+/-12V from car battery

Many audio circuits need a symmetric power supply of ±12 V. If such a circuit is to be used in a car, the -12 V line must be somehow derived from tbe + 12 V car battery. This is done most conveniently by a switch-mode supply, in which the inversion is achieved by capacitors or inductors. When a fair power output is needed, inductors are normally used. Such supplies are often fairly complex: the present one is not nearly so. It can provide an output current of 100 mA and even up to 300 mA if a ripple greater than the nominal 50 mV is acceptable. If necessary, this larger ripple can be countered to an extent by giving C2 a larger value.