amplified version of B \$\endgroup\$
    – sarthak
    Commented Jan 23, 2019 at 18:27
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    \$\begingroup\$ I dislike "can I" questions because I have no clue what your level of skills is regarding electronics. There are many ways to add and subtract signals. Often an opamp is used. \$\endgroup\$
    – Bimpelrekkie
    Commented Jan 23, 2019 at 18:29
  • \$\begingroup\$ duckduckgo.com/… \$\endgroup\$
    – D.A.S.
    Commented Jan 23, 2019 at 18:30
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    \$\begingroup\$ In theory, yes, but in practice any difference in level or phase will cause "leakage" if uncompensated. And audio tends to be perceived logarithmically so even a small percentage of imperfection can be substantial. \$\endgroup\$
    – Chris Stratton
    Commented Jan 23, 2019 at 19:25
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    \$\begingroup\$ I tried a circuit with a differential amplifier some time ago And that's exactly the type of information you should have included in the question as that shows you did try something and also what skills you already have. Even if it didn't work, include the circuit, maybe only a small thing needed to be changed. \$\endgroup\$
    – Bimpelrekkie
    Commented Jan 23, 2019 at 19:44

4 Answers 4

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You've been given 3-amp and 2-amp solutions. Let me present the 1-amp solution, the differential amp.

schematic

simulate this circuit – Schematic created using CircuitLab

This assumes equal value resistors. It has a lower input impedance than a 3-amp circuit, but that's probably not an issue for you.

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  • \$\begingroup\$ This is why they are called operational amplifiers (op-amp), because they are used to do mathematical operations. \$\endgroup\$
    – Joe Mac
    Commented Jan 24, 2019 at 0:13
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A pair of inverting amplifiers will do the job.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. An inverting subtractor circuit. Note that power supply decoupling capacitors are not shown.

  • OA1 inverts the A signal to give -A on the output.
  • OA2 inverts the sum of the two signals fed into it so the output is -(-A + (A + B)) = -B.
  • To allow trimming of levels to get maximum elimination of the A signal replace R5 with a 4k7 in series with a 10k pot and trim until the A signal is at a minimum.

Note that both A and B are inverted so their relative phase will be correct even though inverted from the original. For audio this should be satisfactory.

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I have two audio signals, A and A+B on two different wires. I would like to know if there is a solution to electronically retrieve the B signal alone.

You have a computer so download a free wave editor and input the two audio signals to your PC and save them as a stereo wave file. Next open the wave editor (I use Wavelab but it isn't free) and then open the wave file and use the tools you normally get to manipulate audio.

The easiest tool is one that inverts the A channel. Next, play the stereo file (with A inverted) as a mono file and you'll get B coming over your speakers.

Is there any IC or any kind of circuit that could allow me to do that kind of substract ?

Well you don't need an IC other than the ones in your PC.

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  • \$\begingroup\$ Yeah but i specifically asked for an electronic oriented answer because I won't embed a computer in my project. But thanks. \$\endgroup\$
    – Vincent Perez
    Commented Jan 23, 2019 at 21:07
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    \$\begingroup\$ You did say “any kind of circuit” and although this immediately implies a circuit it doesn’t disclude a system that contains a circuit of some kind. I just wanted to give wider options. Think laterally I was told decades ago. @VincentPerez \$\endgroup\$
    – Andy aka
    Commented Jan 23, 2019 at 22:05
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An instrumentation amplifier would do the job, use a gain of one and you'll be able to subtract B from A+B

If you put A+B on the positive terminal and A on the negative terminal you'll get A+B-A=B. Keep in mind that you need to make sure that the instrumentation amplifier doesn't distort your signal and that you keep the signals outside of the common mode range of the amplifier.

enter image description here Source: https://hackaday.com/2016/03/18/beyond-measure-instrumentation-amplifiers/

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