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I bought an amplifier online, and it has three wires for AC input (12V). However, there is nothing on the board about which wire is GND and which is V+ and V-.

The other connectors are for the audio channels (it has 3 channels).

enter image description here

How can I use my multimeter to find out which wire is which?

Is the distinction between V+ and V- necessary when dealing with AC?

I noticed that two of the wires are connected, since my ohm-meter detected resistance between them. However, since this is an amplifier I would think the circuit isn't closed before I connect speakers?

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  • \$\begingroup\$ AC doesn't have V+ and V-. It has Hot, Neutral, and Ground \$\endgroup\$ – DerStrom8 Sep 24 '17 at 13:38
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    \$\begingroup\$ Steady on, @DerStrom8: that could be misinterpreted as saying that it's OK to connect directly to mains (which isn't the case). It looks more like a 12-0-12 AC transformer input, or similar. \$\endgroup\$ – Transistor Sep 24 '17 at 13:39
  • \$\begingroup\$ Those 4 diodes look like a bridge rectifier. check the traces and see which of the teminals connect to them. Those will be the AC in. \$\endgroup\$ – ratchet freak Sep 24 '17 at 13:39
  • \$\begingroup\$ @MadsPH: Add in the link for the datasheet of the board. Put it in your question - not in the comments. Note that the PCB is clearly marked AC-IN, not DC. \$\endgroup\$ – Transistor Sep 24 '17 at 13:40
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    \$\begingroup\$ No, it's not 'neutral' until grounded. As there are three terminals it's likely to use a centre-tapped transformer, hence my 12-0-12 comment. The centre would probably be grounded. \$\endgroup\$ – Transistor Sep 24 '17 at 13:57
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You are learning why we always recommend "no datasheet - no sale". It's a better way to live.

As discussed in the comments it looks like it is designed to be supplied by a centre-tapped transformer. You now need to figure out a suitable voltage.

  • Read the capacitor voltage rating from the capacitors.
  • Check the maximum input voltage specifications for the amplifier chip.

Pick the lower of the two above. The maximum AC input will be \$ \frac {V}{\sqrt 2} \$ since the rectified voltage will be \$ \sqrt 2 V_{AC} \$.

Finally, check the power rating and minimum speaker impedance from the chip datasheet and calculate what RMS output voltage is required to generate that power. DC voltage will be given by \$ \sqrt 2 V_{RMS} + a \; few \; volts \$ to allow for voltage drop across the amplifier. See how that matches the other calculations.

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  • \$\begingroup\$ Thank you very much for your help! Even though there was no datasheet, it did come with some recommendations for speakers and power supply. That said 12V AC. Given your pointers, I think that's reasonable. The chip is TDA2030A, which sets Vs to +/- 16V on the datasheet. The capacitors are rated for 25V. \$\endgroup\$ – MadsPH Sep 24 '17 at 14:35
  • \$\begingroup\$ That all sounds about right. I recommend you study the TDA datasheet to understand how it all works. \$\endgroup\$ – Transistor Sep 24 '17 at 14:45

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