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I'm powering (hoping to power?) two boards from one 12V AC source. One is a preamp requiring 12V AC in (easy so far) and the other is a scavenged audio board requiring +12/-15V DC.

I'm using the circuit below to double + rectify the AC into ±18VDC, then into a 7812 and 7915 which measure the expected +12/-15V at output.

The preamp powers up fine, and the audio board powers up fine, but when I connect them via the audio board's audio output, I get sparks and smoke.

Any ideas? Is it because I'm using the AC leg as ground for the DC circuit, and so the DC voltages are fluctuating with ground but staying the same 'distance' from ground, so they meter correctly?

schematic

simulate this circuit – Schematic created using CircuitLab

Possibly-not-100%-correct schematic for the preamp: Preamp schematic

Pic of the preamp: Preamp photo

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  • \$\begingroup\$ Since you put the capacitors, out1 and out2 will stay at a constant voltage (assuming an infinite load impedance) with respect to GND. \$\endgroup\$
    – next-hack
    Commented Oct 11, 2017 at 19:59
  • \$\begingroup\$ Yep, out1 and 2 are meant to be DC, I'm wondering if there's a problem connecting an AC circuit powered from AC1/2 to a DC circuit powered by Out1/2 + GND \$\endgroup\$
    – buildsucceeded
    Commented Oct 11, 2017 at 20:04
  • \$\begingroup\$ Ah, now I understood your question. Can you show the circuit diagram of the preamp? Has it got another insulation transformer? \$\endgroup\$
    – next-hack
    Commented Oct 11, 2017 at 20:06
  • \$\begingroup\$ @next-hack done! \$\endgroup\$
    – buildsucceeded
    Commented Oct 11, 2017 at 20:09
  • \$\begingroup\$ sorry I was traveling, however I agree with Peter Bennet's answer :) \$\endgroup\$
    – next-hack
    Commented Oct 11, 2017 at 21:22

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One side of the AC source is connected to the preamp ground. If you connect the other side of the AC source to ground in your +/- power supply, you will short the AC source when you connect the grounds together.

If you swap the two AC wires on either the preamp or your +/- supply, so that the same wire from the AC source is connected to both grounds, all should be well - but measure the voltage between the two grounds to be sure!

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