Symmetrical supply in cars

There is a problem if you want to use a car battery to power a circuit which requires a symmetrical supply voltage. Having abandoned the idea of mounting a second battery in your car for that purpose, take a serious look at the circuit discussed here. It converts the car battery voltage into symmetrical ±12 V or even ±15 V rails. Since the converter is able to supply a continuous output current of about 0.5 A, and a peak current of up to 1 A, it is eminently suited to powering control amplifiers and small power amplifiers.
Component list
Resistors:

R1;R4 = 1kΩ

R2 = 2kΩ2 2W

R3 = 10kΩ

Capacitors:

C1;C9;C10;C12;C14;C15;C17 = 220µF

C2;C5;C8;C11;C13;C16 = 100nF

C6 = 1µF MKT

C7 = 220nF

Inductors:

L1;L2;L3 = SFT10-30 or SFT1030

(40µH)(TDK)

Tr1 = SFT12-50 or SFT1240 (see text)

Semiconductors:

D1;D2;D3 = BYW29-100

IC1 = LT1070 (Linear Technology)

Miscellaneous:

K1 = 2-way PCB terminal block.

K2 = 3-way PCB terminal block.

F1 = fuse 3A (slow) w. PCB mount holder.

Metal case, e.g., Hammond 1590S, 110x82x44mm.

Insulation set (washer and bush) for IC1, D2, D3.