from the signal conditioning circuit 98 are fed respectively to the inputs of the class a amplifiers 96 97 via the respective coupling capacitors 99 100 for amplification a portion of the signal current passed via the capacitor 110 is rectified by the diode 118 its output being smoothed in the long time constant circuit comprising by the resistor 116 and capacitor 117 thereby providing a slowly varying control signal which follows the peak input audio signals this control signal is applied to the base of the transistor comprising the variable impedance circuit 114 resulting in the d c current levels in the respective transistors 79 80 and their respective associated voice coils 46 48 to change upwardly and downwardly as the peaks of the signals sought to be amplified vary upwardly and downwardly respectively as a result high d c currents flow in the power transistors 79 80 and the voice coils 46 48 only when needed to avoid clipping the transistors 79 80 virtually being off when no signal is to be amplified however when the switch 115 is in its alternate position or upper position as shown in fig4 the d c current levels would not be changed automatically but would be determined by the setting of the second potentiometer 128 which changes as a result of changes in the setting of the first volume control potentiometer 129 to which it.