Yesterday I was building a headphone amplifier, and was looking for a differential voltage source. Something small, preferable a wall adapter. Any fix voltage in the interval +/-5V ... +/-15V would be just fine. Expected current 100...150 mA.
Most wall adapters from the scrap box were from old mobile phones, single voltage, 5V or less, with glued cases and no way to adjust the voltage. Still, there was one with a screw-fastened case, single voltage, 8.4V/500mA:
It has a 4 wires RJ9 female connector (like a LAN connector, but with 4 pins only), which was perfect, because I needed at least 3 wires, but preferably 4. Even more, the schematic was very hacker friendly:
All it needs to add a negative voltage was another rectifier circuit, like the one in red:
Even more, the PCB had an un-populated area in the lower voltage part, more then enough to add the 4 red components.
That was too nice to be true!
When something is too nice to be true, most of the time it's because it's not!
After the red part was added and everything was double checked, it was time to plug it in.
Does it worked? No.
Instead of +8.4 / -8.4V, the output was +8.4 / -25.2V. Exactly 3 times more negative voltage then expected.
Can you spot the mistake?
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