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In our project studio, we have several pieces of gear that use 9 volt AC wall-wart type power supplies. (When you have 16+ wall-warts, I think the term "infestation" is applicable).

I thought it logical to assume that by purchasing a large current capacity transformer, I could replace the power supplies. I tried an 18VAC center-tapped transformer, and balanced the loads between the legs of the secondary. The hum rendered the project useless.

I thought at first that the hum was induced by the transformer field, so I move it to 20 feet away, with no improvement.

Any ideas on how I can make this work?

Is the transformer actually center tapped (only three conductors from the secondary) or is it dual secondary (four conductors on the secondary side)? I don't think you could do what you're describing with a center tapped single secondary because you'd get a ground loop between the center tap and the 'bottom' side of the lower secondary.

If it is a dual secondary transformer, I'd check that the secondaries are connected in phase with each other. It might not make any difference depending on how the equipment you're connecting to it works, but it certainly can't hurt. Dual secondaries in parallel should work fine.

The transformer is center-tapped... Never considered the ground loop aspect. I should be able to verify that theory if I hooked up some of the units to just one leg of the secondary. If no hum, then your analysis would be confirmed.

You would think that someone would manufacture a power supply for these situations. If one is out there, I haven't been able to find it.

is this what you want?

http://www.geofex.com/Article_Folders/Spyder/spyder.htm

No, the outputs of that are 9V DC, not 9V AC.

oh, well whatever.

look on digikey for these part numbers:


MT7269-ND
MT7270-ND
MT7271-ND
MT7272-ND

It's what you need, depending on current requirements.