Use the circuit shown we can reverse or flip the switch amplification without inversion. Voltage gain depending on the position of the switch. This may be +1 or -1. Changing circuit has two versions, A and B. I personally prefer B
type, because there is no non-inverting input voltage level is uncertain at the moment. In one version, when the switch is flipped, with the non-inverting input is temporary floating, which may lead to unpredictable voltage indu1ced through the wiring of the switch, so that the hard click or murmur. This phase inverter circuit provides a polarity control is useful, for example, many channel audio power amplifier supplied (or queen Only) This function flips stage situation with a speaker wiring or installation error, because it is easier push a switch, instead of re-connecting cable.