TAPE HISS REDUCTION CIRCUIT
 
    The circuit in Fig. 1. is used to either boost or cut frequencies. When making a recording, point X is wired to point R so that treble signals are boosted by 10dB, and then during playback, point X is wired to point P so that the signal from the tape, including the hiss, has the treble cut by an equivalent amount.
    
The circuit values are such that the overall frequency response, from record through playback, is fiat over the range 2OHz—2OkHz. Thus the output signal after playback is identical with the input signal before recording, but the hiss is cut by 10dB.
    RV1 sets the gain of the circuit to be unity at low frequencies (<500Hz); RV2 is adjusted so that the collector voltage of 03 is half the positive rail voltage. When this is set, the circuit will function without apparent distortion with an input voltage ot upto 1.5V r.m.s.
    If monitoring during record is not required, the same circuit may be used for record and playback, with X switched between P and R as necessary.
    If monitoring during record is required, two circuits are needed, one with X wired to A and the other with X wired to P.
    For stereo, two circuits are required.