Electronic – Mains transformer blew up amplifier, incorrect description in wiring instructions


I am assembling a power amplifier from a kit I purchased from Velleman (Module: VM100). With the amplifier I purchased this mains toroidal transformer – .

The amplifier came with these instructions:

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Here is a photo of the PCB, you can see the male connectors are labelled as such (the manual states Y: yellow, R: Red, B: Blue, G: Grey):

enter image description here

  • Yellow : 25-30V AC
  • Red : AC-0
  • Grey : 25-30V AC-0
  • Blue : AC

I connected the transformer as described, however when I powered up the transformer the amp blew up. I suspect this is because I have connected the transformer to the amplifier incorrectly, and on closer inspection of the Transformer I found the following wire labelling:

enter image description here


  • 0…..240V 50-60Hz (Brown Blue)


  • 0…..30V-2A (Red Yellow)
  • 0…..30V-2A (Black Grey)

If I am reading this correctly I should have connected this transformer to the PCB is as follows:

Connect the following wires to mains:

  • Brown = 0V (Neutral wire)
  • Blue = 240V (Live wire)

And the following to the PCB:

  • Red = AC-0
  • Yellow = 25-30V AC
  • Grey = 25-30V AC-0
  • Black = AC (not Blue = AC)

Is this new reconfiguration correct?

Best Answer

You are correct in what you have determined, the connections should be wired as you have described and not per the documentation. The only exception would be to use brown for live and blue for neutral to conform with wiring standards.

The transformer you are using is different from the one they have picture/documented, in that the black and blue wires have swapped purposes.

As a word of caution to others finding this question: the manual does state to be careful of different colour schemes.

Warning in the manual

The net result is you will have connected mains directly to the amplifier, most likely destroying some or all of the circuit.