tube it is then inputted into the two output end filter circuits 44 and at last the signal is outputted from the left and right output ends 26 28 the class a current amplifier 14 has no effect of voltage amplification and its other characteristics include large inputting impedance low outputting impedance with the input signal and the output signal being in phase the signal amplifier circuit 11 can further comprise a voltage amplifier cascading with the class a current amplifier 14 to obtain advantages of both a voltage amplifier and a current amplifier a heating voltage of the filament electrode 58 of the double triode 40 is 6 3v and is supplied by the filament heating circuit 18 an operating voltage of the plate electrode 52 of the double triode 40 is 115v which is supplied by the voltage booster circuit 16 capacitors of the two input end filter circuits 44 and 46 and grounded resistors connected to the cathode 54 of the double triode 40 can be adjusted according to user preferences adjusting the capacitors of the input end filter circuits 44 and 46 allows for changing frequency response of the filter circuits 44 and 46 adjusting the grounded resistors connected to the cathode.