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Old 3rd January 2011, 05:35 PM   #1
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Default Wall Adapter - Dual Unregulated Output

Hello,
I'm looking for AC wall adapter that has 2 outputs or center tap, so that I would be able to make +/-12 or +/-15V on the board. Power is around 8-15W. It should be like wall transformer with 2 output windings. No internal stabilization, does not matter if it's AC-DC or AC-AC. Could not find any of those in Digi-Key or Mouser. Have you seen something like this from reliable sources?
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Old 3rd January 2011, 05:43 PM   #2
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Never seen one myself. If you want to use a wall wart in split supply projects you either need to use a halfwave voltage doubler scheme with an AC adapter, or a DC adapter and supply splitter.
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Old 3rd January 2011, 06:00 PM   #3
gruni is offline gruni  Germany
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they definitely exist. ive got a printer from hp, it has a wallwart supply marked 0/16/32 but of course it can be used 16/0/16. its about 30 watts i think.
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Old 3rd January 2011, 06:05 PM   #4
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I'm splitting one 18 VDC wall transformer (car race salvage) with two 8V 1n5344 diodes, 2 each 22 ohm current limiters in, suitable capacitors, inside my mixer. The signal ground is fake, the middle of the two zeners. Works great, really cut the hum in my mixer by getting the power switch and unshielded transformer outside the steel box. Lots of HP printer 30V one voltage transfomers at my local charity resale shop. Watch the light ones, they are switchers and might howl into audio projects.
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Old 3rd January 2011, 07:40 PM   #5
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Quote:
Originally Posted by indianajo View Post
I'm splitting one 18 VDC wall transformer (car race salvage) with two 8V 1n5344 diodes, 2 each 22 ohm current limiters in, suitable capacitors, inside my mixer. The signal ground is fake, the middle of the two zeners. Works great, really cut the hum in my mixer by getting the power switch and unshielded transformer outside the steel box. Lots of HP printer 30V one voltage transfomers at my local charity resale shop. Watch the light ones, they are switchers and might howl into audio projects.
That's the reason why I prefer to take AC out of the box. I also don't want to increase the size of PCB (for PCB mountable transformer) or the size of the enclosure.
But then, I'll have to create virtual ground and balance both parts in case negative and positive will have different current. 2 Zener solution will work, but with relatively small current. Opamp with 2 transistors can handle this better, but in any case, any current disbalance will cause one of transistors to handle difference current and this is not very good thermally.
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Old 3rd January 2011, 08:27 PM   #6
LAJ is offline LAJ  United States
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Default I got one from...

an old Bell 300 modem. It was 12-0-12 VAC. I now use it as a power supply for an unbalanced to balanced converter (using a 5532 op amp).
But since you probably can't find that anymore, I'd get a DC style wall wart and bust it open. It'll probably have a full wave (non-bridge) rectifier setup. Separate where the rectifiers combine, turn one around around, and run the 3 wires out.
To open a wall-wart, get a small ball peen hammer and tap the seam all the way around on a concrete floor. It'll eventually open up. Reglue it with airplane cement.
Good luck.
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