Playback amplifier with high interference immunity

A method of improving the interference immunity of a playback amplifier is proposed which has already been successfully applied to the Mayaka-202 portable tape recorder. The input stage of the amplifier, consisting of a linear one-transistor circuit and an operational amplifier with frequency-dependent negative feedback, is placed separately from the next stages in the immediate vicinity of the playback head. It may even not be necessary to shield the printed-circuit board which carries the amplifier input stage alone, shielding being necessary when all amplifier stages are mounted on one printed-circuit board. The interstage connecting cable is now 300 mm long but carries a 15 to 25 mV signal rather than a 0.3 to 0.5 mV one so that the relative pickup level is reduced by a factor equal to the transistor gain, while the cable to the playback head is now only 30 to 40 mm long so that the interference level drops by as much as 14 dB. The method is equally applicable to tape recorders with feedthrough channel and to plain tape phonographs.