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:

enter image description here

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?

  • \$\begingroup\$ Can you specify more about what blew up? \$\endgroup\$
    – RobinSt
    Apr 7, 2019 at 11:25
  • 1
    \$\begingroup\$ WARNING you have reversed Live and Neutral in the above. Live is brown, Neutral is blue. \$\endgroup\$
    – user16324
    Apr 7, 2019 at 12:31
  • \$\begingroup\$ @RobinSt The amplifier PCB initially worked however after a minute the components begun to burn \$\endgroup\$ Apr 7, 2019 at 13:44
  • \$\begingroup\$ @BrianDrummond I was aware of this when I wrote the comment, however as described on the transformer (0.....240V 50-60Hz (Brown Blue)). Does this not imply that Brown is the Neutral? \$\endgroup\$ Apr 7, 2019 at 13:49
  • 1
    \$\begingroup\$ No. See EU colour code. \$\endgroup\$
    – user16324
    Apr 7, 2019 at 13:55

1 Answer 1


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.

  • \$\begingroup\$ Thank you for your answer, much appreciated. My mistake was to assume I could follow the instructions directly as I purchased both items from Velleman. I clearly should have confirmed the wiring of the transformer this before following blindly. \$\endgroup\$ Apr 7, 2019 at 13:41
  • \$\begingroup\$ How is heat transferred to the heatsink? is any thermal compound used? \$\endgroup\$ Apr 7, 2019 at 15:11
  • \$\begingroup\$ yes there is a small amount of thermal compound used, why do you ask? \$\endgroup\$ Apr 7, 2019 at 15:24

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