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I'm designing a PCB in KiCad and I'm new to this so I have two doubts. The circuit is an audio amplifier.

1) I want to be able to put a signal generator at the input for measuring, and an audio jack for listening to real music, too. I came up with this:

enter image description here

The problem is that I don't know how to connect the jack - which pins go to ground and which one goes to the input. How is this done?

2) I have a similar problem but with the speaker. I want to be able to measure on a load resistance, but also I want to connect a speaker to the circuit to be used when the circuit is working.

enter image description here

But here, also, I don't know how to connect the speaker. How can this be done?

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  • \$\begingroup\$ For part 1) That jack socket symbol looks like a mono jack, and effectively has only has two connections. You can tell which is ground from the source by plugging a jack cable into it, and measuring the voltage between ground and the different metal parts of the jack plug, being careful not to short between parts of the plug with the multimeter probes. You can check the jack socket, if it's not obvious, by plugging a jack cable into it and checking for continuity between the parts of the jack plug, and the socket's pins. \$\endgroup\$
    – gbulmer
    Commented Sep 17, 2016 at 16:11

1 Answer 1

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enter image description here

Figure 1. Various jack arrangements.

  • J1 is wired for single input. When the jack is disconnected it shorts the input to ground to silence it.

  • J2 and 3 are wired in series to give priority to J2. Put a plug in J2 and it disconnects J3. Pull J2 out and J3 becomes effective or grounded if both plugs are out.

For the output you could consider the arrangement J4 which will connect the output to R1 if the jack is unplugged.

[From OP's comment:] At the output I'm not using a jack but a speaker (i.e. two terminals). I guess that the positive is connected to the output and the negative to ground, but I'm not sure. Is this correct?

Yes.

Strictly speaking audio doesn't have a + or - since it's an alternating current. However, for stereo amplification it is important to wire the speakers in phase so that they both push and pull together. Getting them backwards results in cancellation of the bass and a slightly strange void in the middle of the stereo image. For this reason the terminals are often marked + and - on each channel and the 2-core cable will have a marker or stripe on one of the cores.

Convention is that that if one leg of the speaker is connected to amplifier ground then that would be marked as -.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 2. Single ended and bridged amplifier setups.

Bridge-mode amplifiers often seen in car stereos use two amplifiers working in opposition on each channel to double the voltage output and get up to four times the output power. This is necessary because the power supply is limited to 12 to 14 V. In these cases neither input is connected to ground but + and - would still be used to indicate correct phasing.

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  • \$\begingroup\$ Thanks for your answer, it was very helpful. However, at the output I'm not using a jack but a speaker (i.e. two terminals). I guess that the positive is connected to the output and the negative to ground, but I'm not sure. Is this correct? \$\endgroup\$
    – Tendero
    Commented Sep 17, 2016 at 16:46
  • \$\begingroup\$ See d'update ... \$\endgroup\$
    – Transistor
    Commented Sep 17, 2016 at 17:07

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