the 100 kilohertz pulse rate of the clock 92 combined with the 32 bit counting capacity of counter 89 results in a pulse width modulated signal of approximately 3 kilohertz frequency on line 51 to the digital to analog converter 52 while a preferred manner of sensing a reverse feed signal has been described other methods also suggest themselves thus logic circuits may be devised and implemented which are responsive to an absence of forward feed or zero feed which are characterized by a binary 0 or low voltage in the msb 85 also logic circuits may be devised and implemented which are responsive to specific code words for reverse feed referring to fig5 a power supply circuit 100 is indicated which may be connected to the ac house mains via a transformer not shown supplying 12 volt 60 hertz to the power supply the 12 volt ac supply undergoes full wave rectification and filtration to provide u00b1 15vdc to the power amplifiers and also to provide through voltage regulator 74 u00b1 7 5 vdc to the bight and feed position potentiometers 68 39 and 68 respectively and to manual balance control potentiometer 75 as well as u00b1 7 5 vdc to the digital to analog offset voltage dividers 101 and 101 39 in the digital to analog converters 52 and 52 39 for feed and bight respectively see also fig1 though not shown the power supply 100 also provides u00b1 7 5 volts dc to lsi 50 as previously stated all the bight components.