Simple power supply is bugging me. Help!

[brett]

I've got an AC (240V) to AC (24V) wall-wart that I'm converting to a bipolar supply (+/- 12 to 16VDC)
The rectifier is one of those bridge KBL04 (??) types, followed by 2 x 2200uF caps.  (output here = 35V)
Then, to generate a "ground" or "earth", I have 2 x 1000uF caps in series between the rails. 
However, the junction of the caps is nowhere near mid-point of the rails.  It is 10V below the + and 25V above the -. 
When I use this supply for a little amp, and crank it, the rails both sag (to +6 and -19V).

So, should a supply like this normally balance itself just by the action of the caps between the rails?
The "earth" is also connected to my signal generator (wall-wart powered) and scope (240V with true earth).

What am I missing?? (besides the brains to work this out)

[Sir H C]

The caps are an AC only path so any dc current can start to unbalance your arrangement.  You need some path to that center point.

To use a wall wart to create such a supply, the usual technique is to have one side be the ground, and then the other line in to go to + or - for the two rails.  This requires 2 diodes, but since you charge the caps at 1/2 the rate (50Hz in Europe IIRC), you will have to make them bigger.

[Paul Perry (Frostwave)]

Sir H.C. is right.
I'd just go half wave on each side.
You can still use the bridge, but half of it won't be connected.
All my +-15v rail boxes are run from half wave rectifiers, followed by regulators.
Your only problem will be that there is a lot of voltage being dropped across the regulators, which will limit the amount of current you can draw.

[brett]

Thanks
I'll try two 1N4004s straight out of the supply, then create a ground at the junction of 2 x 1k resistors tied from rail to rail.  Some smoothing and everthing should be sweet.
(I tried the DC flow thing with the 2 x 1ks  in the system above, but it didn't give much improvement.)
Regulators aren't needed for the power section, just +/-9V for the pre-amp and driver (a TL072).