I have recently bought a ready-built DAC with WM8741 and CS8421 uppsampling from eBay, and gave about $ 100USD. What was missing was an transformer; 2 * 15 volts and 2 * 9 Volt and a box to build it in.
I thought at first that I could fix my own toroidal transformer where I found the correct voltages from winding. Tested through with several transformers, but what happened was that the four rectifier diodes BYV27 to 9 Voltage was so hot that it began to smoke and I had to replace a couple of them with new ones. It is also heard a buzzing sound from the transformer.
But the DAC one does work, so I get sound from it when it is connected.
I have now got a recommended R-Core transformer. However, there was no difference, the same thing happened.
What I found when I looked at the card, was that the negative side of rectifier diodes for 9 Volt was connected to ground, which was not the case of 15 Volt. The negative side only connected to capacitors, but not directly to ground.
I tried then to cut the lead of the 9 volts to ground, and now did not diodes get so hot. However, I get no sound now, even though the LEDs are lit and all the ICs have power.
Only when I connect the cutted line to ground, I get sound while the rectifier diodes get hot and the transformer starts to growl.
Can the DAC card to be nonfunctional, or have I possibly done something wrong when I connect the transformer? Have connected all four cables from the 0 Volt cables at the transformer to 0 volts on the card, and each for 15, 15, 9, 9 Volt. That is, I have have taken a cable from each power and pairing them together to get 0 volts. Is´nt this the right way? I tried first to connect only 15 Volts 0 Volts cables, but that did not get 9 Volt to work on the board.