My new "Big A Power Supply" has transformer hum

[Dr Ron]

I purchased a used 12 volt / 2 amp power supply and rewired it to have 9 volt regulated outputs for effects. I named it the "Big A" because it can supply a lot of juice! It works great, but the power transformer makes an audible hum that seems loud enough to shake the metal box.

The schematic is pretty straightforward - AC into the transformer, with a diode pair at the output going through a current limiter to the 12V+, and the center taps to the 12V-. There are two big electrolytic caps across the 12VDC terminals. I removed a 390 ohm resistor that was also across them. These components were already there. I added a current limiter to a 9V regulator and 480µ cap, plus three output jacks.

I'm wondering what's up with that hum.
Is it normal / nothing to worry about?

[Gus]

First thing to do is to find out is it caused by the transformer or is it caused by the circuit connected to the transformer?

[maarten]

Sounds like the metal core is vibrating: make sure you tighten all the bolts and nuts, of both the iron core of the transformer itself (if not welded) and of the connection to the the box in which you have it installed.

Maarten

[Thomeeque]

The schematic is pretty straightforward - AC into the transformer, with a diode pair at the output going through a current limiter to the 12V+, and the center taps to the 12V-. There are two big electrolytic caps across the 12VDC terminals. I removed a 390 ohm resistor that was also across them. These components were already there. I added a current limiter to a 9V regulator and 480µ cap, plus three output jacks.

What exactly do you mean by those "current limiters"? Those two fuses?

[Thomeeque]

I'm wondering what's up with that hum.
Is it normal / nothing to worry about?

You could try to measure current sucked from mains with no load at U2 (e.g. bridge SW1 in OFF state by ampermeter), if there are no some leaks..? Or just check, if the transformer gets hot..