PCBgogo

I just designed a switching PSU that converts 120V AC to 42V DC, and is capable of 20amps.
Just realized I need to create a dual supply for an audio amp I am creating, so I want to convert my single supply design into a dual supply.
Every dual supply schematic I look at, the common ground is connected to the center tap of the transformer.
But my single supply PSU does not have a transformer on the high voltage side, so I'm just wondering,
where would the common ground point even be, and is it even possible to create a dual supply without a transformer?
Do I need to add a transformer, or a second bridge rectifier, for a dual supply? I was trying to draw something up with a second bridge, but I just don't see how I would do it.
Or could I just invert the voltage after the buck converter/capacitance multiplier?
I would add a transformer, if I could find one that can do 20 amps on the secondary, but they are all like $100, unless its only 12V.
On aliexpress they sell a +/-35V dual PSU for around $50 CAD, so I know its possibly to make it fairly cheap, but I'm just not seeing how right now.
So if anyone has a transformer recommendation it would be very much appreciated. It needs to be 120V(or 120/240) on the primary, and be at least 40/42v 20a on the secondary(or should it be 90, with a center tap?).
If anyone can push me in the right direction, that would also be greatly appreciated.
I just feel like I'm missing something, and I really don't want to have to buy the one on ali just to figure it out...
Oh and as for adding an inverted buck converter, how do I make an inverted capacitance multiplier with a mosfet, would I use a P-channel?
My single supply uses a darlington for its capacitance multiplier, but finding a BJT capable of 480 watts is tricky, so for the dual supply design, I'm going to change it to a mosfet.

Thanks for any help you can provide.123

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