The limitation of car
supply voltage (12V) forces to convert the voltages to higher in order to power audio amplifiers.
In fact the max audio power x speaker (with 4 ohm impedance) using 12V is (Vsupply+ - Vsupply-)^2/(8*impedance) 12^2/32 = 4.5Watts per channel, that is laughable...
For powering correctly an amplifier the best is to use a symmetric supply with a high voltage differential. for example +20 - -20 = 40Volts
in fact
40^2/32 = 50 Watts per channel that is respectable.
The
transformer must be of correct size in order to carry the power needed, on the net there are many charts showing the power in function of frequency and core size for a given topology. My transformer size is 33.5 mm lenght, 30.0 height and 13mm width with a cross section area of 1,25cm^2, good for powers around 150W at 50khz.
The windings , especially the primary must be heavy gauged, but instead of using a single wire it is better to use
multiple wires in parallel each insulated from the other except at the ends. This will reduce resistance increase due to skin effect. The primary and secondary windings are centertapped, this means that you have to wind 5 turns, centertap and 5 windings again. The same goes for the secondary, 10 turns, centertap and 10 turns again.